I have a metal lathe with a broken part. The part is not available, so I have to machine one up from scratch.

As this part is the lower half of the crossfeed assembly on the turret cutter, it has to be accurate, which is no problem for straightness and tolerance. However, as ways and gibs have to be able to hold lubricant (so the parts don't just slide on each other and wear out), there must be a standard for surface finish that's preferable for this type of assembly.

I can't seem to find a clear specification for what to use, so what would be an appropriate finish type for those conditions?

  • $\begingroup$ 'Frosting' or 'flaking' is the term for the final roughness added to the way to improve oil retention. There are lots of articles about that available that should help you. $\endgroup$ – Ethan48 Jan 16 '16 at 0:39

The traditional way to achieve this is to machine your part to the correct geometric tolerance and assemble it with a layer of marking blue applied to the mating surfaces. Work the part backwards and forwards a few time and then disassemble then inspect and remove any high spots with a scraper.

An alternative would be to use fine abrasive paste and lap the surfaces to their final fit/finish.

As you say you don't necessarily want a mirror polish surface as this won't hold oil well but similarly you don't want any ridges or ripples which will impair accuracy and accelerate wear so the goal is a flat surface with scratches in it rather than the troughs/ridges from as as-machined surface.


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