Thick glass can substitute for granite

i'd say grind within .001" and scrape the rest,
of course you can go finer but you have wiggle room at a thou, and you can correct a mistake and still have meat.
there will be lots of checking flatness and parallel, you may even devise a holder that keeps both pieces in alignment while scraping.
it could speed the operation and assist in supplying symmetry to both blocks:)
 
Keith Foor said:
OK, so here is a question.... and I hate to continue hijacking this thread. But i think it sort of applies. What dimension should I grind to on a 123 block that I am going to scrape in? Obviously I am going to take off some more metal as I scrape. So where do I stop grinding to do that? Should I leave 2 tenths, or 5 tenths,,, more,,, less I just don't know. I was thinking 5 which gives my 2.5 ten thousands per side to scrape but if that's not enough I will end up small on them. Now of course I can scrape them BOTH down until they are the same, but smaller than 1 by 2 by 3 but I would rather try to actually hit the mark dead on. Or at least as dead on as I can with what I am working with.
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What's the accuracy of your grinder? You'll need to leave that much plus however much you plan to scrape off.

How do you plan to check the dimensions while scraping?
 
If it were me, I'd trust my grinding long before I'd trust my scraping. On size, squareness, and finish. And besides that, on keeping them identical.
 
Plan is to grind them and turn them while grinding them once I get them in close. I saw this technique used by Don Bailey of Suburban Tool YouTube fame. The machine I am using to grind on is a T&C grinder and not a surface grinder. So again, this might not turn out too well because the grinder may not hold the level of tolerance needed to do it. And again, this is why I want to try doing all this with a couple chunks of cast iron and not a cross slide or other part of my lathe or mill.

I already know that I have to retram my mill as it's cutting .003 deeper towards the frame across 2 inches. So I am already tapered slightly. I still have material to remove with the mill so I will be tramming the head before I do any more cutting. I have considered making one of those dual gauge tramming setups but without knowing I can get everything squared up and even it seems pointless to try at this point. So that is a project for later.

I figure I can get the head trammed in to .001 or so and then square up the blocks and get them down to 1.030 or so for grinding and then leave some for scraping.

I will try to get some pictures and video of all of this and keep everyone in the loop. I will be starting a thread that I will post a link to in here once I get on the project. Renter moved out of my other house. Doing some work on it and getting it ready to occupy again so the machining is on the back burner.

I will also be doing a full build of a 15 HP rotary phase convertor from start to finish with part numbers / specs of all parts for duplication in your shop. Working on the idea of starting a youtube channel for projects like this. And linking it back here to the forum. Will need to discuss that with the powers that be first to make sure it's ok.
 
Keith Foor said:
I figure I can get the head trammed in to .001 or so and then square up the blocks and get them down to 1.030 or so for grinding and then leave some for scraping.
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How are you going to maintain accuracy while scraping? Seems to me that with the usual process you could end up with perfectly flat but non-parallel sides.
 
I was under the assumption that the grinder would bring it into parallel. Now that might not be the case but it is what I was thinking
 
Yes, grind first then scrape. Assuming that the grinder will make them parallel but not quite flat. Am I not thinking right here?
 
When you scrape, odds are that you will lose parallelism.
 
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