Calipers & Micrometers

A generic question re accurate vs inexpensive measuring tools. Encountered a problem this weekend measuring bullets, but the same issue impacts anything requiring accuracy, including telescope parts.

I don't have expensive measuring tools. Have an Ace Hardware ancient made-in-usa 1" micrometer that did not cost much a long time ago. Also an old made-in-switzerland nylon-fiberglass 6" dial caliper, and a newish Harbor Freight 8" digital caliper that is built impressively good for the low price. And a couple of inexpensive dial gauges.

Maybe the nylon-fiberglass dial caliper and steel digital caliper have drifted with time, but I usually use them for relative measurements where repeatable numbers are more important than absolutely accurate numbers. The dial and digital calipers are both quite repeatable. I thought I was getting better numbers out of them in the past, but maybe I never checked closely enough and they have always been off.

Was measuring a nice bullet from a reputable manufacturer that is supposed to be 0.355" dia. The HF digital caliper kept telling me 0.350. The swiss dial caliper kept telling me 0.352. So I dug out the ancient cheap slightly-corroded General/National micrometer, which measured EXACTLY 0.355. Not 0.3545 -- Not 0.35525 -- Exactly 0.355 as was the spec on the part. Spooky close to the expected value that probably is not an accident.

Spent awhile web-searching. Most comments from machinists say the Harbor Freight digital calipers are about as accurate as more-expensive Japanese digital calipers (perhaps the same factory nowadays). Guys who had directly compared HF digital calipers to their pricier calipers and got the same readings within 0.001".

Couldn't find any web info on how to calibrate a capacitance sensing caliper. Took it apart, blew it out, reassembled and messed with the physical tracking adjustments, but it remains consistently 0.005" off.

If the distance is 0.1", it measures 0.095. If the distance is 1", it measures 0.995. Repeatable as you please, just 0.005 short on every measurement (compared to the old micrometer which appears dead-on).

Assorted questions come to mind:

1. Since this HF caliper looks spookily similar to mid-price Mitutoyo calipers, does anyone know what calibration procedure would be performed to fix a Mitutoyo with this problem? I can't find web links of ANYONE having this issue with HF digital calipers. Web reports give the impression that they are either accurate or they don't work at all and you throw em in the trash.

2. Would one buy the same problem (or worse) with mid-price stainless dial calipers? How much would you have to spend to get a stainless dial caliper accurate at or below 0.001" absolute? Reckon anybody but China makes calipers nowadays? Read web reports that some recent Mitutoyo calipers are Chinese nowadays. I don't have a beef with China or any other nation. Would just like to get an accurate caliper . Preferably without spending a fortune.

3. Are modern mid-price micrometers likely any better? There are relatively inexpensive micrometer group sets nowadays (my lonely ancient micrometer only goes up to 1" max, and would at least like to measure 8" occasionally).

My old rusty micrometer probably cost less than $25 a long time ago, and seems dead-on accurate to at least a thousanth. Would a modern similarly-cheap micrometer be likely the same accuracy, or would it be the same crap-shoot as with modern inexpensive calipers?

Maybe it isn't a crap shoot and I'm the proud owner of the only inaccurate digital caliper in the world. Am hoping to avoid becoming the proud owner of MANY inaccurate measuring tools in the search of something better .

Thanks!

Update: I disassembled the HF digital caliper a second time and cleaned the sensor with a dry microfiber cloth (the minimal manual instructions had recommended compressed air so that is what I used the first time).

The sensor is just a tiny circuit board with an array of parallel copper stripes. It could get scratched if abused sufficiently, but does not look terribly delicate.

This got the HF digital caliper to agree with my old micrometer to within 0.0015" to 0.002" (the half-thousandths indicator on the digital caliper isn't real repeatable). This matches the >200 mm absolute accuracy spec in the one-page manual, though I'm measuring distances < 25 mm and the spec sheet does not indicate whether to expect better accuracy at short distances.

Perhaps it is now operating within spec, and for many things a missed 1 or 2 thousands isn't bad as long as it reads the same every time. Perhaps this is remarkable that such an inexpensive gadget could be absolute accurate within a couple of thousandths, and so remarkably repeatable.

But the questions remain about possible 'affordable' alternatives that have absolute accuracy at least to 0.001". The old cheap micrometer is marked in thoursandths, but is easy to guestimate in-between markings, so it seems accurate at least to 0.00025. Maybe 0.001 is asking too much in a caliper? What about in a 4" cheap micrometer?

Thanks again.

James,

Not owning any digital measuring devices, I can't help you in that respect. I do own an SPI (Swiss made) green plastic 6" dial caliper and it's served me well for nearly 30 years for quick ball park measurements. It works fine for bullets. When I want better accuracy, I pull out my 33 year old Mitutoyo 6" dial caliper. The advantages to the dial calipers are a.) no batteries and b.) they are easily zeroed are instantly adjustable.

For critical measurements, there is no substitute for a good micrometer. I've got four of them - all Starrett, and all are way older than I am. The advantage to these high quality micrometers is that since they are adjustable for thread wear, they last almost forever and the thimbles can be re-zeroed.

Rather than settling for a new cheaply made micrometer or dial caliper, I'd suggest searching the used tool stores (or relatives) for oldies but goodies, clean them up, check them and put them to work. Old instuments made by Starrett, Brown & Sharpe, Lufkin and not quite so old Mitutoyo's are far better made than the Chinese stuff and are a great value.

If you want to measure a part to three decimal places of accuracy, then you need a caliper or micrometer that reads at least four decimal places. Good, fast, cheap, pick any two.