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

[John Hasler]

Thats what I'm doing when I'm measuring the stock after I cut and zeroing the mic on it. I really doubt the stock is changing in diameter from the pressure of mic anvil, especially with torque limiting thimble.

I'm suggesting that you make a seperate project of determining the repeatability of your mic. For example, measure two different dimensions of a reference object such as a gage block alternately 100 times (writing the measurements down) and then do statistics on the results.

 

[george wilson]

For comparative purposes ONLY,his mike does not have to ACTUALLY be reading true diameters. He's only using it for theoretically consistent readings.He still needs to protect it from the heat of his hands. That mike does not look like it has sufficient insulation. Even if it did,if I was using it often, I'd still use gloves. I use gloves when using my Starrett Master Machinist's Level. If I don't,it will warp ever so slightly and not give me true readings over the length of my lathe. Getting my lathe level is where I tend to go a bit overboard! Fortunately,my HLVH has built in 3 point contact,and does not need leveling. It stays true to itself with no twisting. My 16" lathe needs leveling,though.

 

[12bolts]

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.

Whilst you are making the cut the work is heating and expanding. The tool will take off more from the dia from start of cut to finish of cut. At the level of accuracy you are aiming for it would be measurable.

and I still saw variability around 0.0005". These cuts are back to back, same workpiece, same cutter, same temperature, same location on the ways, etc..

Nope not the same temperature from cut to cut. When I am aiming for press fits, close sliding fits, or just as good as I can make it. I will take the last cut after the work has cooled.

The machining can be that accurate. Thermal expansion is irrelevant. Nobody is claiming they are machining something and that it suddenly becomes immune to thermal expansion.

So is the thermal expansion irrelevant, or not?

Thats what I'm doing when I'm measuring the stock after I cut and zeroing the mic on it. I really doubt the stock is changing in diameter from the pressure of mic anvil, especially with torque limiting thimble.

Nope but the thermal expansion of your work will change due to friction.

Nah. Holding the mic for 5 seconds by the plastic isnt going to change the measurement.

If you think you can take an accurate measurement in less than 5 seconds from pick up to put down of the tool you are mistaken.

Cheers Phil

 

[spaceman_spiff]

Whilst you are making the cut the work is heating and expanding. The tool will take off more from the dia from start of cut to finish of cut. At the level of accuracy you are aiming for it would be measurable.

You're exaggerating the effects.

The cut I'm making, in 6061, at about 10 thou radial at 850 rpm, is around a 1/25th horsepower cut. Maybe 30 watts. The block of aluminum I had chucked up is maybe 300 grams. Specific heat of aluminum is about 0.9 J/g/K, and the cut lasts about 8 seconds. So in that time, maybe 240J goes into the cut. The majority of that heat goes into the chip. But even if 100% of it went into the workpiece, which it certainly does not, that would raise the temperature by about a whopping 1K or maybe 2F. Resulting in an expansion of maybe 0.000011" inches since the section I'm cutting is about 0.7" diameter. But really, almost all the energy goes into the chip, and the workpiece is cooled by the air moving around it. So the expansion is more like in single digit millionths of an inch.

Plus, I'm making the measurements at roughly the same time after the cuts anyways, so most of the change resulting in expansion drops out of the relative measurement anyway.

So is the thermal expansion irrelevant, or not?

The idea that thermal expansion is not relevant in this case and also that nobody is claiming that machining makes things irrelevant to thermal expansion are not mututally exclusive or even related.

If you think you can take an accurate measurement in less than 5 seconds from pick up to put down of the tool you are mistaken.

Cheers Phil

I dont think it, I know it. The mic is already within 10 thou of the diameter because I just zeroed it before cutting. Its slides right over the workpiece in a second and I've done it dozens of times now. 5 seconds is a little long actually.

According to Mitutoyo I could hold the BARE frame for 120 seconds with my bare hand and it would only expand by 0.00008", and they say adding the plastic insulator would significantly decrease that.

For anyone interested in some juicy precision measurement tips, check out:

http://www.mitutoyo.com/wp-content/uploads/2013/04/E11003_2_QuickGuide.pdf

 

[BillWood]

Hello,

When trying for these high levels of accuracy would a dead centre from the tailstock into the loose end of the workpiece help to prevent minor deflection at the loose end of the workpiece ?

Or are you doing all of the tests very close to the supported chuck end so this doesn't matter.

Bill

 

[Shadowdog500]

I dont think it, I know it. The mic is already within 10 thou of the diameter because I just zeroed it before cutting. Its slides right over the workpiece in a second and I've done it dozens of times now. 5 seconds is a little long actually.

According to Mitutoyo I could hold the BARE frame for 120 seconds with my bare hand and it would only expand by 0.00008", and they say adding the plastic insulator would significantly decrease that.

For anyone interested in some juicy precision measurement tips, check out:

http://www.mitutoyo.com/wp-content/uploads/2013/04/E11003_2_QuickGuide.pdf

Thanks for the useful link for a mytutoyo micrometer!
Did you upgrade to a mitutoyo micrometer?

If not, do you have any reference material From Harbor Freight that indicates how long you can hold your $30 micrometer before you get measurable expansion?

Chris

 

[SG51Buss]

...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?

Yep, sure does. Doing the first cut and second cut with the bit under load sure sounds like apples-to-apples to me. Doesn't have to be a precise angle since you're using feed/cut ratio.

That 0.0001065" feed per 0.001" compound travel is a 0.1065 ratio, 6.114°, or a 83.886° compound angle.
Just for reference here, a true 0.1 ratio is 5.74°, or a 84.26° compound angle.

Do you think it would be worthwhile to find what the true compound angle is, measured without any cutting loads? And if it differs from your derived value?

Leaving the compound angle as-is, you could chuck-up and true a reference rod, zero TIR at the chuck, find the TIR a few inches out and rotate the chuck to split that in half, so that the vertical plane of the test rod is parallel to the spindle. Then, with a dialgauge, measure the infeed value over a long compound feed.

Similar to way I setup my 8° compound angle: http://www.hobby-machinist.com/show...-ER-32-collet-chuck-to-an-Atlas-Craftsman-618

Of course, the compound feed is the hypotenuse, and the infeed is the SIN leg...

 

[spaceman_spiff]

Thanks for the useful link for a mytutoyo micrometer!
Did you upgrade to a mitutoyo micrometer?

If not, do you have any reference material From Harbor Freight that indicates how long you can hold your $30 micrometer before you get measurable expansion?

Chris

You mean besides the fact they are both made out of identical or nearly indentical materials? Multiply it by a factor of 10 and you'd still have an irrelevant expansion at the time scales we're talking about. And there is no reason it would even be a factor of 10 let alone 2.

 

[higgite]

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

John, and others who haven't tried it, if you're into smooth finishes with little to no extra work, you need to try a vertical shear tool. Beautiful finish on aluminum, steel and brass, even on my bench top lathe. For example, last week I made a brass snap cap with a tangential tool and wanted a nice, smooth finish on it, so I emery clothed it. Needed another one and remembered I had a shear tool in the drawer that I haven't used in a while. I did the last .002" DOC finish cut with the shear tool. Didn't need emery cloth. And the tool itself is super easy to grind.

On topic, I didn't set the compound angle to 84.26 degrees, or 5.74, depending on your reference axis. I didn't need that much precision. )

Tom

 

[John Hasler]

John, and others who haven't tried it, if you're into smooth finishes with little to no extra work, you need to try a vertical shear tool. Beautiful finish on aluminum, steel and brass, even on my bench top lathe. For example, last week I made a brass snap cap with a tangential tool and wanted a nice, smooth finish on it, so I emery clothed it. Needed another one and remembered I had a shear tool in the drawer that I haven't used in a while. I did the last .002" DOC finish cut with the shear tool. Didn't need emery cloth. And the tool itself is super easy to grind.

Tom

Thanks. If it ever gets above 20F in my shop again I'll try it.