Is It Practical To Scrape Older Starrett Steel Straight Edge Back To Decent Tolerance?

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Playingwithmetal

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Hey. I have yet another question,and always appreciate your insightful responses. I got a older 13.5 inch Starrett beveled straight edge. It is 3/16" thick and under inspection has a large bow in the middle. I'm considering scraping it flat again and using it again. I am a novice scraper and have only done a few projects using scraping. I would be using carbide have scrapers. I am considering what the appropriate way to go about holding the piece down. I am also unsure if the re clamping process over the repeated scraping cycles is likely to warp the straight edge. Wish I had a bench mounted permanent magnetic Chuck for such thing. Any input and or real world experience would be greatly appreciated.
 
Sure, you can scrape the reference surface in. It is is merely bent then you need to straighten it out. If it will be a reference surface you need a surface plate that is in current calibration, or you will only transfer it's faults to your straightedge. You can clamp it on the wide faces between two steel bars in a vise to scrape it. The bars need to be straight, but not dead nuts straight. You will only be clamping it tight enough to scrape it and it is a hardened tool which will spring back after it is released, so long as you have reasonably straight clamping bars and do not gronk it to death in the vise. It can also be lapped in. Maybe you should define what you mean by " a large bow in the middle." Is it a few tenths, a few thou, or a few sixteenths? And I assume you mean a bow on the reference surface. I don't think you want to scrape the wide sides flat, unless it is just a few tenths...

Edit: that might be problematic anyway because as you scrape the flat sides you will be letting the stresses in the metal out and may end up chasing your tail, you scrape, it warps, you scrape some more, it warps some more...
 
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The chasing my tail part is one of my main concerns. I should have elaborated on the bow in the straight edge. It is about 2 .0005 of a bow in the center and its main contact points are on either end. I have a calibrated grade B plate that is +- .0001 across the 24"x18" surface. That I used to check the straight edge. I plan to scrape both the wide reference and narrow surfaces.
 
The grade B plate is fine, you know the result will be somewhat less accurate than the plate, and as long as you know the useful level of accuracy, it will be a fine tool. It will look pretty with the scraping crosshatching on it! Have you scraped hardened steel before? It works fine, but is pretty slow work...

Edit: Understand that scraping the sides of something that thin will not be permanent. It will continue to move, quickly and temporarily with temperature, and permanently with time. The tool was not meant to be used as a precision surface on its flat sides. Just make it look nice on the sides.
 
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I have never scraper hardened steel before. I have taken a few strokes on the surface and it is not as hard as I had expected. Is there anything particular I should know about scraping hardened steel?
 
here's a trick to try before you start doing any major scraping,
you'll need a small pair of clamps and a coat hanger or string.
clamp the straightedge in a corner and hang the clamped work , from the clamp, to any convenient location
bring the temp of the straightedge to around 70*F, or leave it in your house for a few days.
simply firmly smack the straightedge with rawhide or other soft faced hammer a couple times in different places on the straightedge while hung.
this will reduce any residual stresses that may be stored and possibly causing the irregularities.
it may save you some tail chasin'
best of luck,
lmk if it helps out!:)
 
That's a cool tip. I will try that. I think I'm going to build a wooden base with a frame that fits the straight edge. The one side will have the reverse beveled edge to support that side. The sides of this box will have to be just a touch lower the the straight edge surface I will be working. That way I won't have to clamp it and I should stay put while scraping and make the cycles move faster with no clamping procedure each time.
 
Playingwithmetal said:
I have never scraper hardened steel before. I have taken a few strokes on the surface and it is not as hard as I had expected. Is there anything particular I should know about scraping hardened steel?
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Carbide is harder than steel, so it will cut it, but you will be taking off quite thin shavings. You will need to play the the angle of tool to work and perhaps the angle of the scraper edges as well. And the scraper will need to be kept honed really sharp or you will just be polishing it.
 
You say this straight edge is only 3/16" thick? How wide is it. Is it a knife edge straight edge? If it's the Starrett straight edge I'm thinking of I doubt you are going to get anywhere trying to scrape it. Those are ground to straightness. And the .0005" you are reporting is probably standard allowance for these straight edges. You can try to scrape it, don't think it's going to work.
 
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