Why does my 303 part warp?

[jwatte]

I'm making a Decora-style wall outlet cover in 303 stainless.

The side "walls" are 2.4 mm wide, and the "wall height" is 4.4 mm, the face plate is 2.2 mm thick.

Here's a picture:


And a bit of the sketch of the Fusion 360 model:


I make this out of 3/8" flat bar 2.5" wide 303 stainless steel on a home CNC mill, using a vise as workholding, and flood coolant.
The face is machined with a facing operation with a 3/8" 4 flute end mill on the front side, and scooped out with the same mill and an adaptive operation on the back/inner side.

Once it comes off the vise, it's square. Right after, I use an orbital sander on the front to give it a more brushed look (WD40 and 120 grit sandpaper,) and after that, the plate is warped, as if the front face has expanded, or the side "walls" contract. There's about a 0.5-0.8 mm height difference between the edges and the center -- very pronounced!

The sanding is not particularly heavy, and the part isn't particularly warm after sanding.

What makes it warp like this, and what can I do in my design or machining to avoid this happening?

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[Tarzan63]

If the part is flat and not distorted before sanding it, then you are creating too much heat with the sander. Maybe just use scotchbright by hand.
if it actually distorts during machining, then you need to rough both sides leaving say .02-.03 per side , then finish both sides after rough ops. Also, on the finishing ops, clamp lightly so you don’t bend or put stress into it.

[jwatte]

Interesting; thanks!

I've previously learned that thin "finishing cuts" aren't great for stainless because of work hardening.
Is this perhaps less of a problem for 303?

[Tarzan63]

Using dull cutters or tools that are not on center will cause overheating the material and work hardening.Although,yes,you do want to take a decent depth of cut while roughing. I cut a lot of stainless and typically leave .01 for finish, then use a .0039R to .0156R cutter for finishing. A positive rake insert helps too. I usually use a .032R insert for ruffing with a strong chip breaker with a heavier feed. Too light of feed while roughing also generates heat and distortion.

303 is one of the easiest to machine stainless steels there is. You really shouldn’t have a lot of problems with it. Though machining thin walls in almost any material takes a little finess in workholding and machining it.

[mactec54]

I'm making a Decora-style wall outlet cover in 303 stainless.

The side "walls" are 2.4 mm wide, and the "wall height" is 4.4 mm, the face plate is 2.2 mm thick.

Here's a picture:


And a bit of the sketch of the Fusion 360 model:


I make this out of 3/8" flat bar 2.5" wide 303 stainless steel on a home CNC mill, using a vise as workholding, and flood coolant.
The face is machined with a facing operation with a 3/8" 4 flute end mill on the front side, and scooped out with the same mill and an adaptive operation on the back/inner side.

Once it comes off the vise, it's square. Right after, I use an orbital sander on the front to give it a more brushed look (WD40 and 120 grit sandpaper,) and after that, the plate is warped, as if the front face has expanded, or the side "walls" contract. There's about a 0.5-0.8 mm height difference between the edges and the center -- very pronounced!

The sanding is not particularly heavy, and the part isn't particularly warm after sanding.

What makes it warp like this, and what can I do in my design or machining to avoid this happening?

Stress relieve the blanks before you machine them, you may have to do it again after machining as well, the sanding is not the problem

[jwatte]

Thanks to both of you, I'll look into both of those suggestions!

MatWeb says stress relieving temperature is 400 to 750 degrees Farenheit for 303, so step 1 is using an oven at 550 and see if that helps. (I don't have a furnace, and sending it out isn't in the cards here...)

[mactec54]

Thanks to both of you, I'll look into both of those suggestions!

MatWeb says stress relieving temperature is 400 to 750 degrees Farenheit for 303, so step 1 is using an oven at 550 and see if that helps. (I don't have a furnace, and sending it out isn't in the cards here...)

That may not hit the needed Temperature to do enough good, but worth a try

[djr76]

I machine 303 all the time. Just straighten it in an arbor press.

[deadlykitten]

Once it comes off the vise, it's square

hi / after you take the part out of the vise, even if it is square, leave it like that and check again after 1-2 hours; if it is still square, then you need to review your 'sanding' procedure, more precisely, during sanding, try to reduce forces, heat and duration ... just saying, i don't know how you are 'sanding' exactly; do you clamp the part and sand, or part is free during sanding ? kindly