Why are good metric micrometers so hard to find?

[Rex Walters]

Unless you have a climate controlled hobby shop I don’t think any mics are going to help hold that tolerance.

I’m not trying to hold an absolute tolerance - I’m trying to achieve a fit. I care only about how my parts and pins measure at the current temperature in my shop.

I can definitely feel a difference between a shaft that’s a few tenths smaller than a hole, and one that’s a full thou smaller. I know darn well that you can, too, Jim!

Again: I aim work to thousandths of an inch, but to do so well requires measuring equipment that can do better than that. I don’t know how to say it more plainly.

99% of the time I don’t care about a thou of slop, but sometimes it matters (bearings and spindles come to mind).

Lastly, for the size of mostly steel parts I work in, you’d have to heat one up an awful lot to have it expand by half a thou.

 

[Rex Walters]

Let me try to summarize what I think the experienced pros like Jim are trying to say:

In the inch world, hobbyists rarely if ever need to work to a tolerance better than about a thou (aka +/- 0.0005” or +/- 0.01mm).

I agree with this heartily, but with a couple caveats:

Sometimes, particularly finicky stuff might require tightening this up to a tolerance of about half that or +/- 0.00025” (0.005 mm) — a half thou tolerance.

Also, even working to a tolerance of a thou is easier with tools that can reliably measure with better precision.

 

[Aaron_W]

Keep in mind precision metrology tools have always been expensive. It wasn't until the early 1990's that hobby grade equipment even started to become available. North America used imperial measurements extensively until about that time. Congress attempted to make metric measurements the standard in the US by 1974. There was such a huge backlash a voluntary conversion didn't start until the early 1980's.

While the vast majority of products made in North America are now built with metric dimensions and use metric fasteners most quality measuring tools are still expensive. The manufacturers are still trying to supply the professional market. Once that market is saturated, they'll introduce less expensive models that are more appealing to the hobbyist.

It seems like the USA and UK have long been the hotbeds of hobby machining / model engineering with examples of relatively inexpensive machines and machine tools going back to at least the 1900s. There was a pretty healthy hobbyist market prior to the start of WW2 with makers like Atlas, Adept, Drummond, Myford, South Bend etc all making small lathes targeted towards "home machinists".

After WW2 you start to see some hobby machines coming from Europe and later Japan, but those tended to be much more expensive machines. In the 1970s manufacture moved to Asia, but the markets remained strongly focused on the USA and UK.
People tend to think of the Emco Unimat as kind of a toy lathe but, it was never a "cheap" machine. In 1972 Sears sold them for $149 ($1120 in 2024) or about 1/2 the price of the much larger Atlas 6x18" which Sears sold for $289 +motor.

Like the USA, the UK has not made a complete transition to the metric system, it is still in a limbo state with both systems in wide spread use.

Anyway with the USA and UK being the biggest markets and both being "inch based" countries, it makes a lot of sense to me if there are more quality hobby grade measuring tools based on the inch standard. Historically there was a larger market for inch based than metric, and there is still a strong market for inch based with the USA being the 3rd most populous nation in the world (well behind China and India).

 

[Janderso]

Even if you can measure at a resolution of 0.001 mm it doesn't mean it is accurate at 0.001 mm

That's .0000393" That's in the millionths realm right?
I think tenths of an inch is possibly achieved on purpose by some on this forum.
My Sharp mill can get within a thou or two then the surface grinder can finish within tenths.
My lathe can turn a respectable .001"
Anything beyond that is in a very high end climate controlled shop with very precise equipment.
This is only my opinion based on my limited experience. I may be full of poo.

 

[Janderso]

Like the USA, the UK has not made a complete transition to the metric system, it is still in a limbo state with both systems in wide spread use

Domestic automotive production has switched to the metric system.
I was working on a neighbor's 2000 Jeep the other day. Some components were imperial and some metric.
I think most of us baby boomers struggle with the metric system.
Strange considering it's so much easier than imperial. imho

 

[Oscar Lazo]

It seems like the USA and UK have long been the hotbeds of hobby machining / model engineering with examples of relatively inexpensive machines and machine tools going back to at least the 1900s. There was a pretty healthy hobbyist market prior to the start of WW2 with makers like Atlas, Adept, Drummond, Myford, South Bend etc all making small lathes targeted towards "home machinists".

After WW2 you start to see some hobby machines coming from Europe and later Japan, but those tended to be much more expensive machines. In the 1970s manufacture moved to Asia, but the markets remained strongly focused on the USA and UK.
People tend to think of the Emco Unimat as kind of a toy lathe but, it was never a "cheap" machine. In 1972 Sears sold them for $149 ($1120 in 2024) or about 1/2 the price of the much larger Atlas 6x18" which Sears sold for $289 +motor.

Like the USA, the UK has not made a complete transition to the metric system, it is still in a limbo state with both systems in wide spread use.

Anyway with the USA and UK being the biggest markets and both being "inch based" countries, it makes a lot of sense to me if there are more quality hobby grade measuring tools based on the inch standard. Historically there was a larger market for inch based than metric, and there is still a strong market for inch based with the USA being the 3rd most populous nation in the world (well behind China and India).

Well that now addresses the inch vs metric side of my question a bit more.

 

[Janderso]

Let me try to summarize what I think the experienced pros like Jim are trying to say:

In the inch world, hobbyists rarely if ever need to work to a tolerance better than about a thou (aka +/- 0.0005” or +/- 0.01mm).

I agree with this heartily, but with a couple caveats:

Sometimes, particularly finicky stuff might require tightening this up to a tolerance of about half that or +/- 0.00025” (0.005 mm) — a half thou tolerance.

Also, even working to a tolerance of a thou is easier with tools that can reliably measure with better precision.

Hey Rex,
Are you getting use to grits?

 

[Oscar Lazo]

Over temperature, everything is growing and shrinking, including the micrometer. Everything is rubber, moving and distorting, and our common sense notions of the world don't hold true. The smaller we look at things, the weirder it gets.

Think we are merely wanting to ensure that things fit correctly together, (at some common temperature) which is a subtly different issue. This is a relative measurement, which doesn't require long term stability, only short term. Think that is the difference. Even so, it requires good technique to get meaningful results. It's all too easy to be fooled. In the microwave and mm wave world, it was very difficult to get accurate absolute measurements, but relatively easy to perform very accurate relative measurements.

Absolute metrology is hard - that takes temperature control and sophisticated measurement equipment.

Speaking of controlled temperature workshops. What sort of environment-requirement would that be? I have got an inverter ac in my shop (which is really not optional here). I have measured temperature swings (in the ambient air) of +- 3 deg C (37 F). throughout the day. For a piece of 40 mm mild steel that 6 degree swing would actually be about 2.5 micrometers (0.0001''). That is very close to the accuracy of the Mitutoyo mics I have in mind. Do you think that would make my shop sufficiently stable to make the Vernier measurements meaningful?

 

[WobblyHand]

Speaking of controlled temperature workshops. What sort of environment-requirement would that be? I have got an inverter ac in my shop (which is really not optional here). I have measured temperature swings (in the ambient air) of +- 3 deg C (37 F). throughout the day. For a piece of 40 mm mild steel that 6 degree swing would actually be about 2.5 micrometers (0.0001''). That is very close to the accuracy of the Mitutoyo mics I have in mind. Do you think that would make my shop sufficiently stable to make the Vernier measurements meaningful?

I personally wouldn't trust one day's measurements of part A, to the next day's measurements of Part B. Too many variables that can change. But I would trust measurements of part A and B made minutes apart to relatively high resolution. Would I trust it to 1 micron? Haven't measured my repeatability in a while, but in general I'd not trust a single measurement. However, I'd be inclined to trust the average of 10 measurements each between parts. But this requires stable temperatures between measurements, and no drafts or breezes, and being careful not to heat the part or the micrometer with our hands. In a closed area, the temperature is usually stable enough for these measurements over short time intervals, say 5 or 10 minutes.

Of course, that assumes that our measurement errors are random. At least for me that seems to be true. I find it tough to measure repeatedly to 0.0001", but easy to measure under that value when taking averages. It takes a while, but allows pretty precise measurements.

 

[Nesse1]

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