Fabricating a gauge quality cylindrical square?

[xthumpx]

Hello All! This is my first post here. I have lurked around quite a bit, but I have a more specific question that I have not been able to find a 100% clear answer too. So, my goal right now is to make a nice cylindrical square for the shop I work at. We just had our 18x24 granite surface plate resurfaced to grade A, and are now looking to get some more tools and gauges for inspection. From what I've read so far on the net, cylindrical squares are made by taking a solid bar (or thick walled pipe) chucking it up in a lathe, center drill both ends then either hold the bar with only a dead center on the tailstock, or turn it between dead centers. with the tail stock having a half dead center. you take a cut on the face of the bar then take a cut on the tail stock end all the way up to the center drilled hole then undercut the end leaving 0.375" ring of material around the circumference of the bar end. Then, a parting tool is used on the head stock end. the face at the tail stock end of the bar should now be very close to 90 degrees if the machine itself is in good condition, and in proper alignment. the other end that was cut by the parting tool should be close, but from what I have read the parting tool can give somewhat less predictable results as far as accuracy is concerned. Anyways, after all that you harden and temper the steel then either use a tool post grinder or a cylindrical grinder to get the necessary finish. So, assuming one does everything right on a well built machines, and measured everything making sure there is no taper you would have a cylindrical square with +/- 0.0001" accuracy. So, I guess my question is what is the next step after that to get it more precise assuming you could measure to those tighter tolerances? Does one lap it, or can one spot it on a surface plate, and perhaps file or scrape the high spots like on flat surfaces while being careful not to work a taper into it ( I'm not sure if this can even be done as I have not read any info on such methods)? Just curious really. I'm one of those people who have a thirst for knowledge, and experience. To make sure not to confuse anyone I'm aware that there are probably many more ways for turn a cylindrical square on a lathe, and to grind to good precision, but I'm more concerned with A) methods of verifying a cylindrical square (means of self checking), B) methods of how a more accurate and precise grade cylindrical square is made after initial turning, grinding.

Thanks in advance.

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[mariojl]

You're on the right track, that's the way to make a cylindrical square; when you grind the circumference, you also MUST grind the face ( the mentioned 3/8 flat). As long as you grind a perfect cylinder and clean up the face, your part is a "SQUARE", no doubt about it.
Lapping will get better finish, but won´t add too much for precision.
Mario

[underthetire]

Lapping will help a lot for accuracy. Problem is, without cap gauges or air LVDT's its going to be hard to really measure how flat and square it really is. Those gauges will cost 3x just buying a certified cyl square.

[xthumpx]

I figured some sort of special tooling is required. I guess that just comes with the territory of gauge making and precision measurements. Though, it still amazes me how much was accomplished with limited knowledge and resources when I read books on this subject dating back a hundred years ago. What would be some measuring techniques that could be employed without those specials gauges that would still produce accurate and repeatable results?

[underthetire]

Difference is, 100 years ago they weren't cutting tenths or measuring it. You can always get close, and that's pretty much what they did. Today we can measure microinch range (really from the 60s on to some extent) and 100 years from now who knows. Whole different world measuring sub tenths, let me tell you.

[xthumpx]

What would a lab have to do to certify and/or correct a cylindrical master square?

[underthetire]

Depends on the grade. Ours has to be shipped to a special government facility because of the grade ( and cost ), others can be checked on a good cmm/indicators and lapped back in, just like they do surface plates. We have the most awesome straight edges/squares money used to be able to buy.

[bhurts]

This short article will shed some light on what we could read on a Mic 100 years ago. History of the Micrometer | eHow

Ben

[hanermo]

The article is crap.
It references nonsense, like measuring to one millionth of an inch with an old mechanical micrometer.

Todays best, note best, micrometers measure to about 0.2-0.5 microns, or 0.0002 - 0.0005 mm == 20 millionths of an inch in accuracy.
Look up Mahr extramess (0.2 microns), or Mitutoyo MD25M 0.5 Microns (0.1 resolution).

Analog gages measures tenths of microns.

Std gage blocks are lapped to 1/100 of micron accuracy.

Lapping is a good useful way to make stuff very flat.
It does *not* involve sandpaper.

For flatter than that, optical methods can be used.
An optical flat can indicate wavelengths of light, about 10-100x less than the indicators.