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Advice: Re-scraping a diamond bedded surface plate

thanos

Plastic
Joined
Oct 2, 2019
Location
Canukistan
I'd like to start by saying that I've done my due diligence searching forums and web. It's well discussed, however the posts I did manage to find seem to lack follow up or context regarding the usefulness of each method.

I'm in the process of reconditioning a Starrett crystal pink surface plate more or less following the Robin renzetti process. I scraped my cast iron surface plate in to a reasonable degree, measuring about one micron repeat reading with a makeshift repeat-o-meter. This is where things began to deviate from the plan. After realizing I had a lot of work to do on the granite using the 10 micron diamond I had on hand, I hastefully switched to the next grit up I had which was 200 micron. While it still took a few hours to knock down all the high spots (granite was really bad), I wish I had waited a few days for a more mild grit to arrive. Realizing now that I need to shift gears and use a finer grit to remove some of the deep scratches left by the 200 micron, I'm struggling to get it out of my cast iron plate. Everywhere I've read recommends re-scraping, which I'll touch on momentarily, using razor blades to pop out the diamonds, trying to transfer it into a piece of aluminum, burning it out or machining it out.

At first I tried the razor which seemed to work at first, however after stoning the cast iron surface it felt like new diamond was exposed as the stone was likely breaking the matrix around the decked off diamond, or exposing diamond which I charged at full depth or multiples thereof. Next I tried stoning with carbondum, which seem to break down rather than do anything to the diamond, I stoned and razor scraped for hours but nothing came of it. Next I tried using a resin bond diamond wheel as a flatstone to try and fight fire with fire, however this did not seem overly fruitful. Next I tried rescraping, however my carbide 620/2525 insert lasted about a tenth of the pass before the edge was visibly rounded and I any further efforts to scrape cause the blade to just skate over the surface. Attempting this project at the office and my sharpening setup being at home, I got frustrated and torched the surface trying to burn the diamond out. Little success there. I continued trying to stone it and razor it. I also tried making a lap which I charged with the roughest (10 micron) diamond and I had, and using that to attack the surface. Hours later, I tried it all, no joy.

I disappointedly realized the only way through this was to re-scrape hard and deep. I brought my work home and spent a day scraping and resharpening, making about four or six deep passes over the whole surface. Now I'm in to day four, trying to re-scrape this thing for flatness but noticing my inserts still aren't lasting. I get maybe half a pass before needing to be resharpened which is mind boggling. I'm worried that I'm screwing up my master in this process, but I'm also getting exceedingly frustrated. I can still see surface pitting from where the diamond likely was embedded. Under a scope or loop I can't see any diamond sitting proud of the surface. I have been avoiding setting this thing up on a mill and trying to fly cut a quarter millimeter off the top, largely due to its size, the mess it will make flinging diamond and cast iron all around the shop. But also have a hard time believing that any insert on a face mill or fly cutter will last given my experience with the scraper. I can't see than any form of grinding with an aluminum oxide or similar wheel is going to be ideal either. I don't think I could ask a grinding shop to tackle this in good conscience without informing them that it's full of diamond and I'm sure that will only result in a hard pass. I'm also struggling with thought that should I get through this and use this same plate again for a finishing grit, I'll likely have to suffer through this again. I've been looking for a smaller cast iron plate to dedicate or at least reduce the amount of time it would take to iterate through this surface, but the stuff isn't really common in my neck of the woods.


I was avoiding making this post because it feel somewhat shameful and over-discussed, but I can't think of a better community that could offer advice or sympathy based on real experience, or give me an idea on how long the struggle's going to last? At the very least the very least we can share a laugh at my expense. Any ideas on how to better get through this?


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I apologize for any spelling of grammatical mistakes, this was painfully typed out and proofread on the phone.
 
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A friend of mine made a copper lap and loaded it with diamond. After some use it needed truing and so he handed it over to the optical shop to have them face it with their fancy single-point diamond turning lathe. He then got a call asking him WTF the lap was made of because they couldn't cut it.

In the end I think they succeeded by taking a deeper cut on the lathe to get below the embedded diamond.
 
Thanos,
Just a couple of thoughts.
First, regarding your inserts, I suspect that you have some confirmation bias: you rounded the edge so much that your resharpening was not sufficient at removing all the rounded edge. They stop cutting relatively quickly, but it is no longer due to diamond grit being present, but because they are not too sharp to start with.
The alternative explanation would be that, by torching the plate, you have flame-hardened it.
I don't think it is very likely and you should check under the scope the inserts after sharpening and when you feel they become dull (if diamond grit is still there, you should notice ~200 um wide grooves on the cutting edge).
Second comment is that, one of the most brilliant idea of Robing in reconditioning the granite plate was to glue some heavy duty aluminum foil to the "lapping plate" and charging the foil (which would be replaced when graduating to a smaller grit).

I hope it makes sense.

Paolo
 
Thanos,
Just a couple of thoughts.
First, regarding your inserts, I suspect that you have some confirmation bias: you rounded the edge so much that your resharpening was not sufficient at removing all the rounded edge. They stop cutting relatively quickly, but it is no longer due to diamond grit being present, but because they are not too sharp to start with.
The alternative explanation would be that, by torching the plate, you have flame-hardened it.
I don't think it is very likely and you should check under the scope the inserts after sharpening and when you feel they become dull (if diamond grit is still there, you should notice ~200 um wide grooves on the cutting edge).
Second comment is that, one of the most brilliant idea of Robing in reconditioning the granite plate was to glue some heavy duty aluminum foil to the "lapping plate" and charging the foil (which would be replaced when graduating to a smaller grit).

I hope it makes sense.

Paolo
That all make sense and is good insight.

Truthfully I do believe I'm sharpening these blades adequately. And I did take a break to scrape a bench center in between this commotion, that cut very differently. I got through that endeavor without resharpening. Also I use the same method and blades to scrape this in before I introduce the diamond. It was a different story then. Probably went through three edges. I don't have access to a scope until next time I'm in the office but I will look closer for those grooves with the magnifier I have access to now. I'll upload photos when I get back inside, I took images of a fresh blade under the loupe, as well as a mostly used one.

I don't think flame hardening could have done that much, it was more of a spot check with a propane torch. I don't think I did it long enough, after noticing to some oxidation I opted to stop. But it certainly possible some of the diamond provided some carbon and case hardened it locally.

I do think you raised very valid point about wear patterns on the insert. I'm confirming the presence of diamond because I'm able to see scratches when "lapping" a piece of carbide such as the scraper blade against the surface plate. The same rough pitch as I left in the granite using the 200 micron. Albeit, after a few days of scraping there are much fewer scratches for a given amount of lapping.


Edit:
I forgot to add that I am trying the aluminum method next. I am struggling to get a decent bond to a crummy old stone plate that I have. Perhaps because I've been trying to use adhesives that I can solvent away in future. I'm sure epoxy would work, but so far getting some hollow spots when I tried CA, and contact cement.

I appreciate the insight and the time you spent responding.
 
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Rethinking and doing some back-of-the-envelope calculations, it makes sense that you have still some diamond there: 200 um is very close to 0.008". I don't know how heavy do you normally scrape. But, in order to remove 0.008", you need quite a few passes (40x if removing 0.0002"/pass, 16x if removing 0.0005"/pass).

If you have a well-trammed mill, flycutting would make sense. You need to remove at least 0.010" in a single pass, in order for the tool tip not to get in contact with the diamonds.

Paolo
 
Some photos for fun and reference: Shot with the phone through the loupe (scale in 1/100s of an inch).

"New" blade (directly from above)::
New Blade.jpg
Used blade (angle due to still mounted):
UsedBlade.jpg

Used blade 2 (directly from above):
UsedBlade2.jpg

Pitting in surface: (line in top left is in optics)
Pitting.jpg
 
This seems like the ideal job for a metal planer. A sturdy one can make a cut deep enough to get under the embedded diamond, cleaning the surface and avoiding excessive tool wear at the same time. Not many of those machines around, and I'm not sure how valuable a cast iron plate is where you are (they are less than the cost of the metal here) but you didn't seem to worry about diamond lapping with it...

Diamond lapping is very effective, but requires dedicated plates. Not a problem for a professional plate lapper, but unless you have a bunch of spare plates laying around, not ideal for the home gamer. Some of the guru's have had partial luck with aluminum foil, but it wouldn't take much to end up with cross contamination. Based on that, despite dramatic loss of efficiency, non-diamond abrasives that break down and become inert, are likely a better choice (maybe after sacrificing a plate to the diamond gods to knock out the worst of the issues.) Our advantage over professionals is time, impatientience is our enemy.

If you follow Kinetic Precision (PFG Stones) he's been making paired dedicated laps for each grit size following Robin Renzetti's guidance.

Another point to be made is that these laps traditionally have relief grooves. Using a planer surface may not be ideal anyway. I'd dedicate the contaminated plate to a coarse diamond grit, make a second lap with nice grooves and use "Time-Saver" or similar self-neutralizing abrasives with it, and just take your time.
 
I doubt anything will work reasonably on granite but diamond. It’s already PITA even with diamond. IMO reconditioning a granite plate is quite difficult in DIY manner. It Sounds to me the only reasonable (but not economical) solution is to have multiple plates, at least one per grit.
Besides of this issue, I would not go into granite plate reconditioning w/o having an autocollimator.
 
Besides of this issue, I would not go into granite plate reconditioning w/o having an autocollimator.
That's actually my next project. I started building one a few weeks ago. Right now I have it partly roughed in on a optical "rail" as a proof of concept. Bunch of glass and a moncromatic source projecting. I have all the glass, lenses, screens, beam splitters on hand. Still need to order some micrometer heads. Next steps are to put the cross-hair reticle(s) on micrometer driven flextures for zeroing out. If it works on the bench the plan is to build it into a tube. In writing these depositions I'm starting to suspect I may be a sucker for punishment. It amazing complex optics are, they don't work the way they teach you in school. It seems any small step in progress requires a leap of learnings. That's what I'm after.

Thanks for all the replies all. Enjoying reading them..
 
If you can fit it on the mill, then cutting the remaining embedded 200µm diamond off is a fairly simple task. I made a similar mistake on one cut when lapping my 900x1200mm surface plate and was extremely annoyed when I realised how much work I'd made for my self by ploughing grooves into the surface. I would suggest about 40-50µm is the most that you should need. You probably don't need clearance grooves in the lap until you get down below 12µm. After that they become increasingly important because the lap will load up and stick (DAMHIKT).

If you haven't embedded the diamond very solidly in the cast iron, you'll find that it gradually stops cutting and it's possible to move to finer grades without too much damage once you've got rid of the 200µm boulders and gone to something a bit finer.

An autocollimator, differential level etc really is vital. Without it you have no idea what's going on and it's easy to go wrong even when you do have good measurements!

You can remove a lot of granite quite quickly by using Timesaver garnet based lapping powders on the lap. It doesn't last very long at all, but it cuts very aggressively to start with and doesn't embed. Don't bother to press/roll it in, just apply it with an alcohol carrier and a paint brush.
 
Without derailing into the details and difficulties of building your own Autocollimator, I would mention that my Nikon's use a single micrometer on the graduated reticle that moves it at a 45° angle to the X and Y planes. You can't do both measurements simultaneously, but it greatly simplifies the measurement assembly and is worth copying if you get that far.
 
Hi Thanos, thanks for posting, what a nightmare! Can you put the plate on your mill and remove a few tenths of the surface in one pass to start over? Or will the cutting edges of the tool get destroyed? Cheers, Bruce
 
I remember having similar problems with embedded diamond, I put the plate in a shaper and took a relatively deep cut, say 5-10 thou with a high rake HSS tool. The tool needs to stay in the work, you need to get under the diamond.
 








 
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