Lathe tool height

[mikey]

I use one of these things. It's quick, easy and accurate enough for what I do.

A tool height change of about .01" moves the bubble one line. I havn't had seen a facing cut nub in a couple years.

What happens if you have to change tools with the work in the chuck?

 

[Ken226]

What happens if you have to change tools with the work in the chuck?

I'm not sure I understand.

I set up the tools once, right after I buy it. After I set it's height, I don't mess with it.

Most of my turning tools havn't had the adjustment locknut loosened in years. They still check out on-center with the bubble tool and still don't leave a nub.

I just re-checked them a couple weeks ago after having the saddle off of my lathe.

 

[mikey]

I'm not sure I understand.

I set up the tools once, right after I buy it. After I set it's height, I don't mess with it.

Oh, okay. If that works then all is good.

For me, I find that my tool post does not repeat center height reliably so I check it and reset it if needed, every single time I put a turning tool in place. I use Aloris and Dorian tool posts and mostly Aloris tool holders so the quality is there but the repeatability that is said to exist is not nearly as good as others seem to think.

 

[Ken226]

Oh, okay. If that works then all is good.

For me, I find that my tool post does not repeat center height reliably so I check it and reset it if needed, every single time I put a turning tool in place. I use Aloris and Dorian tool posts and mostly Aloris tool holders so the quality is there but the repeatability that is said to exist is not nearly as good as others seem to think.

You peaked my curiosity. I've never actually checked my height repeatability.

I'm gonna go out a put the same tool on/off about a dozen times and take some measurements on the height.

 

[Ken226]

I took 8 readings after zeroing the indicator.


It was done with a .0005" dial indicator, so the numbers below are me guessing by interpolating the position of the needle between the graduations. So, basically not very accurate, but probably a pretty small numbers anyway.

Anyone interested can watch the video and guesstimate his own numbers, and averages if they want. I tried to keep the face of the indicator in-frame.

For me, that'll work sufficiently that I'm comfortable not re-adjusting the tool every time I use it. You may have to watch it full screen to see the indicator face markings.

 

[RJSakowski]

Quite frankly, I don't get too technical about this stuff. I get my tool at center height because that is where they cut most accurately and where the geometry seems to work best. My suggestion to you is to make a tool height setting gauge for your lathe that is dead on the spindle centerline for you specific lathe and use it every time you place a turning tool on your tool post. This applies to all turning tools, carbide and HSS. Once the tool is set to the proper height then what you measure is what your tool cuts. That is, if you measure the work and then dial in 0.005" DOC then you should have a 0.100" reduction in diameter.

If you think about it, if the tool is too high then you are rubbing the work with the area of the tool just under the cutting edge; in order to cut you must force the tool into the cut. If the tool is too low then the work will try to climb onto the work, again not good. All this can be avoided with a little care in mounting the tool.

Actually, forces on a tool below center height will tend to push it away from the part, decreasing the depth of cut and reducing the cutting forces. This, along with lack of rigidity, will cause chatter. OTOH. a tool above the center line, along with lack of rigidity, will dig in deeper which will tend to stall a smaller lathe.

As to cutting what the dial indicates, any error will result in removing less material than indicated which I would take up with a final pass anyway. I personally set my tools up to the centerline ot close to it but I don't check the height every time I change tool holders. As said before, every lathe has its own idiosyncrasies and the operator has to work out what works best for him or her.

(BTW, you missed a decimal place in the last sentence in the first paragraph)

 

[RJSakowski]

I took 8 readings after zeroing the indicator.


It was done with a .0005" dial indicator, so the numbers below are me guessing by interpolating the position of the needle between the graduations. So, basically not very accurate, but probably a pretty small numbers anyway.

Anyone interested can watch the video and guesstimate his own numbers, and averages if they want. I tried to keep the face of the indicator in-frame.

For me, that'll work sufficiently that I'm comfortable not re-adjusting the tool every time I use it. You may have to watch it full screen to see the indicator face markings.

Great R&R study. I doubt that any of the commonly used methods for setting tool height can set the height to better than a thousandth.

 

[Ken226]

This thread piqued my interest in another thing I saw mentioned somewhere a few posts back.

How much loss of accuracy is there between the dialed infeed and the actual depth of cut for a tool that is below center?

so, I sketch a 1/2" circle, and 4 horizontal lines connected to the edge of the circle. Each one .01" long to represent depth of cut. The first is "on center", from the edge of the circle going in towards center. Another .005 below center. And the third, .01" below center, and the fourth is .015" below center.



I then created and dimensioned 4 new circles, each concentric to the first but intersecting the end of the horizontal lines. They show the diameters at of a theoretical part @ .01" depth of cut and each tool height below center.



Obviously, on center, a .01" cut taken from a 1/2" shaft results in a shaft 0.48" in diameter.

If the tool is .005" below center, a .01" depth of cut results in a part diameter of 0.480004"

If the tool is .01" below center, a .01" depth of cut results in a part diameter of 0.480017"

If the tool is .015" below center, a .01" depth of cut results in a part diameter of 0.480085"

Here is the sketch:





I'm trying to check my work by calculating the geometry using a different method, but the software in my computer really really wants to round these tiny numbers.

 

[Parlo]

I took 8 readings after zeroing the indicator.


It was done with a .0005" dial indicator, so the numbers below are me guessing by interpolating the position of the needle between the graduations. So, basically not very accurate, but probably a pretty small numbers anyway.

Anyone interested can watch the video and guesstimate his own numbers, and averages if they want. I tried to keep the face of the indicator in-frame.

For me, that'll work sufficiently that I'm comfortable not re-adjusting the tool every time I use it. You may have to watch it full screen to see the indicator face markings.

Why did you choose the top of the locking pin as the reference? It is the only part on the whole toolpost assembly that is not fixed or locked. The top position of this pin has no bearing on the insert height. Surely any other part of the tool or holder would produce more reliable results. I would also suggest that the travel of the plunger is decreased to a few thou to prevent side load and any chance of moving when sliding the tool underneath it.

 

[Ken226]

Why did you choose the top of the locking pin as the reference? It is the only part on the whole toolpost assembly that is not fixed or locked.

Because I didn't have the foresight to pm you first and get your input. Joking.

On this particular tool, the stud is pressed into a hole in the holder and the seat/insert slide down onto it. It's the only part around the insert seat area that actually IS fixed.

Maybe Shars changed the design since I bought mine and yours is different?

As it is, the results were + - less that 5 ten thousandths. Close as I could tell, the extreme spread was -.0002 / +.0005 with a standard deviation of +-.0003.

If the methods you mentioned would have got me significantly better results, I'd have needed a .0001" indicator to see it anyway.