What is a gap bed lathe and also an engine lathe? I have always heard those terms and never asked till now.

Steve E.........

In the really old days of machining, many (most all?) lathes were powered by belts, which were driven off of large, usually overhead, shafts that were in turn hooked up by LARGE gears and a waterwheel to water power from a river or flume, or some such. They did not each have their own individual source of power. Lathes that do have their own individual power, provided by an "engine", are engine lathes...at least that is how it was explained to me...

As to gap-bed lathes, most lathes are described by a set of dimensions which describes the length and diameter of the work they will hold, without the work banging into the ways when the lathe is turning. For instance, a 9x36 lathe would hold work up to a yard long and 9" in diameter. The diameter of the work they will hold is determined by the distance between the center-line of the spindle and the nearest edge of the ways.

Some lathes have a section of the ways that can be removed...usually right in front of the chuck...to make it possible to work on larger diameter pieces. Removing that portion of the ways leaves a "gap" in the ways through which the larger diameter work can turn...hence, "gap-bed" lathes.

With a gap-bed lathe, one wants to look at two descriptions of the diameter of work for which it is suited...one diameter being that of the material which can be turned with the ways in place, and the other being the diameter which can be worked at the gap in the ways.

quote:

Originally posted by Alberta Canuck:
For instance, a 9x36 lathe would hold work up to a yard long and 9" in diameter. The diameter of the work they will hold is determined by the distance between the center-line of the spindle and the nearest edge of the ways.

What he said, except for 2 details, when they give those dimentions, it's to the bed ways, and to the end of the bed. You must subtract 2x the height of the carrage from the diameter and the length of the tail stock W/tooling from the bed length. Example, on the 9x36 mentioned above, you can face a 9" piece, but you might only be able to turn a 6" shaft (over the carrage), and while you could handle a 36" piece with a steady rest, you might only be able to turn a 24" piece "between centers".

Now there's a couple excellent replies...good job Alberta Canuck and Tailgunner!!!

quote:

Originally posted by Tailgunner:

quote:

Originally posted by Alberta Canuck:
For instance, a 9x36 lathe would hold work up to a yard long and 9" in diameter. The diameter of the work they will hold is determined by the distance between the center-line of the spindle and the nearest edge of the ways.

What he said, except for 2 details, when they give those dimentions, it's to the bed ways, and to the end of the bed. You must subtract 2x the height of the carrage from the diameter and the length of the tail stock W/tooling from the bed length. Example, on the 9x36 mentioned above, you can face a 9" piece, but you might only be able to turn a 6" shaft (over the carrage), and while you could handle a 36" piece with a steady rest, you might only be able to turn a 24" piece "between centers".

Hmmmm. I agree with most of what is said here. It's just that on MY lathe(s), and most others, the ways are higher (closer to the chuck) than the bed is. So, the measurement of the diameter work which can actually be turned is from the nearest thing that can be hit by the work while turning (the ways) to the centerline of the spindle bore (the "free" radius), times 2. So, when I talk about a 9" diameter, that's what the example I was giving would hold if I was describing it to someone.

It IS true that commercial ad descriptions would not be refering to that same distance, but the description they give is not really a useful one to me, for most purposes.


Likewise, the length of work the lathe will hold is not always the length from the chuck to the tailstock. These days many "gunsmiths" prefer lathes with spindle bores larger than most of the really long items they will be turning or otherwise cutting.

As a good example, I refer one to the turning of barrels. Most rifle barrel blanks these days are 1-1/4" or less diameter, while many spindle bores are from 1-1/2" to 1-5/8" diameter. With those spindles, it is quite possible to do turning work on barrels as long as 48", even in a 36" lathe....because the barrel is passing partly through the headstock, not just projecting from the chuck to the tailstock.

So, again, without knowing the diameter of the spindle bore, the description of lathe length given in the ad specs is not always truly useful to gauge the length of work that can be done on that lathe.

I know everyone familiar with lathes here already knows that. Am just throwing it in for those folks who are not familiar with lathe work.

Thank you Gentlemen, That answered my question. As usual the guys on here can answer the questions posted(most of the time LOL) with a wealth of knowledge.

Steve E......

yes, a gap bed lathe is an engine lathe.

jeffe

read this web site:
http://www.lathes.co.uk/page2.html

The diameter of a piece of stock varies in respect to wether it is being cut in the chuck alone or between centers. If between centers it must clear the carrage. and that height must be subtracted from what the diameter of work in the chuck alone as facing a piece might be.