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Old 6th Apr 2019, 10:26 pm   #1
The Philpott
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Default Brass shaft loose in brass bushing.

In this case it's an Avometer, where the brass shafts of both the rotary selector knobs have become loose in their brass bushes- to avoid contamination (and possible damage) i am contemplating building up the shafts by some means rather than trying to work on the bushes (which are embedded into the meter, and probably made of a hard brass that is notoriously difficult to drill or ream.
Electrical continuity is required twixt the shaft and bush.
How would you approach this...? I cannot simply replace the knob/shaft assy. as it appears the majority of the wear is in the bush.
Thanks.
Dave
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Old 7th Apr 2019, 12:25 am   #2
Bazz4CQJ
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Default Re: Brass shaft loose in brass bushing.

I don't have a clear picture of the components involved, or the degree of build up you need. If it's only to compensate for wear, could nickel plating do it? The materials are not expensive from the usual online places and lots of advice on how to do it from Google.

B
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Old 7th Apr 2019, 9:57 am   #3
The Philpott
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Default Re: Brass shaft loose in brass bushing.

Immersion is not an option, as the shaft is bonded to the knob.. I would also need to protect the cross-drilled hole as this is very tight already. The bush is bonded into the meter so any attention to that is risky...however i recall that it's possible to reduce a bore diameter slightly to take up clearance (is this achieved by honing or is there another term for it?)

I was even contemplating silver dip but suspect it would take yonks to build up enough metal.

The wear is mainly in the bush rather than in the shaft- fitting an alternative shaft (in good condition) does not reduce the clearance.

Dave
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Old 7th Apr 2019, 10:04 am   #4
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Default Re: Brass shaft loose in brass bushing.

Maybe members with clockmaking skills could offer suggestions. Re-bushing of pivots is quite a common activity.

Ron
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Old 7th Apr 2019, 10:13 am   #5
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Default Re: Brass shaft loose in brass bushing.

A difficult one because both brass parts are bonded into plastic.

One solution would be to machine away the original bush and fit a new one. Alternatively bore out the bush, press in a sleeve and then ream the sleeve's internal diameter to suit the shaft. I have no idea though whether the case could take the machining forces.

I've seen worn shafts knurled to increase their diameter then turned down to suit a bush,
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Old 7th Apr 2019, 3:38 pm   #6
The Philpott
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Default Re: Brass shaft loose in brass bushing.

Knurled- that was the other term lurking in the back of my mind. I did a trial assembly and ultimately the lateral wobble proved non-critical to successful operation so it was a case of-if in doubt leave it alone. (It seems end-float was affecting continuity and that has been resolved.) Thanks for your comments.

Dave
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Old 7th Apr 2019, 3:48 pm   #7
mark_in_manc
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Default Re: Brass shaft loose in brass bushing.

If it were mine, I'd give the shaft a gentle squeeze in the vice between two bits of scrap steel rod about the same OD as the bush ID, at the right point along the shaft length. The aim would be to send it oval, and restore a non-rattly fit. It's not the most elegant solution but it will work and after all, this is not a high-speed bearing If trial fits are quick and easy I'd try to creep up on it, rather than overdo it.
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Old 7th Apr 2019, 10:58 pm   #8
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Default Re: Brass shaft loose in brass bushing.

I would avoid attempts at any machining on the shaft or bush due to their relation with the plastic. If you distort the shaft you could crack the knob. Also avoid any heat.

I think you may be able to find some very thin shim brass or other material the would wrap around the shaft and take up the slop. Iv'e seen some that is thinner than alfoil. Try to get an estimate of the space required to fill and start hunting for shim material. Check out PSME (precision scale model engineering in the USA).
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Old 7th Apr 2019, 11:06 pm   #9
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Default Re: Brass shaft loose in brass bushing.

In the absence of any other promising options, are the concerns about the plating route real show-stoppers? The cross-drilled hole could be filled with something (perhaps a wax?) to answer that issue. Perhaps some material could be found to create a protective layer on the knob which could be peeled off later on. Alternatively, plating can be done using a swabbing technique and I've done that once and it works surprisingly well.

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Old 9th Apr 2019, 10:46 am   #10
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Default Re: Brass shaft loose in brass bushing.

Excuse my ignorance here, but is the 'fit in the hole' the problem?
My Model 8 has seen very little use and has nearly 1mm side play in both selectors. I never noticed until I just tried it.

Yours clearly has had much more use, but surely you would need to rotate those knobs an awful lot of times to wear the holes?
Could the real problem be the bush face worn allowing a little 'end float' rather than fit?
If so, I wonder if shims between the knob and captive bush might help?

Alan
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Old 9th Apr 2019, 10:48 am   #11
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Default Re: Brass shaft loose in brass bushing.

I think the problem has been resolved as reported in post #6.
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Old 9th Apr 2019, 8:22 pm   #12
The Philpott
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Default Re: Brass shaft loose in brass bushing.

Indeed, since the end-float was cured (by fitting an alternative knob/shaft and associated components) the 1000ohm range now zeroes perfectly and waggling the selector does not cause the needle to shift at all. The side play was not, it seems, a contributory factor.
The variance in avometers of this era (1940's) was apparent as always- both the AC and DC rotary selectors which i rejected were tried in the parts donor meter (a Model 48A) and their diameter was too large to slide into the bush.

It was logical that end-float was the cause of poor contact in the leaf switch, had I thought it through properly in the first place. Interesting discussion though. Thanks all.

Dave
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