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303 Stainless Twisting While Machining


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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. 

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Skinning the material on the back side can help sometimes too with thinner stuff (skim one side, then flip, rough, re-vise, finish if a re-vise alone won't get it). CR steel has a lot of tension in the material at the surface. Relieving the tension in one area usually causes distortion because the forces within the material are no longer in equilibrium. 

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40 minutes ago, Ewood42 said:

Skinning the material on the back side can help sometimes too with thinner stuff (skim one side, then flip, rough, re-vise, finish if a re-vise alone won't get it). CR steel has a lot of tension in the material at the surface. Relieving the tension in one area usually causes distortion because the forces within the material are no longer in equilibrium. 

yes the skin on this material holds stress, sometimes real bad.

it can also be a bit hard, you want to get under it and even maybe conventional cut. might eat cutters

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Rough +.1 min maybe +.2 then stress relieve then semi finish +.02 then stress relieve again. Then finish and yes more operations and maybe more than someone was thinking, but no shortcuts to making a good quality part. It takes what it takes and that is the cost it takes. I would not Step down and make sure I am suing 3 flute high shear tools to pull the stress and heat out using HST toolpaths would be my only suggestion. Might think about cutting lose and running each as a part and not as s set to also allow movement to happen that will not when they are locked into a bar like that.

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303, she's for me.  304, she's a...

I have unfortunately ran way more 303 and 304 than I wish to admit.  Rough and flip.  At that size I would think you could skim one side, flip, rough, relieve and finish.  However, as Ron pointed out it takes what it takes.  I have had good luck with 303 and I have had bad luck with 303.  I had similar parts that I did in 4 ops.  The last op being a finish op with the parts fully seated in soft jaws.

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20 hours ago, gcode said:

and a file that makes perfect parts in one batch of 303 may make trash in a different batch of 303 next week

+1 to that. Though as far as consistency between batches, I think I've had more trouble with 304. Cuts great, until you hit a dirty spot in the steel and there goes your edge.

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On 6/22/2018 at 12:25 PM, Ewood42 said:

+1 to that. Though as far as consistency between batches, I think I've had more trouble with 304. Cuts great, until you hit a dirty spot BALL BEARING in the steel and there goes your edge.

That is what I've always heard.  Hit a ball bearing, because 304 is the garbage of stainless.  I truly have no idea how 304 is made but since day one of my career I have heard it was the garbage of stainless.  My very first job ever (on my own that is) was 304.  Old timer trained me, then at the end of Friday on week one handed me a router and said this is your frist job.  I saw the material was 304SS and researched all weekend.

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3 minutes ago, jlw™ said:

That is what I've always heard.  Hit a ball bearing, because 304 is the garbage of stainless.  I truly have no idea how 304 is made but since day one of my career I have heard it was the garbage of stainless.  My very first job ever (on my own that is) was 304.  Old timer trained me, then at the end of Friday on week one handed me a router and said this is your frist job.  I saw the material was 304SS and researched all weekend.

:blink:

 

That is most definitely a new one! Never found a ball bearing, but we had a couple samples analyzed once when we found hard spots in some material. The hard spots were areas that had higher concentrations of alloying elements, but holy crap, a ball bearing of all things... wonder if they were adding the ball bearings to adjust the composition and just didn't let it cook long enough. Would have had to throw it in just as they were pouring the slab.

 

You're talking still round, not something that just got rolled into the steel, right?

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6 hours ago, jlw™ said:

No, I always assumed --even then-- it was a figure of speech.

304 should be pretty good as it is food grade stainless in some applications.

It isn't as good as 316 for food grade and it isn't as consistent as 316 or 303 free machining either, in my experience.

I wouldn't call it garbage because it is high chrome and nickel makes it expensive. But i have cursed it and called it worse because speeds and feeds one batch were 20% slower next time around

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