Denham Lathe crash :>(

Stupid me had an episode while setting up the lathe to do a 6tpi internal thread and crashed the toolpost into the chuck. This snapped the end off the mandril that takes a feed from the chuck spindle out of the head for running the treading spindle and the power feed.

Is there someone who could face, drill and ream the end to take a new piece, either to press fit, silver solder or some other way of reparing it for me?

The largest diameter of the shaft is 1", the diameter of the bearing surfaces that run in I think a Babbit bearing is 0.75" and the broken bit is 0.5" with a 3/16" keyway that I could do on my mill.

Overall length is approx. 5"

I live in Ross-on-Wye, Herefordshire but could mail the bits if anyone can help please?

Thanks

Roger

Try visiting the Hereford Society of Model Engineers at Broomy Hill in Hereford.

In my experience, they are a very friendly and helpful bunch.

They will have a running day on Sunday 13th August.

You may be able to contact them through their website.

Am surprised that you are not a member already!

Howard

Ouch

… Howard’s suggestion sounds good to me.

MichaelG.

That might have been nasty enough to damage the chuck as well.

I would find out first just how hard it is and if it is machinable, bore the main shaft and maybe thread it and then fit or screw a new small end into it. Loctite and possibly pin the joint. If your saddle cannot move without the broken part, it is possible to use the tailstock to push it and then back off the tool and pull it back manually. The compound, if aligned dead parallel to the spindle axis can also work for short lengths. The end which goes in the bearing bush should be made a tiny bit big and then rubbed down at low spindle speed with some wet and dry on a file to get a nice finish. Care should be taken not to let the file hit the moving chuck. The pinning can be done using the mill.

I thread internally with a left hand bar with external inserts in it in reverse. This cuts a right hand thread away from the chuck and is much safer. Make sure the chuck is nice and tight if it is a screw on one and use the slowest speed the lathe can do. Locking the chuck on the spindle is good, but Smart & Brown model A lathes are screw thread and their manual allows reverse if slow speeds are used and care is taken not to use big cuts.

Edited By old mart on 01/08/2023 15:13:35

Yep, love to do that , please see below

That's what was necessary in order to retrieve the bit of shaft with a 40 tooth gear attached to it, so no chance of machining any time soon besides my spindle bore is 3/4" and I would be machining with at least a 4" stickout with no support, not a good scenario for accurate work.

Roger

Nope, would need more than 30rpm and a small toolpost to damage it, didn't even mark the piece of metal it was holding.

Strangely the power feed has a clutch built into the spindle but the leadscrew doesn't, I suspect it might of had a shear pin in it's early life but was replaced with one that wasn't a shear pin!!!

Roger

I am a member even though I haven't visited for quite a while but their lathes don't have large bores except for possibly one that isn't that accurate any more, also they are committed to building a couple of 7 1/4" locos.

Roger

Size not a problem except for maybe a watchmaker's lathe. Hold thick end in chuck with operation end in fixed steady. Perhaps best to locktite in oversize piece and turn to size to ensure concentricity.

And you're going to run the fixed steady on a bearing surface?

If you want to send me a accurate diagram i can make you a new shaft complete and post it on?

Roger, since you have an ape as your avatar maybe you should be aware that a mandrill is a primate and what you have snapped is mandrel. Snapping the end off a mandrill makes a different type of mess

Martin C

Haha, I knew it didn't look right but my meds have been messing my head lately so I knew someone would put me right

Oh and that ape is a silverback gorilla..... watch out

Roger

Roger -

If I can offer use of a lathe to make a fresh one or whatever repair you think is appropriate let me know. I'm in Ruardean, less than 10 miles from you. I have sent you a PM with contact details.

Rgds Simon

When you get the lathe repaired, you should research the possibility of a frangigle link that is common tith most lathes. Plastic gears in an accessable part of the drive train or a little drive pin like Smart & brown lathes have. It would lower the likelyhood of a major crash.

You could repair the shaft using a mill if you had a rotary table.

Shock horror. using a bearing as a bearing will never do!

Maybe rebuilding with an aluminium key would help as someone advised me on a recent thread wrt an engine flywheel.

Sorry Martin had to do this

4/487 is the part broken

snipped out of the circa 1937 user manual

Apologies can be considered

Roger

I must investigate if this is a just a taper pin or meant to be a weak link

Doesn't give the pin at the end of 4/316 a number so maybe replace it with some cheddar cheese might work?

Roger

FWIW, When i did some unintentional metal mangling, During the strip down for repair, the roll pins came out.

As soon as the lathe could be used, totally manually, I made two (For the Leadscrew, and for the Feed shaft ) brass pins, 5 mm OD 2.5 mm ID to act as replacement shear pins if I was stupid enough to do it again!

On a friend's Raglan the feed s did not work, The shear pin had failed. Replacement cured that problem!

Definitely worth having something, easily acessible that breaks easily, and is easiloyb replaced, rather than an expensive and complicated repair job!

If the worst comes to the worst, remodel a knitting needle to act as a shear pin.

Howard

I totally agree, I'll take out the taper pin and fashion up a brass replacement and see how it holds up when threading, the motor is a circa 2kw one and I thread at 30rpm so it'll be interesting to see if it holds up against that kind of torque.

Roger