Better to use the topslide graduated dial to measure the amount the tool is moved along for parting or grooving etc.

On some lathes (Myford etc) you can put a graduated handwheel on the leadscrew for the same purpose via moving the whole carriage.

Or you can make a carriage stop with an incorporated micrometer unit. If you are a metric worker, you can buy those micrometer units dirt cheap on line. It.s basically a micrometer thimble and screw unit without the U shaped body to hold the anvil. Sorry, not allowed to post links to show what I mean.

Edited By Hopper on 16/03/2021 10:19:42

Maybe it is my nasty suspicious mind, but is it possible that the dial indicator has metric internals and an imperial dial? Approximately 0.020" is suspiciously close to 0.4mm, and I wouldn't put it past some makers in certain countries to do this. After all, you would get the same error using the compound slide on an "imperial" mini-lathe.

Brian G

There is a very helpful note on the Mitutoyo website discussing the accuracy of their gauges. They state that for a plunger-type dial indicator with a range of 10mm, the total indicated error over the full travel of 10 dial rotations should be within +/- 12 microns (about 1/2 thou). I have a 0-1 inch Mitutoyo dial gauge, and 1 must say I have never been aware of any significant errors over quite wide travel ranges provided I have set it accurately and rigidly mounted. Maybe other makes are less accurate?

It depends upon what is being used to make the measurement. Here are two instruments:

On the left is a dial indicator, on the right a dial test indicator (DTI). The dial indicator should be accurate, assuming that the shaft is aligned with the direction of movement. The DTI is not intended to give accurate readings but simply an indication of movement from an arbitrary zero.

I've just checked the dial indicator on the left with a 1mm slip gauge and surface plate, the reading was 1.006mm. It would probably be more accurate if I'd cleaned the surface plate first.

So a dial indicator should give accurate measurements of distance. If it doesn't then something is wrong with it or the setup.

Andrew

Edited By Andrew Johnston on 16/03/2021 11:29:58

I use a 0-2" dial indicator (as in Andrews post above) on an adjustable rod to measure carriage travel towards the headstock on my lathe. It has always been spot on for me with no errors noted.

Chris

This thread prompted me to check a dial indicator currently set up in my workshop. Over its maximum travel of 0.4" it was less than a thou. out against a slip gauge.

Which I suggest scores D minus compared with a micrometer, and C minus compared with a caliper!

Good luck to anyone who happens to have an indicator that maintains accuracy across its range, but as a breed they are untrustworthy over a distance. The way they are used mitigates against accuracy too. A micrometer works in a stiff frame and often has an insulating pad to minimise temperature errors. It also has a precision ground thread, and a high-grade parallel anvil, polished and hardened. In comparison, an indicator is held in a wobbly stand, at any old angle, has a gear train of dubious precision, a wobbly mechanical movement, a rounded probe, and the dial needle is behind glass, which is classic parallax error territory.

DTI's work extremely well as comparators, and measure well enough over short distances too, but they're not for metrology! Of course for home workshop purposes, one as applied by Greensands might be 'good enough' : I don't suppose many of us really work better than a thou/0.02mm – ish, in which case a DTI is in the zone.

Be fun to have everyone on the forum make a rod exactly 31.4159mm long at 20°C and have them all measured professionally to see how scattered the result is! Only for interest though because roughly 31.42mm is generally good enough for most practical work. Chasing accuracy unnecessarily may be a sin, but why not enjoy it? Have fun!

Dave

I have three dial indicators and just out of curiosity I just checked them. Two are 1" travel of Chinese manufacture and one is a 1/2" travel UK "Oldak". One Chinese is spot on and the other is 1/2 thou low using a 1" slip gauge. The 1/2" one reads 1 thou over using a .500" slip gauge.

John

I made a stand so that a large diameter plunger type could be used as a Comparator. Measurements are made as differences from a stack of slip gauges. Or rather by making up a stack which will return the reading to the original Zero.

Howard