How Metal Stock in CNC Milling Machine: From Raw Bar to Finished Part
This guide explains how metal stock in cnc milling is selected, sawn, clamped, and cut. It is written for engineers and buyers who need to judge stock form, allowance, and workholding before releasing a job.
In this article
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Key takeaways
How metal stock in cnc milling is defined
Metal stock is the raw input to a milling machine: bar, plate, sheet, tube, or a near-net blank. It arrives with a mill certificate, a hardness range, and a saw-cut face that is not yet a datum. The machine does not care where the material came from. It cares whether the stock fits the vise, whether the faces are flat enough to locate on, and whether the allowance covers every finished surface.
Stock form is chosen before toolpaths are written. A 200 × 150 × 40 mm bracket is usually cut from plate. A 25 mm diameter shaft with milled flats comes from round bar. A part with deep pockets and thin walls may start as a near-net forging or casting so the cutter removes 2 mm instead of 20 mm.
Machinists care about three numbers on the stock: the allowance per face, the flatness of the locating face, and the hardness. Aluminum 6061-T6 at 95 HB cuts fast and moves little. A 4140 pre-hard plate at 28–32 HRC will pull a deep cut and needs lower feed. Stock choice quietly sets the cutting parameters you can run.
One more point that trips up new engineers: stock is bought to the finished envelope plus allowance, not to the finished envelope. If the drawing calls for a 100 mm plate and you buy 100 mm plate, you have no material to face, no material to square, and no way to hold ±0.005 mm. Size the stock first, then release the CAD.
Stock forms and what each one suits
Plate is the default for prismatic parts. It gives you two parallel faces to clamp and a stable base for the first op. The trade-off is grain direction and internal stress. A hot-rolled plate can move 0.2–0.5 mm after the first facing cut releases residual stress, so rough first, stress-relieve or rest, then finish.
Round bar suits turned-and-milled parts, bushings, and anything with a rotational axis. Bar stock is cheap per kilogram and easy to feed into a bar feeder for lights-out running. The catch is the milled features: a round bar in a three-jaw chuck is not a rigid base for heavy side milling. Use a collet block or a mill-turn center instead.
Near-net stock, meaning forgings and castings, is the answer when the finished shape is far from a rectangle and the material is expensive. Titanium and Inconel parts often justify the tooling cost. You remove less metal, save cycle time, and keep the grain flow. The downside is that the as-cast surface may carry 1–2 mm of variation, so the first op must establish a clean datum.
Sheet and tube are for thin parts and structural frames. Sheet under 6 mm will chatter if you clamp it flat without support, so back it with a sacrificial plate or use vacuum fixturing. Tube needs a mandrel or soft jaws to stop the wall from collapsing during the cut.
Allowance, sawing, and squaring the stock
Sawing is the first machining operation, even if it happens on a bandsaw. A good cut leaves 1.5–3 mm per face for small parts and up to 5 mm on large plate where distortion is likely. Cut too close and the facing pass runs out of material on one corner. Cut too generous and you pay for chips.
Squaring follows sawing. Face the bottom, then the top, then the two long sides, then the ends. Each pass references the face cut before it. After squaring, the block should hold 0.05 mm flatness and 0.05 mm squareness, which is enough to locate on for the real operations.
For thin or long parts, check flatness again after the first roughing pass. Stress relief happens when material leaves, not when the saw cuts. If a 500 mm plate bows 0.3 mm, face 0.15 mm, let it rest, then face again. Chasing it with a single heavy cut will just bend it further.
Do not skip the deburr between operations. A raised burr on the locating face is a 0.05 mm error that shows up as a taper on the finished part. A few seconds with a file or a chamfer tool saves a scrapped batch.
Workholding: how the stock is held matters
A vise is the fastest way to hold prismatic stock, but it is not neutral. Tightening a vise bows a thin part upward in the middle. For anything with a wall under 5 mm, support the part under the cut or switch to soft jaws machined to the stock profile.
For the second op, flip the part and locate on the faces cut in op 1. Use a stop so every part sits in the same position. If you are running 10,000 pieces, a dedicated fixture with toggle clamps beats a vise on cycle time and repeatability.
Five-axis machines change the arithmetic. With a Ø400 mm rotary table, the stock can be tilted so the tool reaches five faces in one setup. That removes the re-clamp error that normally appears between op 2 and op 3. On a 16-machine 5-axis cell, this is the main reason setups drop from four to two.
Magnetic chucks and vacuum plates work for flat, non-ferrous stock. They hold well in light cuts and poorly in heavy ones. If the part is 300 mm wide and 8 mm thick, a magnetic chuck plus a support plate under the pocket is more reliable than four clamps at the corners.
Tolerances, finish, and what the stock allows
Metal stock sets the ceiling on accuracy. A cold-rolled bar holds size better than a hot-rolled plate. A ground plate holds flatness better than a sawn one. If the drawing asks for ±0.005 mm and the stock moves 0.1 mm when you face it, no cutter path will fix that.
Surface finish follows the same logic. As-machined faces land at Ra 1.6–3.2 μm. A fine finish pass with a sharp insert and a light radial step gets you to Ra 0.8–1.6 μm. Below Ra 0.8 μm you are usually polishing or lapping, and the stock's inclusion content starts to show.
Material grade matters for finishing. 6061 anodizes evenly. 7075 can show a cloudy patch when the grain is exposed. 17-4PH holds a polish well after heat treatment. Tell the machine shop which faces are cosmetic, because the stock layout decides whether that face is cut from the plate surface or from the core.
Heat treatment after roughing is common on steel parts. Leave 0.3–0.5 mm per face, send the part out for hardening, then finish. Skip the allowance and the part will come back oversize with a scale layer you cannot cut without losing the tolerance.
Step by step: from stock to finished part
Six operations in the order a shop actually runs them
- 11. Read the drawing and pick the stock formCheck the largest envelope, the thinnest wall, and the tightest tolerance. Pick plate for prismatic parts, bar for rotational parts, near-net for expensive alloys. Target stock volume within 1.3–1.6× the finished volume.
- 22. Add allowance and order the sizeAdd 1.5–3 mm per face for parts under 200 mm, 3–5 mm for larger plate. Add 0.3–0.5 mm per face if heat treatment follows roughing. Confirm the mill certificate and hardness range before release.
- 33. Saw and square the blockFace bottom, top, both long sides, then the ends. Hold 0.05 mm flatness and 0.05 mm squareness. Deburr every edge before the next op.
- 44. Rough, then let the part restTake 60–70% of the allowance in the roughing pass. For long or thin parts, re-check flatness and face again if it moved more than 0.1 mm.
- 55. Establish datums and finishCut the datum faces in op 1 and reference them for every later cut. Use a 5-axis setup with a Ø400 mm rotary table when the part has features on four or more faces.
- 66. Inspect before unclampingCheck critical dimensions in the fixture while the part is still located. Record the reading, then remove and inspect the free state. Report both numbers if the drawing is ambiguous.
Stock form comparison
Use the row where the finished shape matches the middle column
| Stock form | Best for | Watch out for |
|---|---|---|
| Plate | Prismatic brackets, housings, covers | Internal stress moves the part after facing |
| Round bar | Shafts, bushings, mill-turn parts | Weak base for heavy side milling in a chuck |
| Near-net forging | Titanium and Inconel structural parts | As-cast surface varies 1–2 mm; needs a datum cut |
| Cast blank | Complex housings with deep pockets | Porosity can appear after 2 mm of cleanup |
| Sheet and tube | Thin panels, frames, enclosures | Chatter and wall collapse without support |
Pick the stock before you pick the toolpath
Stock form, allowance, and the first datum decide whether the part holds ±0.005 mm. Get those three right and the rest of the job is routine.
Common questions
How much allowance should I leave on sawn stock?
For parts under 200 mm, leave 1.5–3 mm per face. For larger plate where distortion is likely, 3–5 mm per face is safer.
If heat treatment follows roughing, add another 0.3–0.5 mm per face so the finishing pass can clean up the scale and hold the final tolerance.
Can the machine cut a part straight from an irregular blank?
Yes, but the first operation has to create a flat, square datum. A casting or forging that varies 1–2 mm across the surface cannot be located reliably until one face is cut.
After that datum exists, the rest of the part is programmed normally. Expect to remove 2–3 mm from the as-cast surface before you reach clean metal.
Why does five-axis machining use less stock?
With a Ø400 mm rotary table, the stock is tilted so the tool reaches five faces in one setup. You avoid the extra material that three-axis workholding would need as a clamping tab.
Fewer setups also mean less re-clamp error, so the true position between features on different faces stays tighter.
Does stock hardness change the cutting parameters?
Yes. Aluminum 6061-T6 at 95 HB runs fast with high rake cutters. A 4140 pre-hard plate at 28–32 HRC needs lower surface speed and a stiffer setup.
If the mill certificate shows a hardness spread wider than 5 HRC across the batch, expect to adjust feed per part rather than running one fixed program.
When is near-net stock not worth it?
When the finished shape is close to a rectangle, or when the annual volume is low. Tooling for a forging or casting only pays back when you remove enough metal to save real cycle time.
For one prototype or a 50-piece run in aluminum, cut from plate. Keep near-net for expensive alloys and for parts with deep pockets and thin walls.
How do I inspect a part that will move after unclamping?
Measure the critical dimensions in the fixture first, while the part is still clamped to the datum. Then unclamp, let it rest, and measure again in the free state.
Report both readings. If the drawing does not say which state applies, ask the designer before the run starts rather than after the parts ship.
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