Surface plates Re: Newbie (New Member, New to me South Bend)

Gregg Eshelman

Cast iron gets used for high precision surface plates. In 2014 a huge one made by Gorton for use in their factory was sold on eBay. According to the data plate on it, it took seven years from casting through various stages before their craftsmen *started* the final scraping step. That master plate apparently became the foundation for all the testing at Gorton, possibly for the rest of their existence.

Dunno if the person who bought it hauled it the 420+ miles home or if the museum in Racine, WI accepted it as a donation or loan.

From: "Edward Draper eddie.draper@... [SOUTHBENDLATHE]"
Sent: Monday, March 6, 2017 8:40 AM
Subject: Re: [SOUTHBENDLATHE] Newbie (New Member, New to me South Bend)

There are 3 issues here, which should not be confused with each other, namely mass, stiffness and damping.

If something rings when you hit it, it doesn't contain much damping.  If it just goes thud or squelch, it has lots.  Concrete damps much better than steel, as, incidentally, does cast iron.  All 3 together can help if your machining problem is vibration.

If your machining problem is persuading your machine to maintain alignment despite a built in natural tendency to do otherwise, then the only necessary parameter in its mounting is stiffness, so it can be forced into shape rather than it forcing its flimsy foundation into its preferred shape.  Beware wood, as while it is thermally stable, its shape changes with humidity, to quite a significant extent, so it and high precision are mutually exclusive.  To be honest, for a hobby lathe, a rectangular framework of substantial angle iron, having diagonals in both directions in every plane, attached close to the holding down points for the machine, ought to be stiff enough to provide the required result.  I would recomend bolting rather than welding, as welded articles change shape over years due to their working stresses stress relieving the welds (unless you have a big enough and hot enough oven in which it can be stress relieved!).

Granite (the type of rock does matter) is used for high precision surface plates as it is more stable than the common alternative, cast iron.  It can also be used for the guide ways of the sorts of machine tools that are part of a building.


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