Thomas CNC Machining: Basic Guide
A plain explanation of what Thomas CNC machining means on the shop floor: computer-controlled multiaxis motion that removes material from metal or plastic. Written for design engineers and buyers who need to decide between 3-axis, 4-axis and 5-axis work before they send a model out for quote.
What Thomas CNC Machining Actually Means
The name is a shop-floor label rather than a separate machine category. Thomas CNC machining describes multiaxis work done on four-axis or five-axis computer-controlled machine tools, where the tool or the part can rotate while cutting. The spindle follows a toolpath generated from the CAD model, and the extra rotary axes let the cutter reach faces that a three-axis setup cannot touch without re-fixturing.
In practice the process is still subtractive: a cutter removes material from a solid block, casting or bar. On a three-axis mill the tool moves in X, Y and Z while the part stays fixed. Add a fourth axis and the part can index between cuts. Add a fifth and the tool can stay normal to a curved surface for the whole pass instead of stepping across it.
That difference matters most on parts with compound angles, deep pockets on multiple faces, or blended surfaces. A hydraulic manifold with ports on five sides, an impeller with twisted blades, or a bone plate with an organic underside all fall into that group. The geometry is not exotic. It is simply hard to reach from one direction.
The trade-off is setup time and programming effort. Multiaxis toolpaths take longer to verify, and simulation is not optional. If a part can be reached in two or three orthogonal setups, a three-axis machine usually runs it cheaper and faster.
Three, Four and Five Axis: Where Each One Fits
Three-axis machining is the baseline. The workpiece is clamped once, the tool works from above, and every feature must be visible from that direction or reachable with a long enough cutter. It is the fastest option for plates, brackets, housings and any part with features on one face plus a few drilled holes.
Four-axis machining adds rotation around one axis, usually A or B. The part can be indexed to four sides without being unclamped, which removes the positional error that comes from re-fixturing. Shafts with cross-drilled holes, cylindrical housings with milled flats, and parts that need work on all four sides are the classic four-axis jobs.
Five-axis machining adds a second rotary axis, so the tool can tilt relative to the part. Simultaneous five-axis work keeps the cutter engaged at a constant angle, which gives better surface finish on curved geometry and lets short, rigid cutters reach deep pockets. Positional five-axis work, where the table tilts and locks, is a cheaper middle ground: it improves access without needing a fully simultaneous toolpath.
The choice is not about prestige. It is about how many setups the part needs and whether any surface is unreachable. Fewer setups mean fewer datum shifts, and datum shifts are where most dimensional errors creep in.
How Accuracy and Surface Finish Are Controlled
Tolerance is set by the machine, the fixture and the thermal state of the part, not by the axis count alone. On stable setups we hold ±0.005 mm (±0.0002 in) on critical features. That figure only applies where the drawing calls for it, and it needs to survive inspection, so we plan the measurement method before cutting.
Surface finish follows the same logic. As-machined surfaces typically land at Ra 1.6–3.2 μm. A finishing pass with a smaller stepover reaches Ra 0.8–1.6 μm, and fine finishing with a light radial engagement can reach Ra 0.2–0.8 μm. Chasing a finer number than the function requires adds cycle time for no benefit.
Material behavior matters as much as the program. Aluminum 6061 and 7075 cut cleanly and hold tight tolerances well. Stainless 316L and 17-4PH work-harden, so the cutter must keep moving and the feed must not be too light. Titanium TC4 (Ti-6Al-4V) and Inconel generate heat at the cutting edge, which means lower surface speed, more coolant and a stiffer setup.
Thin walls are the usual failure point. A 1 mm wall on a 100 mm part will move when it is unclamped, no matter how good the toolpath is. If the drawing allows, leaving a roughing allowance and finishing after stress relief keeps the part where the model says it should be.
What to Check Before You Send a Model
Start with reachability. Look at every face that carries a tolerance or a sealing surface and ask which direction the tool comes from. If a feature needs a cutter longer than four times its diameter, expect chatter and plan a different approach or accept a looser tolerance.
Then check the datum strategy. A part that references one face on the drawing but gets flipped three times in the shop will accumulate error. Consolidating to one or two setups on a four-axis or five-axis machine often costs less than the inspection time saved.
Material and quantity drive the rest. Prototypes and small runs are almost always machined from bar or plate because no tooling is needed. Above roughly a few thousand parts a year, casting or forging a near-net shape and machining only the critical surfaces can cut both material waste and cycle time.
Finally, be explicit about finishes and marking. Anodizing, electroless nickel, powder coating, bead blasting and laser marking all change dimensions slightly, and laser marking needs a minimum character height of 1.5 mm to stay legible. Those details belong on the drawing, not in a later email.
Setup and Machine Selection by Part Type
Use this as a first pass when deciding how many axes a part really needs.
| Part feature | Typical setup | Why |
|---|---|---|
| Flat plate, holes on one face | 3-axis, one setup | All features reachable from above |
| Housing, work on four sides | 4-axis, one setup | Indexing replaces re-fixturing |
| Cross-drilled shaft | 4-axis with rotary table | Angular holes machined in one pass |
| Impeller, twisted blades | 5-axis simultaneous | Cutter stays normal to the surface |
| Manifold, ports on five sides | 5-axis positional | Fewer datums, better access |
| Deep pocket, long reach | 5-axis with short cutter | Tilt avoids long, flexible tools |
| Large frame, 4,000 mm long | 3-axis or 4-axis gantry | Size, not geometry, drives the choice |
When Multiaxis Is Worth It, and When It Is Not
If the part has features on more than two faces, compound angles or surfaces that must blend, choose Thomas CNC machining on a 4-axis or 5-axis setup. If every feature is reachable from one direction, a 3-axis job will run faster and cost less, and the extra axes buy you nothing.
Common Questions
Is Thomas CNC machining a different process from regular CNC machining?
No. It is the same subtractive process. The term is used to describe work done on multiaxis machines, mainly four-axis and five-axis, where the part or tool rotates during the cycle.
The cutting mechanics, tooling and inspection methods are the same as any other CNC milling or turning job. What changes is how many faces you can reach in a single setup.
Which materials can be machined this way?
Aluminum grades such as 6061, 7075, 2024 and 6082, stainless 303, 304, 316L and 17-4PH, alloy steels including 4140 and 4340, copper and brass, titanium TA2 and TC4, Inconel, magnesium, and plastics from ABS and POM through PEEK and carbon fibre.
Material choice affects feeds, speeds, tooling and sometimes the number of setups. Very gummy plastics and work-hardening stainless need different strategies than aluminum.
How many setups should I expect for a complex part?
Most multiaxis parts we quote run in one or two setups. A single setup is possible when every tolerance-bearing feature can be reached by the rotary axes without unclamping.
Two setups usually appear when one face is a flat datum that must be machined first, or when the back side needs a feature the rotary travel cannot cover.
What tolerance can actually be held in production?
±0.005 mm (±0.0002 in) on critical features, measured with the method agreed at quoting. That is a capability limit, not a default, and it applies to features the process can actually reach.
Looser tolerances on non-critical surfaces keep the part affordable. We check 100% of parts before shipment and can supply inspection reports on request.
How do finishing operations affect the final dimensions?
Coating and plating add or remove a thin layer. Anodizing builds oxide on the surface, electroless nickel adds a measurable deposit, and bead blasting rounds edges slightly.
If a coated surface is a fit or seal, call out the pre-plate dimension and let the finisher work to it. Laser marking needs at least 1.5 mm character height to remain readable.
Can multiaxis machining handle prototypes as well as production?
Yes. There is no minimum order quantity, so a single prototype and a 10,000-part run use the same process route, just different fixture and inspection planning.
For prototypes, machining from bar or plate avoids tooling cost entirely. For larger volumes, a cast or forged near-net blank often reduces both material waste and cycle time.
Send the Model, Get a Setup Plan
Upload your CAD file and we will return a quotation with a free DFM analysis inside 12 hours, including the setup count we would use and any feature that needs a drawing change.
12-hour quote100% inspectionNDA on request
More From the Shop Floor
We publish setup notes, tooling trials and inspection data from the factory floor.
