How to Secure Metal Stock in CNC Milling Machine

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  Key takeaways
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  Why secure metal stock in CNC milling decides the result
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  Which workholding method fits your stock
• 4
  Clamping force, torque, and the mistakes that follow
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  Fixtures, tabs, and thin-wall parts
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  Step by step: securing stock on the table
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  Workholding method comparison

Every cutting force travels from the insert into the part, then into the workholding, then into the table. If any link in that chain flexes, the cutter wins. The part deflects away from the tool, the chip load drops, and the tool rubs instead of cutting. On a 12 mm carbide end mill at 3,000 rpm that shows up as a taper of 0.05 mm over a 100 mm wall, plus a corner radius nobody asked for.

The damage is rarely just dimensional. A loose part can lift off its parallels when the jaw pressure releases, then drop back after the cut. The next pass cuts air. Tool life falls because the insert is hammering instead of shearing, and the operator hears a sound that changes pitch between passes.

Safety is the other half. A 20 kg steel block that leaves the vise at 8,000 rpm carries enough energy to destroy a spindle and a door. We treat any setup that moves under a light tap with a dead blow hammer as a stop-work condition, not a judgement call.

The parts that punish poor workholding most are thin-wall aluminum, long shafts, and anything with a finished surface on two sides. Those are also the parts where a second setup costs the most time. Getting the hold right on setup one is cheaper than blending a taper out by hand.

A machine vise is the default for rectangular, square, or plate stock up to about 400 mm long. Bolt it to the table with two T-nuts, indicate the fixed jaw within 0.01 mm, then seat the stock on parallels and tap it down before final torque. Standard jaws hold 6061 and 1018 without marks. For a finished face, switch to soft jaws machined to the part profile.

Step clamps and toe clamps suit large plates that exceed vise capacity, or parts where you need the full top surface open. Clamp directly over a solid section of the stock, never over a pocket floor, and keep the clamp screw close to the part edge. A clamp that sits 30 mm away from the contact point bends the plate and springs back after the cut.

Vacuum chucks work on thin non-ferrous plates where no clamp can sit. They need a flat, non-porous surface and a seal groove around each part. Aluminum plate from 1.5 mm to 12 mm holds well. The limit is side load: a vacuum gives you even normal force but almost no resistance to a heavy radial cut, so roughing passes stay light.

Dedicated fixtures make sense past roughly 50 parts, or for castings and forgings with no flat reference. A fixture machined from 6061 or tool steel locates on a datum you control, so setup time per part drops to seconds. For a one-off, that cost rarely pays back. For a 10,000-part run, it is the only method that holds cycle time.

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  Regular stock, low volumeMachine vise with parallels and soft jaws if the surface is finished.
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  Large plate, open top faceStep clamps over solid material, not over pockets.
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  Thin non-ferrous plateVacuum chuck with a seal groove; keep radial engagement low.
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  Castings, forgings, high volumeDedicated fixture located on a machined datum.

Clamping force is not a case of more is better. On a 150 mm vise with a 250 mm handle, one hand-tight pull delivers roughly 15–20 kN. That is enough to hold a 100 mm steel block against a 12 mm roughing cut at 2 mm axial depth. Going to two hands doubles the force and starts bending thin stock.

Use a torque wrench until you know the vise. On most 150 mm vises, 40–60 N·m on the screw gives a repeatable hold without crushing the part. Aluminum plate under 10 mm thick will bow visibly above 60 N·m. If you see light under the parallels after tightening, back off, tap the part down, and retighten in two stages.

The parallel check is the one most operators skip. Seat the part, tighten to about 30 percent, tap it down with a dead blow hammer, then finish the torque. Slide a 0.02 mm feeler gauge between part and parallel. If it enters, the part is not seated and the first pass will cut deeper on one side.

Heat moves the stock too. A 200 mm aluminum plate can grow 0.05 mm over a 20 °C rise. For tight-tolerance work at ±0.005 mm, rough the part, let it cool, then take the finish pass. Clamp force and thermal growth stack in the same direction on long parts.

Below about 3 mm wall thickness, no vise can hold the part without deforming it. The answer is a fixture that supports the wall from inside or below, plus tabs that keep the part attached to the parent stock during the finishing passes. Tabs of 0.5–1.0 mm thickness and 6–10 mm width hold a thin wall flat and come off with a hand file or a light pass.

Machine the fixture from 6061 or a mild steel plate, and locate it with two dowel pins in reamed holes. Pins give repeatability of 0.01 mm or better when the fixture comes off and goes back on. A fixture located only by bolts will shift when the bolts are retightened.

For second operations, soft jaws machined to the finished profile of the first side are usually faster than a full fixture. Cut the jaw pocket 0.05 mm under nominal so the part sits with light interference, and mark the jaws left and right. Swapping them is a common cause of a 0.1 mm step between operations.

If the part has no flat face at all, a 5-axis setup with a dovetail or a machined boss on the stock gives you a grip without touching the finished surfaces. This is where the 16 simultaneous 5-axis centers we run earn their keep on complex geometry.