Meddings Driltru Handwheel (Star Wheel) Stiff

I have a Meddings Driltru floor-standing pillar drill. I've noticed that the quill becomes more difficult to move when the weather is hot (such as today!). As I do a lot of work with small drills, this is causing a problem!

Having removed the handwheel, spring and shaft, as well as the quill assembly, I found that the problem is the plastic (nylon?) bush that the handwheel shaft runs through. I have removed the 2BA grub screw under the bush that, I assume, sets the tightness of the bush on the shaft.

In the parts diagram, the plastic bush is labelled 15 - Activating Gear Bush, and the shaft is labelled 13 - Activating Gear and Shaft Sub-Assembly.

My question is, what can I do to make the shaft run smoothly? The shaft is 5/8" in diameter. Should I run a 5/8" reamer down the bush?

Many thanks,

Mike

I had exactly the same thing. I opened up the bush slighty by boring it on the lathe as I had no reamer to hand, but a reamer should work too.

Bikepete: Did this work without creating too much play?

Would a complete replacement bush in metal (perhaps oilite running on the shaft) be a long term option to fix it forever? Maybe the original was cost-engineered.

" Under dry-as-molded (DAM) conditions, polyamide, or nylon, usually contains 0.1 ~ 0.3% water. At room temperature and 50% relative humidity (RH), type 6 polyamide could eventually absorb 2.75% water. Every 1% moisture increase in nylon may result in 0.2 to 0.3% increase in its dimension "

In other words, drying out the bush should cause it to shrink.

Neil

Quite right Neil. A bush that shrinks will tighten on it's shaft.

Peter.

Just for interest, I reamed the bush very lightly from both sides, polished the spindle (it was lightly scored) and reassembled. This improved the problem. Once the weather had cooled down, the problem went away altogether, which definitely points to the fact that the nylon bush is tightening onto the spindle when the weather is hot.

I hope that helps anyone with a similar problem.

Mike

.

Not quite, Peter

Nylon [for example] is hygroscopic, and will increase significantly in volume when it takes-up water

As the outside diameter is constrained it applies pressure to the inside diameter.

... Hence a Nylon bush that expands in this way will tighten on the shaft.

Expansion and contraction with temperature is a somewhat different issue, and less significant in the present context because every component is affected in a similar way; and by much smaller amounts than you get with hygroscopic expansion.

MichaelG.

Perhaps it's HDPE.

As I've been given to understand, HDPE is less affected by humidity.

ATB

Daniel

There are a multitude of materials that a bearing bush could be made from. Nylon would be an unusual choice for this application and HDPE even more so. If Meddings wanted to specifically use a polymer bush they would have been best served by consulting Igus (or just choosing one out of the Igus range).

I know of a company that converted a Porsche 911 to full racing specification and took great pride in showing the complete set of high precision nylon parts they had machined for all the suspension. The first track outing was a disaster as the suspension virtually locked up in the wet conditions!

Ian P

The comments about nylon remind me that some time ago I helped a sailing friend to overhaul a cutlass bearing. I don't remember the details of this but note a suggestion elsewhere that the bearing surface of the plastic material involved is fluted.

.

Perhaps it is ... I currently have no way of knowing.

MichaelG.

Apologies Mike, I must have got called away and didn't spot this reply at the time. Seems you're now sorted but FWIW mine doesn't have play that I've noticed, but it is a little bit 'sticky' again when I tried it just now - perhaps I didn't take enough off for this hot weather!