Groove/Cutoff tool is p!ssing me off...

[Japazo]

Our company purchased a nice CNC Lathe a few months ago and we have slowly been accumulating tooling for it. In an attempt to save a little money in the short term, I purchased a Dorian/Newcomer slot-grip style grooving/cutoff blade and toolholder. It works ok for the minimal amount of shallow grooving that we use it for. The problems arise when we try to cut off anything more than 3/4" in diameter.

On anything that size or larger, the tool leaves a significant taper on the cutoff surface. The back side of the workpiece after being cutoff always has a dome shape, while the remaining stock has a less-noticeable concave shape. We have tried re-positioning the bar, indicating the blade to near-zero, and even shimming the tool-holder to way over compensate for the taper and STILL it will not cut correctly. I am at my wits end with this thing. I know other people are using them successfully so what is the logical procedure for making this thing work?

Any tips, hints or suggestions would be greatly appreciated! Thanks ahead of time...

[txcncman]

What insert are you using? What speed? What feed? What material?

Are you absolutely sure your stock is not walking in the chuck/collet while parting off?

P.S. I hope you are not facing into the wind.

[Japazo]

What insert are you using? What speed? What feed? What material?

Are you absolutely sure your stock is not walking in the chuck/collet while parting off?

P.S. I hope you are not facing into the wind.

lol...

uh, we use gtn-3 inserts with a .122 groove width. We typically run anywhere from 250rpm (right before the cut breaks) to 1000 rpm depending on material and diameter. For soft materials we cut at about .004" per rev. Tougher materials are usually around .0015" to .002" per rev.

A particularly annoying example was some 1 3/8" Ultem plastic we cut recently. It had a .450 thru-hole. After playing with the setup for a few hours the best we could come up with was a .007" length difference from the center of the part to the OD rim. If this were some kind of exotic material I might be ok with that, but it's plastic! It seems like nothing we do has a considerable effect on the taper.

[txcncman]

Ok. You did not confirm or deny that the material was walking. I suspect it is. Try a test where you check the end of the bar stock position before using the tool. Then use the tool to cut, but do not cut off. then recheck the bar stock position. Since you mentioned plastic, this leads me to believe you have a work holding issue.

[Japazo]

No, the material is not walking. First of all, this happens on all materials including mild and stainless steels. We have had this happen with up to 300 psi of clamping pressure on stainless steel. Also, we're clamping using exact-sized bored jaws so the entire face is gripping. When we start to run some more stainless today I'll double check, just to confirm, but I'm 99.9% sure this isn't a work holding problem.

I'm leaning more toward deflection of the blade as it cuts, but cant figure out how or why it is deflecting...

[neilw20]

It sounds like the work is bending away from the tool, and so cutting at an angle.
Have you thought of moving the Z by the amount of the error during the cut.
To prove the bend theory, can you cut off using a live center.
If it changes then it is bending.
Plastic is awful for that.

I often just program in some 2 axis moves to make problems go away.
So the tailstock is off a bit? Just cut a taper and it ends up straight.

[txcncman]

No, the material is not walking. First of all, this happens on all materials including mild and stainless steels. We have had this happen with up to 300 psi of clamping pressure on stainless steel. Also, we're clamping using exact-sized bored jaws so the entire face is gripping. When we start to run some more stainless today I'll double check, just to confirm, but I'm 99.9% sure this isn't a work holding problem.

I'm leaning more toward deflection of the blade as it cuts, but cant figure out how or why it is deflecting...

Ok. You are correct. You win. I will shut up.

[neilw20]

Sometimes fiddling the tool height can help.
A little high, and it bends down to the correct height.
Putting a little high or low can often allow you to steer a skinny tool.
Too much side clearance can allow it to twist, making one side cut a bit more, and steer off.
On bad materials I have often used a tool narrower than the cut requires and do a bit on one side then move out and move z a bit and do some on the other side.
Then the tool tends to steer towards where you have already cut, always forming a dome, which is easy to do a spring cut on.
At high speed it seems to work well. Lots of little light cuts only as deep each time as the tool is wide.
Then a single cleanup pass if needed after the cutoff.

[Japazo]

Sometimes fiddling the tool height can help.
A little high, and it bends down to the correct height.
Putting a little high or low can often allow you to steer a skinny tool.
Too much side clearance can allow it to twist, making one side cut a bit more, and steer off.
On bad materials I have often used a tool narrower than the cut requires and do a bit on one side then move out and move z a bit and do some on the other side.
Then the tool tends to steer towards where you have already cut, always forming a dome, which is easy to do a spring cut on.
At high speed it seems to work well. Lots of little light cuts only as deep each time as the tool is wide.
Then a single cleanup pass if needed after the cutoff.

hmm, I'll have to run a scribe test to see if it is running considerably higher or lower than center. I think we're pretty close to center, but haven't done a qualitative test to confirm.

Does it make a difference that we almost always do a direct cutoff with no relief cuts? You're talking about what the CNC world would call a deep grooving cycle, correct? We can try a few cutoffs like that and see how it goes. It just doesn't make sense to me that doing a full plunge cut through the material with an indicated tool would cause it to dome. I guess I have some more fiddling to do!

[RICHARD ZASTROW]

Some cut-off tools/inserts are formed with a slight angle on the face of the tool. The purpose is to drop off the parted piece then finish the cut by cutting off the "nipple" left on the barstock. When the barstock is advanced, there's no "nipple" to affect the length of the next piece when bumping against the stop.

This angle can cause the tool to "walk" resulting in an angle or dome on the cut off surface.

Dick Z

[txcncman]

Some cut-off tools/inserts are formed with a slight angle on the face of the tool. The purpose is to drop off the parted piece then finish the cut by cutting off the "nipple" left on the barstock. When the barstock is advanced, there's no "nipple" to affect the length of the next piece when bumping against the stop.

This angle can cause the tool to "walk" resulting in an angle or dome on the cut off surface.

Dick Z

He already said he was running the GTN-3 insert. Not an R or an L.

[PixMan]

The problem is probably related to the fact that the GTN/GTR/GTL series of inserts and blades are the biggest pieces of crap made with replaceable inserts.

Iscar came out with that design over 30 years ago, the patent is long gone. Now other companies (including bottom-of-the-barrel Chinese sources) make the inserts. They were never particularly good, just OK. As copies, they truly suck.

Everything you wrote about the speed and feed sounds right, so unless you have excessive distance between the tip of the insert and the body of the holder it SHOULD work fine. However, that design affords the inserts to keep moving back in the pocket as pressure is applied. Eventually the spring clamp action of the steel no longer holds and it's worse on sub-standard holders and/or insert tolerances.

I suggest you find a Sandvik 151, Valenite VSG, Walter FX, Kennametal A2 or other brand in single ended, or if you want the very best go with a double-ended length insert system.