303 Stainless Twisting While Machining

I am working on a job that is quickly becoming a nightmare. Wanted to pick all of your brains to see if you had any suggestions because I have hit a wall.

Cutting 303 cold drawn stainless steel in a haas vf4ss mill. Blank size is 0.3125" x 2.00" x 6.00". Trying to fit 4 rectangular brackets out of the blank. Brackets are 0.2756" x 0.2835" x 5.4331". There is a 1.4961" x 0.1575" deep step in each part that is being milled out. We are running the job in 2 operations. First op is to rough and finish the blanks 0.230" on the depth of the part, face the top, drill and tap thru holes. Second op, if I ever get a good part off, is to saw cut the blanks and load them into a fixture to finish the size and blend the profile from the bottom of the part.

I cannot get these parts to stop bowing on me. They are curling along the length of the part in two directions. I have tried to slow my feeds and speeds and take smaller depth cuts. I have tried to rough the parts leaving .02" of stock a side and then finish. I have tried, as a test, just roughing out the blank and leaving the step and holes for later, saw cutting the part and found it to still be bowed.

Spoke to my supervisor today. I have two things to try this morning. I am going to attempt to rough each bracket with .02" of stock on each one, unclamp the part in the vise and let it spring, then reload and finish the profiles and face the top. If that doesn't work I am going to try to cut down to 3 parts per blank using the same process.



If there is something I am missing please let me know. Thanks guys.

303 cold drawn can be some angry material depending on the source. When we face issues or anticipate issues we send it over to the heat treat shop for normalizing. This is no guarantee to fixing but it sure puts a few more cards in your hand.

You need to take the "bark" off both sides as equally as possible. You are releasing the stresses that were put in when it was rolled at the mill.

Agree with BSC on taking the bark off. Also IMHO Ganging four of these long thin parts out of one blank is a bad idea.

Knew it would be long and skinny

I always have started thick, with at least 2 passes to each side. First pass to take the skin off, second pass to make it straight.

thoughts:

maybe the stock you are starting with has a section that is prone to movement. Maybe starting with like 3/8 square bar would be better? Maybe?


I tend to do what you are doing only in aluminum[lucky for me] and my CAM has a pretty easy bit to trace the outline and leave tabs, so while it would be hard on end mills milling rather than sawing, I wonder if you face both sides, then trace the parts out of the stock, maybe it would stay straighter?

I just made some parts 303 s.s
This was my process, stock 12x3x1/4.
11.8 long 2.44 w .157 thick
Rough in 2.44 equal amount both sides left .05
First pass with shell mill to remove scale part had .3 camber after pass.
Flip force down in Jaws skim back side. Part came back mostly straight, hand bent part as good as I could by eye, flip flop part till had both sides cleaning up lite passes .005-.007 . .012 left on thickness.
Finish width, then ran first op Witch cut ends to length skim top did features flip backside finish part done. That just what worked for me. Every part is different . The more like Swiss cheese the harder it is to get flat with no curl. In the past I have traded 316 for 303 it's a little more stable just tougher on tools try to stay away from 304 it's not as predictable.
Sorry about the book
Best of luck.

Rough them, mill around the part and across the top just take a chip on 5 sides. Send them out to bright anneal and you will be all set. I had hundreds of parts 22 inches long 3/4 square bar. They needed to be milled to size all around and then a pocket inside leaving a 1/8 wall. The curled up like crazy. Bright anneal solved the problem. We roughed them first annealed then finish machined.

Make Chips Boys !

Ron

Rough, straighten, finish, straighten again if needed.
I often use a small arbor press with stops/shims set for how much I need to push to get something back to being fairly straight.

need to have part stress relieved or annealed first. quite common to machine warpage curl and on unchucking it just warps some more. also quite common to straighten part on a press and the next day it warped back a bit.
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longer the part to thickness and width the more it will warp