How to set two leg type calipers..?
The old lathe books tell of using a basic (two leg type) caliper to test for diameter and this works well for me if I have some reference to set the calipers too. But if I dont have a reference how do I set the caliper accurately?
Yes, I know about internal micrometers but I am not sure these are what I want, maybe, but I dont have one, yet.
The Artful Bodger
For Woodworking I would think placing one leg on the end of a scale and setting the other on the desired dimension would be close enough. Using an internal micrometers would not be easy. Cutting reference blocks on a table saw using the fence as a stop and checking with outside micrometers would work.
Jim Connell, DeLand FL.
Daytona Beach is near us.
You havenít begun learn until to learn until you learn how little you know.
what's wrong with a scale? every old Starrett, and Brown & Sharpe catalog shows how. Its a really good way to check diameters of stock while in the rack. Done it many times.
Originally Posted by The Artful Bodger
Internal mics are normally checked with an external micrometer (i.e. you don't trust them), so when you have both types you can get rid of the calipers
Originally Posted by gvasale
My eyesight is not what it used to be and the old books and catalogues talk about developing a 'feel' for the use of calipers which surely indicates the potential accuracy must be approaching that of a micrometer?
Jim, I had metal working in mind but making refererence blocks is a good idea, thanks.
Peter, I suppose my real problem is that I tend to view precision instruments such as micrometers somewhat in awe and like to leave them safely in their little case until they are really, really needed!
Ha ha, perhaps I should just buy some cheap micrometers for rough and ready work!
I guess it all depends on what tolerances you hope to achieve. Many of the books which explain this tool technology make reference to the "transfer of measurement." Presumably you would start with the most precise reference you have in the shop. Say, for example, you have a micrometer. You can position the spindle to a known diameter. Lock the spindle. Hand adjust an ID two leg caliper using the micrometer as the reference. Now carefully transfer that setting to an OD, two leg caliper using the ID caliper as your reference.
The reality is, though, when these were used in the time of the texts, I question how much was actually being measured as opposed to fitted. What I mean is that it is entirely possible to work without any measured values as long as you begin with a finished piece. You may have a hole that needs a close sliding fit with a shaft. You don't care what the bore size actually is while making the shaft---only that it fits the bore with a sliding fit.
In today's world, if you wish to continue using calipers I might suggest buying a used, inexpensive, low priced gage block set. Any tolerance will do--even damaged blocks. They will still far exceed the tolerance capabilities of a caliper such as you mean. Then you can make a stack and set an OD caliper directly from it. For setting an ID caliper adjustment, make the stack and place a 1-2-3block at either end. Now set the ID caliper from between the blocks.
I don't doubt someone here will point out that "so and so" works to very exacting levels of precision only by feel and a round leg caliper. I wouldn't put it past human ability, but accurately measuring is a whole other thing entirely. Round leg calipers truly shine with comparative measurements--not quantitative measurement.
Once upon a time parts were made to fit their mates vs a tolerance. This is still a valid home shop approach - for example a shaft or piston could be turned to fit a bore by picking up the bore with inside calipers and using them to compare the outside calipers. Skilled men could get good fits with this comparative technique; the nominal dimension being initially set with nothing more than a scale.
Originally Posted by The Artful Bodger
I don't know anyone who doest that though, disposable incomes are higher, there's technology bringing the used market to our doorstep and then there are cheap imports. If someone truly can't afford $20 for a used Starrett or china mic, I'll give 'em one
Arthur, almost all my home shop activity is to 'fit', even when I make one part to a drawing I tend to fit the mating part to it (there being no real assurance the part I made is the size in the drawing anyway). For example if I am boring a change gear I made for my lathe (a recent project) I set my internal two legged calipers to an existing gear.
Thanks Mcgyver but I do actually have a micrometer or two, its just that I thought there must be some recogised way of setting the two leg calipers, which seems to be gauge blocks (as Arthur said).
I think I have my answers, thanks everyone.