3" 4-jaw: no good for 4" stock

[tylernt]

Various sites on the web such as mini-lathe.com and littlemachineshop.com claim the 3" 4-jaw can theoretically hold 4" stock with the outside jaws. Not my 3" 4-jaw! If I only engage the jaws by a single thread, I measure about 3.8" across the inside of the jaws, meaning you'll only get a very tenuous hold on 3.75" stock, with 3.5" being a more reasonable proposition.

You might be able to expand the jaws on a 4" ID, but you're still hanging on to the jaws with about 1 thread.

Fortunately I was first turning 4" stock down to 3.5" before boring it, so I drilled the center with a drill press and tapped it. Then I could thread it onto an arbor (mandrel?) chucked in the 3-jaw and add tailstock support to turn the OD before chucking the stock directly in the 4-jaw.

Turning 4" stock on a 7x10 mini-lathe is quite the trick though... had to bolt my compound on further outboard with just one bolt. Luckily I didn't break the swivel disk right out of my cross-slide doing such a foolish thing, but the motor tends to stall at low RPM, probably long before cutting forces reach destructive levels.

[sch]

That is one solution. In general even with reversal of the jaws the maximum clamping diameter clamping on the outside is a bit less than the diameter of the chuck body. If your lathe has room for a
4" diameter work piece, then you could mount a larger chuck, according to minilathe.com you could put a 5" chuck on. http://www.mini-lathe.com/Mini_lathe/Chucks/chucks.htm

[Mike Burch]

From the picture, the lathe is a small Sieg, and I would guess it's a C2.
My Sieg C3 (same radius, longer bed) came with a 3" three-jaw and a 4" four-jaw. I have wondered about getting a bigger three-jaw, but the problem with a 5" chuck is that if the jaws extend much past the diameter of the chuck, as they will when holding anything of serious dimensions, they will foul the bed.
The other problem is, as Tylernt found, that the cross-slide cannot be withdrawn far enough to accommodate anything big anyway, so the whole exercise becomes somewhat pointless.

[tylernt]

The other problem is, as Tylernt found, that the cross-slide cannot be withdrawn far enough to accommodate anything big anyway, so the whole exercise becomes somewhat pointless.

Yup. I have some 2" square steel I plan to mill into a compound replacement though. I will drill mounting holes in a variety of locations to allow more OD "reach"... just no time at the moment.

[mattthemuppet]

interesting to know! I just got a 3in 4-jaw, although to be honest my "lathe" is so feeble and flexy it would just laugh at me for trying to turn something so large

[tylernt]

interesting to know! I just got a 3in 4-jaw, although to be honest my "lathe" is so feeble and flexy it would just laugh at me for trying to turn something so large

Turning 4" down to 3.5" @ .005" DOC was an exercise in patience, to be sure. I'm sure the machine has the rigidity for .025" DOC (at least, it will once the compound is eliminated), but the motor has no torque below 300RPM and keeps stalling.

I don't plan on turning 4" stock often, but if I did, I'd look at gearing down the motor.

[danlb]

When I was faced with a similar situation I addressed it a little differently. I also have a 7x lathe and needed to turn down a larger disk.

In my case I bought a 4 inch chuck and it came with a 4.5 inch back plate. I drilled the bolt pattern so I could mount it backwards on the spindle and that let me turn the recess to match the spigot. Once it was mounted I needed to turn down the outside.

The tool post was the stock 4 position turret.

To get the room that I needed I placed a left hand tool in the position of the toolpost that was nearest me. The tool was pointing straight to the left as if it was a boring bar. That gave me the ability to turn 5 inches. The tool was brazed carbide so I was able to run it a bit faster and it only stalled a few times.

Dan

[Arthur.Marks]

I don't plan on turning 4" stock often, but if I did, I'd look at gearing down the motor.

Home Shop Machinist, Vol. 32 No. 3 May-June 2013, p.28: "Additions and Modifications to a Mini-Lathe: Increasing Torque With a Smaller Motor Pulley."

[Dr Stan]

There's a solution available in Boise

17" LeBlond

http://boise.craigslist.org/tls/3805043119.html

[Evan]

There is a saying: You can turn small work on a big lathe but you cannot turn large work on a small lathe.

My wife's grandfather used to make her sewing machine needles on his 24" x 8 foot lathe.