CNC Workholding 101: Vises, Clamps, Vacuum Tables, and Fixtures

The part moves 0.001 inch and the whole job is scrap. I have seen this happen more times than I can count. The G-code was perfect, the feeds and speeds were optimal, and the tool was sharp. But the workholding was not up to the job and the part shifted mid-cut. Workholding is the foundation of every CNC operation. You can have a perfect program and a top-of-the-line machine but if the part is not held securely nothing else matters.

Workholding refers to anything that holds your workpiece in place during machining. It might be a vise bolted to the table, a vacuum pulling from below, clamps pressing from above, or a custom fixture shaped specifically for your part. Bad workholding costs you in three ways. First is scrap from shifted parts that are no longer within tolerance. Second is poor surface finish from vibration caused by insufficient clamping rigidity. Third is lost cycle time because you have to cut less aggressively when the part is not secure.

Getting workholding right is the single highest-leverage improvement you can make in your shop. It costs less than a new spindle and often makes more difference in part quality. This guide covers the four main methods so you can choose what fits your parts and budget.

CNC Vises

Category	Example	Purpose
Type A	Example 1	Purpose 1
Type B	Example 2	Purpose 2
Type C	Example 3	Purpose 3

If you are machining rectangular metal parts a vise is your default workholding. It is fast, repeatable, and clamps with predictable force. The workhorse of most shops is the CNC precision vise. Brands like Kurt, Chick, and Orange dominate this category for good reason. They deliver consistent clamping force with minimal jaw lift and excellent repeatability.

A 4-inch Kurt VersatileLock delivers 1,050 to 7,500 pounds of clamping force depending on how hard you torque it. At 10 foot-pounds of torque you get about 1,050 pounds of force. At 60 foot-pounds you get the full 7,500 pounds. A 6-inch vise handles most job-shop work and an 8-inch vise with hydraulic assist can deliver over 11,000 pounds for heavy production work.

There are several types of vises for different situations. A standard machinist vise with an Acme screw is fine for light work and drilling but lacks the precision needed for production milling. A CNC precision vise with 0.0005 inch repeatability is essential for any shop doing tolerance work. Hydraulic vises deliver consistent clamping force automatically for high-volume production. Pneumatic vises allow quick-change setups between jobs using shop air. Self-centering vises handle round or symmetrical parts by centering them automatically. Five-axis vises have a low profile design that avoids collisions during multi-sided machining.

Hard jaws are the serrated steel jaws that come standard with most vises. They grip well but leave marks on the workpiece surface. Use them for roughing operations or when the surface finish on the clamped area does not matter. Soft jaws are machinable aluminum or steel blanks that you cut to match your part profile exactly. They distribute clamping pressure evenly across the part surface, prevent marring, and let you grip irregular shapes that hard jaws cannot hold securely.

To make soft jaws correctly follow these steps. First bolt the soft jaw blanks to your vise and tighten them securely. Second face the top surface of both jaws so they are perfectly parallel and at the same height. Third grip a spacer between the jaws that is the same width as your part. The spacer ensures the jaws open to the correct width after machining. Fourth machine the negative profile into the jaws using a contour toolpath that matches your part shape. Cut the profile about 0.001 to 0.002 inch deeper than the jaw height. Fifth remove the spacer. The profile now matches your part exactly and the jaws will clamp evenly.

A good rule of thumb for clamping force is that your clamping force should be at least three to five times the maximum cutting force your tool generates during the operation. For a half-inch end mill taking a 0.050 inch depth of cut in aluminum you need roughly 500 to 1,000 pounds of clamping force. This is well within the range of a 4-inch precision vise at moderate torque.

Pro tips for vise workholding include indicating the vise after bolting it down. A vise that is 0.001 inch out of square will make every single part crooked. Use parallels under the part to ensure consistent Z-height across different workpieces. Blow chips off the parallels before loading each part because a chip under the part throws off the height. Watch for jaw lift as you tighten the movable jaw can lift the part slightly. Tap the part down after clamping to seat it firmly on the parallels. Never climb-cut into a gap where the tool path approaches an unsupported edge of the part. The cutting forces can pull the part out of the jaws.

Clamps

Not every part fits in a vise. For large plates, odd shapes, or workpieces that need full edge access clamps are the answer. A good clamping kit covers most situations you will encounter.

Strap clamps are the most common type. A bolt presses down on one end of a steel strap while the other end presses on the workpiece. The key rule for strap clamps is that the support heel block should be slightly higher than the workpiece and the clamping bolt should be positioned closer to the part than the support. This creates a mechanical advantage that holds the part firmly.

Toe clamps press sideways into the bottom edge of the part leaving the top surface completely clear. They are ideal for parts that need machining across the full face without any clamp interference. Step clamps use a stepped design to accommodate different material thicknesses. They are quick to adjust and good for one-off setups with varied stock sizes. CAM clamps and toggle clamps use spring-loaded or cam-action mechanisms for rapid manual clamping. They work well for light operations and jig work where you load and unload parts frequently.

T-track and modular clamping systems use a grid of T-slots in the machine table or a fixture plate to position clamps anywhere you need them. This is the standard approach for router tables and large-format machines where part shapes vary widely.

Clamp placement rules are simple but important to follow. Clamp close to the cut within one to two inches if possible. The closer the clamp is to the cutting force the less leverage the part has to lift or shift. Distribute clamps evenly around the part with at least one clamp per corner for rectangular parts. Clamp onto solid material never over a pocket or thin web because clamping over empty space distorts the part. Lower clamp bolt torque for thin parts because over-tightening thin stock warps it and when the clamps release the finished surface springs back out of flat.

Vacuum Tables

Vacuum workholding uses atmospheric pressure to hold the part down. It is the best method for thin flat parts that cannot handle clamping pressure without distorting. A vacuum pump pulls air out from between the part and the table while atmospheric pressure at 14.7 PSI at sea level pushes down on the top surface of the part.

The holding force is calculated as surface area in square inches times vacuum level in inches of mercury times 0.49. At 20 inches of mercury on a 10 by 10 inch part the calculation is 100 square inches times 20 times 0.49 which equals 980 pounds of holding force. That is enough for most light to moderate cutting operations.

Three main types of vacuum pumps serve different needs. Regenerative blowers provide 4 to 10 inches of mercury with high air flow measured in CFM. They are best for MDF spoilboard systems where some air leakage through the porous material is expected. Rotary vane pumps provide up to 29 inches of mercury with moderate CFM for maximum holding force on non-porous materials. Venturi air-powered vacuum generators provide up to 25 inches of mercury with low CFM and are best for small parts and occasional use. They are the cheapest option at $100 to $500.

MDF spoilboard is the standard vacuum approach for routers. Seal the MDF edges with shellac or epoxy to prevent air leakage. Face the spoilboard flat with a surfacing pass. Lay the workpiece on top and the vacuum pulls through the porous MDF to hold it. Gasketed sealing uses rubber O-ring cord pressed into routed grooves to seal directly around the part. This approach wastes less vacuum and achieves higher holding forces.

Do not use vacuum workholding for porous materials like untreated wood and foam because they leak vacuum and cannot hold reliably. Do not use vacuum for heavy roughing operations where cutting forces exceed the available holding force. Do not use vacuum for very small parts under about 2 square inches because the surface area is too small to generate useful holding force.

Custom Fixtures

When a vise clamps or vacuum will not work you build a fixture. A fixture is a custom device that holds a specific part in a specific orientation for machining. The fundamental rule of fixture design is the 3-2-1 locating principle.

A free body in space has 12 degrees of freedom or 6 directions times 2 ways each representing translation and rotation. To constrain the part completely you use three points on the primary plane which prevent rotation about X and Y and translation in Z. Two points on the secondary plane prevent rotation about Z and translation in X. One point on the tertiary plane prevents translation in Y. That is six constraints for complete location. Then you apply clamps to hold the part securely against these locators.

Three golden rules of locating apply. First locators are fixed and do not move. Second clamps press the part into the locators they do not position the part. Third never over-constrain the part because more than 3 plus 2 plus 1 creates binding and inconsistent location from part to part.

Steel fixtures are best for production runs where wear resistance and repeatability matter. Aluminum fixtures work well for prototypes and light production where lower cost and easier machining offset the shorter wear life. Three-dimensionally printed nylon fixtures are increasingly popular for short runs because you can design and print a custom fixture in a single day without machining.

A fixture plate with a grid of threaded holes and dowel pin holes lets you build fixtures repeatably. You bolt the plate to the machine table once and build fixturing on top of it. Zero-point clamping systems use a pull-stud mechanism to locate and clamp fixture plates or vises to the machine table with sub-micron repeatability. They reduce setup time by up to 90 percent with repeatability under 5 microns. Entry-level systems start around $1,000 to $3,000. For a small shop running one-off parts zero-point is overkill but for any shop doing repeat setups it pays for itself quickly.

Selection Guide

For rectangular metal blocks use a CNC precision vise. For flat sheets use a vacuum table or clamps. For odd shapes use strap clamps or a custom fixture. For thin parts under one-eighth inch use a vacuum table or double-sided tape. For delicate finish surfaces use soft jaws or vacuum to avoid clamp marks. For high-volume production use zero-point clamping with dedicated fixtures. For round parts use a self-centering vise or collet block. For prototypes use 3D-printed fixtures that you can create quickly and cheaply.

For more on workholding options see our [CNC Workholding Options](

G90 G54 G00 X0 Y0
G01 Z-0.1 F10
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/posts/cnc-workholding-options/) guide and our CNC Beginner’s Guide for machine setup fundamentals.