CNC bevel treatment: a guide to precise manufacturing
A bevel is a slanted face cut into an edge. This page covers how the angle, edge width and tolerance are held on a CNC machine, and which parts should not be beveled at all. Written for engineers and buyers who need to put a number on a drawing.
What CNC bevel treatment actually removes
A bevel is an inclined plane cut across an edge or corner. The cutting tool follows a programmed path, so the resulting face is a controlled surface rather than a hand-filed guess. On a turned shaft the bevel usually appears at the end face; on a milled plate it runs along the outer profile.
Compare that with a chamfer, which is a symmetric edge break at 45° in most drawings. A bevel is not limited to 45°. It can sit at 15°, 30° or 60°, and its width can vary along the cut if the part needs it. That flexibility is why bevels show up on weld preps, gear teeth, valve seats and optical mounts.
The operation itself is simple to describe. A cutter with the right included angle, or a ball nose tool on a 3-axis path, sweeps along the edge and removes the material between the two faces. On a 5-axis machine the tool can tilt, so the bevel face stays perpendicular to the spindle and the surface finish improves.
How angle, width and tolerance interact
Three numbers define a bevel: included angle, leg width and the tolerance band on both. On a 30° bevel with a 2 mm leg, a 0.5° angular error shifts the edge position by about 0.017 mm. That is small. Push the leg to 10 mm and the same 0.5° error moves the edge 0.087 mm, which is well outside a ±0.005 mm callout on the adjacent face.
So the wider the bevel, the tighter the angle control has to be. We usually hold bevel angles to ±0.5° on milled edges and ±0.25° on turned edges under 50 mm diameter. Where the bevel meets a sealing face, the leg width matters more than the angle; that leg is measured from the theoretical sharp corner, not from the burr.
Tolerance on a beveled edge is not the same as tolerance on a flat surface. The tool deflects more when it engages a corner, and the entry and exit of the cut are the two places where the angle drifts. Inspectors should check the bevel at both ends of the cut, not only at the midpoint. A single midpoint reading can pass a part that fails at the exit.
Tool selection and the setup that follows
For a straight bevel on the outside of a rectangular part, a dovetail cutter or a chamfer mill with the matching included angle does the job in one pass. The tool diameter must clear the leg length; if the leg is longer than the cutting edge, the shank rubs the wall and the finish turns ugly. We keep a set of 60°, 90° and 120° chamfer mills on the floor for this reason.
On curved or tapered edges, a ball nose tool on a 3-axis path gives a bevel that follows the contour. The trade-off is stepover marks. At 0.1 mm stepover and Ra 0.8–1.6 μm target, the scallop height stays under 0.5 μm, so the marks disappear under a light polish. Coarser stepover leaves visible lines that no anodize can hide.
Setup matters as much as the cutter. A part held in a vise with 40 mm of unsupported overhang will chatter on a bevel cut, and chatter shows up as a wavy edge. Support the overhang, keep the tool flute length short, and run the bevel as the last operation when the part is already stress-relieved. Cutting a bevel first and then removing 3 mm from the opposite face will move the edge.
Material behavior at the beveled edge
Aluminium 6061 and 7075 cut clean bevels with sharp corners. The edge holds at ±0.005 mm without much effort. Stainless 304 and 316 work-harden at the tool tip, so a bevel cut with a dull cutter raises the local hardness and the next pass cuts differently. Change the insert before the edge starts to shine.
Titanium Ti-6Al-4V is the hard case. It conducts heat poorly, so the cutting edge runs hot and the bevel face can smear. We run titanium bevels at lower surface speed, flood coolant, and a positive rake tool. The bevel width is usually kept under 3 mm on titanium because wider cuts push the tool and the angle wanders.
Plastics behave differently again. POM and PEEK hold a bevel well but chip at the exit if the feed is too high. ABS and PC soften with heat and leave a fuzzy edge. For these, a single finishing pass at 0.05 mm radial depth leaves a clean bevel and avoids the melt line that shows after bead blasting.
Measuring a bevel without cutting the part
A bevel is hard to measure because the datum is a theoretical corner that no longer exists. The practical method is to measure the leg width with an optical comparator or a vision system, project the two faces, and calculate the angle from the intersection. This takes about 90 seconds per edge on a comparator and gives repeatable numbers to ±0.02 mm on the leg.
For production runs, a profile projector with an overlay works better. Draw the nominal bevel on a transparent sheet, overlay it at 20× magnification, and check the deviation. If the bevel drifts, the operator sees it immediately and can correct the tool offset before the next part. CMM touch probing can measure a bevel too, but the stylus radius has to be compensated or the reported angle reads low.
Surface finish on the bevel face is measured with a portable skidded gauge. The probe must sit on the inclined face, not the adjacent flat. A reading taken across the corner mixes two surfaces and reports a number that means nothing. On a Ra 0.8–1.6 μm callout, the bevel face is usually the last surface to reach spec, because it gets the least tool engagement.
Bevel, chamfer and radius: which one to specify
Use this when the drawing still has a choice to make.
| Feature | Typical angle | Best for | Watch out for |
|---|---|---|---|
| Bevel | 15°–60°, not fixed | Weld preps, gear flanks, valve seats | Angle error grows with leg width |
| Chamfer | 45°, sometimes 30° | General edge break, fastener lead-in | Harder to blend into a curved edge |
| Radius | Full or partial arc | Fatigue-critical edges, handles | Radius tool must match the arc exactly |
| Sharp corner | 90° as drawn | Datums, mating faces | Burr risk; deburr may break the datum |
| Compound bevel | Two angles on one edge | Thick weld preps, deep grooves | Needs two setups or a formed tool |
Where the bevel decision lands
If the edge carries a load or feeds a weld, specify a bevel with a controlled angle and measure the leg. If the edge only needs to stop cutting hands, a 0.5 mm 45° chamfer is cheaper and easier to inspect. Do not specify a bevel where a chamfer does the job.
Common questions on CNC bevel treatment
What is the difference between a bevel and a chamfer on a CNC drawing?
A chamfer is an edge break, usually 45°, and its only job is to remove the sharp corner. A bevel is an inclined functional face, often at 15° to 60°, and its angle and width are called out because they carry load or guide a weld.
If the angle is not dimensioned and the callout just says 'break edge', the shop will cut a chamfer. If the angle is dimensioned, expect bevel treatment.
How tight can you hold a bevel angle?
We hold ±0.5° on milled bevels and ±0.25° on turned bevels under 50 mm diameter. The limit comes from tool deflection and from how the part is held, not from the machine's positioning accuracy.
On a bevel wider than 6 mm, ask for ±1° instead. Chasing ±0.25° across a wide face adds cost and rarely changes how the part works.
Can a bevel be added after heat treatment or anodizing?
After heat treatment, yes, but the cut is harder on the tool and the edge may chip on hardened steel above 45 HRC. We usually cut the bevel before hardening and leave 0.1 mm for a finishing pass afterward.
After anodizing, no. Cutting through the coating leaves a bare edge that corrodes differently from the rest of the part. Bevel first, then anodize.
How is a bevel inspected on a production run?
Optical comparator or vision system, measuring the leg width from the theoretical corner and calculating the angle from the projected faces. First article plus periodic checks through the run.
A CMM can do it, but the stylus radius must be compensated or the angle reads low. We keep comparator overlays at 20× for the parts that run often.
What surface finish should I call out on a bevel face?
Ra 1.6–3.2 μm is standard for a functional bevel on aluminium or steel. Ra 0.8–1.6 μm is available when the bevel seals or slides, and it takes a slower finishing pass.
Ra 0.2–0.8 μm on a bevel face is possible but expensive, and it only makes sense on a sealing or bearing surface.
Does a bevel always need a deburring step afterward?
Almost always. The exit point of the cut leaves a fine burr even when the bevel itself is clean. On stainless and titanium the burr is small but hard to see, so it survives a visual check.
We deburr by hand on edges that matter and tumble the rest. If the bevel is a datum, tell us, because tumbling can round the edge you just measured.
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