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Meddings Driltru Handwheel (Star Wheel) Stiff

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Mike Donnerstag25/07/2019 17:34:28
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90 forum posts
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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

Bikepete25/07/2019 17:41:09
221 forum posts
16 photos

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.

Mike Donnerstag25/07/2019 17:54:25
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90 forum posts
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Bikepete: Did this work without creating too much play?

DC31k25/07/2019 18:39:59
52 forum posts

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.

Frances IoM25/07/2019 18:55:14
637 forum posts
24 photos
bit surprised if nylon as this is hygroscopic and could change dimensionally with humidity
Neil Wyatt25/07/2019 19:56:27
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" 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

Peter Tucker25/07/2019 21:06:01
179 forum posts

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

Peter.

Mike Donnerstag23/08/2019 14:00:36
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90 forum posts
9 photos

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

Michael Gilligan23/08/2019 14:27:49
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13820 forum posts
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Posted by Peter Tucker on 25/07/2019 21:06:01:

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

Peter.

.

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.

Daniel23/08/2019 14:57:27
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234 forum posts
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Perhaps it's HDPE.

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

ATB

Daniel

Ian P23/08/2019 15:12:22
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2145 forum posts
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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

ega23/08/2019 15:52:15
1228 forum posts
103 photos

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.

Michael Gilligan23/08/2019 19:19:11
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13820 forum posts
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Posted by Daniel on 23/08/2019 14:57:27:

Perhaps it's HDPE.

.

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

MichaelG.

Bikepete24/08/2019 10:17:58
221 forum posts
16 photos
Posted by Mike Donnerstag on 25/07/2019 17:54:25:

Bikepete: Did this work without creating too much play?

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!

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