Milling aluminium - Strategy to minimize warp/spring

Let me know if this is posted in the wrong place and I'll move it. I'm a hobby machinist with an older 3 axis CNC mill converted to Linuxcnc. I've made quite a few parts successfully over the past few years, but still lack experience.

This project is a turbo inlet plenum for my project car. It's by far the biggest, highest material cost, thinnest wall component(s) I have attempted so far, so I thought I'd ask for some advice before I turn these billets in to scrap.



Overall part length is 450mm and I'll machine the two halves from two AL 7050 plates starting at 65mm thickness. Part wall thickness varies but I've gone down to 2.75mm in places and I'm worried this thing is going to warp/spring. There's a perimeter of M6 screws which will hold the two halves together. A 2mm O ring chord seals the two halves together.

I'll describe the dark grey part, but they are both pretty similar:
I was planning to try and do the inside and outside faces in two ops, then machine the four holes in the inlet flange later with the two finished halves screwed together

OP1:
Stock screwed to a 25mm plate which is then clamped down.
Hog out all the internal material
finish all the internal faces
Drill / mill inlet port holes
drill and tap the perimeter holes
Drill and ream the two 4mm dowel holes
Mill O-ring groove.



Op2:
Part now flipped and screwed to the 25mm base plate using the perimeter of bolt holes. Positioned on two 4mm dowels
Rough out and finish all external surfaces
Finish 4 inlet ports
Mill 4 O ring grooves.



Question is, how much do you think this thing is going to spring when I unscrew it?



I could try:
Op1: Rough out inside faces then Drill and tap perimeter holes
Op2: flip part, rough and finish all external faces
Op3 flip part back again and finish machine internal faces.
BUT I'm worried that during OP3 the chatter will be horrendous - 2.75mm walls singing.

So yeah, how should it really be done?
Thanks in advance.

FWIW just leave out the hobby machinist part.

I personally haven't made parts like that, but I've seen quite a few that have. I think you will be ok on just the two op setup. I have seen a lot of very similar parts for similar applications and they are done in the same way you are describing. I don't think you will have much issue with movement. Plus, if the holes are drilled in place and the dowels line up, any small flex can just be pulled out when you align the halves and bolt them together. Personally, your method seems pretty reasonable and if it was me, I would just go for it.

It starts with good aluminum plate that's been strain stress relieved. Cheap metal costs you if you get unlucky with internal stress or porosity (some Russian 7075 has given me issues).

That's a fairly stiff part from a geometry standpoint, so that helps with setup plans. But while you could do them in two ops, I think I'd take the hit for a little extra setup and plan on four, with a wait between roughing and finish to allow a little time for movement.

You could do this by alternating the two parts - R1 inside, R2 inside, R1 outside, R2 outside, F1 inside, etc. A little time lost to the extra setups, but granted, more risk if you don't get the registration right for finishing.

Post pics when you're done...

It starts with good aluminum plate

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Key word, Plate. Definitely buy sawn plate, it has a lot less internal
stresses than extruded bar stock. I've learned that lesson the hard way,
several times.

where do the stresses in 7000 series aluminum come from? it is annealed and precipitation hardened after being extruded, right? is that process not uniform?

dian said:

where do the stresses in 7000 series aluminum come from? it is annealed and precipitation hardened after being extruded, right?. is that process not uniform?

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Depending on vendor, you could get residual stress from rolling operations, and if there's not good uniformity of heating when age hardening you might get irregular properties from the batch.

I've seen some major variances from a vendor that I got extruded 6000 stock from - what a pain that was...

i was thinking that solution treating would be a complete anneal. then its heated for several days, so should be pretty uniform, but just speculating here.

i will have to do some parts from 7075, so i wonder if there is any skin to take off etc.

Milland said:

I've seen some major variances from a vendor that I got extruded 6000 stock from - what a pain that was...

Click to expand...

a bit off topic, but this year 6082 we see here has been all over the place, some parts anodize with noticeable yellow hue, some clear gray, some have extrusion "bar end" defects, some have abnormally large grain size near the skin of extrusion, there was some bad material from time to time in previous years, but nothing like this year

on topic - I'd definitely leave the outside finishing as a very last step, that outside perimeter bolted down is quite stiff structure, and even if that hole will have a M6 bolt go through it, don't drill it to 6mm (or 6.1), drill and tap it for M6 thread that you can use to hold it down for last op, since this is one off, you can then even use hand drill to remove that thread and drill it to final size

I'd also leave plenty of material after roughing, like +2mm, sometimes these U shapes open up considerably, especially considering the quality of the material as of late

the part with the round outside, leave some bosses on it (similar, but not as large as the ports that get bolted to the engine head) to have something to hold on to the part with, cut them off in the last finishing op when they'll be sticking up

dian said:

i will have to do some parts from 7075, so i wonder if there is any skin to take off etc.

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I like to plan for it. I usually have the luxury of not being under time pressures or getting every last cent out of a job, so multiple setups and thicker stock work for me when appropriate.

If these are critical parts you may want to get the material analysed before putting time into the job. The lack of testing and material control really screwed over Orbital Sciences when it came to a fairing separation strip that turned out to not meet spec. Multiple missions lost along with 100's of millions of dollars.

NASA Uncovers 20-Year Aluminum Scheme That Caused 2 Failed Missions - ExtremeTech

From that link: "NASA lost around $700 million on the failed Glory and OCO missions".

Last place I worked we did a lot of 7075 parts that would start out as plate and end as thin walled highly contoured parts.

We would rough maybe leaving .20" a side and then send it out for stress relieve.

I've got a large 7075 part I'm running in a few days, I don't have time to stress relieve so I'm going to flip it multiple times during roughing before I finish the part.

tom_R2E3 said:

BUT I'm worried that during OP3 the chatter will be horrendous - 2.75mm walls singing.

.

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In similar situations I've put a layer of thixotropic (doesn't sag) epoxy on the opposite wall before finishing. That will stiffen the wall, and after you've finished you can use a heat gun to warm up the part and remove the epoxy.

I don't know what facilities you have, but this is how I generally handle such parts (milled or turned) in a ''get it done / shade tree / one man and his neighbours cat, hole in the wall job shop.'' - aka the sort that PM purists HATE more than the IRS

Rough to at least .1'' / 3mm OS all over. ...............................some places that might move more (complex thick to thin junctions etc etc) will need extra material

Leave at least 24 hours in a warm place (on top of my central heating boiler is good)

Re set and establish a fresh datum to work from.

Often I have to play with speeds and feed ''the book'' S&F's can be miles out for unstable parts.

Thin parts that vibrate etc, fill with molten candle wax - damming up around the outside if necessary, others I wrap in masking tape - etc etc

i found this:

"7075-T6 is a very high strength, solution treated and "artificially" aged (i.e. furnace-aged at a temperature above room temperature) aluminum alloy. The Tx51 tempers are subjected to a mechanical "stress relief" that can be thought of as "stress aligning". The material is stretched (at the mill) during the straightening after solution treatment and prior to aging. The stretching operation aligns the hot/cold working, straightening and quenching stresses in the direction of the stretch. While this method does not eliminate the stresses, it does allow for control of those stresses. During machining the "movement" of the machined surface becomes more predictable since the randomness of the residual stresses have virtually been eliminated.

The temperature required to provide any meaningful stress relief would be greater the aging temperature originally used to achieve the T6 strength level, and would therefore result in "overaging" of the material -- significantly decreasing the mechanical properties. This is the reason that you are having a hard time finding stress relief schedules for solution-treated-and-aged aluminum alloys, and that thermal stress relief is not recommended for heat-treated aluminum alloys. Reheating of previously heat-treated aluminum alloys is also subject to other potential problems."

also:

"The best and safest way to avoid distorsion is to let the part rest for about 2 weeks before final machining. Then the final machining should be done with sharp tools and small feed. This is the original recommendation from the ASM metal handbook (revision 8) that I used more than 30 years ago and it worked fine for very accurate parts."

vibratory stress relieve is recommended (even increases fatique life).

Good advice above-component depth is to advantage too. I've machined a few cranks out of solid(black m.s)Rough leaving 1/16 on,and let it sit for a week. Also ring it with my nylon hammer. Btw Sami-looked around Whitmores. Tempted by 1or2 things but now out my price bracket!

Thanks everyone for the constructive input! Some great information and useful tricks - I would never have thought of using epoxy to increase the resonant frequency of the part.

I can't say much for the quality of the material, other than it came from a very high end aerospace machine shop. It's been on the floor of my garage for the last eight years waiting for a project.

The more I look at the job, the less worried about distortion I am. Having said that, the parts are for me and I have time on my side so I'll program it for four Ops on each part. Will post photos when it's done (probably over the Christmas holiday).

Thanks!

Hello again, a little more feedback on this project:

I finished it over Christmas and new year and did each half of the plenum in two setups. I had no problem with the aluminium springing / warping. All in all it went really well with no serious mistakes. Surface finish on the outer surfaces could have been better, I used 0.8mm stepover on part with a 12mm ball nose. I think I was too impatient and should have taken more time on the machine, next time I'd go with 0.5 or 0.4mm. In the end I hand finished the outer surfaces to remove the machining marks so it doesn't really matter.

Here's a couple of photos as requested:

temp - Google Photos