Exploded a GTN3 holder

[mikey]

The metal removal rate and therefore the load on the tool decreases as the diameter reduces when using a constant rpm and constant feed per rev. If the feed was excessive it would show at the beginning of the cut where the forces are greatest. I suspect the blade was too weak for the operation and a deeper blade and possibly wider would have been more suitable.

Look, given that his spindle speed was constant, the cutting speed at the point of cut was continually reducing as the cut proceeds. As he gets closer to the center, cutting pressure at the tip of the tool increases. Given that he was using auto-feed, that tool tip is being fed under increasing pressure but it is not cutting because the cutting speed is too low. A machining center controller would have increased spindle rpm to accommodate this but that wasn't possible here so the blade broke.

This was about cutting conditions and had nothing to do with the blade.

 

[Parlo]

Look, given that his spindle speed was constant, the cutting speed at the point of cut was continually reducing as the cut proceeds. As he gets closer to the center, cutting pressure at the tip of the tool increases. Given that he was using auto-feed, that tool tip is being fed under increasing pressure but it is not cutting because the cutting speed is too low. A machining center controller would have increased spindle rpm to accommodate this but that wasn't possible here so the blade broke.

This was about cutting conditions and had nothing to do with the blade.

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.

 

[Janderso]

I haven't had the balls to part with the auto feed yet.

What lathe do you have?
If your set up is right and you have a rigid lathe, it’s a piece of cake.In my experience

 

[Janderso]

Look, given that his spindle speed was constant, the cutting speed at the point of cut was continually reducing as the cut proceeds. As he gets closer to the center, cutting pressure at the tip of the tool increases. Given that he was using auto-feed, that tool tip is being fed under increasing pressure but it is not cutting because the cutting speed is too low. A machining center controller would have increased spindle rpm to accommodate this but that wasn't possible here so the blade broke.

This was about cutting conditions and had nothing to do with the blade.

I’m sure you are right. Those type of cutters flex quite a bit so aren’t rigid. A 4” piece of SS is a lot to ask for that cutter imho.
Watching videos of machining centers with the ever changing spindle speeds is amazing.

 

[Janderso]

4" diameter 316 SS, 135 rpm, I set the auto feed for the least amount of chatter(still some fine vibration) as the part was going the cut became silent, going well. Very quickly there was a big vibration, and before I could shut things down, the GTN3 holder snapped. I had previously parted with the same insert, I'm wondering if the bit should have been changed before attempting another 4" SS cut. My setup for parting is to indicate the blade with a dial indicator </= .0005 at least in and out deflection of the tool post. I use the auto feed to try, and stay ahead of the work hardening, and plenty of cutting oil. I also move the blade out of the holder, and hand spin the chuck to get to fresh metal. The piece of the of the insert that was imbedded in the cut was hard to remove, and it took awhile to get it out too.

That’s a lot to ask of that cutter. I learned my lesson on the one that I broke. They are fine for lighter cuts in my experience.

 

[Parlo]

I’m sure you are right. Those type of cutters flex quite a bit so aren’t rigid. A 4” piece of SS is a lot to ask for that cutter imho.
Watching videos of machining centers with the ever changing spindle speeds is amazing.

Do you have any proof or references that tool loading increases as metal removal rate decreases.
I agree that the tool is not strong enough for the application.

 

[mmcmdl]

Those Iscars like to cut and they will do just that . SFM is the issue . If not a CNC , the operator has to be aware of it adjust speeds and feeds . These cutoff tools aren't made to push or rub material off , they're made to cut . I've pushed some to , and past the limits with excellent results .

 

[Parlo]

Those Iscars like to cut and they will do just that . SFM is the issue . If not a CNC , the operator has to be aware of it adjust speeds and feeds . These cutoff tools aren't made to push or rub material off , they're made to cut . I've pushed some to , and past the limits with excellent results .

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.

 

[MrWhoopee]

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.

 

[mmcmdl]

Some of my older Iscars had coolant passages directly to the insert which helped greatly . They almost need a separate flood or heavy drip line for optimal cutting which most hobby shops don't use . ( including myself these days ) . A squirt bottle filled with oil works good but it ties up your " emergency exit " hand .