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What causes this chip shape?

Nerdlinger

Nerdlinger

Stainless
Joined
Aug 10, 2013
Location
Chicago, IL
Please see the attached pic of the uncurled chip. It is sharp on one side and jagged on the other. This material is 8620 but I have also run into this with 4340. Is that what happens when the feed rate is too low so the chipbreaker on the insert can't do it's job? I was at 300SFM and .002IPR (for some reason) and kept increasing the feed rate until it went away (.007IPR) but I like to try to "know" what actually happened there...

Thank you!

Chip.jpg
 
When you flex it, does it brittle fracture, or is it ductile and takes a number of bendings to break?

I'm guessing (WAG) that it's stacked cold-welded* chips, polished by rubbing slowly on the insert top. CarbideRobert would be better to judge this...


*Relatively speaking, of course.
 
.312" down to .247" with .016" rad insert. I think it's a Kennametal PF chipbreaker. Like I said, it went away after I increased the feed to .007ipr but am just wondering if the theory behind it is: too low feed rate prevents the chip breaker from curling the chip. Thanks!
 
Milland said:
When you flex it, does it brittle fracture, or is it ductile and takes a number of bendings to break?

I'm guessing (WAG) that it's stacked cold-welded* chips, polished by rubbing slowly on the insert top. CarbideRobert would be better to judge this...


*Relatively speaking, of course.
Click to expand...

I'll see if I can dig a few up and try but I think they crack.
 
Carbide Bob will give a thesis on this. I’ll give you the tech guy for distribution answer you nailed it! To low of feed you’re probably just rubbing the metal off instead of getting shear action.
 
Nerdlinger said:
I'll see if I can dig a few up and try but I think they crack.
Click to expand...

Yeah, I'd think so. So my supposition is the low feed leads to thin, ductile chips which don't flow well over the insert (maybe impacting chip breakers at the wrong place), leading to rubbing and stacking/folding of chip on chip.

So you get a series of short segments cold welded to each other, so they will break when flexed. Jagged on one side, smooth on the other. When you feed harder, the thicker chip flows over the tool differently, allowing the chip breakers to function properly and breaking the chips correctly.

Again, just my interpretation...
 
is the stock heat treated or is it annealed?
are the chips showing sign of heat EG turning blue, all heat in the chip and not the tool?
 
reason I ask about the stock if it's heat treated, I was taught at an young age heat treated to machinability harden to
27-36 HRc will be easier to machine, and get a better finish, annealed is to gummy. and with the right feeds and speeds
short cycles, rough cut with heavy DOC and slow feed, then finishing low DOC higher SFM.
but not an expert here so don't banish me.
 
Since CarbideBob hasn't showed up, I'll stick my neck out.......:cool:

It looks like "razor wire" to me, which means the chipbreaker isn't doing its job (feed too low for the chipbreaker).

Kind of like fine finishing with a flat top insert, ever try that..?? :willy_nilly:
 
ChipSplitter said:
Since CarbideBob hasn't showed up, I'll stick my neck out.......:cool:

It looks like "razor wire" to me, which means the chipbreaker isn't doing its job (feed too low for the chipbreaker).

Kind of like fine finishing with a flat top insert, ever try that..?? :willy_nilly:
Click to expand...
no to chicken,

just observations
but looking at the stringy chip, looks thin and is not discolored, from heat.
looking at the OP set it is run from a collet with small dia. bar stock, which is cool
question#1 why is this chip not creating enough heat?
question#2 will increase in feed rate solve #1 or ?
question#3 is the depth of cut correct feed rate is not enough.

back to square one on recommended feeds and speed for the material and that insert?
 
I'm not sure about the one side sharp and one side jagged but not breaking as has been mentioned is very likely the combination of your DOC and feed, and maybe speed too. You've got a good DOC but the feed is low, which creates a wide, thin chip that is flexible enough to not be broken by the bend imparted by the chip breaker. A similar thing can happen if you make a thin chip the other way, with low DOC and reasonable feed. A more square cross section of chip will be less flexible and tend to break easier trying to bend at the same radius.
 
You're cutting 8620. Pretty sure it's made out of melted VW buses and M-16's from the 60's. Predicting a chip in almost any cut parameters is going to be difficult.

R
 
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