Let's say I wanted to have made a grey iron casting about 36" long to hand scrape for use as a straight edge. This would be annealed by the foundry after casting and then shipped to me. Wouldn't something like this (as I notice things like V blocks, 123 blocks, parallels, etc) be hardened before using? Wouldn't the quenching introduce stresses in the casting that would be detrimental to accuracy?
PS - I realize there may be other solutions to this problem, like a buying a piece of precision ground steel. I am just trying to get an understanding of how heat treating works in this application.
Your gray iron casting would not be heat treated. Precision levels and straight edges that are made from gray iron or ductile iron are scraped and used soft. The other items you describe are generally made from steel (tool steel or other alloys) and heat treated, but are precision ground after heat treat. Your thinking is correct, you can't do precision work and then heat treat. Items so handled do not stay as they were. For example, 17-4 PH stainless shrinks about a thou per inch when heat treated. Most tool steels and other steels that are quenched when heat treated tend to warp some, a result of uneven cooling and stress relief in heat treating.
Don't count on a foundry automatically annealing a casting. Unless it is a part of the specification of the order, it's doubtful they would, and even less likely that the typical foundry would have the facility to do so. I'm thinking you'd end up taking it to a heat treat facility to have it done.
But it is also true that when one desires to produce an item that is hard, for wear purposes, the material of choice is not an iron casting.
Granted one can produce hard cases on castings by chilling rapidly, but isn't it true in general that heat treatment of tool steels is typcially carried out in a much more controlled condition, and with a much more controlled alloy, than a casting operation would allow?
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Mostly true. Controlling alloy with induction melting would not be any problem, so if one desired, it is possible to cast a specific alloyed iron, and industry does it all the time.
Heat treating cast iron is no big deal, it's done regularly. Think flame hardened ways. Big difference is that the ways are generally ground after heat treat, as we all know. Scraping them would be an arduous task, though certainly possible.
Yep, I agree. Fine quality tools that must endure abuse and retain high precision are generally heat treated by closely controlled furnaces. I recall my heat treater, who used to heat treat to mil specs, had a controller that held his furnace temp to ± 1°. That was a requirement for the spec. as I recall. It's been years, though, and my memory tends to be the second shortest thing I possess. :-)
is it a straight edge or a flat you are after? A straight edge is relatively inexpensive. flats on the other hand....
I received from a mentor and friend a 30" B&S camelback and 2 scraped flats, each about 24". they are flat surfaces on bars of a profile as shown below. ___ /___ | 2"
3" They are hand scraped on all four sides and the shape is such that they can be used on dovetails. When I got them i smeared some blue on and checked the three against each other and they are perfect, a lot of work went into these. All are definitely cast iron and soft. His primary use (and what Ive used them for) is to check and scrape machine ways as they can be man-handled into position. Its much easier to use a surface plate for scraping smaller pieces.
He wanted to generate them and therefore need three. Generating flats this way can give tremendous accuracy. A 30" piece will get twisted by the force of holding the work in a surface grinder. I'm am amateur; so the there may well be industrial techniques im not aware of that equal or surpass generating a hand scrape flat, however that my Starrett precision level is hand scraped gives credence to the argument that this is the right way to create a true flat. You could just make 1 using a surface plate as a reference. He either didn't have one or wanted the result to be independent of his surface plate's accuracy. That's two of the best tool makers (starrett, B&S) telling you you don't need hardened surfaces for precision inspection tools.
My understanding is that cast iron is the preference because of its stability. He'd use words like seasoning rather than heat treatment in describing their preparation. I've heard stories of lathe beds being buried for years and tool makers cycling heat treated pieces through the oven to the freezer at home over and over to season and let the stresses work themselves out. I'd be curious what a more knowledgeable persons has to say about this - what's good practice, what's overkill, what's an old wives tale. You don't want to put the time into making a flat that's half a thou next time you pick it up 3 years from now.
My friend spent some time with me both hand and power scraping. He's from the era when a pro was trained in scraping and regularly practiced it, which, i suspect from talking to the trade, is not the case today. its really not that hard to do (although your arm will feel like its going to fall) and is a useful skill to learn. for example, casting get some file and scraping to create reference surface so it can be held without twisting it.
Ah, scraping, causes me to blabber....hope it helps
No, you definitely would not want to harden a grey iron casting, especially AFTER scraping it in! You really can't do this with cast iron. Even if you hardened it and scraped it, it would start warping due to the trapped stress. I'm working with a lathe top slide that had been hardfaced 30 years ago. It warped from this application of a few thousandths of metal under thermal stress, and is now unwarping as I grind the hardfacing! The stress was held for 30 years, and is now being relieved!
If you want to make a straightedge for hand scraping use, you want to make it light so it doesn't sag under its own weight. I have a Michael Morgan straightedge, and although it is pretty good, I judge it to be too heavy, and therefore it can sag over irregular surfaces. It is only 25" long, but weighs about 30 Lbs.