303 Stainless Twizzler

[Mjölnir]

we've got some 1" x 1" x 54" 303 stainless bars that finish at approx 3/4 x 7/8 and has a 3/8 slot down the middle depth about half way through. bars came stress relieved and bowed about .100. we've been trying to flip flop them around machining as equal stock as possible on all sides. we got them close to reasonably flat, i'd be happy with .03" at this point, and they just popped to about .2" bow. i'm beginning to think this isn't even possible with this material but i've been told they've been made before, before i started here. even my shop manager who was here at that time is stumped. any ideas?

[Del.]

Can you cut them with High Speed endmills?

[Mjölnir]

7 minutes ago, Del. said:

Can you cut them with High Speed endmills?

that crossed my mind briefly, it would take forever on parts this long.  we do have some high speed corncob rougher's in the tool cabinet. we've been using bull nosed carbide as sharp corner tools break down down quickly. i suppose at this point it can't get any worse using high speed tools. or can it? 

[Del.]

Just now, Mjölnir said:

that crossed my mind briefly, it would take forever on parts this long.  we do have some high speed corncob rougher's in the tool cabinet. we've been using bull nosed carbide as sharp corner tools break down down quickly. i suppose at this point it can't get any worse using high speed tools. or can it? 

I certainly think it would reduce the bow.

[nickbe10]

Are you clamping them down and flat while you machine? If the bow got baked in during stress relief this might be an issue.

Have you got the ability to and enough stock to machine them in the free bowed state? So now you would be cutting the "bow" out of the picture. If you don't have enough stock for this can you straighten them first?

The problem with 303 is that it is consedered  "low grade" material so you never really know what's in there (unless you get the composition tested), and therefore what might be causing the problem.....

[Mjölnir]

we're machining the bars in a free state and had mostly machined the bow out them. when sizing them by taking equal stock off opposite sides, they bowed twice what they originally were. my stainless experience revolves mostly around injection molds and that's usually 420 which is literally fun to hardmill, it comes out gorgeous. are any of the 300 series stainless steels considered not "low grade"?

[nickbe10]

Given your procedure I am with Del on this one. Use high speed to make your finish cuts. I machined a bunch of thrust reverser rails from 348 stainless many years ago and ended up in the same place.

One explanation for the behavior I was given was that chrome atoms were being dragged/torn out of the matrix leaving a large void when using carbide. As the remaining atoms tried to fill the void by re-bonding it had a tendency to pull the part into a bow. HSS is sharper so will give you better shear characteristics and you will not create the voids, or at least not so many........It definitely worked for me in the 348 (which has more chrome) and was able to hold +/- .002 over 18 inches. It is still one of the most challenging processes I ever worked on. Fairly simple geometry, turning a square bar into H section. But getting the sequencing repeatable was challenging, and I had several +/-.002 tolerances on the part to hold.

Be prepared to change your finish tools more often than you might expect to keep and up front sharp shear action.

[crazy^millman]

2 hours ago, Mjölnir said:

we're machining the bars in a free state and had mostly machined the bow out them. when sizing them by taking equal stock off opposite sides, they bowed twice what they originally were. my stainless experience revolves mostly around injection molds and that's usually 420 which is literally fun to hardmill, it comes out gorgeous. are any of the 300 series stainless steels considered not "low grade"?

316 would be my 1st choice if I were forced to use a 3 00 series for the parts. 303 or 304 would be my last choice.

[crazy^millman]

DP

 

Edited January 19, 2018 by C^Millman
Double Post no reason to repeat myself.

[cncchipmaker]

Are they bowing after you cut the slot in them or when you are trying to cut them flat? Sounds like you may have to rough everything out, straighten, then finish the parts in a free state.

[Rstewart]

Rough, send out for stress relieve again, then finish with HSS.

[mkd]

HSS corncobbs have worked some roughing flatness magic in alum for me.

maybe the same in 303?