Lapping granite?

[Lucky7]

I’ve lapped cast iron and steel. No experience lapping granite. Or measuring flatness to extent needed for a grade A surface plate. Am semi seriously thinking about getting a granite straight edge for use as a master for re touching longer cast iron straight edges. No space here for a surface plate bigger than the 30”x48” I have. Thinking granite at least (but could be longer) 76” long 8” wide 18” deep. Want grade single A as finished result. Easy to hang on the wall, out of the way. (Yes, I know what it will weigh)

Since granite straight edges like this are not off the shelf items, how crazy am I to consider getting a big, cheap, out of spec surface plate, cutting it into slabs sized for above needs and lapping them. Ideally sell the extra bits to like minded individuals. Easier/cheaper just buying ready made to spec from one of the big companies?

This is a somewhere between pie in the sky dreaming and willing to write the check today for me. Thoughts?

 

[PeterT]

Robin Renzetti did a couple videos on his YouTube channel re resurfacing granite surface plates -abrasives, procedure, measuring etc. Not exactly what you are proposing but might give you a feel of what's likely involved.

 

[Mcgyver]

A man sized project! Have you a autocollimator or interferometer (and all the fixin's)? I think you'd need either that or something better than A to compare to. I've not lapped granite but I suspect it's an exercise of localized removal in specific spots as you map and remap with the instrument.If you've got that stuff, maybe doable, but I'm feeling tired thinking about it .

 

[Lucky7]

There’s a few pieces of metrology kit I’d still need to find, not to mention get proficient at using. Yes, I’ve watched Robin’s, Lance’s, Oxtools, and others youtubes on this subject.

Wondering if anyone here has experience in granite lapping. Is surfacing a long straight edge more difficult than a square surface plate? I’d imagine yes, based off of experience rescraping several iron surface plates and making from rough castings eight+ (almost out of fingers to count) straight edges.

 

[Dabbler]

You could cut your plate into thirds, and use the 3plate method to create a precision surface... if you want to do that much work.

 

[wmetfab]

Im a lapidary stone worker I polish large pieces.
I use diamond pads and rotary tools, they make dips if you hang out too long in one spot.
you could use silicon carbide grits of various mesh till its as flat and smooth as you want.
Its possible but its a lot of work. A lot

Heres my polishing rig. It was about 65 hrs to build out of scraps and the milwaukee power head is worth $3,000+ this piece of jade had a deep groove 1/2" it was 13.5 hrs to go from the cup wheel to 1500 grit.

 

[wmetfab]

Here's a video if 2 guys lapping a granite plate with a cast iron plate and grit