Why not linear motion style rails on a lathe?

[CnoEvil]

Hi Guys,

Newbie here, catching an interest in machine work. I've been reading the posts here and at the practical machinist website trying to learn something about machine work. It started with the desire to build a 3 axis cnc router for woodwork...but then I got to thinking that I might like to build a metalworking lathe too.

I have bought a couple of instruction books for building lathes: Gingery's "The Metal Lathe", Rama Machines,"Metal Lathe" (Using Truck Pistons?) and Bench Lathe Manufacture reprint from Lindsay's.

I notice that the carriage on lathes typically run on flat metal ways, but I've been wondering why not use a linear motion setup with bearings or v-guide wheels. Why not? Would would be the reason or reasons that such an arriangement wouldn't be useful on a home-built metal lathe? Not stiff enough? Too much slop? Vibration?

Thanks,
Mark

[Magic9r]

Given that traditionally constructed lathes work perfectly well I think you are looking at the cost to achieve a given ridgidity for your answer.
Regards,
Nick

[CnoEvil]

That's kind of what I thought the problem might be. I was just toying with the idea anyway. I'd most likely be best served by sticking with what has proven to work.

[DR]

Linear rails are a good method.

Take a look at the number of new factory made CNC lathes using linear rails, there are quite few. You still need a rigid base to mount the rails on.

Generally speaking, for a super heavy duty machine conventional way are a bit more rigid.

[dicks42000]

OK, here are my thoughts.
Lathes have been made for hundreds of years. Many different makers in dozens of countries have come & gone. Don't you think they might have successfully produced a lathe using linear slides, ball bushings etc. if it met all the requirements for economic production, precision, durability, rigidity, simplicity, etc...? Economics is a lot like evolution, not everyone believes in it, but it does seem to work.
Geometry dictates there are only a few ways to guide a cutting tool along & across a central workpiece. History shows flat bed lathes (ala Atlas etc.), round bed lathes, lathes with round ways, lathes with prismatic V ways. Not many attemps at creating cheap, light weight lathes have worked. Even the Atlas flat bed lathes (cheap & easy to produce, reasonable for light & HSM type use, easily rebuildable, etc.) didn't last. As a means of precision guidance of a carriage, prismatic V ways seem to have won out.
A rigid structure almost always will be heavy, especially in cast iron which is cheap in bulk, casting is an easy means of rough fabriction of the necessary shapes, has good wearing properties, and good vibration damping characteristics.
Rigidity & mass are a machinists friend, especially when horsepower goes up as the need/ desire for faster metal removal rates increased. Compare a 12 inch Atlas lathe with a Colchester Student or 13 inch Dean, Smith & Grace lathe. The Atlas would be driven by a 3/4 HP motor, the Colchester or DS&G would like 5 to 7 1/2 HP quite happily and weigh 5 to 6 times as much.
Even my 13 in. SouthBend weighs something like 4 times what my Dad's Atlas did. It happily handles 2 HP....
Except for interests sake, there is no point in trying to build a lathe "ala Gingery". As others have said, even a cheap Chicom mini-lathe is better than what you could make at home without other machine tools. Why re-invent the wheel....South Bend did publish plans in their "Machine Shop Projects" series about building an 8 in. bench lathe. Never seen one, though.
Those are my thoughts. Hope it helps.
Rick

[Mark Hockett]

Mark,
My CNC lathe and CNC mills use linear guide ways. They seem to work well. The mill has a 750lb table capacity. This is not a very close up pic but you can kind of see the ways,

[CnoEvil]

Hi Mark,

It's great to see that someone has used linear rails on a lathe. It kinda gives me hope that my idea wasn't too far fetched. You see, I figure if I build my own lathe one of my biggest problems might be getting the ways true and might even require a trip to the machine shop to have it ground true. If I were to use linear rails I could work out my adjustments in the rails myself. What kind of bearings does the tool carriage on your lathe run on?

Of course I realize that the base would have to be pretty stout, but even if I stick with the tried and true traditional ways, it would still have to be stout. I was planning on using flat bar stock to build it since I don't think casting iron is within my reach presently and aluminum just doesn't seem like the thing to use.

Mark

[EDMTech]

Hi Mark,

It's great to see that someone has used linear rails on a lathe.
Mark

Actually, pretty much EVERYONE in the CNC industry uses LM guides (linear ways or recirculating ball ways as they are also known) on nearly all of their machines, and have for years. Box way machines are still available but are few and far between, generally being reserved for the heaviest duty work. There are of course exceptions to this rule (I believe Fadal still uses all box ways) but linear ways are still the most popular system.

Edit: I should note, I am referring to the rectangular style ways, not round style ways like Thompson shaft. Thompson shaft is not a very good choice as supporting it for it's entire length is difficult, where an LM guide has a bolt every few inches to keep it where it belongs.

[CCWKen]

I think much of the change over is due to the advances in forming, casting and forging technologies too. Far less rigitity is need for the light machining required. All but gone are the days of hogging out parts from raw stock.

[Spin Doctor]

Properly designed they should work quite well. But properly designed they will in likelyhood be more expensive than traditional bed designs. If you are really thinking about building your lathe and want to go with some sort of linear rail design then take a look at this one

http://www.lathes.co.uk/scinta/index.html

or any of the other lathes on the Machine Tool Archives site

http://www.lathes.co.uk/page21.html

There is one that uses round bars for ways that look to be at least 50mm in diameter. Just can't think of the make off the top of my head. Also for a really good bar lathe you need some way to adjust the guide bushings