Tool Height On A Lathe

chevydyl said:
cut off tools must be dead on center, cutting tools its said that a thou above is good, but on center is good, never below, it causes dig and pulls the tool into the work, that's why you go a thou above
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Hmm-I think that is backwards-if the tool is above center, and it digs in, it will be pulled down, and into the larger part of the work diameter below the tool.

If the tool starts out below center, and it digs in, it will be pulled downwards and out of the work.(the part of the work below the tool moves away from the tool)
 
awander said:
Hmm-I think that is backwards-if the tool is above center, and it digs in, it will be pulled down, and into the larger part of the work diameter below the tool.

If the tool starts out below center, and it digs in, it will be pulled downwards and out of the work.(the part of the work below the tool moves away from the tool)
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He was right.

When you are turning below center, the workpiece wants to grab and climb up onto the toolbit...worst case scenario. Your depths of cut will be off, depending on how far off center.
 
Correct me if I'm wrong but jbolt was not stating that the cutting tip of a cutt off tool must be below center. He is stating that most cutt off tools (integrated QCTP type or mounted in a QCTP turning holder) generally sits lower on the tool post compared to how high a typical turning tool would sit at center height.

Franko, to answer you question, how much lower really depends on the type & size of cutt off tool used. Larger the blade (height) tge lower it would need to sit.
 
awander said:
I just did some checking (instead of just shooting off my ..pen) and it looks like the conventional wisdom is that I was wrong.
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That doesn't jive with my experience. When it bit is high and it snags, it is pulled deeper into the work, usually abruptly and sometimes with a bad result, such as gouging the work, breaking the tool or dislodging the work from the chuck.
 
I measured some of my AXA style tool holders and the seat for the tool bit varies in height from .440" to .470". The Aloris, Phase II, & Shars spec. is 7/16" or .438". The Dorian spec. is .375".

The Grizzly G0602 spindle center is .920" above the compound rest. This means a 1/2" tool bit in an AXA holder will be .018" above the centerline, minimum, unless a Dorian tool holder is used or one of holders from the other manufacturers is cut down.
 
darkzero said:
Franko, to answer you question, how much lower really depends on the type & size of cutt off tool used. Larger the blade (height) tge lower it would need to sit.
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What I am trying to determine is how much more material would I ever want to remove from my turntable seat for the maximum lowering of the tool I might require. The answer seems to be enough so the lowest the tool can go is about 1/8" below center.
 
Franko said:
That doesn't jive with my experience. When it bit is high and it snags, it is pulled deeper into the work, usually abruptly and sometimes with a bad result, such as gouging the work, breaking the tool or dislodging the work from the chuck.
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I think it depends on the tool geometry; rake and clearance. Raising a tool has the effect of increasing rake but decreasing clearance. If there is minimal clearance, the tool will not dig in because of interference that arises. If there is generous clearance, the tool can dig deeper. The rigidity of the lathe and the work also comes into play. As the tool starts to bite the tool deflects downward and the work tends to deflect upward. A more rigid setup controls those deflections better so there is less chance of digging in.
 
Rigidity is definitely good. The time it happened to me, my ways had gotten a little loose. It wasn't something that I checked very often. When it happened, I actually saw the tool moving and was thinking, "that can't be good."

The next moment before I could shut it down, Bam! Gouged part pulled crooked in the chuck and broken insert, in an instant.

I'm sure I'm the only idiot in the world who has done such a stupid thing.
 
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