DIY lathe design question

Stefants

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I’m at the thought exercise phase of building a lathe. It’s not because I think it’s the cheapest way to do things or that I think it’ll yield the best machine possible, but because I enjoy the process. I’ve built a wood lathe from wood previously and I’m happy with the result so far. I’m curious to try out the concrete lathe concept that’s out there and see just what’s possible first hand. Maybe it’ll work, maybe it’ll be more a learning experience than a machine build :). With a couple kids and limited time, we’ll see when/if I get out of the thought exercise and actually build something...but for now, I have a design question.

The vast majority of lathes I see are designed such that the spindle is located pretty much right between the two ways. Between the radius of the work plus the tool extension beyond the toolpost, this puts much of the downward cutting force on the front way...or pretty much all of it if the downward force ends up in front of the front way. It seems that setting the spindle back somewhat would locate the cutting forces more symmetrically between the two ways, particularly for parts of any appreciable size. This would result in the carriage being forced down against both ways more uniformly during a cut, rather than putting the majority of the load on the front way or potentially even twisting forward (lifting the carriage off the back way in the worst case scenario). Is there a reason moving the spindle back relative to the ways is not more common?

A couple pictures taken from freely available downloadable (old) books (Lathe Design, Construction, and Operation by Oscar Perrigo and Lathe Bed Design by Joseph Horner):

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I saw reference to an offset spindle once, but the concept didn’t seem to stick.

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Is there a reason it isn’t more common? Or is it more common than I think it is? Thanks!
 
The carriage will have more of an internal bending moment of you move the force away from the front way. I’m not sure how significant that is.
 
Hello @Stefants,

First, Welcome to the site!
You are among like-minded people here.

Second, I have no idea. I've never even thought about it.
However, I am already "watch"ing the thread, because I know it will generate many good response.

-brino
 
Just hazarding a guess here.
1) Most lathes were developed for general-purpose use, so the design allowed for both forward and backward rotation of the spindle, so the back way and front way should be in the same position in relation to the spindle.
2) Moving the spindle center from the center of ways (which generally act as the feet) would cause problems with faceplate work - the heavy, counterweighted chunk of metal clamped to the edge of the faceplate would cause the lathe to walk towards the front way, if the spindle was centered over the front ways instead of between the ways.
 
This smacks of re inventing the wheel; there are, I am sure plenty of reasons that lathes have evolved in their present form.
 
You brought up an interesting point, so I went and checked my lathes. The Hercus (Australian SB9) has the spindle centred, but the Swedish Storebro Bruk Ornmaskiner has the spindle set toward the back rail - somewhere around 60/40.
 
And without that understanding you can't know whether it was just "historical engineering"(well that's the way we've always done it) or if their was a sound reason, Certainly something I've not thought about before.
 
It’s definitely worth picking a knowledgeable community’s brains about this before diving in and doing something obviously silly. I appreciate the input. I do suspect there’s a reason it doesn’t seem to be common practice...I just wish I knew exactly what it was. I don’t think it’s a notion without merit. I’d like to suss out any undiscovered checks, though. It’s always dangerous to see the possible upside and not be able to identify the downside.

Thanks for checking your equipment, Hawkeye. An existence proof more recent than 100 years ago (the approximate age of the books I posted pictures from) is useful and encouraging.
 
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