Micrometer question

[ghmerrill]

What micrometers do any of you guys use for measuring ID of tubes (cylinders, slides, whatever)? Generally something like the Votaw one? Required resolution? A lot of things I'm seeing are 0.001", but the Votaw is better and not hideously expensive.

[ghmerrill]

Thanks. Indeed, 0.001" is pretty small. Part of my concern of course is that if there is a "claimed" resolution of "0.0005" vs. a "claimed" resolution of "0.001", I'd be more inclined towards the former (modulo how expensive the difference is) since it might indicate a real resolution closer to "0.001" -- and maybe a bit more wiggle room in making measuring error. Also, it's like ... one of my rules has 1/32" marks on it, which of course I never use. But it does making getting 1/16" increments closer to right. And there's always the problem of measuring how well your measuring tools measure. I'm always leery of cheap measuring tools, though I do have a couple that seem quite good. I guess an approach of "Close enough not to be too oversize,and then lap to fit" is a reasonable way to go rather than trying to get it "exactly" right in the first place.

[Rick Denney]

I have a nice vernier caliper (an old Mauser) that reads to the nearest .0005. But that doesn't mean it's accurate at that reading. It's reasonable at 0.001, though. The precision is controlled by the reading mechanism, either a vernier scale or a dial.

The advantage to these is that they knife probes for measuring inside dimensions. There is some technique to getting a good measurement, but in many cases one must take three separate readings and average them to improve accuracy.

Measuring holes with a micrometer, however, is downright nasty. It usually requires a T-guage, carefully fitted into the hole, and then measured with an micrometer. Again, technique is involved and multiple measurements improve accuracy.

Measuring the outside of the inner tubing is probably close enough, but it will be slightly smaller (a couple of thousandths, maybe--Joe can confirm) than the inside of the outer tube, to provide room for lubricant.

Piston valves are honed to tighter tolerances than slides, and require more accuracy in measurement.

Rick "who learned out to read a vernier scale with old surveying transits" Denney

[ghmerrill]

I have some nice old Mausers too, but unfortunately none of them are micrometers.

Does anyone reading this happen to own one of these: http://www.fvfowler.com/pdf/2304/2304_68.pdf? The specs on it seem astounding and you can get it for under $100.

[ghmerrill]

Never mind. Those particular Fowlers seem to have a real bad rep.

[Doug Elliott]

Mitutoyo products are excellent and reasonably priced.
That Fowler looks great for its very limited purpose, but I've never been happy with any Fowler I've bought.

[Dan Schultz]

Measuring parts only goes so far. Generally speaking... .001" is quite close enough. That sounds pretty close but what you can't measure with conventional stuff from Harbor Freight and the likes... is surface condition... the finished, straighteness, and such. When you measure something... all you get is the measurement at that particular point. That doesn't mean that a 1.000" diameter piston will fit into a 1.002" diameter casing.

I've found that digital calipers might be OK for measuring diameters are OK for measuring the outside of something... they are pretty 'hit and miss' at getting inside dimensions. Of course... you CAN'T measure inside with conventional micrometers without the aid of telescopic guages.

In the case of fitting pistons.... that's what lapping compound is for.

That being said... the precision of your measuring instruments is no better than the devices you use to check them with. Most digital calipers are not precise through their entire range.

[ghmerrill]

Some really good points by Bloke, Doug, and Dan. I confess that I often to "overbuy" in terms of measuring tools. I'm not all that enamored with the digital stuff (and don't at the moment actually own a digital measuring device), but I am enamored with reliable precision. The stuff I've gotten from Grizzly seems to have been just fine, but they don't seem to have their inside micrometer in stock for the forseeable future (probably on a boat from China at the moment). And I'm not going down the rathole of telescopic gauges, however tempting that may be.

Having looked around right now, I think the question is whether I want to spend around $50-$75 for something to play with that will probably be "good enough" or around closer to $200 for a Mitutoyo. Maybe I'll go cheap this time until I have a better feel for what I'm doing.

[Doug Elliott]

The Mitutoyo 6" is under $150 and you might find it on sale for less. I don't know what your problem is with telescoping guages... they're pretty much necessary for lots of things and there are cheap ones available that sort of work OK.

[ghmerrill]

The Mitutoyo 6" is under $150 and you might find it on sale for less. I don't know what your problem is with telescoping guages... they're pretty much necessary for lots of things and there are cheap ones available that sort of work OK.

Mitutoyo definitely comes highly recommended (short of the high-priced stuff). I'm a little surprised I don't see more decent ones on eBay since I've picked up some quite good (but old) Starrett tools there in the past for relatively little. But I guess micrometers are a bit different.

My concern about the telescoping gauges was that they would be too much more expensive and would be overkill. But I'll take another look. I can justify stuff like that since my oldest kid has decided that he wants to become a competent machinist and he would use it too.