Accurate measurement

Bah humbug .

As above. A vernier caliper is not a digital caliper and vice versa. A dial caliper is neither.

Similarly, a live centre is one that rotates with the work, normally in the headstock, but one with bearings that is mounted in the tailstock is more properly called a rotating centre.

Calmly... Deep breaths... The nasty inexactitudes are gone now!  Ahh, that's better.

Repeating everyone els's comments:-

Calipers are quick to use and good enough for much work. Micrometers can be two to ten times as accurate for hand held ones and better than that for precision bench micrometers. Because of their respective constructions, micrometers tend to be far better than calipers for measuring the diameter of larger round pieces.

PS:- Anyone need a five foot Sylvac digital caliper? It was lying all forlorn in the scrap compound at work, calling out to me.

Edited By Mark Rand on 08/02/2018 23:17:16

I have a Mitutoyo vernier caliper, no dials, no batteries; also a mechanical Moore and Wright micrometer; also a good quality steel rule. Somewhere there is a piece of string with knots every cubit, but that doesn't get used much. The steel rule gets used for anything that needs to be within about +/- 0.5mm, the verniers for anything that needs to be +/- 0.1mm, the micrometer for anything less than that.

cheers

Bill

Just an observation.

We generally make bits to fit other bits.

Comparison is easier than absolute measurement. So guages are your friend.

The outside of things is easier to measure than the inside. (Shafts are easier than bores) so there is a preferred order to making things to fit.

Don't make stuff to a better fit than it needs,

regards Martin

The level of accuracy will depend on how the device has to function.

Slip gauges are accurate to millionths of an inch. They have to be, otherwise they would be useless as a near absolute standard of size (But only under very closely controlled conditions) For our purposes such accuracy is not needed.

For most purposes, a 6.5mm hole will suffice for a M6 bolt, to hold two pieces together..

For greater positional accuracy, you would use a fitted bolt, in a reamed hole. Or the two parts would be dowelled, with the bolts merely holding the part together.

For a guide rod, needing to be a lubricated sliding fit in its mating part, a 25 micron clearance would suffice for most purposes.

For the power cylinder of a Hot Air Engine, the piston would be a lapped fit (like the plunger and barrel of a Diesel fuel injection pump, or an Injector Needle to its Body.)

You can make things too accurately. Years ago, after a rebuild, when King George V ran out of Paddington, it ran a hot box within a few miles. Insufficient allowance had been made for the fact that a steam engine gets hot, and more so, as it runs. The old WD 2-8-0 side rods used to clank as they moved off, but quietened as everything warmed up after a few miles

An internal combustion engine will run happily when unloaded. Apply full load, and if the Piston size and shape are incorrect for the bore (which may distort as the engine heats up) and a catastrophic seizure can result.

The converse can be an engine that is difficult to start because of lack of compression, but once running and warm, performs quite well.

Horses for courses.

Howard

I use a dial caliper for most of my work, I find that it is accurate enough for me as long as I wipe the jaws each time I use it and pinch on the outside of the jaws to ensure that they close properly on the work. I find that they agree with my micrometers.

I used to use a Lidl digital caliper in the mistaken idea that repeatability not absolute accuracy was the most important factor. Then I realised that they don't score in either.

Edited By norman valentine on 09/02/2018 19:52:01

One thing micrometers have over calipers is repeatability as it is much easier to control the amount of pressure on the measuring surfaces with a device that uses a thread to control movement even more so if they have a friction clutch like a mic

For the OP try a little exercise - mount a dial indicator up on your lathe toolpost so it can take a reading of the saddle and compound slide movement , the nib shoud be in contact with the face of the chuck .

Now zero out the indicator and then move the carriage towards the chuck say .100 or 1mm depending in what measuring system you use and zero out again .

Move the carriage back a little then try to move it back into zero - you will find this difficult to do even with a carriage stop as there will always be some difference in the reading from zero .

Next lock the carriage and do the same procedure using the compound slide ( make sure its travel is parralell to the bed of the lathe ) you will find you can easily bring the dial indicator back to the pre set zero .

That is the difference between a straight linear movement and a linear movement controlled by a thread .

A caliper uses straight linear movement and a mic uses a linear movement controlled by a thread which really works a bit like a gear ratio .

At what temperature do you all measure. Instruments are only accepted as true indicator in controlled temperature environment. If you want consistency then you have to keep the work place constant, temperature doesn't matter so long as the calibration is done at that temperature.

Forgive me if this digresses too far from the original topic, but it may be of interest to some:

**LINK**

https://www.nist.gov/news-events/news/2016/02/thinking-outside-cuvette-measure-dimensions-skinny-glass-container-pml

MichaelG.