# Looking for a scraping block



## macardoso (Apr 9, 2021)

As the title suggests, I’m looking for something to use to bridge my high point while measuring scraping. I am currently using a 0.2” thick standard gage block, but I find the cross section to be a bit small at times, especially during roughing.

I’ve seen some of the guys on YouTube using shop made round blocks, knurled on the rim, hardened, and precision lapped for flatness and parallelism. Are these available commercially or can anyone think of something bigger than a gage block but smaller than a 1-2-3 block that is flat and parallel on opposing sides?

Without any good reason, I think it would be cool to have one that was a circle with a contact area of 1 in^2. This would be roughly a 1.13” diameter.


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## extropic (Apr 9, 2021)

If I understand you correctly, you want a precision parallel to place between a DTI and a scraped surface. Is that correct?

I can't think of a commercially available 1.13" precision disc, however . . .
What about using a square (not rectangular pattern) gage block. They are close to the desired dimension across the corners. Obviously, flatness and parallelism would be excellent. Square gage blocks do have a central hole through (with generous countersink on both sides).

You could cut a chunk off a large HSS (lathe) tool bit. Check the parallelism first (assuming you don't have a surface grinder). Quality USA HSS tool bits might be flat and parallel enough for your purposes.


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## macardoso (Apr 10, 2021)

extropic said:


> If I understand you correctly, you want a precision parallel to place between a DTI and a scraped surface. Is that correct?
> 
> I can't think of a commercially available 1.13" precision disc, however . . .
> What about using a square (not rectangular pattern) gage block. They are close to the desired dimension across the corners. Obviously, flatness and parallelism would be excellent. Square gage blocks do have a central hole through (with generous countersink on both sides).
> ...


That is correct.

I've looked at square gage blocks but the hole in the center would drive me nuts I think! I've looked for ones without the hole, but no luck.

Not a bad idea using a tool bit. Maybe I can find a parting blade that is flat on the sides? Cut off a chunk from that.


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## Wheelcock (Apr 10, 2021)

Think one of the smaller parallel face optical flats used for checking mics would work great. 30mm (1.18”) USSR ones are usually $20-40 on Ebay and even scratched up would probably work great for this with the right indicator tip


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## macardoso (Apr 10, 2021)

Wheelcock said:


> Think one of the smaller parallel face optical flats used for checking mics would work great. 30mm (1.18”) USSR ones are usually $20-40 on Ebay and even scratched up would probably work great for this with the right indicator tip


That’s really an excellent idea. I see a 30mm one for $22. Good chance I’ll drop it on my surface plate and chip it, but that is probably cheaper than a metal one even if I could buy it.


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## francist (Apr 10, 2021)

Hey, these are pretty close. Dies from a punch press I think, I got them from a brother-in-law when he worked at a wood stove manufacturing place. They punched the holes for handles and levers in the steel plate with them. A bit more than 1.3” diameter but they’re nicely ground, I use one of the male dies as a cylindrical square. If you have a similar industry around you might check for spent ones — mine came from the scrap bucket.

-frank

Edit: ahh, disregard. Was thinking the opening in the face would be perfect for mounting an indicator say, but that’s not what you were after.


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## Optic Eyes (Apr 11, 2021)

Most guys actually working doing rebuilding had several sizes, just ground tool steel works fine


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## Richard King 2 (Apr 12, 2021)

When I used to scrape Moore jig bores I used a device like what your describing.  I still have it someplace.  I'll look for it.  It was turned from a 4" diameter cast iron tube and a small steel bar.  It had 1" walls and it was about 1 1/2 high.  we used a 3/4" round steel bar that was 4" long and milled flats 1 1/8 flats on the bottom on each end, then attached it to the top of the tube with cap screws.  Then we scraped the bottom of the tube to 40 PPI.  Then set that on a surface grinder and groin a flat spot in the middle top of the bar.  Then set that on the top of the Jig Bore table to indicate the parallelism of the table top and spindle.   That die punch fran shows would work if you could fill the slot with steel or cast iron and grind it flat.


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## macardoso (Apr 12, 2021)

Thanks for all the ideas!

Part of the issue for me is a complete lack of precision grinding capabilities. I have a mill (in very poor shape) and a lathe (decent), but no surface grinder.

I ended up going with one of the early comments and bought a 30mm optical flat ($20 from Russia - free shipping). It is cheap enough that I don't feel terrible abusing it for my needs. Until that arrives, I will continue to use the gage block I've been using and will likely use it when I need to measure within a tight area.


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## macardoso (May 4, 2021)

Optical flat arrived from Russia today - looks great for the application. As long as I don't drop it, it should be good for a long time. It is about 3/4" thick which is a bit thicker than I'd prefer but will be fine.


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## macardoso (May 5, 2021)

One fun experiment, the Coefficient of Thermal Expansion of optical glass is approximately 8.0 compared to that of tool steel (5.8). The thermal conductivity is roughly 1.0 W/(m*K) compared to that of tool steel (20).

I guess this means the glass block is 20x slower to pick up heat from my hand, but will expand 1.4x that of a similar block of tool steel.

In all, that means if the block warms up to the temperature of my hand from room temperature, it will grow in thickness by 0.7 tenths. Not bad Steel would only grow by 0.5 tenths but it would happen faster.


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