Tool Height

[richz]

When setting your tool height should you be on center or below?

 

[8ntsane]

The best is on centre
To hi wont cut, too low will, but could dig in, and cause you to wreck your work piece.
Get it in centre with facing cuts, look for that little nub on the end, raise the tool cutter till it just dissappears and cuts clean with out it. Then swing your tool post 90 degree, and your safe for turning too.

Lightly pinching a 6 inch ruler will get you really close. but a facing cut will tell you the real story.

 

[richz]

I have read some stuff on determining tool height. One you would turn a round down to say .200. Then use a height gage to measure to the bottom of the round and add .100. This would seem to be the exact center. I guess my question should have been how close to center. Should it be exact or is a few thou below ok.

 

[Tom Griffin]

Nothing is exact, but the smaller the diameter of the work, the more critical tool height becomes. Just eyeball the nose of the tool with the point of the center and it'll be fine.

Tom

 

[pdentrem]

1200
Sounds like the lantern is flexing and your are compensating for that. I had the same problem but with a QCTP of the piston style and had to do the same thing. I have since changed to a wedge style and problem solved.
Pierre

 

[8ntsane]

every now and then if i`m having a hard time getting a good finish, i confess, i`ll go just a touch over center.
but that`s with the lantern post and Armstrong holder.

i agree though. best just to stay on center.

1200 If your going above centre, the cutting edge wouldnt contact the work. I would be thinking your below centre to start with, and that would cause a bad surface finish, and by raising the cutter up wards a hair , your closer to really being on centre.

About the only time I see going a touch above centre works, is with flexable tooling like boring bars. They tend to deflect down wards under a cutting load, so in that instance, setting a touch hi, does work. Even still, a more ridgid B=bar would be a better answer to that problem

 

[Tony Wells]

Going below center presents a negative (or moreso) top rake angle to the work. On a large diameter, this can actually help tool life on interrupted cuts. This assumes, of course, that you have the rigidity and horsepower to run negative rake tooling in the first place. And I don't mean enough that you could actually see it, but would be measurable. Generally, the tooling would bought or ground with the desired top rake angle already on the tool. Of course, below center on turning long, slender work is a definite no-no, as the work will tend to climb up and over the tool edge. It springs back, and repeats....this creates a nasty chatter and ruined finish. This is also a problem with parting. Yes, below center a bit will require less feed pressure than trying to outguess the deflection downward by going above, but as you near center, it becomes problematic as the work climbs up.

 

[burnrider]

Good thread. Those of us still improving are learning in this thread.

 

[Tom Griffin]

Gordon,

Have you read the literature 1200rpm has provided? It recommends setting the tool slightly above center to compensate for the angle of the tool holder. By doing that, it makes the cutting edge of the tool more tangent to the surface of the work and therefore less likely to be pulled in. Under those conditions, there is certainly nothing wrong with doing it that way. It's just that most of us are accustomed to using tool holders these days with zero back rake where the tool should be on center.

I must admit that I've never heard of that technique either, but I guess we are never to old to learn a new trick.

Thanks 1200!

Tom

 

[Tom Griffin]

1200rpm,

I used to belong to a woodworking forum and had to quit because of a situation that started similar to this.
Someone asked a question and I responded with a perfectly valid response. Another member attacked me in a similar fashion because he didn't like my answer.
He then started attacking me every time I responded to anything. After a week or so, I quit responding to questions and he started sending me emails questioning my experiences and qualifications to answer any questions posted by others.

I finally had to quit and haven't been back since. It was just a forum and I didn't need the grief. What made it worse was that he was a moderator.

On one hand, there is valid discussion and on the other, there is someone who wants to be right by any means.
I'm not accusing anyone of anything. It's just that I have seen this before and it doesn't always have a happy ending.

I hope we don't have the same issues here.

Steve,

I can assure you that will not happen here.

We work hard to keep everything civil and informative and welcome the alternative point of view. The nice thing about machining is that there are many ways of doing things that provide the same end result, and unless it is something dangerous or reckless, there is no single right or wrong way to do it.

Tom