Dovetail Tolerance

chevydyl said:
Ken, how would I season iron in alaska lol
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Yeah, not funny, isn't it?

Wish i could trade you some of your cold weather for some of our hot weather we get here.

Seriously, one way would be to build a fire in a barbeque pit, you have them up there don't you? and set the piece of cast iron in the bed of coals. Come back in a couple of days, remove and it should be ready to go. Of course, I know you guys don't have a supply of Mesquite wood like we do down here. Coal works too! Don't let it get too hot, it could melt cast iron!
 
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I read a Navy report on stress relieving cast iron written back in 1948. They were primarily concerned with heat treating. That probably resulted from the need to get weapons systems out of manufacturing quickly. They did look at the aging effect. They first tried aging indoors at the foundry. Little relief was found. On a hunch that it had more to do with being outdoors, they tried heat cycling for 0F to 220F and achieved minimal reductions, but only cycled a few times. They also tried simulating corrosion by acid etching the surface with some success. One other thing they found was that dumping the mold quickly generated less stress in the castings. Taken together, I would suppose that the stress is injected by differential cooling in the mold. Removal from the sand and environment conditions outdoors allows the metal to relax. The stuff we normally get for machining is a different animal from mold cast iron. Durabar is hot extruded and cooling is slower and more controlled. It should contain less stress to start with. Alaska gets some pretty good temperature swings from winter in the interior to sitting in the sun in the brief summer. Like the Navy, I don't have years to play around with years of sitting it out by the garden, so I'll go with the recommended heat treatment... or at least as close as I can get. The stability of Ken's straight edge is encouraging.

I didn't find any empirical information on ringing the casting, and I've never heard of Ford hanging engine blocks from the overhead and beating on them with big mallets. I will rap my proto straight edge a few times just in case though.

I have enjoyed the discussion, although I still don't know what bridgestone's QA people had for +/- specs on their dovetails... or any of the other manufacturers either.
 
Baithog said:
....snip....I have enjoyed the discussion, although I still don't know what bridgestone's QA people had for +/- specs on their dovetails... or any of the other manufacturers either.
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Each manufacture pretty much establishes the tolerances they want it to be for the machining part of it. Once that is done, if they specify scraping, then that is a totally different level of specifications. Most follow Dr. Slenger's (sp) book of accuracy in a machine tool. This is the book you use to determine the result accuracy and from that determine what measure to take to correct by scraping and rechecking.

For me, if I can get the readings to hold within a .001" total indicator reading or better, I generally call it good.

Ken
 
4gsr said:
The King Way alignment fixture is a nice tool to have. Rather it is original or home made. But the way you are using it in your pictures, using the tailstock ways as your datum plane, you may/can get false readings, and you probably are. Not trying to create an argument here, just saying. How do you know that the tailstock ways are straight without any wear? Did you confirm they were straight? or just assume they are thinking they did not appear worn as the carriage ways were? If you did confirm they were good and straight, how did you verify this?

Ken
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Well I had .008 drop at the headstock on the original bed using the least worn ways to measure the carriage ways on, the ones that the carriage rode on were really worn. I also checked the dovetail ways using the carriage ways to measure from. The new bed I got from Joe @ Plaza machinery was only worn .006. Its only a little time to check all of the ways in as many configurations as I could think of and write down the numbers on the bed with a sharpie. I can visualize it that way. I don't have a granite surface plate large enough to measure the bed so I did the best with what I was able to make and the project of making the way alignment tool and actually figure out how to use it. It was a fun but somewhat time consuming project and slightly expensive buying used indicators and getting someone to fix them. Sometimes it not the end game, its the path to get there!!! I am the same way with antique airplanes... Tim
 
The easy way to check for wear on any bearing surface on a lathe bed is to check against an unworn surface. It does not necessarily need to be a way surface. The areas between the ways are fine to use. Some people say no, don't do that, in 99% of the cases I've seen over the years, its fine to do so. I reground a bed using the unworn surfaces as a guide for the sled that held the grinder.
 
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