CNC Fabrication of Extruded Profiles: Machining a Part That Was Never Flat

An extrusion is straight because it is stressed

An extruded profile leaves the die with a frozen-in stress pattern. The outer skin cools first and sets, the core cools later and tries to shrink against a skin that will not move, and the result is a section that is held straight by an internal balance of tension and compression.

That balance is what machining destroys. Cut away one wall and the remaining section is no longer in balance, so it moves. The movement is not a machining error; it is the extrusion relaxing into the shape it would have had without the removed material.

The practical consequence is that a machined profile has to be designed for the stress release, not just for the final geometry. A profile that is straight as extruded and true after machining is a profile whose walls were balanced before the cut.

Distortion tracks the wall you remove

The distortion over a one metre length rises with the depth of the cut: 0.15 mm when 1.5 mm of wall is removed, 0.25 at 2 mm, 0.45 at 3 mm, 0.70 at 4 mm and 1.00 at 5 mm. The relationship is close to linear with depth, and it is much steeper on a thin-wall hollow section than on a solid one.

The number that matters is the ratio of removed wall to remaining wall. Removing 1 mm from a 3 mm wall leaves 2 mm and a big imbalance; removing 1 mm from a 10 mm wall leaves 9 and almost none. That is why a solid profile machines true and a hollow one does not.

There are three ways to deal with it. Machine symmetrically, so that material is removed from both sides and the balance is preserved. Machine in stages, with a stress-relief pause between them. Or anneal the profile before machining, which relaxes the frozen-in stress so that the cut releases nothing.

Table 1 — Distortion against depth of cut, and what to do about it.

Fixturing a profile that wants to move

A profile cannot be clamped like a block. Clamping force distorts a hollow section, so the part is machined true in the fixture and springs back when it is released. The result is a part that measures correctly on the machine and wrongly on the bench.

The usual solution is a support fixture: a machined nest that matches the profile's outside shape, with clamps that close on the nest rather than on the part. The nest takes the clamping load and the profile is only located, not squeezed.

Table 2 — Fixture types for a hollow profile, and what each one does to the part.

For long parts, the fixture also has to deal with the profile's own straightness, which is typically 1 to 2 mm per metre as extruded. A nest machined straight will hold a straight part and leave a bowed one bowed; a nest machined to the profile's actual curve holds it consistently but produces parts that are all equally curved.

Feed rate has an optimum, not a maximum

Machining PVC and PP produces a burr at both extremes of feed rate, for different reasons. Too slow and the tool rubs, generating heat that softens the material and smears it. Too fast and the cut is interrupted, the tool chatters and the edge chips.

Measured on a PVC profile with a 6 mm two-flute cutter, burr height is 35 micrometres at 1000 mm/min, 25 at 2000, 40 at 3000, 65 at 4000 and 95 at 5000. The optimum is around 2000 mm/min, and it is a genuine optimum rather than a compromise between two acceptable ends.

Spindle speed shifts the optimum but does not remove it. Higher speed reduces the chip load at a given feed, which reduces the rubbing at the slow end, but it also raises the temperature. On PVC the practical range is 12,000 to 18,000 rpm with a single-flute cutter, and on PP it is lower because the material is softer.

Coolant, chip removal and clamping marks

PVC and PP are machined dry or with air blast rather than flood coolant. Flood coolant on PVC leaves a residue that is hard to remove and can stain; air blast removes the chip and keeps the edge cool without wetting the part.

Chip removal matters more than cooling. A recut chip welds to the edge and produces a tear, and on a deep pocket the chips have nowhere to go. A vacuum shoe or a directed air blast is part of the tooling, not an accessory.

Clamping marks are the third defect. A profile clamped in a vice will show a witness line at the jaw, and on a visible face that is a reject. Soft jaws, a machined nest, or clamping on a face that will be trimmed are the three options, and the third is usually the cheapest.

What to specify

A machined profile is specified by the final geometry and by the process constraints that the geometry has to respect.

• Wall thickness before and after machining, so the stress release can be assessed.
• Whether the profile is annealed before machining, and at what temperature.
• Flatness or straightness after machining, and over what length.
• Edge quality: burr height limit and whether deburring is included.
• Which faces may carry clamping marks.
• Tolerance on hole positions, which is set by the fixture, not the machine.

The pre-machining wall thickness is the item that gets left off and causes the most trouble. A drawing that gives only the finished wall does not tell the machinist whether the cut is a light skim or a stress-releasing operation, and those need different fixtures.