# What do you user gage/ jo blocks for in your home shop?



## HMF (Jun 26, 2011)

Is a set of gage/jo blocks something a home-shop machinist typically would need?
What would I use them for?

Thanks!


Nelson


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## randyjaco (Jun 26, 2011)

Nelson
I am sure others will disagree, but I have absolutely no NEED for gauge blocks in my shop. I don't have a surface plate nor other precision metrology equipment. If I did have the blocks, I'd probably only use them as standards for calibrating my calipers, but I already have mic standards that serve that purpose.

Randy


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## Tony Wells (Jun 26, 2011)

They are primarily an inspection tool, and can be used to set a sine bar or sine plate. I have a set that I rate only for shop use, since the cal date is out. I stack them for ease of setting a depth stop on the mill, for instance. Since center drills or spotting drills are short, you need more quill travel, but when drilling to a fixed depth, a block makes a handy stop. If also counterboring during the same operation, there is another "fixed" depth. Of course, this is helpful for production work. 
The standard use for gage blocks (I don't like calling them jo blocks for some reason) is on the surface plate along with a height gage using a test indicator to verify length of a part in the vertical plane, or verify step to step dimensions. They are useful, with proper accessories, to set bore gages, other gages such as Gagemaker thread measuring instruments, etc.. 

Of course, I have seen, and done myself on occasion, people use them to provide go/no go gages for splines and keyways. That's not what they are for, but they work, but it is hard on them.


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## Tony Wells (Jun 27, 2011)

Well, that is an industry standard accepted approach. You should have resolution one order of magnitude greater than your tolerance. That means that you should have 0.000010 resolution to accurately, and repeatably measure to a Â± 0.0001. Typical shop grade, or grade B gage blocks are only good for 0.000050 anyway, which means that unless you have the actuals on a report, you don't really know what you blocks are. That is one error vector in blocks used for comparator setup However, around 1975, grade B was officially obsoleted. New manufacture blocks per ANSI carry a different grading system altogether. Grade B and "economy" sets are still sold, since there are no real laws to govern it. The best blocks now are down to Â±1 Âµin. That would be grade 0.5, IIRC.

When I need that kind of accuracy, I use a bench micrometer, or if it's a height or length (far less common), I have an amplified electronic test indicator. If I remember, I'll put up a picture.

Mumbles, I'm glad that's you. I haven't the eyesight or patience for splitting tenths any more.


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## Tony Wells (Jun 27, 2011)

To me, the 10:1 rule applies more to the calibration of gages rather than actually measuring of parts, but that's what has been specified in internal quality documents from my customers. In the case of qualifying bearings, direct measurement with a comparator gage, as you are doing, makes sense. In the case of an instrument that has substantial range, like a a 1" indicator, or micrometer, it does make sense that a more accurately calibrated standard be used. Perhaps an example of measuring a gage block with a scale would make the point. How accurately is the scale marked? How accurately can it be read, with parallax error? And graduation width? Obviously that method would not be acceptable. That's because you can't read the scale to even an equal resolution to the expected accuracy of the block. Or compare a gage block to a gage block. If you want to verify the size, would you take a reading with a dial caliper on a 1.0000" block and then measure a 2.0000"? No, because the dial caliper simply does not have the accuracy or resolution to make that measurement.
In your case, you can visually split the tenth with a dial, and that, in effect is applying the rule anyway. If it were digital, you couldn't do that. It would read as much as 0.000049" before tripping to the next 0.0001". That's probably one reason that people still stick with the 10:1 rule. Also a factor is your expected range of travel. If you set your snap gage, then measure a part far from the standard, your indicator accuracy comes into play. That's why snap gages have such a small range of travel, normally.

It's interesting that the difference in overall performance between using a 10:1 and a 4:1 is only 15%. Seems that 4:1 is far more practical, but then, who makes odd increments of resolution like that? 

http://www.transcat.com/PDF/TUR.pdf


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## Tony Wells (Jun 27, 2011)

You're right, Bill. We did leave the home shop a while back. But, I consider more information than necessary a plus, not a minus. 
And I think we're in climate controlled environments now, besides. Not my shop. That's one reason I don't do that sort of work much.


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## Tony Wells (Jun 27, 2011)

I wish there were better QC in factory assembled motors. Long time ago, I did mess with smaller 2 stroke engines, and found that too many of the needle/roller bearings were dreadfully poor in their selection of rollers. It's a wonder more of them didn't scatter out during races.
Mumbles, you sure are making it tempting to set back up for that type of precision though. I do miss it at times. The shop is heated and cooled, but I have no direct control over RH, and no certified system for logging that either. Would be fun though.


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