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Mini-Lathe Repair

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SillyOldDuffer07/12/2018 10:26:40
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Some interesting comments on this, including one suggesting the lathe should be sent back. I'm far from convinced that all the criticisms are valid, particularly when they address 'faults' that Ron hasn't actually encountered, or compare one way of doing things with another without taking into account the design philosophy.

Plastic gears snapping during a crash is a well-known mini-lathe problem. The same gears do not appear to wear out in normal use, and the absence or presence of oil does not appear to cause any problems either. (Unless you know different!) There is reason to believe that the gear - it's always the same one - is deliberately intended to break : it's a mechanical fuse. The design fault is that the gear is inaccessible and takes a few awkward hours to replace.

Are plastic gears only used because they are cheap? I suspect not: a key advantage on a table-top lathe is that they are quiet making it possible to use the machine inside the house. Is oil necessary inside the headstock? Only if your lathe uses old-fashioned plain bearings, in which case it's essential.

Actually what's going on with these lathes is "Value Engineering". This is really what matters in manufacturing as a way of earning a living, not making 'nice' kit. Any fool can look at a design and increase its cost by insisting on the best possible solution to every feature whether or not it's needed. The problem with doing so is that lifting 'quality' increases the cost to the customer. Less obviously, and much more dangerously, inappropriate quality savages profits. Myford fans have to explain why the old company had to cease trading despite their lathes being so good: the answer is in the accounts. Ditto most of the other makes listed in lathes.co.uk Good machines, rotten economics.

It's much more difficult to engineer products that work satisfactorily at a competitive price whilst making a profit. This is more-or-less what the Chinese have achieved with hobby machine tools. The challenge for Western engineers is to produce something better than, say, a Super 7 for under £800 at the factory. No point is whinging about oriental successes - get your act together and out-do him.

Dave

 

 

 

Edited By SillyOldDuffer on 07/12/2018 10:28:36

Ron Laden07/12/2018 10:49:03
669 forum posts
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Removed the gears from the head and you can see in the picture the small gear lost 4 teeth plus it broke at the keyway as Jeff said, the larger gear lost one tooth.

Jason,s idea of replacing the timing belt with a multi vee belt sounds a good mod to me and certainly worth a try. I will see if I can search out a belt and pulleys that would suit. If anyone knows of a source of small multi vee belts and pulleys it would be appreciated.

Ron

dsc06281.jpg

Neil Wyatt07/12/2018 10:56:01
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Posted by Andrew Johnston on 06/12/2018 20:30:24:
Posted by SillyOldDuffer on 06/12/2018 20:24:23:

The case for the prosecution is these lathes are built down to a price, with a simplified design..........

And it costs money to add grease. smile

Andrew

"Acetal is dimensionally stable, is resistant to most chemicals, and does not absorb moisture. However, it does require continuous lubrication when subject to high loads. ... The advantage of nylon for use in gearing is that it is typically self-lubricating"

www.machinedesign.com/materials/plastic-gears-are-future

Ron Laden07/12/2018 11:11:58
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Hi Neil

Just so I am on the right footing do I take it that the gears are nylon and if so is lubricant detrimental to them..?

Ron

Neil Wyatt07/12/2018 11:20:05
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The metal vs nylon debate is interesting.

I think my gear lasted around 12 years or more, and was subjected to awful abuse. I am pretty sure that when they go it is because they are subjected to loads well beyond what the original designers anticipated for the lathe, such as crashing a tool into the chuck jaws or taking deep interrupted cuts trying to 'round off' irregular work.

They seem to fail either quickly in the hands of a beginner or after a decade or two. What probably saved my gears from my beginner's errors was having the toothed belt a bit loose so it would slip with a terrifying howl rather than break a gear.

Looking at the the cost of replacement gears, the difference for a pair is about £14, so I imagine it would add no more than this to the cost of a machine.

I also am sure the factories would happily fit them as standard if requested by importers.

It's worth thinking about how frequent broken bull wheel gears are on secondhand Myfords - probably mostly caused when changing chucks - to see damage in this area can be.

Neil

ega07/12/2018 11:23:03
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I think someone mentioned Nylatron. I have successfully used gears made from this material in a larger lathe.

JasonB07/12/2018 11:25:52
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Gears are Acetal according to ARC site. Who also suggest a light application of grease.

Andrew Johnston07/12/2018 12:42:01
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Looking at the picture posted by Ron the problem, as has been pointed out, is one of fundamentally poor design. The gear that has broken has a very thin boss which clearly wasn't strong enough. It's not easy to tell but the gears also appear to be 14.5 degrees PA, which is also bad, as the teeth will be thinner, and weaker, at the root.

I'd disagree with SoD on a number of points. There's nothing wrong with cost reduction exercises but they have their limits, and I doubt the factories churning out mini lathes pay much attention. If you're going to provide a weak point you don't do it where you have to dismantle half the machine to get to it. You put in a simple, accessible, shear pin.

The late, great JohnS took the view that one of the big problems with the old Myford company was old fashioned, and expensive, manufacturing techniques, reliant on old school skilled workmen. The odd thing is those techniques are often venerated on this forum, but judging by the response on the has anyone bought a new Myford thread far fewer people are, or were, prepared to pay for them. As for many of the manual industrial lathes of the past, industry has changed and very few are now required. So a shakeout was inevitable.

Andrew

JasonB07/12/2018 13:26:22
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I think it is a modified form due to being a 12T gear, if you look at the photo on ARC's site the larger one looks more like 20pa as do the pair it runs with rather than being 14.5pa.

 

They have not helped themselves by putting the keyway between two teeth where the gear is at it's thinnest

Edited By JasonB on 07/12/2018 13:27:50

JasonB07/12/2018 13:35:50
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Ron, the other option is to bypass the layshaft entirely and pop a big 3-phase motor on it that drive sthe spindle directly like this bodger haswink 2

Chuck Pickering07/12/2018 13:36:51
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Has anyone needing these gears tried 3d printed ones? I

I used Fusion 360 and it's spur gear add in to design and print every gear in my 7x lathe.

My 7x is almost 20 years old, and I haven't broken any of the headstock gears, so haven't tried 3d printed ones there. But, I have printed various change gears, including a 90/20 combo gear for extra fine feed and a custom banjo to accommodate it, and a replacement motor pulley that work fine.

Ron Laden07/12/2018 14:16:40
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Posted by JasonB on 07/12/2018 13:35:50:

Ron, the other option is to bypass the layshaft entirely and pop a big 3-phase motor on it that drive sthe spindle directly like this bodger haswink 2

Wow, that is some motor, its a bit too scary for me though, plus 3 phase and associated electrickery that scares me even more.

Ron Laden07/12/2018 15:42:31
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The plot thickens !

I was about to order a replacement set of the plastic gears, I was on the Amadeal site (they supplied my lathe) when I noticed that their metal gear set looked different. Looking at the spec they had quite a lot more teeth than the plastic ones, 50% more in fact, so I gave them a call. They said that their metal gear sets have had the number of teeth increased to reduce the noise associated with the old set which were just a copy of the plastic ones. They claim that the new metal set is still not as quiet as the plastic ones but not far behind and a vast improvement over the old metal set.

The plastic layshaft gears are 12T/20T and the spindle gears 21T/29T which in the Amadeal new metal set have become 18T/30T and 31T/43T. So thats put the cat amongst the pigeons as I am thinking of giving them a try, though I wouldnt do it without the mod to the belt drive.

Neil Wyatt07/12/2018 16:29:35
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Posted by Ron Laden on 07/12/2018 14:16:40:
Posted by JasonB on 07/12/2018 13:35:50:

Ron, the other option is to bypass the layshaft entirely and pop a big 3-phase motor on it that drive sthe spindle directly like this bodger haswink 2

Wow, that is some motor, its a bit too scary for me though, plus 3 phase and associated electrickery that scares me even more.

And look at the state of that welding!

Ron Laden07/12/2018 20:52:26
669 forum posts
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Would anyone know of a source/supplier for the multi vee belts and pulleys, it is only a small belt 102mm diameter/320mm long. I was thinking a J2 section which from what I,ve read is 2.34mm rib centres and 4 ribs would give me a belt approx 10mm wide. I found a couple of belts at near enough the right length but they are only 2 ribs which is too narrow. From what I,ve seen so far pulleys could also be a problem but I guess I could make my own.

Just wondered if anyone knows of a supplier that offers a good range of belts and pulleys in the smaller sizes.

JasonB07/12/2018 21:00:32
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13" may be the closest, slightly larger dia pulley would take up the slack if you can't move the motor enough

Ron Laden07/12/2018 21:13:57
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Posted by JasonB on 07/12/2018 21:00:32:

13" may be the closest, slightly larger dia pulley would take up the slack if you can't move the motor enough

Brilliant Jason, thanks a lot, there is plenty of motor adjustment so I think that belt will be ideal.

Ron

Ron Laden08/12/2018 07:58:20
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I have ordered the new type metal gear set and a poly vee drive belt to replace the timing belt. I will need to make a motor drive pully and the layshaft pully. The motor pully is fitted with a grub screw and the layshaft pully has a keyway and secured with a circlip, I plan on copying that on the two new ones which will save any mods to the layshaft.

Having no experience of multi vee belts or cutting keyways, this is what I am thinking but please shout if I have it wrong. I am guessing that the depth of the rib grooves in the pully will need to be cut just shy of the depth of the belt rib, too deep and the belt at the base of the ribs will contact and the ribs wont..?

The keyway on the existing pully is 3.9mm wide and 1.9mm deep, I was thinking of drilling a 3.9mm hole before I put the hole in for the shaft and positioned to give a depth of 1.9mm from the edge of the shaft hole if that makes sense..? After the shaft hole is in I would then use a square needle file and square out the keyway, it will be around 13mm long and the material is 6082.

Ron

Neil Wyatt08/12/2018 10:41:50
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> The keyway on the existing pully is 3.9mm wide and 1.9mm deep,

Not 5/32" wide and 5/64" deep by any chance?

Imperial bits sneak into the oddest places

Neil

Ron Laden08/12/2018 10:49:35
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Posted by Neil Wyatt on 08/12/2018 10:41:50:

> The keyway on the existing pully is 3.9mm wide and 1.9mm deep,

Not 5/32" wide and 5/64" deep by any chance?

Imperial bits sneak into the oddest places

Neil

Probably is Neil, I just measured it with the vernier in metric.

 

Edited By Ron Laden on 08/12/2018 10:49:59

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