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

[spaceman_spiff]

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

All my measurements for these tests are relative so checking it against an absolute standard wouldn't mean anything, only when I actually try to hit a specific diameter will the mic accuracy come into play. At that point, it will almost certainly be a spindle surface meant to fit inside a bearing race, so I will use the bearing race itself as the "standard" and shoot for a relative change for that. Never knowing the actual diameter of anything lol

 

[spaceman_spiff]

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

You are correct sah...I will most likely be fitting bearings to a shaft so I will make relative measurements to the bearing

 

[spaceman_spiff]

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!

I have been avoiding locking the cross slide because it moves the tool post slightly when doing so and adds another variable. However it may be worth looking into if the amount it moves is repeatable.

 

[spaceman_spiff]

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

Yeah but my point is, whatever your cutting/burnishing passes do, as long as they are repeatable, then the accuracy shouldn't be any different, even if the finish is inferior.

 

[spaceman_spiff]

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.

Yeah but we're talking about cut accuracy not lifetime accuracy of the surface. If whatever inferior surface thats created by using turning tools is repeatable, why would it be any less accurate than a superior finish surface?

 

[spaceman_spiff]

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.

I think the idea of a superior finish versus an accurate diameter are being mixed up. One does not necessarily come along with the other. A tool post grinder will not correct pitch error in the tool post screw or the tool post ways not being straight.

 

[spaceman_spiff]

Pretty much a big waste of time, especially on a notably imprecise machine, ways, geometry, and unmentioned, spindle bearings. Cutting tools are a large part of the accuracy equation as well; we do not need to have such complication in the (machine's) ability to measure, all that is necessary is an accurate finish cut, leaving a very small amount of stock to (lightly) file and subsequent polishing with abrasive cloth. It may be another matter with ultra precision machines, but turning and grinding is the real answer for more complicated precision parts that turning alone is not likely to give.

Using the compound to increase resolution is a very old and standard procedure. Filing is not an accurate machining process.

 

[spaceman_spiff]

It's interesting that your 0.0002" - 0.0003" errors are consistent. I wonder if that's caused by the rounded edge of the carbide insert. When I first started using insert tooling, I also experienced a similar reduction in depth-of-cut to bit-advance ratio.

Oh, wait. That consistent error may also be caused by not being at a precise 84.26° compound angle. Look at the error to target-feed ratios. For example, in your second test, to get the 0.0071" target, the compound was advanced 0.071"? The 0.0003" error over that 0.071" travel is about a 0.24° angle...

Okay I really want to make sure I understand you here...maybe this is the answer!

I set the compound angle very imprecisely, just turned it until it was around 84 and clamped it down.

Then, I figured out the actual relationship between handwheel and tool feed by measuring it. By turning the stock true, then feeding the compound by 100 thou, cutting and then measuring the result. Which came out to about 0.0001065" feed for every 0.001" of compound handwheel. And then I did the tests based on that.

So when I was trying to hit 0.0071", I rotated the handwheel by 0.0071 / 0.0001065 = 66.7 thou.

But I want to make sure that explains what you are saying..it does right?

Im actually pretty comfortable if I can maintain 3 tenths. It definitely is tantalizing to see if I can get down to 1 tenth but I have a feeling that may be impossible.

However, when I get my ER32 chuck, I'm going to repeat these tests and do everything as close as possible to the chuck and see if that helps. It will be several inches closer to the spindle bearings than the 4 jaw chuck setup I'm doing now. I have to think there is a tenth or so to be had by the reduced runout!

 

[Mark_f]

I love this post, but I ain't steppin in this pile ). It is interesting and the opinions are so varying. One thing I KNOW, my mics have an instruction sheet ( who'd a thunk it?) . They say to handle them by the ends. It says DO NOT HOLD BY THE CENTER OF THE BOW. It says the warmth of your hands will change the accuracy. That being said , the change is slight. I was taught this very early in my training.( we worked in a temperature controlled environment). But I do not believe this is a problem to most of us as I don't know about yours, but my little shop runs a wide range of temperatures, especially this time of year. it was 40 in there today. We want to get cuts as close as possible and I do this by making a close fit. The amount of closeness usually determines whether I need a 2 pound or 4 pound mallet to put it together.

Mark Frazier

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

 

[12bolts]

All my measurements for these tests are relative so checking it against an absolute standard wouldn't mean anything, only when I actually try to hit a specific diameter will the mic accuracy come into play. At that point, it will almost certainly be a spindle surface meant to fit inside a bearing race, so I will use the bearing race itself as the "standard" and shoot for a relative change for that. Never knowing the actual diameter of anything lol

You have missed the point of my questions. Just because your display reads to tenths, or hundredths, does not make the tool accurate to that. Resolution is no indication of accuracy.
You cannot check the accuracy of a tool on something that is only accurate to the same level you are shooting for. A master must be better than what you are checking it with.
Repeatability is another, as well as accuracy across the range. You alluded to some unexplainable errors in your measurements.

Cheers Phil