Need some advice on measuring inside diameters

Good point, Jim.. I'm guilty of having misread mine a time or two, obviously when in a hurry.

Harold

It would be difficult to generalize, but the inherent error in calipers is generally greater than the tolerance one would have to hold to make a proper slip or press fit. While it's not impossible to find calipers that read identically, inside compared to outside, they generally are not properly coordinated. I bought a new Mitutoyo that was returned because the inside jaws were off by .002" compared to the outside jaws. That may be a bit extreme (the amount), but it's not uncommon. Add to that the fact that they are pressure sensitive and most folks make them read what they want, not what they should be reading, they are poorly suited for precision work.

In essence, how would you do fine work, where tenths matter, with a tool that has difficulty discerning a thou?

You think you can work with calipers to your satisfaction? Try making a drill jig with bushings, using your calipers for measuring the size of the bored holes. If, when you're finished, the drills fit the bushings, and the bushings don't fall out, you may have my attention. That would be a tall order for a skilled machinist, and likely well beyond possible for a novice. A good press fit has a narrow window---no more than a couple tenths-----and you can't do it (reliably) with the typical caliper. You'd have better luck with a spring caliper and a micrometer.

Mind you, I'm not making any assertions about you personally. It's a matter of the tool not being suited to the job. Calipers are great for rough measurements, but are woefully lacking in reliable precision. When I was employed in the missile industry, while inspectors used calipers, they were not permitted to reject any features based on caliper measurements.

Harold

Corse if you have a big enough press you don't even need to drill the hole let alone measure it with a caliber.

Few months ago I was adapting a burke head to a horrizontal mill, drilled a 3.25" hole in a 2" thick plate with the radial, cleaned it up with a boreing bar (on the radial,kind of wobbled), kind of checked it with a caliber that had been dropped a few times then pressed the overarm bar thru the hole. Took somewhere between 15 and 20 tons , perfect fit. Welded the ends just to be sure then put the hole thing on the lathe to turn a true face to mount the head. Thought i might need to shim the head to get a good tram but it was dead on.

I know. That's exactly how I do it, or did it, when I actually had time to play in my shop.

I have about 15 spring-leg calipers, all Starrett, including a few hermaphrodites. My uncle rarely used anything else, and I inherited them.

I once had a pretty good feel for using them, but I don't know if I still have the touch.

-- Ed Huntress

Chuckle! Sounds like the way they'd do it when running a jig borer.

You had pretty good luck, Dan. Problem is, it's hard to get repeatability. In a case such as you described, it could have gone a different way, especially when you introduce welding to the equation. Had you not turned the flange, I can't imagine it would have been square.

There are other methods of working that tend to be more reliable. I enjoy saying things can't be done, but what I'm really saying is that they aren't done reliably------it stands to reason that even a blind squirrel finds the occasional nut. Still, your story is quite impressive!

Harold

Lots of good suggestions from the group.

One item that will be helpful is to get one or more parts with known bore diameters, if possible with the same surface finish and of the same material you will be using. This will allow you to "calibrate" the "feel" you will have to develop unless you want to spend big bucks for a dial bore gauge and setting masters.

1/2 thou is very tight and other items such as straightness, taper/bell-mouth, out-of-round, and surface finish may be more important than simply "diameter."

At those dimensions, thermal effects become important, i.e. a part can be in spec when you measure it in an 80 degree room and out when its 60 degrees.

If your budget can stand it, something you may want to consider is an air gauge (setting masters will cost more than the basic gauge). Also, you may want to check into using a honing machine to finish the bores.

GmcD

That's the key to success. Becoming familiar with how it feels and applying it consistently, and often, is the way it works.

It's like riding a bike, Ed. You may be rusty, but it's still there-----just needs some salt and vinegar!

Harold

you may want to consider

Wow! That's *way* over kill for a half thou tolerance. I've used air gages extensively and can attest that they can discern millionths. Great way to measure, and fast, but hardly in the realm of the home shop. Best answer to this riddle is to learn to apply *good* telescoping gages properly. They're as accurate as the hands with which they are held. Best part is they are capable of measuring over a huge range with a minimal investment, assuming one has micrometers in the same range.

Harold

The blind squirl always gets the nuts cause he can smell them and isn't distacted by what he see's , his only problem is avoiding the truck.

The foraging habbits of ants, now that's more relavent. ants deploy to a food source by following chemical clues left by those befor them, the more cues the more likley an ant will follow it on the assumption the source must be very good, Trouble is even a good source will eventualy be depleted or destroyed. Therein evolution has graced the ant colony with a scout, which does not follow the cues left by others. Often he will come up empty and in the short run look foolish, however his evolutionary retention speaks for itself.

Corse we humans are much smarter than ants, we know if someone isn't following the little cues like everyone else they must be wrong. All together now Lets pass another ordinance.

On Mon, 14 Feb 2005 12:29:48 -0800, the inscrutable "Harold and Susan Vordos" spake:

OK, that's what I wanted to know.

Good point.

For my use, a few thou won't make much difference and I'll buy or borrow a mike if I need real precision.

I was employed as a QA inspector for an electronics firm in the 70s and used dial calipers and vernier height gauges. I was taught by an ex-missile industry QA geezer who told me how to get better precision out of an inferior tool. (Most of our stuff was Starrett, but you know what I mean. He was used to being around optical comparators and other nice stuff.) I can read a few tenths difference between two parts using a caliper with a thou face. And having precision blocks helped me get a feel for proper caliper use. I don't "bend to fit" any more.

P.S: My first day there he made me promise I wouldn't use any of the Starrett (or other) tenths mikes for C-clamps. That got a good laugh all around.

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Actually, no. I meant it. It depends on which turns out to fit the user's "feel" better. For some, something with a bit more drag, such as a spring caliper, might be easier to learn to use.

And they (an inside spring caliper matched with an outside one of similar range) were used for transferring bores to pistons, or shafts to bearings before micrometers were available, and a good workman could make a really nice fit using the two.

But it did take time to develop the feel, just as the telescoping gauges do.

Agreed.

It certainly makes sense. And would certainly be good for telling the difference between two measurements on a worn bore.

I'm not sure that he has room for that technique, however. IIRC, his bore size is small enough so the curve limits the swing more than with a larger bore. (I would consider a 3 inch bore to be about the minimum for which this would be a really good way to go.) A six inch bore would make it even better.

And there is still the trick of making sure that it is crossing the bore with no vertical angle involved, which will increase the apparent size. The rocking of either the spring calipers or the telescoping gauges would probably eliminate this problem in use, but you don't have the "feel" working for you with a too-short rod.

Enjoy, DoN.

Slip a piston up from the other end of the bore, and let the bar ride on that.

-- Ed Huntress

And this condition, at least, would show up with one of the three-legged tri-mikes or similar. If it measures differently when you rotate the mic from pointing to where the chuck jaws contacted to half-way between them, you can be pretty sure that this is what you have. (Mark where the chuck jaws contact before removing it, so you know what orientations to try. And this would *not* be caught by the telescoping gauge or the inside micrometer with extension tubes.

But if you are working something this thin, that is an argument for a 6-jaw chuck to minimize the springing. Or even better, turn a ring with about a 1" radial thickness and a slit to put between the jaws and the OD of the workpiece, so the force of the jaws is better spread. Make the ring at least as thick as the length of jaw engagement, and ideally somewhat longer.

I did not consider ellipticality from wear in my suggestions before, simply because he was apparently making this *new*. The spring from thin walls, however, is another matter.

Enjoy, DoN.

[ ... ]

While I have a Starrett set, I also have another set which I prefer. They were made by Lufkin (now out of that business, unfortunately), and are better in really shallow holes than my Starrett ones. The Starrett go beyond the maximum diameter point, so really shallow holes are difficult to measure properly. The Lufkin ones are ground down to the precise half way point, so they will measure anything which has a sharp ridge at all.

Were the Starretts ever made that precisely? Do I just have a later version which is not as nice?

Enjoy, DoN.

My Starrett set was made in the early '50s, and it's very nice and smooth. However, I've used them so seldom that I can't testify to their accuracy.

Most of the stuff they made then was really good quality.

-- Ed Huntress

Words that will strike terror in a father's heart .. "Dad, this funny C-clamp didin't shut all the way so I put it in the vise and fixed it for you." I think it was one of my greatest tests of fatherhood :) That nice old Starret 1-2" c-clamp is still hanging on the wall as a reminder ... Lock the shop! Glenn

That should work -- as long as it is not a blind bore. :-)

Enjoy, DoN.

So, hire a blind machinist...

-- Ed Huntress

snip--

. A good press fit has a narrow window---no more than a couple

That's really the key in this discussion. If your needs are not critical, there's absolutely nothing wrong with calipers, be they vernier, dial or digital. I'm still using the first ones I bought, back in '57, made my Helios. They've been to hell and back, including being lost off the roof of my truck years ago. I got a strange phone call one day from a guy asking me if I owned some calipers with my last name on them. "Sure do", I replied, "they're right out in my shop, in my toolbox."

"Nope", he replies, "I have them right here in my hands. The last letter of your name, the s, is missing. It was then I realized he had my calipers. When I electroetched my name on them the s didn't print.

He found them in an intersection about two miles from my home at that time in Utah.

QC used the same tools, but in addition used fine dial indicators and Cadillac Pla-chek instruments for measurements. They left nothing to chance.

I was taught by an

That's the toughest thing to overcome. When I was grinding, I got used to never looking at the mic while I was checking diameters, trusted my feel exclusively. When you do that, you learn to do it consistently----which is the main reason I keep harping that one can read mic's to .000050", especially if you have good surfaces.

But how many do so? I had a strict policy where my measuring tools were concerned, and I still enforce it, although there's on one to borrow them now. I permit NO ONE to handle my measuring tools. Anyone asked when I worked with others, I told them to buy their own, just as I did.

Harold