Cast iron quality for lapping plate?

Hi all,

I posted this at the bottom of another not directly related thread and am thinking that it may be more feasible to post in a separate thread ... thus, this thread - hope one of you may just briefly be able to help

" ...

If I use a cast iron lapping plate for the lapping machine is there then a special type of cast iron that will be best for my purposes (mainly lapping flat copper, aluminum, and polyurethane plates)?

I have some EN-GJS-500-7C cast iron plate that I would prefer using

datasheet: https://www.masterpiece.dk/UploadetFiles/11934/25/Tasso_Data_Sheet_EN-GJS-500-7C_issue_2.pdf

but if there is a clear difference I could get a plain grey iron plate EN-GJL-150C plate instead (it is softer, possibly more graphite)

datasheet: https://www.masterpiece.dk/UploadetFiles/11934/25/Tasso_Data_Sheet_EN-GJL-150C_issue_2.pdf

Might one of you have some insights into this?

Cheers & thanks for reading and considering,

Jesper

... "

If I'm understanding you correctly, you intend to flatten out copper, aluminum and polyurethane plates, none of which could be lapped using an iron plate/media. You would need to find a plate made of something softer than those materials you intend to cut, which in your case would be difficult and then even if you did find something it would likely be too soft to hold form.
With loose abrasive lapping the abrasive particles need to embed into the softer material in order to cut anything. If the two materials are of the same hardness the abrasives just roll around between the two leaving a very dull finish and it will chew up the lap quickly which is undesireable.
What you'll need is something on the order of 3M lapping film and a granite surface plate, called mounted abrasive lapping. A little spritz of water on the plate is usually all it takes to hold the film on. It is available I believe through McMaster Carr though we order it through MSC at work. It also comes in many micron sizes and aluminum oxide or diamond. The difference between it and sandpaper is the film has a plastic backer and a very controlled thickness whereas sandpaper does not.

Hopefully I've answered your question.
I can assist more if you need it.
Cheers!