plastic intermediate gears or metal for mini-lathe?

Gallery, Projects and General > How do I??

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John Stevenson:
I think someone is trying to pull ones plonker.

Cyclic error on a mini lathe ?

£350 for a mini lathe and £5000 for something that can accurately measure the cyclic error, only to find out that the mass produced stamped out nut is nowhere within any known specs.

Woodsman22:
Hello guys;

My thanks to "Neo Tech", Mike, Stuart and Andrew for your replies to my questions. I am having to rush this reply as I am typing it before having to return to work. So my apologies for any incompleteness in this post.
First, I am not entirely new to the hobby, but refer to myself as a "newbie" as I have NO professional experience with machine tools. I do own a very old Sheldon "metalworker" lathe as well as this little Chinese machine. The Sheldon was bought 2nd hand and has several bull gear teeth that were damaged by the previous owner along with two damaged teeth in the gear at the back of the 3-step pulley that connects to the bull gear on the spindle. In other words- I only dare run it in direct drive, never mind the back gears. The Sheldon was made either in 1939 or 1940 and has leather belt drive. It is old, worn and tired, but I like "old iron" so I bought it.
I suppose I bought the Chinese 7 X 12 because I like a challenge and it does not weigh 400 pounds like the Sheldon does.
Mike stated; "Why do you need super accurate threading?". Well Mike, not "super' accurate, but accurate at any rate. I have bought this mini to help with some (small scale) gunsmithing tasks that I would like to do, and threading the end of a .22 LR barrel to take a muzzle weight or threading the barrel shank to fit a receiver requires some precision, and taps and dies will not do for such work.
Vtsteam stated:
"My guess is that Woodsman is already decided on metal gears for threading, has purchased and installed them, has already read the pros and cons elsewhere, is asking about longevity of a plastic spindle drive gear and actually wants practical advice on installing different spindle bearings after purchasing them and removing the headstock for that purpose."
- well not exactly Vtsteam; you are correct that I have the machine disassembled but I am not committed to making the spindle gears ALL metal (caps for emphasis, not for "shouting"), I was wondering if keeping the intermediate gears plastic was a good idea and Stuart has already answered that one for me (thank you Stuart!). As to any difficulty assembling or disassembling - no worries there, taking it apart was no problem as I have a hydraulic press for that (Harbor Freight- it was definitely a worthwhile purchase). I am wondering though, if the taper roller bearings that I bought from LMS were a good choice since they were only $2.00 more than they charge for new replacement plain bearings? They are labelled "Koyo" brand.
The thread that got me wondering about threading accuracy on the mini is located here:http://mb.nawcc.org/archive/index.php/t-68702.html
The comments made about the mini by Jerry Kiefer were not complimentary, especially where threading capabiity is concerned. I quote him here: "Someone brought up threading on this lathe. Threading on these machines can be a real disaster especially finer threads. They come with cast plastic drive gears that seldom have more than a couple of teeth the same size. In addition they are generally warped to some degree. If thread pitch is 28 tpi or larger, the thread form may be except able in many cases. However finer threads can be a major problem. The first pass is not to bad, but the second and third pass will overlap to various degrees in many cases because of the inconsistent drive gears. In addition the thread pitches are very limited and seldom used in Horology."
Yes, I know that he is talking here specifically about Horological pursuits and stating that the mini is unsuitable for those tasks, but I took note of his comments about the plastic gears with special interest. Could the lathe really be that bad? I guess I will find out if the metal change gears are any better and whether they will make a difference in accurate threading. Somewhere down the line I might like to dabble in horological projects, and maybe if that turns out to be the case and I have the funds I will look into buying a Cowell's for that purpose. Thanks to all who took the time to answer my questions, I really do appreciate the input.

vtsteam:
Seems to me if the gears were inconsistent and produced overlapping threads in fine pitch, that would be taking place almost exclusively in the final gears driving the carriage, if at all. And in fact the lead screw might be more suspect. Or shifting of bearings somewhere. Some kind of periodic backlash in the system. Or maybe slip on the spindle gear itself.

To have overlapping threads would be difficult to achieve in the gears early in the train, particularly with finer threads. Because gears there rotate many times for one rotation of the lead. Inconsistencies in a single gears teeth would be minimized -- similar to the way a dividing head reduces slight inconsistencies in the dividing plate. I'd look towards the end of the reduction system -- towards the carriage. Does that make sense?

Woodsman22:
" I'd look towards the end of the reduction system -- towards the carriage. Does that make sense?"

- Yes it does make sense, Vtsteam and I thank you for helping me to focus in on the possible sources of error. Much appreciated! :thumbup:

John Stevenson:
Error ?

What error ?
Just because someone buy a machine and can't use it, then slags it off on a web site doesn't mean to say they know what they are doing.

For plastic gears to be that far out they give an error they have to be visibly bent / oval / knackered [ delete as required ]

If the second and third passes don't follow then someone hasn't worked out how to use a threading dial or is trying to do metric on an imperial lathe and not keeping the half nuts engaged all the while.

If I get a bit of time tomorrow I'll post a video of mine threading a fine 1mm pitch thread right up to the chuck, 900 revs and standard plastic gears.

Every pass is bang on the previous one.

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