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Ground Scraping Master

SIP6A

Titanium
Joined
May 29, 2003
Location
Temperance, Michigan
If you were scraping a small slide (think watchmakers lathe compound) could you use a ground master? Would the ground surface pick up and transfer the bluing as well as a scraped surface?
 
If you were scraping a small slide (think watchmakers lathe compound) could you use a ground master? Would the ground surface pick up and transfer the bluing as well as a scraped surface?
If its flat to your wanted spec why not, is normal to use a granite table as a master after all. The nice thing about using a reference thats got some decent scraped depth ime is it doesnt suck to the work as granite can, especially if youre aiming for a fine shallow finish.
 
Granite seems to carry the blue well, and not smear. Ground metal surfaces do seem to smear.

The granite seems to have some "tooth" (roughness) to the surface, like good artist's paper. That seems to prevent smearing, possibly by the blue sitting between the tiny points of contact so that it is not all released at once.

Scraped surfaces also have a sort of "tooth" to them, and they work well.

The ground metal has no tooth and the blue is all on the surface. Maybe there is a way to spread it thin enough to not smear?
 
Granite seems to carry the blue well, and not smear. Ground metal surfaces do seem to smear.

The granite seems to have some "tooth" (roughness) to the surface, like good artist's paper. That seems to prevent smearing, possibly by the blue sitting between the tiny points of contact so that it is not all released at once.

Scraped surfaces also have a sort of "tooth" to them, and they work well.

The ground metal has no tooth and the blue is all on the surface. Maybe there is a way to spread it thin enough to not smear?

This is pretty much what I thought before posting.

I wonder if I was to give my master a light glass beading after grinding?

It looking like I would be ahead to master scraping on the small masters before attacking the parts that I have a lot of hours in.

Thank you for your reply.

Todd W
 
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I wonder if I was to give my master a light glass beading after grinding?
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Now THAT is a scary thought for the purists among us......

Thing of it is, it might actually work rather well, if it can be done to an even finish. The ideal is probably similar to fine ground glass, which is probably very much like the granite surface.

I'm not sure I'd want to be the first to try it though! I am not confident in my technique with the blasting gun.
 
It seems like a reasonable proposition to sand blast a ground surface. I've never thought of blasting as a means to remove measurable material, and it's easy enough to check with an indicator before and after.
 
Todd took a scraping class a few years ago. I would suggest checker boarding it in both directions. You could blue up the ground part and check the hinge and record it, then glass bead it and test it again. I'm curious what happens. If it blues up bad re grind it. The scraping and possibly beading will let the blue adhere to the iron. Remember the reason we scrape precision straight-edges to 40 PPI is so they can be used many times before they wear out.
 
Could grind it, stone it, try it on a surface you've scraped already. If things don't work out go another way.
 
This is pretty much what I thought before posting.

I wonder if I was to give my master a light glass beading after grinding?

It looking like I would be ahead to master scraping on the small masters before attacking the parts that I have a lot of hours in.

Thank you for your reply.

Todd W

I think this is an interesting idea and would like to hear how it works out. Just wanted to throw in this tidbit: we sometimes use bead-blasting to straighten thin parts before doing other operations. In principle, it's the same idea as shot peening, where the compressive stresses tend to curve the part concave on the side that gets peened. If you do use a *light* blast and your master is thick enough, it probably won't change the flatness significantly, but if you notice that after blasting, your master is high on the ends and hollow in the center, this would be my first guess about the cause.
 
I think this is an interesting idea and would like to hear how it works out. Just wanted to throw in this tidbit: we sometimes use bead-blasting to straighten thin parts before doing other operations. In principle, it's the same idea as shot peening, where the compressive stresses tend to curve the part concave on the side that gets peened. If you do use a *light* blast and your master is thick enough, it probably won't change the flatness significantly, but if you notice that after blasting, your master is high on the ends and hollow in the center, this would be my first guess about the cause.

That's interesting hearing your experience with bead blasting. I would have expected the opposite reaction. That is, that the bead blasted side would be stretched into convexity. The experience with mill tables that are bowed convex from compression on the underside of the T-slots is to peen the back side to correct it. And rafter squares can be manipulated to make small corrections by peening the inside or the outside of the corner to make the square more more obtuse or more acute respectively.

I'll have to keep that in mind.
 
Does seem the peened side would be convex. You are making that side fractionally "longer", and it seems would have to bend convex.

Anyhow, I had not thought of that result from so minor a treatment, but it is certainly a consideration. No really good way to compensate that, either, because you get the texture and the bending at once. Even doing the opposite side would not do it, because it is not a "controlled" process.

Might have to try it and see how much effect there is.

Not too many alternate methods. Chemical etching would not necessarily be even, nor exactly what is wanted for texture, but might be minimal stress. Lapping against a plate with the right size abrasive could work, maybe.

Scraping remains the best method, perhaps?
 
OK, you gents caught my mistake--Thanks for pointing it out. It's been several months since the last batch of those and I got it backwards. Yep, it goes convex on the side that gets blasted. There's a ton of info about shot peen forming, which is where we got the idea to try it with the bead blaster. For the small corrections we need--typically correcting less than 0.02" straightness on parts made from lengths of 1/8x1.5" D2 bar--a few seconds of blasting after heat-treat is all it takes to get the curve out enough for us to move on with the next steps. Since the flexural rigidity is proportional to the cube of thickness, if you're working with a master that's 1.5" thick instead of 1/8" thick, it'd be 1700-odd times stiffer and I'd suspect a little light beading makes only a negligible change to the straightness.
 
Here's one from left field. I've seen Evaporust leave a greyish finish on treated objects. It looks like very fine surface roughness. If the ground surface doesn't hold the blue correctly, it would be interesting to treat the surface of a test piece for a bit and see if it changes the performance.
 
Assuming the master is made from cast iron, I would suggest creating a very light checkerboard pattern by a variation of pull scraping. The goal is to texture the surface without actually removing a significant amount from it.

The easiest way to do this would be to take a strip of water or oil hardening tool steel 1/16 to 1/8 thick, about 1/2 in wide, and long enough to be easily held. Harden, polish the 1/5 in surfaces, then grind the end flat and at 90 degrees with a method that puts multiple scratch marks across the end going the short way. Hold this at a 70 to 85 degree angle to the surface and pull toward you. The small peaks from the grinding scratch the surface, but with light pressure, the valleys ride above or at the ground surface. The checkerboard pattern reduces any stress induced in the surface- though this would be very improbable.
 








 
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