How Big A Lathe Do You Need?

That's easy:  about one inch in diameter and two inches longer than whatever you have.  (Yeah, it's an old joke ... sorry, but if you don't know it, someone has to be the first to tell you). 

I've posted before about having a Sherline lathe, smaller than the ubiquitous 7x10 or 12 mini-lathes that everyone sells, and more often called a Micro lathe.  The 3.5 x 17" description on their web site is typical of the way lathes are described: 3.5" is the diameter of the biggest piece that will turn over the lathe bed (not the cross slide), and 17" is the long length of the bed.  Sherline, an American made tool, is famously accurate out of the box.  Those "7 by" and "8 by" lathes imported from the Chicoms usually require more work, more futzing around to get good results - although I'll be the first to say I hear they get better every year.  At machine tool shows, a standard trick is for (a very experienced) Sherline representative to pull a new one out of the box, chuck up a small length of half inch steel bar, and in a few cuts turn it down to a .010" diameter nub about 1/10" long.  Then he'll take a home made drill bit and bore a .005" hold down the center of that .010" nub.  The main drawback of the lathe is that out of the box it doesn't handle large diameters.  Sherline responded by making riser block systems that add 2.5" diameter, making it a 6x17 lathe.

For example, here's my lathe at some stage in the past, showing the riser block under the head stock (black block with metallic ends at left).  The riser changes everything.  By raising the centerline, it forces risers under everything - or taller tool posts, etc..  It also makes the lathe a bit less rigid, and rigidity isn't its strongest point to start with.  (Yeah, it's quite the mess.  Bonus props for anyone who can tell what the large roundish thing is at upper right, that roundish thing with squares of darker brown in a lighter brown field.)

The real answer to how big a lathe one needs is what you intend to do with it.  If you want to do large things regularly, get a larger lathe.  If you only need to do them rarely, a small one may work well for you.  Let's look at some specific things:

Barrel work:  this is going to require a large lathe, and you're going to want to look for one sold as a gunsmith's lathe or a tool room lathe.  In any lathe, working on things more than one diameter out of the chuck and unsupported leads to loss of accuracy so most work on barrels; like reaming them with a chamber reamer, squaring the breech face or crowning the end, is done by chucking the barrel with just the little bit you need access to sticking out of the chuck.  The barrel would extend to the left in this view, through the head stock and sticking out for the rest of its length, which means the headstock had better have at least as big a bore as the barrel diameter!  Somewhere around 1 1/4 to 1 1/2" is needed. 

Something a lot of folks might be interested in is making an AR lower receiver from a blank forging, or "0% lower".  (Ooh! A ghost gun!)  This guy used the big lathe at his disposal, along with his big milling machine and metal shaper.  The only reason not to use those tools if you have them is for the challenge.  I can't say I've sat around for a long time and pondered how to do it on a Sherline, but the basic problem is drilling and tapping a large hole in a large chunk of metal.  I've cut holes that big in metal, just not metal shaped like that - which means it's all about fixturing and lining up the cutting before you tool touches metal.  It might be possible to fixture the receiver on the cross slide and advance it into the chuck, boring the hole to size ... and it might be possible to do it on the mill. 

You may want to turn a large flywheel or large cylinder for something, but if you have only one or a few operations, you may be able to carefully plan out some ways to exceed the rated size of your lathe.  I've seen some pretty wild tricks to do that on the smaller lathes where the headstock is rotated 90 degrees from its regular orientation (pointing off the bench in that photo).  Be careful!  Take light cuts, and remember the adage of "measure twice cut once" should start with "think it through three times".  I think the important part of that trick is that a large flywheel doesn't need to be turned to the accuracy of a rifle chamber.  Always remember that a milling machine can do much of what a lathe can do, if you think of the cuts differently. 

For most of the rest of life, cutting screw threads on rods, fixing standoffs, turning most objects (how many tubes bigger than 6" do you work with, anyway?), and most repairs that you'll do in gunsmithing, the Sherline or any of the "7 or 8 by" lathes will do just fine. 

Another old saying in the world of tools is that "a horse can do the work of a pony, but a pony can't do the work of a horse".  If you have the room and money for a big tool room lathe, do it.  Take some classes in it, get any training you can.  Any machine that can cut steel isn't going be slowed down in the slightest taking off a finger.  But the most common mistake is to start a lathe with the chuck key in place (or Tommy bars in the case of the Sherline) and throw it at anything/anyone in line.  They can put a serious hurt on you.