RMweb Gold ikcdab Posted October 23, 2017 RMweb Gold Share Posted October 23, 2017 The title says it all really. The motor I am using is an MFA unit with a 3mm drive shaft. The worm drive is for a 3.1mm shaft, so is a loose (but not sloppy) fit. There is no provision on the worm for a grub screw so I presume it is meant to be an interference fit. There isn't enough 'meat' to drill and tap for a grub, so how to fix the worm to the drive shaft? I assume glue....araldite? Any other suggestions? Link to comment Share on other sites More sharing options...
Graham456 Posted October 23, 2017 Share Posted October 23, 2017 Loctite 662 and 638 are designed for this sort of use and can be unstuck with a quick bit of heat if you burn the motor out 1 Link to comment Share on other sites More sharing options...
bertiedog Posted October 23, 2017 Share Posted October 23, 2017 Although agreeing that Loctite is the answer, 3.9 thou is quite sloppy, and it may give troubles. The best solution is to ream out the worm to a larger size, and loctite a good fitting sleeve on the shaft, and paper it down to a good fit in the worm, and again loctite it in place. By paper I mean abrasive paper of about 200 grit glued to a lolly stick. It should not need a lathe to do any of this, but you will have to find a good fit tube for the shaft. you could re drill the worm hole, but a reamer would be best or get somebody to do it in the lathe more accurately. Either way the final fit is best done by papering with the motor running, with a felt washer on the shaft to prevent dust getting into the motor bearing. The exact size of the reamer does not matter, just larger than the sleeve, probably 3.75mm Stephen Link to comment Share on other sites More sharing options...
bertiedog Posted October 23, 2017 Share Posted October 23, 2017 Looking at the figure of 3.8 (.1mm), it may be possible to paper wrap the shaft to take up the play, fag paper is about 1.1 thou, but my guess would be air mail weight, a piece cut and glued with super glue to the shaft with no overlap. Then a dab of super glue over the whole paper, let it set, then abrasive paper to just true it up, and the worm will press on with loctite. If done with care it should be super accurate. Stephen 1 Link to comment Share on other sites More sharing options...
DCB Posted October 24, 2017 Share Posted October 24, 2017 If I was to fit a 3.1 mm bore worm to a 3.00 mm shaft and it would definitely wobble and then fall off. Unless you are an engineer or watchmaker I would suggest you get the right worm for the motor or the right motor for the worm. Boring the worm oversize is tricky and you could damage the worm in the process, it would be easier to fit the 3.1 bore worm to a 2mm shaft than a 3.00. Link to comment Share on other sites More sharing options...
katwigan Posted October 24, 2017 Share Posted October 24, 2017 Loctite 638 is by their spec's good for up to 0.25mm or 0.010" clearance. A clearance of 0.1mm ( 0.0039") would be well within the capabilities of one drop. Loctite in it's various compounds saved significant downtime in the 35 years I worked as a maintenance fitter and subsequently as Maintenance Manager in Mitsubishi's Iron Foundry here in Adelaide. Just don't get it in things that are supposed to rotate ! Kevan Link to comment Share on other sites More sharing options...
RMweb Gold ikcdab Posted October 24, 2017 Author RMweb Gold Share Posted October 24, 2017 Thanks for all the replies. I'm not sure that my engineering skills are up to reaming or drilling the worm to a larger size at the accuracy required. This worm is for a 16mm scale garden rail locomotive and both the worm and pinion (?) are quite coarse, not fine like you'd find in a 00 engine. So I think that the 0.1mm is less critical than in a smaller scale. So I'm tempted to use the araldite / loctite solution. If the glue is evenly spread around the drive shaft, then the worm should be as near enough central as necessary? Link to comment Share on other sites More sharing options...
Guy Rixon Posted October 24, 2017 Share Posted October 24, 2017 Loctite 638 is by their spec's good for up to 0.25mm or 0.010" clearance. A clearance of 0.1mm ( 0.0039") would be well within the capabilities of one drop. Loctite in it's various compounds saved significant downtime in the 35 years I worked as a maintenance fitter and subsequently as Maintenance Manager in Mitsubishi's Iron Foundry here in Adelaide. Just don't get it in things that are supposed to rotate ! Kevan Sounds good, but what degree of concentricity would you expect with 0.1mm clearance? Link to comment Share on other sites More sharing options...
Mike Storey Posted October 24, 2017 Share Posted October 24, 2017 Thanks for all the replies. I'm not sure that my engineering skills are up to reaming or drilling the worm to a larger size at the accuracy required. This worm is for a 16mm scale garden rail locomotive and both the worm and pinion (?) are quite coarse, not fine like you'd find in a 00 engine. So I think that the 0.1mm is less critical than in a smaller scale. So I'm tempted to use the araldite / loctite solution. If the glue is evenly spread around the drive shaft, then the worm should be as near enough central as necessary? Is this for an Accucraft mod? If so, you may find, ironically, some additional advice on the 45mm group website, if you have already tried the 32mm (16mm NG Association website)? I have seen a number of enquiries on the latter, where similar issues have been described. Link to comment Share on other sites More sharing options...
katwigan Posted October 24, 2017 Share Posted October 24, 2017 Sounds good, but what degree of concentricity would you expect with 0.1mm clearance? Loctite as most liquids do tends to distribute itself evenly, particularly around such a small dia. I should think by sitting the motor vertically that the concentricity would be probably be within original design spec's of the gear. Even if the worm is hard up against one side of the shaft it is actually only 0.05mm or 0.002" off centre ( about half the thickness of a normal human hair ) Kev Link to comment Share on other sites More sharing options...
RexAshton Posted October 25, 2017 Share Posted October 25, 2017 If I was to fit a 3.1 mm bore worm to a 3.00 mm shaft and it would definitely wobble and then fall off. Unless you are an engineer or watchmaker I would suggest you get the right worm for the motor or the right motor for the worm. Boring the worm oversize is tricky and you could damage the worm in the process, it would be easier to fit the 3.1 bore worm to a 2mm shaft than a 3.00. I agree that it's not the ideal situation but the OP wants to know how to do it, not why he shouldn't. I'd go with the paper sleeve suggestion above but it will only work satisfactorily if the pitch of the gear is relatively coarse. Link to comment Share on other sites More sharing options...
RMweb Premium Izzy Posted October 25, 2017 RMweb Premium Share Posted October 25, 2017 I would go along with those who suggest using loctite will work okay. I still have and use a bottle of loctite 601 - I think this was the original formulation - and if you put some into the bore of the worm, you don't need much, and then slide the worm onto the shaft and twist it about, the adhesive will tend to smear itself over all the contact surfaces and even out what gap exists. As it is an anaerobic adhesive - goes off with an absence of air - then as any gap gets bigger the curing time will of course get longer. When set the joint will be rock solid, but usefully an application of heat of some kind to the joined parts - I usually use a soldering iron - will break the bond/soften the adhesive and allow the parts to be separated. You must not get loctite near any motor shaft bearing surface............. Izzy Link to comment Share on other sites More sharing options...
PenrithBeacon Posted October 25, 2017 Share Posted October 25, 2017 I have experience of this problem. I just gave up and bought a High level gearbox. Problems like these are just not worth the hassle. Regards Link to comment Share on other sites More sharing options...
RMweb Premium Torper Posted October 25, 2017 RMweb Premium Share Posted October 25, 2017 I have experience of this problem. I just gave up and bought a High level gearbox. Problems like these are just not worth the hassle. I entirely agree. But if the OP is determined to use the MFA motor with its 3mm shaft, High Level isn't really an option. DT Link to comment Share on other sites More sharing options...
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