Locating and clamping play pivotal roles in any workholding system. Accurate positioning of the workpiece is crucial for precision machining. Carr Lane Mfg. delves into the essential principles of locating and clamping, offering valuable insights into achieving precise and consistent results.
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Workholders must position the workpiece accurately and consistently relative to the cutting tool. Using locators correctly is vital for this purpose. These tools ensure proper referencing of the workpiece, making the process repeatable and successful.
Referencing involves positioning the workpiece relative to the workholder and the cutting tool through guiding devices. Achieving accurate placement within the workholder is crucial for machining precision.
Designing a workholder requires considering the referencing of both the workpiece and the cutter to ensure accurate machining.
Repeatability, the workholder’s ability to consistently produce parts within tolerance, relies on precise referencing capabilities. The ideal locating point is a machined surface for consistency.
“Referencing“ is a dual process of positioning the workpiece relative to the workholder and the workholder relative to the cutting tool. This process is performed by guiding or setting devices.
With drill jigs, referencing uses drill bushings. With fixtures, referencing uses fixture keys, feeler gages, or probes. On the other hand, referencing the workpiece to the workholder is done with locators.
A workpiece has twelve degrees of freedom that must be restricted for proper referencing. Sturdy enough to resist cutting forces, Locators play a crucial role by providing a positive stop for the workpiece, ensuring accurate machining.
As shown in Figure 3-1, the twelve degrees of freedom all relate to the central axes of the workpiece, including six axial degrees of freedom and six radial degrees of freedom.
The axial degrees of freedom permit straight-line movement in both directions along the three principal axes, shown as x, y, and z. The radial degrees of freedom permit rotational movement around the same three axes in both clockwise and counterclockwise radial directions.
Figure 3-1. The twelve degrees of freedom.
We categorize location into three forms: plane, concentric, and radial. Each form has its specific application, with plane locators working from any surface, concentric locators focusing on a central axis, and radial locators restricting movement around a central point.
In most applications, plane-locating devices locate a part by its external surfaces, as shown in Figure 3-2a.
Concentric locators primarily locate a workpiece from a central axis, though this axis may or may not be in the center of the workpiece. The most common type of concentric location is a locating pin placed in a hole.
Some workpieces, however, might have a cylindrical projection that requires a locating hole in the fixture, as shown in Figure 3-2b.
The third type of location is radial, and as seen in Figure 3-2c, the radial locators restrict the movement of a workpiece. In many cases, locating is performed by combining these three locational methods.
Figure 3-2. The three forms of location are plane, concentric, and radial.
Locating from flat surfaces uses solid, adjustable, and equalizing supports to position the workpiece accurately. The 3-2-1 locational method is a common approach, using six locators to reference and restrict the workpiece’s movement effectively.
Figure 3-3. Solid, adjustable, and equalizing supports locate a workpiece from a flat surface.
Solid Supports are fixed-height locators that precisely locate a surface in one axis. Though solid supports may be machined directly into a tool body, a more economical method can be accomplished with installed supports such as rest buttons.
Adjustable supports are variable-height locators. Like solid supports, they will also precisely locate a surface on one axis. These supports are used where workpiece variations require adjustable support to suit different heights and are mainly used for cast or forged workpieces with uneven or irregular mounting surfaces.
Equalizing supports are a form of adjustable support used when compensating support is required.
Although these supports can be fixed in position, equalizing supports typically float to accommodate workpiece variations. As one side of the equalizing support is depressed, the other raises the same amount to maintain part contact.
Figures 3-4 show three locators, or supports, placed under the workpiece. The three locators are usually positioned on the primary locating surface. This positioning restricts axial movement downward, along the z-axis (#6) and radially about the x (#7 and #8) and y (#9 and #10) axes. Together, the three locators restrict five degrees of freedom.
Figure 3-4. Three supports on the primary locating surface restrict five degrees of freedom.
The following two locators are usually placed on the secondary locating surface, as shown in Figures 3-5. They restrict an additional three degrees of freedom by arresting the axial movement along the +y axis (#3) and the radial movement about the z (#11 and #12) axis.
Two additional locators are placed on the secondary locating surface, as shown in Figure 3-5, to restrict an additional three degrees of freedom by arresting the axial movement along the +y axis (#3) and the radial movement about the z (#11 and #12) axis.
Figure 3-5. Adding two locators on a side restricts eight degrees of freedom.
The final locator, shown in Figure 3-6, is positioned at the end of the part, restricting axial movement in one direction along the –x-axis. Together, these six locators restrict nine degrees of freedom. The remaining three degrees of freedom (#1, #4, and #5) will be restricted by clamps.
Figure 3-6. Adding a final locator to another side restricts nine degrees of freedom, completing the 3-2-1 location.
Locating from an internal diameter is efficient, using locating pins and plugs based on the feature’s maximum material condition (MMC). This method ensures accurate positioning by restricting movement in specific directions.
The two forms of locators used for internal location are locating pins and locating plugs. The only difference between these locators is their size: locating pins are used for smaller holes, and locating plugs are used for larger holes.
Figure 3-7 shows that the plate under the workpiece restricts one degree of freedom, preventing any axial movement downward along the -z (#6) axis. The center pin, acting in conjunction with the plate as a concentric locator, prevents any axial or radial movement along or about the x (#1, #2, #7, and #8) and y (#3, #4, #9, and #10) axes.
Together, these two locators restrict nine degrees of freedom. The final locator, the pin in the outer hole, is the radial locator that restricts two additional degrees of freedom by arresting the radial movement around the z (#11 and #12) axis. Together, the locators restrict eleven degrees of freedom. The last degree of freedom, in the +z direction, will be restricted with a clamp.
Figure 3-7. Two mounted on a plate restrict eleven out of twelve degrees of freedom.
Understanding the direction and magnitude of machining forces is essential for fixture layout. Analyzing these forces helps design workholders that properly direct cutting forces and ensure precise machining.
In Figure 3-8, the milling forces push the workpiece toward the solid jaw when clamped adequately in a vise. The clamping action of the movable jaw holds the workpiece against the solid jaw. It maintains the position of the part during the cut.
Figure 3-8. Cutting forces in a milling operation should be directed into the solid jaw and base of the vise.
As shown in Figure 3-9, cutting forces push the workpiece down against the supports in the drilling process. There’s also a force that pushes sideways against the drill, which helps keep the workpiece in place against the locators. The clamps are there to make sure the workpiece stays put during drilling. The most significant impact happens when the drill exits the other side of the workpiece. If the workholder is designed right, these forces help keep the workpiece stable for accurate drilling.
Figure 3-9. The primary cutting forces in a drilling operation are directed downward and radially about the axis of the drill.
When designing fixtures, predicting the strength and direction of cutting forces on the workpiece is critical. This prediction can be an educated guess from experience or a data calculation. A basic formula, seen in Figure 3-10, helps determine the force based on how the cutting tool interacts with the workpiece.
Please note: “heaviest-cut horsepower“ is not total machine horsepower but the maximum horsepower used during the machining cycle. Typical machine efficiency is roughly 75% (.75). The number 33,000 is a units-conversion factor.
Figure 3-10. A simple formula to estimate the magnitude of cutting forces on the workpiece.
The above formula only calculates force magnitude, not direction. Cutting force can have x-, y-, or z-axis components. Force direction (and magnitude) can vary drastically from the cut’s beginning, middle, and end. Figure 3-11 shows a typical calculation. Intuitively, force direction is virtually all horizontal in this example (negligible z-axis component). Still, the direction varies between the x and y axes as the cut progresses.
Figure 3-11. Example of a cutting force calculation.
The main job of a locator is to reference the workpiece and guarantee consistent results. To do this effectively, correctly positioning locators is crucial. Here are the key points to remember:
Always try to place locators on the machined surface of the workpiece. Machined surfaces ensure consistent and stable positioning.
Depending on the workpiece, you might use the entire machined surface or specific areas. Machined holes are particularly effective for positioning since they require fewer locators and provide the most accurate location.
Another reliable setup uses two machined surfaces that form a right angle, ideal for the six-point location method. The most crucial factor in choosing a surface for location is its ability to provide consistent results.
Spacing between locators is critical for accuracy. You should place locators as far apart as possible to avoid inaccuracies and ensure stability. Figure 3-12 shows two workpieces positioned using the six-point method, highlighting the importance of locator spacing. The piece shown in part (b) has locators too close to each other, leading to poor positioning. In contrast, part (a) shows locators well-spaced, achieving accurate placement. Wide spacing helps compensate for any irregularities in the locators or the workpiece, ensuring maximum stability.
Figure 3-12. Locators should be spaced as far apart as practical to compensate for slight irregularities and to achieve maximum stability.
Figure 3-13 illustrates issues arising from placing locators near each other. When the center positions of the locators are off by just .001 “, the impact on the part’s location can vary significantly based on their spacing. In the scenario depicted in (a), this slight misalignment barely affects the part’s position due to adequate spacing between the locators.
However, when locators are positioned as closely as shown in (b), even a minor .001 “ difference can significantly impact the accuracy of the part’s location. Another challenge is when locators are positioned too closely, as demonstrated in (c). This close spacing can cause the part to wobble on the locators within the workholder, leading to instability and inaccurate machining.
Figure 3-13. Positioning locators too close together will affect the locational accuracy.
When placing locators, the last thing to consider is dealing with chips, which are always part of machining. Keeping chips from messing up how the workpiece fits in the holder is essential.
You can manage chips by placing locators without many chips or making locators that lessen chip impact. If neither is possible, use locators designed to be easy to clean, or that can keep chips away by themselves. Figures 3-14 shows different strategies for making locators less affected by chips.
Figure 3-14. Locators should be relieved to reduce locational problems caused by chips.
Coolant build-up can also cause problems with machining accuracy and location. Solve this problem by drilling holes or milling slots in areas of the workholder where the coolant is most likely to build up. With some workholders, coolant-drain areas can also act as a removal point for accumulated chips.
When designing a workholder, always try to minimize the chip problem by removing areas of the tool where chips can build up. Omit areas such as inside corners, unrelieved pins, or similar features from the design. Chip control must be addressed in the design of any jig or fixture to achieve stable and accurate machining.
When creating a workholder, it’s important to avoid using redundant locators, which means not using multiple locators to control the same movement. Doing this can cause problems with the part's position, affect the machining accuracy, and lower overall efficiency. It can also lead to damage from too much pressure on the part or the fixture itself.
Figures 3-15 gives examples of this. Part (a) shows that using two locators on flat surfaces that do the same job isn’t necessary. Because parts vary slightly in size, they will unlikely touch both locators simultaneously.
Part (b) highlights a similar issue with using two locators around a circle – you should only need one.
Part (c) illustrates the problem of using both hole and edge locators simultaneously, which can cause confusion and make it hard to place or remove parts properly.
Figure 3-15. Examples of redundant location.
A general rule of thumb is always to avoid redundant location. The simplest way to eliminate it is to check the shop print to find which workpiece feature is the reference feature.
How a part is dimensioned often indicates which surfaces or features are essential. As shown in Figure 3-16, since the part on the left is dimensioned in both directions from the underside of the flange, use this surface to position the part. However, the part shown to the right is dimensioned from the bottom of the small diameter. This is the surface that should be used to locate the part.
Figure 3-16. The best locating surfaces are often determined by how the part is dimensioned.
Foolproofing prevents improper loading of a workpiece, which is more prevalent with symmetrical or concentrated parts. The simplest way to foolproof a workholder is to position one or two pins in a location that ensures correct orientation, as shown in Figure 3-17. With some workpieces, however, you must take more creative approaches to foolproofing to ensure the workpiece can be loaded correctly.
Figure 3-17. Foolproofing the location prevents improper workpiece loading.
Figure 3-18 shows ways to foolproof part location. In the first example, shown at (a), an otherwise –non-functional foolproofing pin ensures proper orientation. This pin would interfere with one of the tabs if the part were loaded any other way, ensuring it is loaded correctly.
In the following example, shown in (b), a cavity in the workpiece prevents the part from being loaded upside-down. Here, a block slightly smaller than the opening of the part cavity is added to the workholder. An adequately loaded part fits over the block, but the block keeps an improperly loaded part from entering the workholder.
Figure 3-18. Simple pins or blocks are often used to foolproof the location.
One method to help ensure the accurate location is the installation of spring-loaded buttons or pins in the workholder, as seen in Figure 3-19. These devices are positioned so their spring force pushes the workpiece against the fixed locators until clamped, ensuring repeatable locating and streamlining the clamping process.
Figure 3-19. Spring-loaded locators help ensure the correct location by pushing the workpiece against the fixed locators.
To choose the right locator size, look at the workpiece’s features. Use the Maximum-Material Condition (MMC) principle, which is the thickest part of an external or the smallest part of an internal feature. For instance, if a hole’s diameter can be between .500 and .510 inches, aim for the smallest size (.500 inches) to ensure the locator fits well.
Make the locator a bit smaller to fit comfortably. If a hole is .500 inches at MMC, a locator pin might be . inches for a slight gap, ensuring it’s neither too tight nor too loose. Figures 3-20 and 3-21 provide examples of applying MMC to both types of features and selecting locator sizes.
Figure 3-20. Locator sizes are always based on the workpiece features' maximum material condition (MMC).
Figure 3-21. Determining the size of a single locating pin based on maximum-material conditions.
The accuracy of a workholder needs to be higher than that of the workpiece. There are two main tolerances for locators: one for the locator’s size and another for its position. Different methods can set these tolerances, sometimes based on engineering decisions or specific measurements related to the part.
Making tolerances tighter increases production costs significantly. For example, making a tolerance twice as strict could quintuple the cost. The ability to achieve these tolerances, or manufacturability, is also crucial. Depending on how strict the tolerance is, the method to create a hole, for instance, can range from punching to drilling, reaming, and even lapping for the most precise measurements.
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It’s important to remember that not all tolerances can be met by every toolroom; some are too precise to achieve in practice. Not every part of a workholder needs the same level of tolerance. Some parts can have looser tolerances without affecting the overall function.
Using percentage-based tolerances can help maintain a balanced relationship between the workpiece and workholder tolerances. Setting workholder tolerances as a percentage of the workpiece tolerances ensures consistency.
Typically, these percentages range from 20% to 50% of the workpiece’s tolerances, as determined by engineering standards. This approach ensures that tolerances are proportional and manageable.
Locating the workpiece is the primary function of a jig or fixture. Once located, the workpiece must also be held to prevent movement during the operational cycle. The process of holding the position of the workpiece in the jig or fixture is called clamping. The primary devices used for holding a workpiece are clamps. Careful selection of the clamping devices and their location is essential for proper performance.
Selecting the proper clamps for a job involves understanding their two leading roles: keeping the workpiece in place against the locators and stopping it from moving. Locators are designed to handle the main cutting forces during machining, not the clamps.
Clamps focus on keeping the workpiece steady and dealing with any forces that try to move it when cutting tools exit the material.
For instance, the drill’s main forces push down and sideways when drilling, but as the drill comes out on the other side, it can lift the workpiece. Clamps must be strong enough to prevent this movement without having to counteract the main drilling forces.
A milling machine vise shows how locators and clamps work together. The vise’s solid parts act as locators, absorbing the cutting forces, while the movable part is the clamp, securing the workpiece in place. This setup ensures machining accuracy by keeping the workpiece stable and correctly positioned throughout the operation.
Figure 3-22. A vise contains both locating and clamping elements.
Consider the vibrations, stress, and forces they’ll face when choosing clamps. Clamps need to keep the workpiece steady without loosening during operation. Adding a safety margin to the forces helps ensure stability.
It’s crucial to pick clamps that won’t damage the workpiece. Too much force can bend or mark it. Use clamps with rotating pads or softer materials to avoid damage and ensure they’re strong enough without causing harm.
The efficiency of loading and unloading the workpiece also matters. Quick clamping actions help maintain the workholder’s profitability by minimizing downtime.
Proper clamp placement is critical. Clamps should secure the workpiece firmly to the locators without changing its shape. They should absorb any secondary forces without affecting the workpiece’s position. Position clamps over strong support points to avoid deformation, as shown in Figures 3-23 and 3-24.
Figure 3-23. Clamps should always be positioned to direct the clamping force into the supports or locators.
Figure 3-24. The workpiece and its supports determine the number and position of clamps.
Machine operation should not interfere with clamps. Ensure clamps don’t block or collide with machine parts during the machining cycle. Adjusting the cutter’s path can prevent collisions.
Low-profile clamps, like gooseneck clamps shown in Figure 3-25, help keep everything compact. Small contact areas increase pressure efficiency and reduce interference.
Figure 3-25. Using gooseneck clamps is one way to reduce the height of the clamps.
Always clamp the clamping force directly towards the locators or the strongest part of the workholder to prevent distorting the workpiece. Avoid clamping methods that put undue pressure on delicate parts, as shown in Figures 3-26 to 3-29. Use techniques like strap clamps or collets to distribute force evenly and prevent damage.
Figure 3-26. Directing the clamping forces against an unsupported area will cause this cylindrical part to deform.
Figure 3-27. eliminate deformation by directing the clamping forces into the supports under the part.
Figure 3-28. Part features such as holes can be used to clamp the part when possible.
Figure 3-29. When the part can only be clamped on its outside surface, pie-shaped chuck jaws can hold it and reduce deformation.
Determining the necessary clamping force can be a complicated calculation.
In many situations, an approximate determination of these values is sufficient. The table in Figure 3-30 shows the available clamping forces for a variety of different-size manual clamp straps with a 2-to-1 clamping-force ratio.
Figure 3-30. Approximate clamping forces of different-size manual clamp straps with a 2-to-1 clamping-force ratio.
Alternatively, we can determine the required clamping force based on calculated cutting forces. A simplified example is shown in Figure 3-31. The cutting force is entirely horizontal, and no workpiece locators are used, so frictional forces resist the cutting forces.
Figure 3-31. A simplified clamping-force calculation with the cutting force entirely horizontal and no workpiece stops.
When workpiece locators and multi-directional forces are considered, the calculations become more complicated.
The worst-case force situation can be estimated intuitively and treated as a two-dimensional static mechanics problem (using a free-body diagram) to simplify calculations. In the example shown in Figure 3-32, the cutting force is known to be lbs based on a previous calculation. The workpiece weighs lbs. The unknown forces are:
F R = Total force from all clamps on the right side
FL = Total force from all clamps on the left side
R 1 = Horizontal reaction force from fixed stop
R 2 = Vertical reaction force from fixed stop
R 3 = Vertical reaction force on the right side
N = Normal force = FL + FR +
µ = Coefficient of friction = .19
Figure 3-32. A more complicated clamping-force calculation using a two-dimensional free-body diagram.
The equations below solve for unknown forces, assuming that for a static condition:
At first glance, the example above looks “statically indeterminate, “ i.e., there are five variables and only three equations. But for the minimum required clamping force, R3 would be zero (workpiece barely touching), and FL would be zero (there is no tendency to lift on the left side). Now, with only three variables, we can solve:
Solving for the variables,
FR = lbs
R1 = lbs
R2 = lbs
In other words, the combined force from all clamps on the right side must be greater than lbs. With a recommended safety factor of 2-to-1, this value becomes lbs. Even though FL (combined force from all the clamps on the left side) equals zero, a slight clamping force may be desirable to prevent vibration.
Another general area of concern is maintaining consistent clamping force. Manual clamping devices can vary in the force they apply to parts during a production run. Many factors account for the variation, including clamp position on the workpiece, but operator fatigue is the most common fault. The simplest and often best way to control clamping force is to replace manual clamps with power clamps.
The force generated by power clamps is constant and adjustable to suit workpiece conditions. Another benefit of power clamps is their speed of operation: not only are individual power clamps faster than manual clamps, but every clamp is activated simultaneously, ensuring consistency and a secure hold.
We’ll explain some of the most impactful equipment available for supporting multi-axis work and shares advice on how to best match it to your new machine and the work you plan to do. Learn different ways to optimize tool and workpiece setups, maximize spindle uptime and preserve performance.
Maximize Your ROI: Expert Tips for Efficient 5-Axis Machine Operation
What we’ll cover
The purchase of a 5-axis machining center, or even one that works on multiple axes, is a big deal for any metalworking operation. Apart from the financial commitment, it changes how jobs move through the shop and the way people work—and there are a lot of adaptors out there right now. According to the Association for Manufacturing Technology (AMT), orders for 5-axis machining centers grew 22 percent year over year, in the midst of COVID. Machine shops represent nearly half of the buyers, followed by aerospace and transportation sectors, which tend to invest in larger 5-axis machines. To get the most out of such a significant investment, making smart decisions when choosing the equipment that supports the new machine, and the team using it, goes a long way. We’ll explain some of the most impactful equipment available for supporting multi-axis work and share advice on how to best match it to your new machine and the work you plan to do. Read on to learn different ways to optimize tool and workpiece setups, maximize spindle uptime and preserve performance.
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Setting up tools
In order to realize the return on your investment as soon as possible, a multi-axis spindle needs to be spinning as much as possible. In other words, measuring, adjusting, inspecting or any general fidgeting with tools is best done away from the machine while the machine continues making chips. One of the first things a shop needs to maximize 5-axis performance is accurate 3D models of every tool holder for your CAD system. The tooling assembly has to be modeled as close as possible to how it will be put into the machine—body diameters, gage lengths, etc. There is so much movement and rotation that the chances of a collision are real and costly. The problem is, not all STP files are created equal. Imprecise models aren’t uncommon, especially for cheaper tools. Not only are these difficult to simulate accurately, but it makes adjusting and verifying more time consuming; worst case scenario, the programmer fingers through a catalog and draws the tools himself. Accessibility can also be an issue—how easy are the files to access and acquire? As with tool quality, you get what you pay for when it comes to models. Carefully consider model accessibility and accuracy before purchasing and consider ease of use on future projects. With models and tool lists done, the next important step is accurate measuring and verification of the predefined tool specs. The simultaneous actions of 5-axis machining require more strict control of gage lengths, as measured from the face of the spindle and body diameter. This is where offline presetters become indispensable.
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Setting up tools Tap into digital for help
Using a ruler with 1/16 increments, trusting every spec or using inconsistent in-machine probes are not effective ways to measure when a couple thousandths of inch can make a difference. Tool presetters serve primarily as measuring systems for verifying tool geometries match the programmer’s specs—offsets, body diameters, etc.—offline before putting them into a machine’s spindle. This essentially eliminates any of this work at the machine tool, so that valuable multi-axis spindles can work while verification is done at the presetter. Depending on how and where you prefer to perform setups, there are a variety of presetter options. Vision-based presetters allow for much more than just verification. They are the perfect place to assemble tools and compare directly to the catalog, DXF or STP model that was given to the programmer. Last but not least, consider the tools themselves when thinking about setups. Digital heads are a great option for efficient 5-axis tooling setups. Their on-tool digital readout makes it easy for operators to see and make extremely precise diameter adjustments in just about any situation—even if it must be done in the spindle. A corresponding mobile app allows you to look up tool speeds and feeds on the spot. The next generation of digital boring heads is already on the way. These allow for more interactivity and control via an app for even faster setups and data mining.
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Holding parts
Most of the 5-axis machines come with a round, square, or rectangular table or plate. They don’t, however, come standard with quick-change workholding ready to go. There are different bolt hole patterns and pins used to locate and stabilize the workpiece. Having to match these locations with each different workpiece adds another cumbersome step in a process that needs to be closely controlled—before cutting even starts. Add the fact that 5-axis machining is rarely used for processing the same part over and over, and the right workholding can be a difference maker.
Specifically, the ability to use a zero-point system as a primary datum locator for all workholding, regardless of if it’s a vise, magnetic clamping or a dedicated fixture, is indispensable for setup reduction.
While zero-point workholding was initially developed and has long been used for setup-time reduction, the system has had great success in 5-axis machining for another distinct reason: access. A standard vise will block the sides of a part and low-profile vises leave sides more open, but grips only along the part’s lower edge. Alternatively, the option to use a zero-point system to clamp exclusively on the underside of the part leaves the workholding hidden—concealed from all these interferences by the workpiece itself—for a freer state of machining. Pulling a part down is also better than squeezing it, because the squeezing process puts stress on the raw material and the material removal process can compound the negative effects of this stress. What’s more, the closer the part is to the table, the higher the risk of collision with the tool or tool body. Solutions include using a longer holder, which only tickles the part, or raising the part off the table. There are many dedicated workholding solutions dedicated to this, even in confined work zones. 5-axis table adapters, for example, allow 5-axis base chucks to be mounted anywhere they are needed to access the table T-slots or grid holes, all while providing a strong foundation. Another example, the Uniflex System, is made up of a clamping ball and collar that are attached to the underside of the part or fixture. The part and fixture are then lowered on to either a clamping base or a clamping extension. The clamping collar is then rotated to tighten the six bearing balls on to the main ball. If the part or fixture is warped or needs to be set at an angle, the clamping ball can pivot up to 15° in any direction.
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Holding parts Choosing the right workholding
The benefits of 5-axis machines are greatest when workholding is chosen based on the size and shape of the workpiece instead of the limitations of the table interface. Now, some quick advice on choosing the right workholding for a new 5-axis machine. Find a balance between stability and access. The fixture may be large and provide rigid holding but may limit access to the workpiece—and vice versa. If a part is too small for your fixture, consider palletizing multiple parts. If a part is too big for your fixture, consider upgrades if you have a modular system, or add custom pieces, if possible. Know what parts are coming down the line. If the parts are prismatic, most often a vise will do just fine. If the part is round, you’ll need to hold onto an outer or inner diameter on the part. Whether the part will be raw material, a casting or partially finished is also an indicator what the workholding will have to accommodate. For example, a round raw bar from the mill may have an OD tolerance of ±.005”, but if a part is cast, then the size may be ±.02”—could be better, could be worse. If a part has already been machined, then the size is not a variable—it’s a known controlled value. Choose the right chuck and lay it out carefully. Depending on the material of the fixture plate, the part or fixture should not extend too far beyond the diameter of the chuck. With a steel plate that’s at least 20 mm thick, you can generally go 40 mm off diameter without losing stability. If it’s an aluminum plate 25 mm or thicker, 20 mm off the diameter should be about the limit. Thicker plates are an option, but material costs start getting a little higher than most customers want to bear. When it comes to multiple chucks, plate material and thickness are key. General guidelines for maintaining accuracy and rigidity: if the diameter is less than a 138 mm chuck, spacing should max out at about 200 mm; for bigger diameters, we recommend a maximum spacing of about 350 mm.
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The machining process
A common misconception is to always look first to square or rectangular billets for parts that will ultimately resemble that shape when finished. Sure, in most cases this will save a few minutes, but, because of all the movements that you can make accessing five sides, there are more options. Bar stock is less expensive, more readily available and easier to hold than more frequently used billets. And yes, while there will be slightly longer machining time, the material savings and ease of access will far outweigh the cost in the big picture. The right mill, with an HSK or BIG-PLUS interface, will allow very aggressive material removal. A shorter stickout helps too. Indexable mills are usually longer once assembled because a straight shank must go into another tool holder. Alternatives like our Fullcut Mill are capable of high performance in ramping, helical-, shoulder- and plunge-milling operations. Sharing a similar compact design, the C-Cutter Mini can quickly finish jobs—chamfer, back chamfer and even some light face milling. Often times with a standard solid carbide end mill, you’ll see a helix cutting tool used in the Z-direction to get down to a specific height, and then it’s moved in the X and Y to make a pocket. With a Fullcut Ramping Mill, you’re able to move simultaneously in the X, Y and Z directions in order to ramp down and around the workpiece, rather than having a whole series of steps that are then followed by X and Y movements.
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The machining process Reaching difficult pockets: hydraulic or shrink fit?
The ultimate goal is to make every tooling assembly as short as possible, 5-axis machine or not. The shorter the tooling, the less room for error. That said, the unusual angles, sizes and shapes of the parts that require multiple axes don’t always allow for that. We’ve talked about some workholding solutions for this challenge, but you have options in tooling selections too. The choice often comes down to hydraulic chucks and shrink-fit holders. Compared to collet chucks, shrink-fit holders and hydraulic chucks have a smaller nose diameter relative to the tools they hold. This is critical, as these holders allow a further reach into mold cavities or other tricky work envelopes without interference. Let’s quickly explore what to consider when choosing between the two holder types.
Cost – When it comes to the holders themselves, shrink-fit is generally a slightly lower cost. Where the major difference in expense lies is in the equipment needed to heat shrink-fit holders.
Maintenance – Because shrink-fit heating temperatures can approach 600° Fahrenheit, we recommend using dry cutting tools without oil on them. From there, diligent attention must be paid to the holder bores and tool shanks. Any contamination will be baked onto the metal and progressively deteriorate performance. Hydraulic chuck maintenance is straightforward as long as the hydraulic chamber stays sealed. Training, handling and safety – Hydraulic chucks are infinitely simple. A turn of a wrench locks the tool in place. When it comes to shrink-fit systems, there are a few more factors to consider when getting the team up to speed, including safety considerations. Setup – Hydraulic chucks use a simple wrench to lock in the tool, providing the option to swap tools at the machine or offline. Shrink-fit setups must be done exclusively offline where the heating and cooling can be powered. Most heating cycles can be as fast as 15 seconds. Vibration – Potential out-of-balance issues due to a hydraulic holder’s one-sided set screw design are a worry of the past. A good-quality hydraulic chuck has some natural damping characteristics that run every bit as true as a shrink-fit holder, with more consistent clamping tolerances and forces over the life of the holder. That’s not to say shrink-fit holders are ineffective in terms of vibration management. Their runout is five times better than side-lock holders.
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More than 150 machine tool builders are licensed to use and make BIG-PLUS on original equipment, so there’s a good chance a new 5-axis machine is built to achieve its peak performance with BIG-PLUS tooling. If you aren’t sure about your machine’s interface, the easiest way to figure it out is to place a standard tool into the spindle and see how much of a gap there is between the tool holder flange face and spindle face. Without BIG-PLUS, the standard gap should be visible, or about .12”. If it is BIG-PLUS, the gap is half of this amount, or only .06”. These values change depending on 30 taper, 40 taper or 50 taper sizes, but the gap is visibly less than usual. For those who don’t know, dual contact refers to the shank contacting the spindle taper and the spindle face simultaneously. The torque (or moment) exerted by the cutting forces is maximized at the point where the holder and spindle meet. When the holder contacts the spindle face via BIG-PLUS, the effective diameter would be the larger diameter of the v-flange, since this is the new anchoring point of the holder and spindle. You are essentially beefing up the diameter at the point where the reactionary force is greatest. The truth about dual-contact tooling
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All BIG-PLUS tools are dual contact, but not all dual-contact tools are BIG-PLUS—and this really matters when choosing tooling. Only a licensed supplier of BIG-PLUS has master gages that are traceable to the BIG DAISHOWA grand master gages and have the dimensions and tolerances provided to do it right. Non-licensed dual-contact tool makers are guessing or using a sample BIG-PLUS tool holder as their own master gage—a practice that any quality expert will advise against. Unless the tools are marked “BIG-PLUS Spindle System-License BIG DAISHOWA SEIKI,” the use of tooling not made by BIG DAISHOWA or its licensees may result in inconsistent performance or damage to the machine. So, what are the consequences of using unlicensed dual-contact tooling on an expensive 5-axis spindle? If the distance between flange face and gage line diameter are more than specification, little or no face contact occurs; tool holders provide only taper contact and no benefit of BIG-PLUS. If the distance between flange face and gage line diameter are less than specification, there’s face contact only; tool holders float in spindle taper with no positive radial location. Large cutter runout and fretting corrosion on spindle face occurs immediately. Severe spindle damage can occur.
If the gage line diameter is less than the specification, there will be face contact with only minimal or no taper contact. Severe spindle damage can occur.
If the gage line diameter is more than the specification; there will only be taper contact, eliminating all benefits of BIG-PLUS.
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The quickest path to ROI
A 5-axis machine is not purchased in a vacuum. The addition will have ripple effects across a business that need to be prepared for. You can’t simply assume that because a 5-axis machine is a large investment that it will make up for every operator error or up/downstream inefficiency. Similarly, don’t assume the way things were done on an older machine is the best way. Getting creative and seeking out the inventive solutions designed uniquely for 5-axis work can accelerate all-important ROI; choosing the equipment to support the machining center will empower the team to succeed and unleash the full capabilities of the business.
How can we help?
Visit us at www.BIGDAISHOWA.com
Explore our High-Performance Tooling Solutions catalog here.
Find a local representative, get a quote or ask us anything here.
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If you want to learn more, please visit our website Quick Change Fixturing Solutions.
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