The "why" Of Lathe Tool Height

[dogma]

I started learning to use a manual lathe towards the end of the last year and I have seen conflicting advise for the proper height of lathe tools. Often, I see a forum post with advise along the lines of "at center or slightly above" but I have also seen the opposite of "at center or slightly below". I'm curious as to what the technical basis is for the tool height to be above or below center? Is it a function of the type/orientation of the tool or perhaps the rake angle? Is there an authoritative reference for this information, Eg., machinery's handbook?

 

[hermetic]

Well you are right that there are conflicting reports, but IMHO the right one is "at centre, or VERY slightly below" In this position the tool will cut well, if too far below it will tend to dig in. Above centre will have a tendency to cause the tool to drag on the work below the cutting edge, and make a right mess! Imagine a tool ground with very little front face clearance in order to give maximum support to the cutting edge. If you go even slightly above centre you are presenting the front face of the tool to the work rather than the cutting edge.

 

[stupoty]

If it's a form tool like a threading tool the angle changes if it's above or below centre.

Od turning the tool will rub if it's above centre.

Id turning the tool will rub if it's below centre.

Stuart

 

[gheumann]

I'm a "dead center" guy. It seems to me if it is above, it pulls into a larger diameter as it deflects downward, effectively increasing the depth of cut. But someone once told me it depends on the geometry of the tool holding system too - downward deflection COULD move the tool away from the work in which case slightly above would make sense. All I know is that for me, on two different lathes, dead center results in the best performance for parting and turning. I am using mostly CCMT inserts and turning mostly aluminum but it seems to hold true for HSS tooling and other stocks.

 

[JimDawson]

You asked why do we set lathe tool as we do. I still don't really have an answer but I am a center and below guy. Seems to work well for me.

Here a couple of threads that have a lot of discussion about this subject.

http://www.hobby-machinist.com/threads/backlash-or-just-poor-technique.41725/

http://www.hobby-machinist.com/threads/tool-height-on-a-lathe.36515/#post-311124

 

[mikey]

Dogma, there is no authoritative source I know of that will tell you axiomatically that the tool must be set at center height. Most will recommend it, though.

I can remember wondering the same thing about 25 years ago. As there was no internet like we know it today, I experimented myself and came to my own conclusions. Rather than take someone's advice, why not try it yourself and see what results? I'm sure the experience will prove which setting is best.

If you must go beyond the experiential, look up the Merchant Equation and read to your heart's content about why tool geometry is what it is and why the tip must be on center height.

If you do this then maybe you can help me understand how that ol' fox Jim Dawson can get away with setting a tool significantly below center and getting good results! The only thing I can come up with is a scraping/shearing action like you might get with orthagonal cutting from a tool held rigidly in a heavy lathe on a rigid work piece. I'm afraid to try it on my 11" lathe for fear of breaking the compound right off!

 

[Bill Gruby]

Years ago during my apprenticeship I had two mentors. One advocated above center, the other below center. I tried in vain to prove which was the better setting only to find out the truth. If set correctly above or below, neither is better than the other. I practiced till I found which was better for me. I use center to + .003. I have no problems with this for most purposes. Yes, I occasionally go below center to find the better cut, but not often. Below center gives you a slight shaving effect IMHO. Center or above does no. Your choice. Use what you are comfortable with.

"Billy G"

 

[chips&more]

I use mostly certain grades of HSS. I typically do not grind the top of the tool bit. Why, it’s easier to resharpen and does not affect tool height adjustment when resharpened and I really do not notice a difference in finish. And I really do not fuss about tool position relative to the work. I try and eye ball it to center and call it good enough. All this fuss I read about the cutter being high, center or low thing and gotta have Carbide…oh well.

 

[Tozguy]

I started learning to use a manual lathe towards the end of the last year and I have seen conflicting advise for the proper height of lathe tools. Often, I see a forum post with advise along the lines of "at center or slightly above" but I have also seen the opposite of "at center or slightly below". I'm curious as to what the technical basis is for the tool height to be above or below center? Is it a function of the type/orientation of the tool or perhaps the rake angle? Is there an authoritative reference for this information, Eg., machinery's handbook?

I think that, as you say, it is a function of other factors for getting the right chip formation. Starting at centre with a given material and tool, adjusting tool height up or down slightly might help fine tune the set up. This video explains some whys and objectives of adjusting the rake angle.

The idea of experimenting with tool height for a given set of conditions (if need be) makes sense to me. My tools are ground for use on centre but they are not always set dead on unless it is critical for the operation at hand.

 

[Downunder Bob]

In the days before QCTP were widely available, and affordable we had to set out tools as near as possible by packing them up with strips of thin metal plate shims, I always found that if the choice was .010 above center or .010"below then it was usually better to be below. It does become more critical at very small diameters, At larger diameters it isn't really very important, just go with what works.