CNC Machining Custom Parts Guide

• 1
  How a CNC machine actually removes material
• 2
  Material selection drives everything downstream
• 3
  Setup count and 5-axis work: where cost hides
• 4
  What a tolerance callout really costs
• 5
  Geometry that fights the cutting tool
• 6
  Inspection closes the loop between drawing and part
• 7
  Process and tolerance choices at a glance

A CNC machine does not print a shape. It cuts one away. A rotating cutter moves along a toolpath calculated from your CAD model and strips material in passes. Every pass leaves a mark, and the depth of those marks is what you later measure as surface roughness.

The toolpath comes from CAM software, not from the drawing alone. The programmer picks tool diameter, stepover, feed rate and spindle speed. Those choices decide whether a pocket finishes cleanly or chatters. Two shops can quote the same part and produce very different surfaces because their CAM strategy differs.

Cutting force pushes back on the part. Thin walls bend, long slender tools deflect, and deep pockets trap heat. A good programmer sequences operations so the part stays rigid as long as possible, often leaving a roughing stock of 0.3–0.5 mm before the finishing pass.

This is why the same geometry can be cheap or expensive. It depends less on the drawing than on how many setups and how much hand work the shape demands.

Material decides cutting speed, tool wear and achievable finish. Aluminum 6061 machines fast and holds a clean face. Stainless 316 work-hardens under a dull tool, so light passes and sharp inserts matter more than raw spindle power.

Titanium TC4 (Ti-6Al-4V) conducts heat poorly. The heat stays at the cutting edge instead of leaving with the chip, so tool life drops sharply. On a 10 mm deep pocket in titanium, a shop will slow the feed and accept a longer cycle to protect both the tool and the part.

Plastics behave differently. POM and PEEK cut cleanly but move with temperature. A part measured hot can be out of tolerance once it cools to room temperature. Deburring ABS with a scraper is fine; doing that to a polished acrylic face leaves visible scratches.

Pick the material from the function first, then check whether the geometry can be cut in it. A wall 0.8 mm thick in 316 stainless is a different risk than the same wall in 6061.

• 1
  Fast and forgivingAluminum 6061, 6082 and 7075 cut quickly and hold tight tolerances.
• 2
  Wear-resistantStainless 303, 304, 17-4PH and 440C need sharp tools and steady feed.
• 3
  High strength, low thermal conductivityTitanium and Inconel require slower speeds and shorter tool life.
• 4
  Dimensionally restlessPOM, PA and PEEK need temperature-controlled measuring.

Every time the part is unclamped and turned, position error adds up. A part cut in three setups can accumulate 0.02–0.03 mm of stacking error even when each setup is accurate. Cutting it in one setup removes that risk.

That is the real argument for 5-axis machining, not marketing. When the tool tilts and the table rotates, angled faces, undercuts and deep pockets can be reached without re-fixturing. A part that needed four operations becomes one.

Five-axis work is not automatically better. Short, simple parts with open faces often run faster on a 3-axis machine because the setup is trivial and the CAM is simpler. The gain appears when geometry is angled, when features sit on several faces, or when one datum must control everything.

At GreatLight, 16 simultaneous 5-axis centers handle the complex work while 27 three-axis machines take the straightforward parts. Routing the job to the right machine is a cost decision, not a capability one.

A general tolerance block of ±0.1 mm is comfortable on most features. Tightening to ±0.005 mm is possible, but it changes the process, not just the number. It requires a controlled temperature, a finishing pass with a fresh tool, and measurement you can trust.

Tolerance callouts should follow function. A bearing bore needs a tight fit. A mounting clearance hole does not. Applying ±0.005 mm everywhere multiplies inspection time and rejects good parts that would have worked fine.

Surface finish is a separate axis. As-machined faces land around Ra 1.6–3.2 μm. A high-quality finish reaches Ra 0.8–1.6 μm, and fine finishes reach Ra 0.2–0.8 μm. Those numbers come from tool radius, stepover and spindle speed, so they cost cycle time.

When a drawing lists a tight tolerance on a face that also needs a fine finish, expect the quote to reflect both. They are not the same operation.

A cutter is a cylinder with a limited length. A pocket 80 mm deep and 6 mm wide needs a long, thin tool that will deflect. The result is a tapered wall and a poor floor finish. Widening the pocket or splitting it into two shallower steps solves it.

Sharp internal corners are impossible to mill. The cutter leaves its own radius. If the drawing calls for a square corner, someone has to broach it, EDM it, or accept a radius. Designing a 0.5 mm fillet instead of a sharp corner often removes an entire operation.

Deep holes follow the same rule. A hole deeper than about 10× its diameter in stainless needs peck drilling and extra time. Beyond 20× diameter, the risk of drill wander grows and the tolerance loosens.

None of this makes the part unmanufacturable. It makes it slower, and slower is what you pay for.

Inspection is not a final gate. It starts with the raw material certificate, continues through in-process checks during long cycles, and ends with a dimensional report before shipment. A part that drifts halfway through a run is caught while it can still be corrected.

Different features need different instruments. Calipers read outside dimensions but cannot confirm a true position. A coordinate measuring machine can. For a bore with a tight fit, a bore gauge gives a faster and more repeatable reading than a micrometer.

Reports are issued on request. That matters for aerospace, medical and automotive customers who must keep traceability records for each lot.

GreatLight inspects 100% of parts before shipment across a 99.99% qualification rate. The number is only meaningful because it applies to every part, not a sample.