would you use jo blocks as a precision spacer....
.... in a precision vice for an open set up inspection routine?
I know that calibration labs don't even want us to breath around precision gages, jo blocks being one type. But, as a practical matter, is it bad practice for a screening inspection lab to put a stack of blocks between the jaws of a "precision" vice (with ground faces, etc.) in order to use the jaws as say a go or no-go gage? I think about the weight of a fairly large sine plate on a block (distributed on a line basically) as being more risky to the structure of the block than between precise vice jaws. The only pressure necessary would be to keep the blocks in position. I can imagine many variables and the preference of using an adjustable parallel or something else instead of precision blocks. And, it may depend on who owns the blocks.....
......but what do y'all think.....practically speaking??
I think you hit it right on when you say " it may depend on who owns the blocks....."
Originally Posted by MJBOLSTER
It would never be a good practice to use precision gage blocks in the manner you are describing but if you own the blocks and wish to use them this way,who's to say you can't?
How most toolmakers get around this is to have a "working set" of jo-blocks and an "inspection set".
One common accessory for jo-block sets is a pair of endpieces and a connecting rod. You put a stack of blocks between the endpieces and snug the rod to hold the assembly together, forming a snap gage.
This is more commonly used with a set of square blocks, which have a larger face and a hole through the middle for the rod.
I would not be inclined to use a vise due to excessive pressure and possible damage to the block faces.
Or use a clamp set like that shown in the picture with standard rectangular Jo Blocks.
Another option is to use and adjustable "thingy" which can be set to the correct spacing. The local lab workshop where I started work had a cylindrical one comprising drilled base component to which ends of varying lengths could be clamped so as to give the correct length by reference to a depth micrometer working through the drilled centre hole. It was simply shown to me so I've no idea if it was commercial or shop made or even how effective it was. I imagine a there are a goodly number of potentially successful designs for this sort of thing. Odds are that there is a very big devil in the detail if high accuracy is desired.
You could use the wear blocks included in most sets to protect the jo block faces. But as harrytm stated, probably the best way is to have a "working set" and an "inspection set". We use calibration failed blocks for all kinds of spacers and what not when setting up or even when measuring.
Get a $70 set from Enco. If a block gets damaged, pitch it without tears. I don't trust the calibration sheet that came with mine but they are close enough that I can't measure the difference between them and the DoAll set we have at work. The round space block sets (that screw together ) work good for stops and are cheap enough.
The best way to go about mesuring a batch of parts would be to use your gage blocks as a setting standard for either an indicating micrometer or a comparitor stand with an indicator. then you know how much there out and the gage blocks do not see any undue wear.
There's that detail thing again..(putting a stack of my ceramics in the 1953 Bridgeport crankmeister could make my left arm hurt, if I heard a little cracking noise).
For a REALLY cheap snap gage there's nothing like a new lapped parallel clamp. But if your'e measuring something big, beware the spring.
If you HAVE to use a vise, a 3" screwless is a good investment. Going just finger tight on the stack laying in the bottom, and the parallelism is a little more predictable.