Drive dogs for large lumps?

[rsser]

It's said that a MT mounted drive dog for large bowl blanks risks twisting and galling in the spindle taper.

So there are threaded drive dogs to mount on the spindle that have been produced instead.

My question is this: is there anyone out there that has had a problem with a MT mount damaging their spindle inside?

 

[MichaelMouse]

Why take a chance? I know I don't pay as close attention to cleaning the tapers as I could, and that's asking for it. I use the pin chuck, but I imagine it's no great leap for a machine shop that can bore and thread one end to make spurs on the other. Either that or use one of the type that's designed to grab in a chuck.

 

[odie]

It's said that a MT mounted drive dog for large bowl blanks risks twisting and galling in the spindle taper.

So there are threaded drive dogs to mount on the spindle that have been produced instead.

My question is this: is there anyone out there that has had a problem with a MT mount damaging their spindle inside?

I'm with MM on this, Ern.......

I use mine for little chunks of wood, but I'd have serious reservation about using a MT drive dog for a weighty piece of wood......but, I'm sure there are those who have done it for years without a problem.

It will take only one incident to produce galling inside your MT......and, once that's happened, you're screwed. Morse Taper male fittings can be replaced easily, but your lathe spindle is not.

I would think that one moment of lax concentration = bad catch........and, damage is done!

Nice to see you again, Ern. I was reading a lathe forum from the UK the other day, and noticed posts from you on there. Can't remember which one at the moment, but I got there as a result of a computer search.......

ooc

 

[Mike Peace]

Ern, I had heard the same thing so I made a Bowl Driver after seeing a demo by Al Stirt who uses one to help orient the blank between centers to get the best grain pattern. He has plans on his website. Mine mounts in a large set of jaws and is about 7" across.

 

[rsser]

Thanks folks, and hi to you too odie.

My question wasn't really about whether I should use a screw-on dog because I do but about what the case is in favor.

Odie, yes, internal damage would be costly but how likely is it?

Thanks for the pic Mike.

 

[odie]

Thanks folks, and hi to you too odie.

My question wasn't really about whether I should use a screw-on dog because I do but about what the case is in favor.

Odie, yes, internal damage would be costly but how likely is it?

Thanks for the pic Mike.

I'd say the likelihood of galling inside the MT taper is directly related to the odds of doing anything that would cause a complete, or near complete stoppage of the object being turned. For someone like you, who has already attained some level of expertise and experience, the chances of that happening are much less likely than someone who is a novice.

I think many of the currently produced lathes are now manufactured with hardened spindles. One would think a hardened spindle would help to reduce the extent, or eliminate any spindle damage if a slip in the MT happens. My Australian Woodfast lathe is close to 20 years old, and the spindle is not hardened.......so, this would be a consideration for me, while it may be less so for someone else.......

Question: How many MT attachments have hardened male MT tapers? ......and, if any of them do, how will that effect the galling, if a "spin" occurs?

ooc

 

[rsser]

Good questions odie.

FWIW I've had the odd slippage of a MT mounted Jacobs chuck in the tailstock but only because it hadn't been seated properly.

 

[MichaelMouse]

Starting or resistor braking inertia can cause a slip as well, though probably less likely than a catch. Hardened this, malleable that begs the issue of neither but caught in there anyway. Bit of grit, anyone?

 

[Mark Warden]

I think what is missing here is the fact that bite between the spur and the wood would have to be stronger than the bond in the taper. I think it would tear the wood before it could spin the spur.

 

[Bill Boehme]

It's said that a MT mounted drive dog for large bowl blanks risks twisting and galling in the spindle taper. ...... My question is this: is there anyone out there that has had a problem with a MT mount damaging their spindle inside?

That is one reason why I am a bit leery of some of the oversized spur drives. If you stick to spur drives of one inch as the maximum, I think that it is a safe bet that it will auger a hole in the wood before it spins in the spindle socket and galls things all to heck. This is assuming, of course, that there is good metal to metal mating and that both the spur drive taper and the spindle socket are both clean and without rust, free of oil or grease, and have no existing burrs from previous mishaps.

It is possible for you to repair the spindle socket using a MT hand reamer and a cooling fluid such as Rapid Tap to flush out the chips. Following the hand reaming, it is necessary to hone the spindle bore using something like the products made by Spin-L-Mate. I do not know of a cost effective way of reconditioning the taper on a spur drive so the best thing to do is toss it in the trash. While this is an unpleasant thought, just remember that continuing to use a spur drive with a galled shank will continue to damage the spindle bore. And ... each time that the spur drive is reinserted, the damage is made worse and the easier it becomes for the connection to slip until the parts either become frozen together from spinning or becomes useless because the holding friction is too little to be of use. One other thing -- if you continue using the spindle with a damaged socket, it will spread its malady to ALL of your other MT drives.

I think what is missing here is the fact that bite between the spur and the wood would have to be stronger than the bond in the taper. I think it would tear the wood before it could spin the spur.

Maybe or maybe not -- see above. With the really large spur drives, it might be pushing things a bit too close to the edge of the envelope. This is especially true when either surface is not perfectly clean and free of any work hardened metal smearing.

 

[odie]

Some good points here by Bill Boehme.......

A regular sized spur drive could be seen as a precautionary measure over using the "Big Bite" drive, precisely because it's more likely to lose grip and spin in the wood. This would be far more preferable than to spin metal to metal in the MT........

ooc

 

[bonsaipeter]

Drive Spurs for Larger Pieces

Slippage between the internal headstock MT and the external MT of a drive spur driving a large piece of timber can be very real. I've had it happen but luckily there was no sustained damage as I caught the slippage immediately and shut down the lathe. An easy solution to this problem should cost less than $1.00, assuming you have a 3"-4" faceplate. Purchase from a local fastener supply house or good quality hardware store a couple of Grade 5, 1/4" -20 tpi x approx. 1" long screws and a couple of 1/4"-20 nuts. The reason for choosing Grade 5 over Grade 8 is that the Grade 5 is somewhat hardened so as not to bend but are not brittle. Grade 8 is very hard and may "snap" under a heavy load. Grind a chisel point onto the threaded ends of the two screws, insert them through opposite screw holes of the faceplate and secure them in place with the nuts (you may want to add a lock washer to help the screws from loosening). When you tighten the screws in place make sure the chiseled edges are in a straight line passing through the center of the faceplate.

To use simply screw the faceplate onto you lathe's spindle and you now have a very strong spur drive that in no way can damage you headstock's internal MT. Cheap, fast, and easy.

Thanks, Peter Toch

 

[Mark Warden]

Some good points here by Bill Boehme.......

A regular sized spur drive could be seen as a precautionary measure over using the "Big Bite" drive, precisely because it's more likely to lose grip and spin in the wood. This would be far more preferable than to spin metal to metal in the MT........

ooc

Hi Odie the Big Bite attaches to your chuck it's not a MT spur. And for $20.00 it is a great spur and will save your MT

 

[Bill Boehme]

I see that the Texas Spur Drive also threads onto the spindle. Apparently somebody has already figured out that a MT connection can only handle so much torque before it slips. Nevertheless, I would like to keep the amount of torque load to a reasonable level when a large piece of wood is mounted between centers for the simple reason that I do not relish the thought of a 50 pound spinning chunk of wood in free flight in my shop.

 

[odie]

Hi Odie the Big Bite attaches to your chuck it's not a MT spur. And for $20.00 it is a great spur and will save your MT

Hello back, Mark.......

My bad.......I shouldn't have called it the "big bite".......I was actually referring to a bigger bite spur drive that is MT driven. When thinking of it that way, Bill's comments about "pushing the envelope" when compared to a smaller spur drive, are better visualized.

Thanks for the correction.....

ooc

 

[Bill Boehme]

.... Question: How many MT attachments have hardened male MT tapers? ......and, if any of them do, how will that effect the galling, if a "spin" occurs?

Unfortunately, hardening is not the right solution, nor is there a perfect solution. Ideally, the tapered tool shank and the spindle socket both deform a microscopic amount due to the seating force and this large contact area metal-to-metal bond produces the very precise alignment and high torsion friction holding force.

 

[odie]

Unfortunately, hardening is not the right solution, nor is there a perfect solution. Ideally, the tapered tool shank and the spindle socket both deform a microscopic amount due to the seating force and this large contact area metal-to-metal bond produces the very precise alignment and high torsion friction holding force.

So, if that's true......then the bond between the MT surfaces would actually be better between unhardened surfaces?

Many currently produced lathes have hardened spindles, so at least one surface may be hardened. I'm not 100 percernt sure why a hardened spindle would be better, except maybe keeping those threads from damage over the long term......pretty sure it has nothing to do with the MT though.

ooc

 

[Bill Boehme]

So, if that's true......then the bond between the MT surfaces would actually be better between unhardened surfaces?

Many currently produced lathes have hardened spindles, so at least one surface may be hardened. I'm not 100 percernt sure why a hardened spindle would be better, except maybe keeping those threads from damage over the long term......pretty sure it has nothing to do with the MT though.

ooc

There are different degrees of hardening so it probably isn't as hard and brittle as a file nor as soft as a piece of steel that has been annealed but not yet hardened. It sounds more like marketing nonsense than anything else unless they state specifically the Rockwell hardness. You hopefully would never encounter a lathe spindle that has not been hardened to some extent.