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View Full Version : Surface Grinding or Hand Lapping?



stanger53
06-28-2014, 12:24 AM
I rebuild a lot of power steering pumps that basically consist of a "stack" of plates and parts through which the fluid flows. There is no gasket, o'rings or sealer between these pieces, but they are compressed together.Over 1000psi flows through this assembly, without the pressure blowing fluid out the sides from between the plates.

What seals them together is that they are all perfectly flat, parallel, and relatively smooth. Enough so that if you have two of these plates stacked on each other, and there is any kind of moisture between the two, then you almost have to slide them apart like magnets due to their "sticking" to each other.

Sometimes, one face of a couple of the plates gets slightly scored up from the rotor or pumps shaft, so the plate has to be replaced. When I started doing this work 12 years ago, I had a small collection of old plates that had been resurfaced, but now that supply is gone and I have a bigger stack of scored ones.

The plates can be ground down several thousandths without causing any problems with assembly or operation. They appear to be a dark gray steel, but I think they are a sintered steel, because when I wire wheel them to remove varnish and staining, the heat causes them to "sweat" little beads on the surface. They are about 3" in diameter.

So, my question is - how best to have them resurfaced to a flat, parallel, smooth surface? I know I can take them to a machine shop and have them surface ground, but is it possible to achieve similar results by lapping them on a granite surface stone with some emery paper and cutting oil, or will this not give the desired results? Suggestions or opinions?

Jaakko Fagerlund
06-28-2014, 02:20 AM
Test one with hand methods, but I would suggest having them ground on a machine as it is just so easy and fast.

macona
06-28-2014, 02:29 AM
They are probably just cast iron. At high pressure oil has probably permeated the cast.

Find a shop with a lapmaster, that is almost certainly how they were finished before.

http://www.lapmaster.com/machinery/lapping-polishing-machinery/default.html

Surface plate and emery will ruin the part.

Old Hat
06-28-2014, 02:42 AM
Lap in two matching plates of mild steel.
Have them be 150% the perimeter of your pump plates.
Travel the top plate just under 30% over the edges of the bottom plate.
Keep swithching out your orbits or (I prefer zig-zags).
Also swith top for bottom plates.

Once lapped in, do one of you pump plates just to the edges of the lapping plates.
Switch out lapping plates and refine your pump plate on the second plate.
==================

If you think your lapping plates out starting to cup or mound, Go back to top, and repeat.

PS; grooves arn't needed, those are just for film consistency controll for those who
are too liberal with the compound, and too lazy to clear off the laps for fresh compound.

The grooves don't cut,