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greenail

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I've made 3 lapping plates from some store bought 5lb cast iron weights I found on amazon. It has worked out very well so far but I had a few questions.

1. how stable should I expect them to be or is it a crap shoot? I cut 1mm depth grooves in a 10mm grid pattern using my cnc router and it was much easier than I'd expected but I am concerned the stress relieved will cause them to move. How long should you wait after cutting "green" cast iron?

2. after lapping them to 500 grit the groves are very very sharp. The plate shreds a paper towel in seconds and are incredibly difficult to clean. I also understand the groove edges do most of the cutting. Should I deburr and re-lap the plates or leave them "rough".

3. What grit should the plates be lapped to?

4. I found a video of someone who has lapped 3 plates and made some blocks and was able to wring them together. What grit is needed to wring two blocks?

5. I'm planning to make a sphere gauge to test the plates vs a surface plate. I also plan to make a 2 foot twist gauge. What other tests should I use to check the plate? Should I test the contact print from the surface plate?
 

Toolmaker51

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1. Stability is connected to age and cyclic temperature changes. When machine castings were aged, they spent 6 months-2 years outdoors in the company lot.
2. Easiest cleaning will be a nylon or stiff haired brush wet with same liquid [or solvent of] the lapping compound, then stood up to drain dry.
3. What really does the cutting is the surface - being softer than object being lapped. The grit embeds in the iron, copper or brass and wears harder material with every stroke. The grooves are just recesses to let the parts bed down without 'surfing' on unevenly distributed compound. also why only the figure 8 motion can produce true flat. Plates aren't lapped per-se, but planed, ground or scraped into degrees of flat. That kind of flat is 16 to ~25 contact points per square inch.
4. Being able to produce wring-able flats is possible but grit used depends on hardness of gauge material. I've used 8000 grit wet-or-dry [wetted & detergent] to a good looking surface that would not wring. Without temperature and metrology based size control, achieving this will be difficult.
5. Sphere gauge? Twist gauge? You'll have to produce illustrations of some kind. Whitworth 3-plated his way into history with nothing but scrapers and spotting blue. It's been calculated he reached flatness in the millionths, in the 1700's without any corresponding instruments to verify. This is how he generated the early accurate metal planers.
 

greenail

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I posted a video documenting my attempt. I should revisit it and hit them with a few more passes. I also now have an optical flat but i have not had good luck making a light for it that shows up on video. Some LED's have a narrow enough spectrum so you can see the fringes but not well. I should buy a low pressure sodium bulb and film the fringes from the first attempt before I go a grinding again.

 
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