Custom Size Lapping plate

[Rwilliams]

Recently I was assigned the task of developing a custom sized lapping plate of 1 inch thick, 1.875 in width and 6 inches in length. The lapping plate has the specific end purpose to lap the D valve surface in the steam head casting of a full size Westinghouse single cylinder 9.5 inch diameter air pump.

The original D valve surface in the head casting is well worn and no longer flat. The custom size lapping plate will fit in the opening and give enough length to cover the surface that needs some serious attention after many years of use. We estimated that even at the Westinghouse production facility long ago, they had to have some sort of small custom fit lapping plate to make the original pumps steam tight at the D valve surface. Unfortunately such knowledge has been lost to time and one has to resort to guesswork in order to restore the surface to a more useful condition. A steam locomotive is rather useless without a working air pump.

While most smaller lapping plates I have seen or used have small slots with resulting squares of .500 by .500, the small size of the plate needed might work better with squares of only .375 square in order to deal with the smaller surface area that needs to be brought back to a more flat condition.

Does anyone have any practical experience with small special size lapping plates and the most reasonable size of square to be used when making and using them?

The plate will be cast iron.

[GlennW]

I have 11" x 15" plates with 1/4" squares, and also smaller ones I have made for specific jobs that are a diamond pattern.

You can't just make one plate though, as they need to be lapped together to get them flat.

Normally, you groove both sides of one plate and only one side of the other plate.

You then lap them together in different order to achieve flatness.

I'll dig out the info and post it.

[GlennW]

[pete]

Tom Lipton on his Youtube channel showed making a lap using pennys super glued to a block and faced off in a lathe to get the penny's impressions removed and all of them to roughly the same height. Then it was lapped to get it truly flat.The copper works as the soft metal carrier for the lapping compound and the spaces between the pennys worked the same as a grooved lapping block. Might be another method for you? His backing block was round but the same done on whatever shape / size of lap you need for that valve face could be fly cut in a mill to start.

[tornitore45]

Good luck finding copper pennies.

[pete]

I saved a bunch before they got deleted.

[tornitore45]

Sure, I have a drawer with divider. Zinc and Copper. I use them for special washers. They actually cost less than washers at Home Depot.

[NP317]

Yea! They only cost a penny!
~RN