Left hand thread on a Chester drill spindle

My Chester D20 pillar drill has a slightly odd construction, and I'm not sure if I'm missing something or the drill is!

Viewed from above, the drill spindle obviously rotates clockwise. The pulley wheel sits on a tapered cone that in turn sits on a keyed shaft that drives the whole spindle. The pulley wheel is held down by a thin nut (like an old-fashioned jam nut) that screws onto a thread, pushing the pulley down onto the taper. So far, so reasonable.

However, the thread is a left-hand thread (it screws on counter-clockwise from above), so the natural action of the pulley is to push up off the taper and then to unscrew this nut. The first time it happened, I didn't really think it through, I just assumed it hadn't been tightened properly to begin with. It happened again today and I don't think there's been five minutes of operation time since I last retightened it.

Am I doing something wrong here? Is there something missing? I don't understand why you'd specify a left-hand thread without good reason, so what's the good reason?! Should I just find another nut and lock it down?

A photo would be a great help.

Hi Callum have you looked at the handbook on the Chester web site as it gives an exploded diagram and list of parts which may help to identify the item you are talking about. Also have you called Chester service dep. ?

David

Hello Calum,

You don't say if your drill is an old acquaintance which has started developing this habit or a new to you piece of equipment which came with bad manners..

You could try some Loctite on the mating faces of the cone to try and break the habit and back that up with a serrated washer under the nut. The construction of a coned pulley hub coupling onto a keyed shaft is unusual and I wonder if the pulley runs true on it; a wobble there would put cyclic loading onto the nut.

Regards

Brian

I'm not surprised, the spindles of drill mills and Tom Senior light verticals also have lefthand threads for their bearing adjusters.

That would seem to be very similar to the setup on a chopsaw, which has a left hand screw, or nut, clamping the blade in place. Nothing unusual there, I think. As Brian Woods said, a washer should sort it out.

The drill mill has a four speed pulley with a taper fitting and also a lefthand thread nut, but the nut is a substantial cast iron one of about 58mm spanner fitting with almost 25mm of thread length. The taper should never break in use to enable the pulley to unscrew the nut. When I dismantled the mill, I removed the pulley with great difficulty by putting a puller on it and then applying heat from a hot air gun.

You should address the poor fitting taper on the drill. The easy option would be a little Loctite on the taper, but keep it off the nut threads.

Thanks all - I think Brian and old mart are bang on the money, but to answer your questions: here's the parts from the manual

Your can see the relevant parts are 75, 76, and 77. I've had the drill since March, brand new from Chester. It's seen very little use in that time.

To begin with, I had a real problem getting the belts tight enough to not slip under the lightest of loads - I was drilling 3-4mm holes in a bit of cherry and the drill was binding. The belt tensioning arm was quite short, so slipping a lever over it and tightening it up seemed to sort it. Then a few weeks ago it slipped again, which was the first time I discovered the nut had unwound itself. I agree it would make sense if there is any play in the cone then that will be causing bad things; I'll have a look tomorrow and see exactly how it all fits together.

Well, having removed the nut, the pulley was absolutely not coming off the cone, so clearly the solution is just to have the nut tight enough! I put it back on and tightened it down with a bit of welly and we'll see how it runs!