Exploded a GTN3 holder

[mmcmdl]

Yes they need to be pushed hard like most carbide tools.
The problem with programming a CSS is that it will be running at max RPM as the part breaks off!! A capped RPM is used to lower surface speed from a set diameter. A tiny surface speed is unavoidable at small diameters but still cuts.
Speeding up on a manual lathe when getting smaller is ok, providing the trajectory of the part is considered.

Sure , a G96 CSS code along with a max RPM code is used to control this . FWIW , I wouldn't be parting this off totally . I would be leaving an inch or so for the saw to finish the part off . At 3-4-5000 rpm and cutting off , this would make for a pretty cool projectile flying thru the shop or your safety glass . I had a $5000 hunk of Berrilium do this once , not fun .

 

[Parlo]

Something else to consider, which I have observed but have never seen mentioned, is the increased difficulty in getting cutting fluid down into the cut as the depth increases. This is peculiar to parting because of the narrow groove and the tendency for centrifugal force to throw the fluid out of the the groove. The absence of lubrication and cooling in the cut could result in heating and expansion of the material, causing binding. Flood coolant overcomes this problem, as would a squirt bottle.

You're dead right. The original post stated that oil was used, this is definitely a job for plenty of coolant.

 

[NC Rick]

I have some parting holders that look just like that. Fine, fine, fine and then…. That is a hell of a cut. I have never parted 4”SS. 4” aluminum and one needs to be careful as the diameter reduces. I prefer to saw off and face when I could.

 

[mikey]

With a constant RPM. The depth of cut - ie. feed per rev remains constant as the circumference decreases then the metal removal rate decreases. High metal removal imparts more pressure to the tool than a low removal rate. There are no references to minimum cutting speeds.

Still stuck on metal removal rates and formulas, eh. What you seem to be saying here is that with a constant speed and a consistent feed then as the circumference at the point of cut decreases then so does the MMR. Then, since the MMR supposedly reduces under these conditions that this applies less pressure to the tool?

So how do we explain the crushed insert in the pic at the start of this post? Seems to me that tool pressure increased here, no?

 

[Aukai]

For what ever reason the bit buried into the material hard enough to snap the parting blade itself, and it made a big fuss trying to get the bit out. Parlo are you sure as the surface speed decreases, that the feed is the same taking a bigger bite didn't break things?

 

[stupoty]

Something else to consider, which I have observed but have never seen mentioned, is the increased difficulty in getting cutting fluid down into the cut as the depth increases. This is peculiar to parting because of the narrow groove and the tendency for centrifugal force to throw the fluid out of the the groove. The absence of lubrication and cooling in the cut could result in heating and expansion of the material, causing binding. Flood coolant overcomes this problem, as would a squirt bottle.

Yeah I always have this problem with parting , would like flood coolant to help with this.

Stu

 

[Parlo]

Still stuck on metal removal rates and formulas, eh. What you seem to be saying here is that with a constant speed and a consistent feed then as the circumference at the point of cut decreases then so does the MMR. Then, since the MMR supposedly reduces under these conditions that this applies less pressure to the tool?

So how do we explain the crushed insert in the pic at the start of this post? Seems to me that tool pressure increased here, no?

It seems you don't see the correlation between MMR & tool pressure, so regretfully I will resist looking for simpler examples to explain this.

The tool pressure increased because it got hot + dull so the material work hardened, the tool could not cut and broke. The most brittle part is the insert, so chipping is certain.

No coolant leading to work hardening and premature tip wear.
No coolant on a dull tip that was worn on the first part - see above.
Excessive tool pressure on a dull tip.
Incorrect insert grade and geometry leading to premature wear.

 

[ericc]

You can feel the cutting pressure go up. There are potentially two things going on here. First, the correct way to look at the cut is the feed per distance traveled. Not the feed per rev, since this ignores the thickness of the chip being lifted. Imagine unrolling the cut. It is like a shaper whose downfeed is held constant, but the ram speed is getting slower and slower. Eventually, the bit plows too deep and experiences excessive force. The second thing going on is that any misalignment of the tool off center becomes more serious. If the cutter is low, the tendency to roll over the top becomes worse as the stub gets thinner. If the cutter is high, the clearance angle gets worse as the diameter decreases. You can see this by just drawing a picture.

Once, an experienced machinist told me to just keep turning the cross-feed, and turn even harder if experiencing any trouble parting. This works if one is having problems with chatter, but when the diameter gets small, pushing faster and harder doesn't help. Yes, I admit it. I followed the advice blindly (you know how certain those experienced machinists can be), and I got the expected bad results. It reminds me of when I was criticizing an engineer's design. It failed in the simulator, failed on the bench, and eventually failed in the field. He grinned sheepishly and said, "that's just because you're an idiot", but at least that stupid thing never came up again.

Think through what you are doing!!!!

 

[Parlo]

For what ever reason the bit buried into the material hard enough to snap the parting blade itself, and it made a big fuss trying to get the bit out. Parlo are you sure as the surface speed decreases, that the feed is the same taking a bigger bite didn't break things?

It takes the same thickness of chip at all diameters. It removes smaller and smaller amounts of material per revolution as the diameter decreases, lowering the tool pressure as it goes.

 

[Aukai]

I think as you go to the center at the same advancing speed, the rotating speed slows down, and is making a deeper cut. That's how I'm thinking it's working.