heres how to get 0.0001" precision on your ancient hobby lathe

[chuckorlando]

Your not grinding on the surface of the wheel. Just the leading edge.As you advance the part and that edge is used up the portion just behind the leading edge becomes the new edge so to speak. If you wer grinding on the whole flat it would take seconds before the wheel was no longer touching as you burnt off the "cutting surface".

I believe Mr Wilson's point was not that they would not mic the same off the machine. But rather, the sharper taller peaks from the lathe would wear very fast (provided something is wearing on it) thus leaving you .0001 or what have you, under the desired size.

First of all, congratulations on figuring out that trick. That is something a person would normally learn at the elbow of a master machinist and you came up with it yourself. Nice

Grinders definitely create a very fine finish. I don't beleive that, in itself, gives a ground finish any better dimensional accuracy though. Think of the finish as a peaks and valley on the surface of the part. Whether you are talking about a lathe finish of 125 microinches (pretty darn rough) or a ground finish of 10 microinches (pretty darn nice finish) you are still doing your measuring on the peaks of that finish. The very fine tolerances come from the very fine cutting tools (grit of the grinding wheel), the wide contact with the part for even pressure dispersal, the rapidly rotating cutting tools (which makes the grinding wheel essentially one wide tool instead of thousands of tiny tools) and the very fine advance of the grinding wheel usually in the range of ten millionths to a hundred millionths (.000010 to .000100).

Just my two cents,
David H.

 

[Micke S]

A tool post grinder in combination with a hi resolution DRO, or the trick presented in OP, will make great surface finish to the correct diameter. Turning is as said above not good enough for creating a silk lean surface.

 

[george wilson]

That was 1 point I made,Chuck: peaks will soon wear off,or become compressed if going through a hole in a ball bearing,etc.. Soon the turned part will be undersized. Several other reasons also for grinding,though.

Cars shipped by railroad to distant dealers had to have their wheel bearings replaced upon arrival. This was because of the slight but constant jarring of the tracks. Even though the cars had precision ground bearings in their wheels,even that was not good enough. The very minute "peaks and valleys" of the grinding wore enough to cause the wheel bearings to soon become shot.

The solution to this problem was "super finishing",which I think Chrysler invented( It's been a while since I read about super finishing). They started doing this to several of the different parts in cars,such as pistons. It greatly improved the number of miles you could drive before needing a complete overhaul.

I read an old Mechanics Illustrated magazine from the 30's long ago. The title of the article was "How you can get 50,000 miles out of your car". These days that is nothing. It is due to better finishes on parts,better alloys,better lubricants,and other things that we get so many more miles out of cars today. Super finishing is one of the most important factors.

 

[f350ca]

Could you explain this further? I have never been able to get a grip on grinders and why they do a better job as far as accuracy, without the machine they are attached to also being better.

For instance, lets say the finish using the turning tool are relatively rough, but consistent. Then as long as the increase in cut is made relative to the measured finish/diameter combo, the quality of the finish falls out and isn't part of the result anymore.

Another way to look at it is this, I'm seeing errors in the range of 0.0005" to 0.001". The finish is much smoother than that, even just eyeballing it. A 1000 microinch finish is what you get with a saw or flame cut apparently. So as long as the finish remains consistent, I dont see how improving it would increase accuracy.

A couple of reasons I can think of.
No matter how stiff the lathe your using is there is some flex due to the force exerted by the cutter, the light passes made with a grinder exert nearly no force, and that is diminishing as the wheel removes the metal. As the wheel advances into untouched metal there may be some side load, but the very high surface speed of the wheel quickly removes the bulk and the next section of the wheel sees less and so on, till at the end the wheel is rotating above the material and removing virtually nothing.
When using carbide insert cutters there is a considerable radius on the cutting edge. If you try and remove a very small amount of metal say .001 or less your no longer cutting the metal but rather burnishing it as the cut is smaller than the radius.

Greg

 

[george wilson]

As a note: My first lathe was a Sears Atlas 12". It was guaranteed to turn to .001" accuracy ONE INCH from the chuck!! If you're getting 1/2" of accuracy,you aren't too bad off!!

 

[Shadowdog500]

You might want to try combining these techniques with a shear tool. It is supposed to provide a very good finish while consistently removing 1 or 2 thousandths accurately (I've never tried one myself).

http://www.gadgetbuilder.com/VerticalShearBit.html

Thanks for the link, I'm going to try this. Shame the first link to the paper he got the idea from is gone, I would have liked to read that one too.

Chris

 

[MozamPete]

Well, thats where the compound tool post comes in. It will let you magnify its handwheel by MORE than you would ever want!! Yes, you can turn the handwheel 100 ticks and it will only move the cutter forward by .001" if you want!!

I use a similar trick on my old lathe. The hand wheel measures on the lathe are all imperial but I tend to work in metric. So I set the compound to 23.2 degrees and then a thou on the hand wheel gives 0.01mm of movement in the diameter. Saves having to do any inch to mm conversions - I measure using a metric micrometer, figure out what I need to take off (say 0.1mm), then just dial up 100 time that in thou on the compound (10 thou on the compound in this case).

 

[hman]

I just found this thread, and read the whole fascinating thing. Thanks to ALL of you for your expertise! I've heard about the technique previously, but having all the details gone over and hashed out has definitely ben of value.

Just one point that I don't recall being mentioned ... maybe I missed it, or maybe it's so obvious it didn't need to be mentioned. But I'd rather it be redundant than unmentioned:

Be sure the cross slide is locked during the cut.

Thanks again!

 

[12bolts]

......The one I'm using was $30 from horrible freight!......

Does the paperwork with your mic say it is accurate to tenths or has a resolution of tenths?
And have you checked your mic on some standards that are accurate to better than tenths?

Cheers Phil

 

[John Hasler]

Does the paperwork with your mic say it is accurate to tenths or has a resolution of tenths?
And have you checked your mic on some standards that are accurate to better than tenths?

Cheers Phil

I think that for his purposes repeatabiliy to a tenth suffices. He's not making shafts that must fit bushing made in another shop.