Essexs Expert CNC Machining: How Shops Hit Tolerance

• 1
  Why Axis Count Changes Tolerance
• 2
  Which Features Actually Need Five Axes
• 3
  Tolerance Follows the Material
• 4
  What Inspection Data Should Come With the Parts
• 5
  How to Judge a Shop Before You Send the Drawing
• 6
  3-Axis vs 5-Axis: When Each Wins

Every setup adds error. When a part moves from one fixture to another, the datum shifts by whatever the fixture and the operator allow. On a 3-axis machine a complex part may need four or five setups, so those shifts stack up. A 5-axis machine holds the part in one fixture and rotates the table or the spindle head instead. Fewer setups means fewer places for error to enter.

The rotary table matters as much as the spindle. A Ø400 mm table gives enough swing for most pump housings and manifolds, but a long shaft will overhang and deflect. We check overhang before quoting. If the part needs more than about 3× the tool diameter in unsupported reach, we plan a tailstock or a steady rest, or we split the operation.

Thermal drift is the quiet one. A spindle running for hours grows a few microns. On a ±0.005 mm feature that is real. Shops that hold that tolerance warm the machine, cut a test feature, and measure before committing to the run. On a first article we do the same, then lock the offsets.

So axis count is not a spec sheet number. It decides how many times you touch the part, and each touch is a chance to lose position.

Five axes earn their cost on parts with features that cannot be reached from three orthogonal directions. Impellers, turbine blades, ported manifolds, angled oil galleries and sculpted mold cavities are the classic cases. If a feature faces a direction the tool cannot approach without a re-fixture, five axes help.

They do not help on a flat plate with holes. A 3-axis mill with a good vise will run that faster and cheaper. We see drawings arrive with five-axis geometry that turns out to be two pockets and a chamfer. That is a quoting problem, not a machining problem, and it is worth catching early.

Undercuts and deep ribs are the hard zone. A long reach tool small enough to enter a rib deflects, so the wall tapers even when the program is correct. Reducing depth of cut to 0.1–0.2 mm per pass and increasing spindle speed keeps the tool from pushing off. It costs cycle time. There is no way around that.

Thin floors behave the same way. Below about 0.8 mm wall thickness in aluminium, the part starts to move under clamping and cutting force. We plan support, light passes and a stress-relief step between roughing and finishing.

Aluminium 6061 and 7075 cut clean and hold ±0.005 mm on stable features. They also move after machining if the stock was not stress relieved. A bracket roughed from bar stock can bow 0.05 mm overnight. Rough, relieve, finish is the fix, and it adds a day.

Stainless 304 and 316 work-harden. A dull tool rubs instead of cutting, the surface hardens, and the next pass breaks the edge. We keep chipload up and never dwell. 17-4PH in the H900 condition machines well but needs carbide and coolant delivered at the edge, not sprayed at the part.

Titanium Ti-6Al-4V and Inconel are the slow ones. Heat stays in the cut, so tool life is short and surface finish suffers if feed is pushed. Expect longer cycle times and more tool changes. A part that runs in 20 minutes in 6061 may run two hours in Inconel. That is normal, not a markup.

Plastics like POM and PEEK cut easily but spring back. A bored hole can measure undersize right after the tool leaves and come back 0.02 mm later. We measure after the part cools.

A tolerance claim without a measurement is a guess. For any feature at ±0.005 mm we record the actual number, not a pass or fail. A CMM report with the nominal, the actual and the deviation tells you whether the process is centered or drifting toward one limit.

First article inspection is the checkpoint that matters most on a new program. It catches a datum error before 500 parts are cut wrong. We run raw material verification, in-process checks and a final inspection before shipment, and reports go out on request.

Surface finish is measured, not eyeballed. Ra 0.8–1.6 μm is the standard machined range that suits most sealing faces and bearing bores. Ra 0.2–0.8 μm needs a finer stepover or a finishing pass and adds cost. If your drawing calls Ra 0.4 μm on a non-functional face, you are paying for nothing.

For regulated programs, the paperwork chain is part of the deliverable. ISO 9001, IATF 16949, ISO 13485 and ISO 27001 cover different industries, so match the certificate to your audit requirement before the order, not after.

Ask what machine will run the part. A shop that answers with a model number and a travel envelope is telling you it has thought about the setup. A shop that answers with a price is guessing.

Check the maximum processing size against your part. A 4,000 mm envelope covers long rails and frames, but the same shop may not have the floor space to swing a wide weldment. Travel of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm covers most enclosure and manifold work.

Volume flexibility matters more than people expect. A shop set up only for high volume will not touch a single prototype, and one set up only for prototypes cannot hold a 10,000-part run consistent. The ability to move from one piece to a full run without a process change is the thing to look for.

Finally, ask how the shop handles your files. Uploads should be secure and confidential, with an NDA available on request. If that question gets a vague answer, it will get a vague answer later too.