The bottom face of a cold plate has one job: stay in full contact with the processor under it. The Open Compute Project’s cold plate guide says the flatness of that face can affect both mechanical and thermal performance, and asks for it to be measured to ISO 12781-2. The number itself comes from the processor maker’s spec.
The trouble starts when the cover goes on. Friction stir welding takes aluminum to around 400–550 °C at the tool, and the part distorts as it cools: typically the middle stays down and the corners lift, by anything from 0.2 mm to several centimeters. Stirweld, which builds the welding equipment, names cold plates as exactly the kind of part that has to stay very flat.
Brazing has its own heat cycle. One model engineer milled a brass part dead flat, brazed it, and found the edge slightly convex; the reply he got was that a part almost never keeps exactly the same shape through brazing, because the two pieces don’t cool together.
So a face you machine flat before joining won’t be flat after it. Leave stock, finish the face after joining, and hold the part so the hold-down doesn’t bend it. Machining the base before the cover goes on is covered in our guide to machining liquid cold plates; this article starts where that one stops.
Why a joined plate springs back
Stirweld makes the point that this distortion comes from heat, not from the force of the tool. Uneven cooling locks stress into the part, and the stress shows up as bow when the part is unclamped.
That’s why clamping it hard doesn’t fix it. Pull a bowed plate down flat and it looks flat while it’s held. Machine the face there, let go, and the locked-in stress bends it back, taking your new face with it. Grinders see the same thing on magnetic chucks: a warped plate pulled flat by the chuck is still warped when the chuck lets go.
1. Stock on the face, and a light finishing cut
Stirweld suggests designing in extra thickness, up to about 1 mm, roughly equal to the distortion you expect, and milling it off after welding.
That finishing pass is a skim across a wide aluminum or copper face with a face mill or fly cutter. Keep it light: a small depth of cut keeps cutting force and heat low, so the face doesn’t pick up new stress. For the machine, that means a stiff spindle and table that won’t chatter on a wide cut.
Leak-test after the last cut, not before. Facing the thermal side removes metal from the floor under the channels, so a test run before finishing doesn’t cover the part you ship.
2. Hold it the way it lies
A vacuum plate spreads the hold-down over the whole contact area. Schmalz, which makes vacuum clamping systems, recommends it for thin-walled and dimensionally unstable parts in aluminum, copper and brass, because there’s no local clamping force to bend them.
On a bowed part, though, full vacuum can still pull it flat. Two things help. Shim the places where the part doesn’t touch, so it sits in its free shape. And turn the vacuum down to what holds the part against the cut.
Busch, which makes vacuum pumps, describes an aircraft parts maker milling 0.3–8 mm aluminum sheets this way, with the vacuum set high enough to hold them during milling but not so high that they warp.
A gauge on the plate, and a signal to the control if the vacuum drops, keep that lower setting safe. If you run coolant, the vacuum unit needs a separator so it doesn’t draw coolant into the pump.
3. Measure on the machine, then off it
A workpiece probe in the spindle can touch a grid of points on the as-joined face before the first cut. You see how far it has bowed and where the high spot is, and you set the first pass from there instead of cutting air or cutting too deep.
After finishing, the probe checks the face while it’s still held. Then check it again off the machine, unclamped. If the two disagree, the hold-down is bending the part. For scale, Heidenhain lists its TS 260 probe at ±5 µm probing accuracy and 2σ ≤ 1 µm repeatability.
4. Keep the face and its mounting features in one setup
Anything that locates the cold plate against the processor, such as mounting holes and bracket seats, should be machined, or at least probed, in the same setup as the face. That way it references the face you finished, not the one you started with. On the machine side, this is about positioning accuracy, not spindle power.
When you don’t need any of this
O-ring sealed cold plates go together without heat, so joining adds no thermal distortion. You can finish the face before assembly. OCP lists the trade-off: lower operating pressure and lower leak reliability than brazed or welded designs.
How we’d set up a machine for joined cold plates
For finishing joined aluminum or copper cold plates, a V-series machining center is the usual choice: linear guides on all three axes, 12,000 rpm direct-drive spindles available across the range, and positioning accuracy of ±0.005 mm per 300 mm. Tables run from 700 × 420 mm on the V6 to 2600 × 600 mm on the V25, so there’s room for several plates on one fixture.
- A vacuum table, supplied as a complete set with its vacuum pump, available as an option on any of our machining centers, with the brand chosen to suit your budget
- A custom fixture, where ports or fittings on the cover stop the part sitting on a flat plate
- A workpiece probe, available as an option
Every cold plate holds differently: where the ports sit, how far it bows, how flat the face has to be. Send us the drawing through our inquiry form, with a measured as-joined part if you have one, and we’ll work out the hold-down with you. See the full range on our machining center page.
FAQ
Will a face I machine flat stay flat through brazing? Rarely exactly. The joint sets while the two pieces are at different temperatures, and they shrink by different amounts as they cool. If the face has to be flat, finish it after brazing.
How much extra stock should I leave for machining after friction stir welding? Stirweld suggests up to about 1 mm, matched to the distortion you expect. Measure a few joined parts and adjust from there.
Sources
- Open Compute Project, Cold Plate Development and Qualification (manufacturing processes, flatness of the bottom surface, ISO 12781-2)
- Stirweld, 13 actions to solve distortion of an FSW welded part
- Schmalz, Clamping Equipment for Metal and Plastic Processing
- Busch Vacuum Solutions case study, Getting a grip on workholding, Aerospace Manufacturing, July 2020
- HEIDENHAIN, TS 260 workpiece touch probe
- CHANSIN CNC V-series specifications and option list
- Featured image: friction stir welding, photo by DLR, CC BY 3.0, cropped
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