My money's on that particleboard / sandpaper backing not being all that 'flat'. The most he'll be able to achieve on an area of 20mm x 20mm by hand is around 0.25mm, it will be all over the place on a plate of that size! Note a 4 thou warp is 0.1mm, and it's accuracy in this that kills it!
I wouldnt trust anything other than a well calibrated, high speed heavy milling machine at that sort of tolerance.
i pay 20 but you have to dust it yourself we used to use wet and dry on a mirror to do single cylinder 2 stroke heads you lay mirror on table lay wet n dry on that and rub head on it, but wouldn't waste time trying to do any thing bigger by hand.
we used to use wet and dry on a mirror to do single cylinder 2 stroke heads you lay mirror on table lay wet n dry on that and rub head on it, but wouldn't waste time trying to do any thing bigger by hand.
That method works. I've lapped smaller stuff in a similar way. You can equally use metal but the issue in all cases is the flatness of the surfaces that are being rubbed together - a block of 18mm ply will be all over the show compared to a piece of glass or a milled honing block.