Flat Surface

[PeteParticipant @pete41194]

To be fair, the OP’s question was,

“I am in need of more advice! I have been using a sheet of plate glass in my workshop as a flat surface to lap parts in using emery cloth for about 30 years, but recently a freak accident saw the glass in several pieces! I am looking for a replacement and I was wondering if anyone had a better idea for material than plate glass? Has anyone used acrylic sheet or fibreglass board? Would a piece of thick sheet steel work or is it not flat enough? I am not looking for a surface plate, just something flat enough to give me a decent finish.”

As far as float glass accuracy, a quick search mentions .00015″ per inch, or .0038 mm /25.4 mm if that can be believed is an average number.

The OP didn’t provide any information about how large any of his parts to be lapped might be. Smaller parts lapped using smaller areas on any plate used, then the exact degree of flatness might become less important. Youtube shows more than a few videos lapping automotive cylinder heads on glass. Frankly and given the risks with any failure verses the rather low cost of just having it done properly by a professional engine shop. That’s not something I’d take a chance doing. But it seems to work for some.

Since the OP’s previous piece of float glass worked well enough to suit his requirements for at least 30 years, and given how float glass is produced to be almost optically flat, or at least without noticeable waves.Then I’d suggest the same again. I can’t think of any other product that would be as easy to obtain, have the same or less approximate cost, or still be as flat as that simple piece of glass without higher cost, effort, or difficulty in obtaining it locally.

[alecsParticipant @alecs]

And there is not much point in using a micron-precision surface plate for the aforesaid “lapping” with a sheet of emery cloth atop it. Emery cloth, or even paper, is not made to precision thickness so could be all over the place.

For automotive cylinder head “lapping” the flatness required for a good seal is nowhere even close to the tolerances of a surface plate. Ive seen a warped aluminium 4 cylinder Honda motorcycle head brought back flat by rubbing it on the workshop concrete floor  It worked just fine.

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