Copy Milling: How a Master Part Gets Duplicated
Copy milling transfers the shape of one finished master onto new blanks, either with a mechanical tracer or a digitizing probe. This page explains the mechanism, the tolerances you can realistically hold, and the point where a 3-axis or 5-axis CNC program becomes the cheaper route. Written for engineers and buyers who have to decide between the two.
What copy milling actually does
Copy milling is a duplication process. A stylus or probe follows the surface of a master part, and the machine drives a cutter along a matching path into a fresh blank. The master carries the geometry; the machine only repeats it. Nothing is programmed from a drawing, so the first article is the master itself, not a toolpath.
The idea predates CNC. Manual die-sinkers and Keller-type tracers used a hardened template and a 1:1 pantograph linkage to rough out mold cavities. The mechanical version is still around in toolrooms because it is fast to set up for one or two parts and needs no CAD model.
Modern copy milling usually means a touch probe or laser line scanner feeding a CNC control, or a hydraulic tracer head on a manual mill. Both approaches do the same job: read the master, drive the cutter, keep the offset constant.
The practical question is not whether the machine can follow a shape. It is whether the copied shape holds up on the drawing. That depends on how the master was made, how the probe reads it, and how the cutter deflects while duplicating it.
Tracer versus probe: two ways to read a master
A hydraulic tracer works in real time. The stylus deflects against the master, and the deflection signal drives the hydraulic servo that moves the cutting head. Response is fast, but the stylus needs a few tenths of a millimeter of deflection to work, so the copied profile is slightly offset from the master.
A touch probe on a CNC is a sampling device. The control touches the master at discrete points, records coordinates, and either stores them as a point cloud or feeds them to a digitizing routine. Point spacing of 0.5 mm to 2 mm is common for free-form surfaces. Tighter spacing captures more detail and takes longer.
A laser line scanner is the third option. It projects a line onto the master and reads the reflected profile at thousands of points per second. It never touches the part, so soft masters such as wood, clay or foam are usable. Shiny or transparent surfaces need a matte spray first.
All three methods share one weakness. They read what exists. If the master has a ding, a weld repair or a hand-filed blend, the copy carries it forward. There is no design intent in the file, only measured reality.
What tolerance copy milling can hold
For a rigid hydraulic tracer on a manual mill, ±0.05 mm to ±0.1 mm is a realistic band on a smooth profile. The number gets worse on long, unsupported cuts where the quill or ram deflects, and worse again when the stylus force pushes the master sideways.
Probing on a CNC machine removes the stylus force, so the reading step is much tighter. The copied part still depends on the cutting step. A 3-axis machine with a Ø10 mm carbide end mill in aluminum 6061 can hold ±0.02 mm on a shallow cavity, but the same setup in 17-4PH stainless will move more as tool load rises.
Surface finish tracks the stepover you choose. Roughing at 60 to 70 percent of cutter diameter leaves a scalloped surface. A finishing pass at 5 to 10 percent stepover brings aluminum to roughly Ra 1.6–3.2 μm as machined, and a finer stepover plus a ball nose cutter can reach Ra 0.8–1.6 μm.
The master itself sets the ceiling. If the master was hand-shaped and is 0.15 mm off nominal, every copy inherits that error. Measure the master first, and decide which dimensions matter before you cut anything.
Scale, offset and mirror settings
Most tracer and digitizing setups include a scale factor. A 0.5 scale on the pantograph produces a half-size copy, which is how small mold cavities were traditionally shrunk from a large wooden pattern. The tradeoff is straightforward: scale changes amplify or reduce every error in the master by the same factor.
Offset control shifts the cutter path by a fixed distance to account for cutter radius and finishing stock. Set the offset too small and the copy comes out undersize. Set it too large and you leave stock that a second pass has to remove, which doubles the cycle.
Mirroring flips the path for left-hand and right-hand parts. Check the direction on a scrap blank before committing a real workpiece. A mirrored cavity that faces the wrong way is not a small rework job on a mold block.
On a manual tracer, note that scale, offset and mirror are mechanical settings. On a probing system they live in the control and can be changed per part, which makes small batch variation much easier to manage.
Where copy milling fits on a modern shop floor
Copy milling still earns its place in repair and legacy work. A pump housing, a die insert or a linkage arm that has no drawing can be duplicated from the worn original. The copy will be as good as the worn part, and sometimes that is good enough to keep a line running.
It also suits one-off masters in soft material. A pattern maker carves a shape in wood or tooling board, and the tracer cuts a matching cavity in aluminum or steel. No CAD file is needed, which matters when the original design only ever existed on paper.
Where copy milling loses is repeatability across a batch. Every part is a fresh reading of the same master, so drift creeps in. A CNC program from a CAD model reproduces the same path on part 1 and part 500. For a 10,000 piece run, that difference decides the process.
A good middle route is to probe the master once, clean the point cloud into a surface, and cut from that program. You keep the physical reference and gain digital repeatability.
Copy milling versus programmed CNC milling
Use this to pick a route before you commit tooling.
| Factor | Copy milling | Programmed CNC milling |
|---|---|---|
| Geometry source | Physical master or sample part | CAD model and toolpath |
| Typical tolerance | ±0.05 to ±0.1 mm with a tracer | ±0.005 mm on a 5-axis center |
| Best batch size | One to a few dozen parts | One prototype to 10,000+ parts |
| Setup effort | Fast for a single cavity | CAM programming time up front |
| Repeatability | Drifts with each reading | Identical path on every part |
| Complex 3D surfaces | Limited by stylus access | Simultaneous 5-axis reaches undercuts |
| Design changes | Recut the master | Edit the model, repost the program |
| Typical use | Repair, legacy parts, patterns | Production runs, tight tolerance parts |
The short answer
If you have a physical master, no CAD model and only a handful of parts to make, copy milling gets you there without programming. If the drawing matters more than the sample, or you need more than a few dozen identical parts at ±0.005 mm, probe the master once and cut from a CNC program instead.
Questions engineers ask about copy milling
Can copy milling hold ±0.005 mm?
Not reliably on a mechanical tracer. The stylus needs deflection to generate a signal, and that deflection shows up in the copied profile. A probing system reads the master accurately, but the cutting step still has to hold the tolerance on its own.
At GreatLight we hold ±0.005 mm on 5-axis centers when the part is cut from a CNC program, not from a live tracer path. If your drawing needs that band, the geometry should come from a model, not from a stylus following a sample.
What happens to the copy if the master is worn?
Every defect carries over. Wear, a weld repair, an impact dent and a hand-blended corner all appear on the duplicate. Scale settings multiply the error if you are shrinking or enlarging.
Measure the master before you cut. Mark the dimensions that matter, and plan a separate operation for those faces instead of trusting the tracer.
Is probing a master the same as reverse engineering?
Probing collects points on a surface. Reverse engineering turns those points into a usable surface model with clean edges, fillets and datums. The gap between the two is where most of the work sits.
A raw point cloud with 1 mm spacing can describe a free-form blend, but it will not give you a crisp edge or a defined thread. Those features still have to be modeled by hand.
Which materials can be copy milled?
Aluminum, brass, mild steel and tool steel are routine. Aluminum 6061 and 7075 cut cleanly and let the tracer move fast. Stainless 304 and 17-4PH load the cutter more, so feed rates come down.
Soft masters such as wood, tooling board, foam and clay are fine with a laser scanner because nothing touches the surface. A contact stylus will mark them.
How do I get both the physical reference and repeatability?
Probe or scan the master once, clean the data into a surface, then program the part from that model. The master stays on the bench as the reference for inspection, and the machine cuts from a fixed toolpath.
From there, first article inspection compares the cut part back to the master on the dimensions you flagged. If they agree, the program is released for the rest of the run.
When should I stop copying and go back to the drawing?
When the copy needs three or four rework passes, the tracer is costing more than programming would. The same applies when the batch grows past a few dozen parts, or when a design change lands and the master would have to be recut.
A short review of the part drawing usually settles it. If there is a model, or a model can be built in a day, programmed CNC milling wins on both tolerance and repeatability.
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