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

[spaceman_spiff]

Criticism of what I describe here is WELCOME!!! Please, lets dissect this, tear it apart, and make it better.

The good news is: I think I am getting control of diameters, +/- 0.0001" (tenths), on my ancient, worn out Atlas 10F lathe.

I also learned something very important about manual lathes today. That alone is almost more important than being able to cut within tenths.

Heres how I did it, and that special thing I learned:

Notice how the handwheels are graduated in 0.001", and the space between two graduations is about 1/32"? WAY too small to control tenths, accurately anyway. Yes, you can approximate. But seems kind of strange to me that a lathe would have handwheels that coarse. Im sure when even this low grade Atlas lathe was brand new it could cut in tenths reliably. Hmm. Actually..come to think of it the Hardinge HLV-H extreme precision lathe I used at work long ago was also in 0.001". So how the heck is anyone supposed to work in 0.0001"?

Even a DRO or dial indicators wouldn't really help, because you'd still be stuck trying to turn that handwheel just that TEEENY amount!

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!! So essentially you have made it so that each handwheel graduations of 0.001" actually means 10 MILLIONTHS of an inch! Now tenths become cake!

The idea behind this is to just rotate the compound tool post so its a very slight angle instead of parallel to the spindle axis. To be clear, "parallel" to the spindle axis would mean that turning its handwheel would move it towards or away from the spindle.

Now think about that, if its only moving towards or away from the spindle, its not moving the tool towards or away from the work at all, is it?

So if you put it at a very slight angle, it WILL move the tool towards or away from the work. Turns out that at around 84 degrees, for every 100 thou of its handwheel, it moves towards the work by only 10 THOU! Now each tick is 0.0001"! Tada! And if you want you could theoretically reduce that angle even further and probably multiply it by ten times again!

REALITY CHECK

-You are still limited by the accuracy of the threads, ways, etc.. You cannot precisely move the tool by 10 atoms just because you have the slide at an angle. But where that limit of accuracy is is now something you can move on to dealing with, instead of trying to turn a handwheel by some teeny amount.

-The angle readings on the compound tool post are not very fine. You should get the angle to approximately where you want and then do some test cuts to confirm the new tool post handwheel versus actual increase in cut distance, and then do some arithmetic and calculate how many thou you need to turn the handwheel to get what you want. If you dont have a tenths micrometer, now you have an excuse to buy one!!

-You will likely need to rotate your toolpost so the tool is again perpendicular (or whatever you need it to be) to the work, since you are rotating the tool post.

RESULTS!!!

So...does this work or not? The answer appears to be YES!

I spent the last couple hours experimenting to see if I could really make reliable cuts in one shot based solely on the handwheel.

Material: 303 stainless
RPM: about 800
Cutter: carbide TPG322 in a very rigid insert holder and tool post
Lube: none
Diameter of workpiece: about 1/2"
Workholding: 4 jaw chuck. Stickout about 2".
Feed: automatic, about 5 thou per rev
Machine: Atlas 10F from 1944

Cutting notes:

All done in three passes. First two were easily seen. The last pass was BARELY visible taking place..I'm not sure if it was doing anything or not. I have very good vision up close and could just barely see the "line" moving by getting up real close. And to be clear, I made NO adjustments to the handwheels between passes. I just turned the motor off, used the carriage handwheel to move the tool back to the beginning of the cut, and then powered it up and repeated the cut.

I made many test cuts. Some of them came out right, some did not. A few came out about 1 thou off!! But I think I have it figured out now.

The final two cuts I made came out within about 0.0001" of where I intended. I think what was happening with the cuts that didn't work out was that I was not taking deep enough cuts. And I also screwed up the math a few times since my calculator was dying and I couldn't read it. I would get your notepad and calculator handy and keep a log of what you do so you can figure out how deep you need to cut and what RPM/stickout you need to do, etc..

FINAL CUT #1:

Target radial reduction: 0.0117"
Actual radial reduction: 0.01175"

FINAL CUT #2

Target radial reduction: 0.0107"
Actual radial reduction: 0.0108"

(the pic shows the incremental change in diameter)

Please join in the fun and report your results!!! You will need an accurate tenths micrometer to play along. The one I'm using was $30 from horrible freight!

 

[chips&more]

Good job! Just wonder how much hair you have left to fall out! Holding a 0.0001” on any manual lathe is hair losing experience. More like a job for the OD grinder. If I need to achieve a critical dimension on the lathe (I don’t have DRO) I set-up a 0.0001” or better dial indicator and do some praying…Good Luck, Dave.

 

[spaceman_spiff]

Good job! Just wonder how much hair you have left to fall out! Holding a 0.0001” on any manual lathe is hair losing experience. More like a job for the OD grinder. If I need to achieve a critical dimension on the lathe (I don’t have DRO) I set-up a 0.0001” or better dial indicator and do some praying…Good Luck, Dave.

thanks!!

this isnt exactly the end of the battle though..Ill believe that I can really hold tenths once I've made done alot more cuts in a lot more stickouts and diameters..but for now I think it was a small victory

I doubt Id ever need to literally hold 0.0001", but if I can reliably hit +/- 0.0003" I think that would be very useful

 

[Mark_f]

I find this fascinating. I worked many years in an environment that required .0002" tolerances. This is hard to do at home because those kind of tolerances usually require a temperature controlled environment and the tolerance is only guaranteed at that specific temperature. Now that being said, I occasional try to cut some things at a .0002" tolerance at home and the size WILL change with temperature changes, but in the home environment I don't see it as a big problem. BUT in all those years I never thought of trying to make the light finish cuts the way you describe and In my mind it is a great idea! I finish a part to that close a size with a second cut file, hand held and the part spinning in the chuck. This seems to work good if you are good with a file. And yes there is a safety issue using a file on a running lathe. YOU HAVE TO BE CAREFUL AND DON'T STAND IN FRONT OF THE FILE!

But I love your idea and am going to try that on my next close tolerance need. This seems it would be useful on a close tolerance bore. ( can't get my file in there)

Mark Frazier

 

[spaceman_spiff]

I find this fascinating. I worked many years in an environment that required .0002" tolerances. This is hard to do at home because those kind of tolerances usually require a temperature controlled environment and the tolerance is only guaranteed at that specific temperature. Now that being said, I occasional try to cut some things at a .0002" tolerance at home and the size WILL change with temperature changes, but in the home environment I don't see it as a big problem. BUT in all those years I never thought of trying to make the light finish cuts the way you describe and In my mind it is a great idea! I finish a part to that close a size with a second cut file, hand held and the part spinning in the chuck. This seems to work good if you are good with a file. And yes there is a safety issue using a file on a running lathe. YOU HAVE TO BE CAREFUL AND DON'T STAND IN FRONT OF THE FILE!

But I love your idea and am going to try that on my next close tolerance need. This seems it would be useful on a close tolerance bore. ( can't get my file in there)

Mark Frazier

Well since I've posted i've made about a dozen more cuts and have an excel spreadsheet open trying to calculate the error.

Its going to take alot more experimenting before I feel I have a solid grip on whats going on. Sometimes the cut is way off, like 1 to 2 thou! And I'm not sure why. My compound slide gib looks pretty bad. I think this sort of operation also calls for measuring how much the slide angle varies throughout the slides travel. In other words, just because 100 thou handwheel moves the tool 1 thou into the part at the beginning of slide travel does not mean it does so everywhere else in the slide travel.

I think the idea here, is to make it so everything you do is relative to ONE moment in time..one setup, one temperature, etc...to compensate for everything for that instant you make the cut. I dont think holding tenths on this lathe is going to be possible otherwise.

So maybe one way this becomes productive is to know EXACTLY what the handwheel-to-cut constant is for say 20 thou travel of the compound slide, and keep that in reserve for your very final cut, also throwing in there the 1st, 2nd, 3rd, and even 4th finishing passes as optional stopping points as you measure between each.

For me, this isnt actually about "hitting the bullseye" on the first try, fun as that is, its more about being able to get within a very tight tolerance without scrapping the part, and in a reasonable amount of time. For me, reasonable is something like 10 minutes for a given diameter, since I wont be doing production with this and will most likely be making spindles that need very tight tolerance fits into bearings and gears.

So if I can reliably (i.e. ALWAYS) hit +/- 0.0002" without scrapping the part, thats a win, even if it takes 10 minutes per diameter.

First job is to get things dialed in to where I'm not getting these WAY OFF errors every now and then. So I'm going to try and cut only very close to the chuck and with larger diameters to reduce deflection issues as much as possible.

 

[f350ca]

I swing the compound as you describe when I'm using a tool post grinder. The one thou graduations on the cross feed are pretty heavy cuts for the grinder. When trying to take the light final cuts you might have some luck using a HSS bit ground as a shear tool, they're great for shaving those really light passes.

Greg

 

[Karl_T]

I'll just share some of my tips for accurate work on the lathe...

First I installed a quality DRO with the best scale possible on X. It easily reads your position to 0.0002 and its right, lets you see any irregularities or backlash issues. Most important step IMHO.

Second thing is be aware your lathe is made of CHEESE. Cutting forces make it move or deflect. You got to play this consistent for accurate work.

A very rigid machine makes this far easier - the CHEESE moves less. I use a Monarch 10EE for most work. (But years ago I learned to make great parts on a LeBlond built in 1910).

For fine work you can't beat a sharp cutting tool. In most cases a sharpened and honed HSS is best. A sharp tool makes the CHEESE deflect less.

Next be aware that the CHEESE deflects different with different cutting conditions and can get real funky with too light of cuts. Make your last two cuts the same. Just a for example here, but say to ruffed to 0.010 over. Remove 5 thous as first finish pass. DOUBLE CHECK your measurements then go for broke.

If your final cut has any length to it, its going to be larger farther from the spindle. (Remember the CHEESE?) In this case, and if you need the super accuracy; leave at least 2 tenths for file and polish work everywhere. You can't beat a quality fine cut file and stones or emery cloth to get it JUST RIGHT.

Last and most important, PRACTICE make perfect. You won't learn to make quality parts reading about it.

My two cents

karl

 

[SG51Buss]

...When trying to take the light final cuts you might have some luck using a HSS bit ground as a shear tool, they're great for shaving those really light passes...

Ditto. When I need to do sub-mill accuracy, I'll switch over to a razer-sharp, finely honed, tiny tip radius HSS bit. Examined under a 10x eye loupe. AND, my carriage transverse feed rate is down to 0.002". AND, for 1/2" steels, I'll have the spindle down to around 400-300 rpms.

I usually have my compound at 30°, so it acts as a direct-read dial. So, if it's fed-in by 0.001", that'll be the change in diameter, the cut will be 0.0005".

Chasing 0.0001" accuracy is fun. Have you checked the finish cut for taper?

 

[chuckorlando]

I try to watch the .0001 decimal but My end goal is usally to hit the .001 the tenth just tells me when to stop. That being said, when I do want to get it just right, I file or emery cloth. To many variables to try to do it with the cutter. Every tool, compound angle, material, every little change will change whats moving and flexing and ringing.

Just as an experiment, take a cut with the power feed and grab the tool post handle and pull it down and push it up. I bet you can note the depth change instantly. Even if it only moved .0001 thats your whole allowable tolerance.

If you want to hold tenths, you have to think like a tool & die maker and less like a machinist. IMO a t&d maker will lap it in, file it in, make it just right. If the tolerance is +/- .010 they work to .0001 no matter the time. Where a machinist will differentiate between the what needs perfection and what needs to hit a thou.

Broad spectrum statement that does not apply to all. Thats my opinion on the two.

Take spring cuts as well

 

[Shadowdog500]

I believe that Thatlazachinist discusses this in one of his videos. He points out that it does work up to a point, but once you hit the accuracy of the machine increasing the compound angle more than that will buy you little.

I just looked for the video, but the topic was imbedded in the video as a several minute side note, so it was not easy to find.

[UPDATE]. Found it. Start at around 7:00. http://youtu.be/WqTliEGrhL4


I'm also curious about how you did the post cut measurements. I am just a hobby machinist, but I found that fine measurments take a feel. I also found when taking a fine measurment with an expectation of what it should be, I would tend to home in on the number I thought it should be while looking at the scale and subcounciously convince myself that it felt perfect. I did this without even knowing I did it. After reading that this happens to myself (and others) I never look at the scale while taking a measurment anymore. While taking the measurment, I look away from the scale, and adjust the micromoter until it feels right, then I look at the scale to see the result. When I first switched to this method, I found that I really didn't have a feel at all, and had to practice on things of a known dimension until I could measure them accurately. I still practice on items of a known dimention from time to check myself time since I don't do this type of stuff everyday.

Chris