A lathe dog is used to drive the work that is between centers. If You had a shaft between centers without the dog it would be like leaving the key out of a keyway.

Steve

4-jaw?

A lathe dog is a device used to drive a workpiece when turning between centers. And that means, instead of holding the shaft in a chuck, you have a center in both the headstock and tailstock.

The headstock end is of course a "dead" center, while the tailstock can be either a live center or dead.

The "dog" clamps to the shaft, and the leg of the dog engages a slot or hole in either a faceplate or "drive plate".

In this photo, the machinist has clamped a center in the chuck, and the leg of the lathe dog is being driven by one of the chuck jaws.

The technique predates chucks, and so tended to fall out of favor once 3-jaw and 4-jaws came on the scene. But turning between centers still has one solid benefit, in that it's very easy to remove the workpiece, for fitting, measuring or testing, and then put it back in the lathe very close to, if not spot-on, the original zero.

Doc.

Yep, like Doc said. Turning between centers is one of the most accurate ways to turn a part, especially things like a shaft that need to be flipped. Traditionally they are used with a dog drive plate as seen here on my monarch.

Mine used to lay across the entrance to my workshop to stop visitors "Surprising" me while machining.

Regards Ian.

Yessir, and if you're not careful he'll take a bite out of you wherever he is.

Thanks for the replies, but I'm still confused how it differs from a chuck. I guess I can't figure out why I would use it versus a chuck.

Well...


I take a shaft and I bore two holes in it. Then I put a center in the head of the lathe and in the tailstock.

Now I suspend the part between these two parts and engage it with the dog. The dog locks onto the part and transfers the spin from the faceplate to the part.

On paper, I can disengage the dog, take my part out and flip it around (end for end) and still have it keep the same location in the machine.

You really can't do that with a jaw-chuck unless you are very very very good.

Another advantage is you can turn long irregular parts that won't fit in the jaws.


That said... Twice I've used this method in all the hours I've logged on the lathe. I'm more of a collet-chuck/turret-lathe/mass-production dood than a turner of epic parts.

In today's machine shops, lathe dogs have NO place. Nobody cares about T.I.R about a given centerline. It simply is not cost effective, (They Think). We live in a world of Throw-away goods, where the Land-fills ARE a money making proposition. I just wonder what our children are going to do after we have used up Everything.
If you want to make something precision, you use centers and a drive dog, when multiple set-ups are required or when repairing / reworking parts.

"I guess I can't figure out why I would use it versus a chuck. "

If you aren't a well equipped HSM, say you wanted to turn taper between centers. You don't have a taper attachment. The dog drives the work from a center aligned with tailstock offset to required taper. Can't do that with a chuck.
For fussier accuracy (TIR), dogs don't have jaw domination like three jaw chucks.

That might be true in a production shop but in a job shop there is still a place for between centers turning.

I used a lathe dog this weekend on a little project. I recently purchased an 8" H/V rotab, and wanted an easy way to mount a chuck to it for workholding. The rotab has a #3 Morse taper socket in the center, which I verified runs true within a couple of tenths. All my lathe chucks are 1.5-8 threaded to fit my 10" Logan, so I made an adapter which has a #3 male Morse taper and a 1.5-8 thread. In order to check the fit of the taper and set the seated depth accurately, I needed to remove the work from the lathe to test it in the rotab several times. I realized I'd never get the desired runout if I was re-chucking the part each time, so I turned it between centers for the taper portion. After I had the taper fitted, I chucked the part, indicated it in and drilled the 3/8-16 thread for the drawbolt. Then I took the part and mounted it in the #3 taper of my lathe spindle (which also is true within a couple of tenths), and single pointed the threaded end. I was able to thread it for a nice fit on the snuggest of my 3 chucks, which happens to be a Hardinge branded Buck 3 jaw Adjust-Tru. The adapter was made out of some 1.5" Stressproof, which machined like a dream, and came out pretty nice if i do say so. I also had to make a small insert to capture the head of the drawbolt on the underside. The backside of the chucks will seat against the face of the rotab when installed, so they are not cantilevered off of the adapter. I had already verified that the backs were true to the bores. I'll post some pictures in another thread later.

When you use a chuck it is expected the chuck and tail stock center will be aligned. Turning between centers does not require this so you can turn tapers that are actually tapered. Or just turn shafts with a very high repeatability index. Motor armatures, for example.

I think I'm starting to see the light. Thanks!

imparts torque from the faceplate to the piece being turned

the piece is usually being held by a live center on the tail stock side
and a dead center on the headstock side so the only thing making
the part spin is the friction between the deadcenter and the part.

if you crank on the tailstock ram that will get you by for some light cuts
or if if you are doing metal carpentry it might not matter if it stalls and you have to back off.

But if you are taking big cuts, or more importantly cutting a thread
where the piece has to stay perfectly synchronized for each rotation
the the little patch of friction on the deadcenter is not sufficient.

I think lathe dogs harken back to the days before chucks and collets
but are still useful in some cases.