plastic intermediate gears or metal for mini-lathe?

[Woodsman22]

Hello everyone;
   This is my first post and I am new to model engineering/home shop machining (forgive me if this is not the right forum for this post but I am not sure where it should go). I have a Chinese 7 X 12 mini-lathe which I am converting to metal change gears and I am converting the spindle gears to metal as well. However, from what I have read online many people seem to think that going "all metal" is not so good and that it is best to keep the intermediate shaft gears as plastic. I am unsure of what I should do at this point as I have the lathe apart and have the complete set of metal gears, but if the lathe is going to be very loud then perhaps keeping the plastic gears for the intermediate shaft is a good idea? My primary reason for converting to metal change gears is that the plastic ones are variable in dimension (so I have been told) and therefore threads cut using them vary in tolerance. I want this little machine to be as accurate as possible. Having said that, I note that the intermediate shaft power take-off cog wheel is plastic as well and no metal version of it exists so far as I know. Does this affect threading quality? Please forgive the basic questions as I am a newbie. By the way, I am changing to taper roller bearings as well. I just got the whole headstock taken apart, so before I proceed I would greatly appreciate input/advice from those more knowledgeable than I (which is probably everyone!

[Joules]

As your new to model engineering I would suggest you put it back together and get using it.  The periodic error is going to be minimal especialy whilst you learn to cut threads.  Start out with brass as its soft and forgiving, avoid alloy as its too easy to gaul and lock up if both parts are alloy.  Treat the plastic gears gently and think if you break a plastic gear, what would have broken if that gear had been metal.

           Joules

[Woodsman22]

Thank you for addressing my questions in such precise detail. Such generosity in imparting knowledge is surely the hallmark of a great teacher and mentor; one with much experience no doubt.

[vtsteam]

No need for sarcasm. You asked for advice. That will likely come in a range of opinion.

[NeoTech]

The small chinese machines will vary in accuracy, and the change gears will prob. be the least of your worries there. But yeah sure, changing them up is not a bad idea. I have a large chinese/german machine.. And welll lets say all the primary bearings (8 of them) is 6004-RS2 type.. thats what you find in wheel axles.. and the vibrations is horribad. The spindle bearing is the same type ford uses for their wheel mount. And the accuracy of that is.. not even funny to talk about.

The drive screw on the machine leaves things to wish for. And i have been having todo modifications to the whole drive train to get into repeatable 1/100th of a mm. So just saying.. The change gears is your least problem when it comes to accuracy.. For material choice, well i would go with brass, its soft, doesnt expand to much from friction and is easy to make parts out of..

I do see this "i have a chinese machine and i want it to have a high accuracy" threads quite often on forums. And to be honest, learn on your chinese machine and if you get bit by the bug - buy a used machine from a known high accuracy brand.

my 2 cents..

[lordedmond]

cricky that must be a small machine 6004-RS2 has a 20mm bore ?


Stuart

[drmico60]

Hi Woodsman22,

The plastic change gears may vary in dimensions due to temperature changes but simple size change will not alter the accuracy of thread cutting. If you have a 25 tooth gear driving a 50 tooth gear then the ratio will always be 1:2 irrespective of the size of the gears. The only thing that could affect the accuracy would be if the tooth spacings varied on the gears. This is not very likely since they will have been moulded in a precision master mould. The same will be true for the main spindle gears and the intermediate shaft gears.

The main reason for changing to metal gears is that some users have broken the plastic gears. This usually happens as a result of misuse. In the early days of my lathe I broke my 80 tooth change gear by driving the carriage into the headstock. If that had not happened something else somewhere would have broken or failed (perhaps the electronic speed controller due to overload). The gear was cheap to replace whereas other damage may have been much more expensive (new controller board is circa £80, new gear circa £5). For this reason I do not believe it is a good idea to have metal change gears.

There may be some justification for metal gears on the spindle since I have heard of these breaking under load but even here I would be tempted to put the metal gear on the spindle but keep the layshaft one in plastic in order to provide a weak link to break in the event of a serious jam up. I would keep the layshaft in plastic since it is less hassle to replace than the spindle gear if it breaks. I actually planned to do this on my lathe and ordered the metal gear for the spindle. I did not fit it immediately on arrival and some months later when I went to fit it I noticed that the metal gear was cracked. There is not much metal between the top of the keyway and the root of the gear teeth. So much for greater robustness of metal gears.

Another reason for keeping with plastic gears is that they are much less noisy than metal gears and they require little lubrication. If you decide to fit metal spindle gears then also think how you are going to get oil or grease to them. Perhaps drill some holes in the headstock close to the gears to permit access for spray grease. Thread the holes and block them with screws to prevent access of dirt during normal use.

Why do you need super accurate threading? The geometric relationship between the spindle rotation and the carriage movement through the gears ensures sufficient accuracy for most purposes.

Mike

[awemawson]

Bigger professional lathes usually have a 'shear pin' in the drive chain to the lead and feed screws. Usually made of soft brass and just comprising a collar pegged to a shaft with the shear pin. Later Colchesters had a spring loaded detent mechanism for the same purpose.

Andrew

[vtsteam]

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.

If so, Woodsman, it would be helpful if you gave specific details of your bearings, and state of disassembly of your headstock, photos would be very helpful, and a description of the specific problem you think you are facing in installing those bearings, if any.

Then you will get practical advice. This is an extremely helpful forum.

[lordedmond]

Hi
I did have a 7 X 14 lathe a good few years ago ( is it that long how time fly's )
IMHO  replace the change gears and the bearings but retain the plastic speed shift  gear in the head stock if only for your ears the gear that you can buy are poor and are soft and poorly formed
, with these in you have a safety for a crash ( its not if its when ) they will break and are cheap to replace with metal ones in the next weakes bit will fail and may not be so cheap.

as you have stated you are new to ME but are you new to lathe work ?

as for info try here I am swear that it may not be you lathe but they are pretty generic

http://www.arceurotrade.co.uk/machineguides/Mini-Lathe-Angular-Contact-Bearing-Change-Guide.pdf

also plenty of other info on the site  no link with the from only a customer

Stuart

[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!

[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.

[vtsteam]

I believe it.

Even if you wanted to, you'd have your work cut out for you to make a gear train that overlapped threads. It could only happen in the final pair, AND they would need to be heavily eccentric. AND non modulo. If that bad, then they'd be a problem to mesh-- would probably bind or skip. It would be a real challenge to make it happen.

A bent lead screw and loose carriage gibs wouldn't even do it, It would Just give variable depth.

Slip is the only way I can think of, and because the gears are keyed at the power end and the drive end, slip anywhere in the train isn't possible.

Well, excluding slip of the human kind.     Been there.

[NeoTech]

cricky that must be a small machine 6004-RS2 has a 20mm bore ?


Stuart

Small. nah its a good size 320x900mm, a gunmakers lathe commonly called.. but all the axles for the drivetrain internally runs in those crappy ass bearings.. The spindle bearings is 50mm id

[Woodsman22]

"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."

     Well John, you and Vsteam could very well be right about Jerry not knowing what he is doing. I, as a novice am hardly in a position to say one way or another. However, here is an article about Jerry Kieffer and his work at the Sherline website.:

http://www.craftsmanshipmuseum.com/Kieffer.htm

 It seems that they have a high regard for the man's capabilities, and he has done some remarkable work. You can judge for yourselves.

[Woodsman22]

And by the way, the model Harley Davidson motorcycle engine he built can be seen on this page:

http://www.craftsmanshipmuseum.com/Kieffer4.htm

     The image of the engine at that page can be enlarged, and the tiny gears and overall workmanship are impressive, at least they are so to me. I would never say that that man does not know how to use machine tools.

[Joules]

Woodsman22, may I suggest you look on this forum to see what other members have achieved with these lathes as is.  If you think the 7x12 is so bad why did you buy one rather than a secondhand Sherline.  Take sometime to get to know people here, they are an amazing source of knowledge.  Yes some of it can be flawed, who doesn't make a mistake, but understand most speak from experience that they are willing to share freely.  Ask questions, listen, ask more questions, you will soon gain respect Grasshopper... 

[DMIOM]

Woodsman22, may I suggest you look on this forum to see what other members have achieved with these lathes as is.  If you think the 7x12 is so bad why did you buy one rather than a secondhand Sherline.  Take sometime to get to know people here, they are an amazing source of knowledge.  Yes some of it can be flawed, who doesn't make a mistake, but understand most speak from experience that they are willing to share freely.  Ask questions, listen, ask more questions, you will soon gain respect Grasshopper... 

Joules - it might be worth reading the original the original post.  Woodsman22 doesn't say he thinks its that bad - he was doing exactly what you said - he was asking, because he has found comments about the gears on-line and was asking for other, more experienced, feedback.

Dave M

[vtsteam]

     Well John, you and Vsteam could very well be right about Jerry not knowing what he is doing.

I can't see anywhere above where I've said that. I thought I was talking about gear trains and overlapping threads. 

[vtsteam]

BTW, setting the topslide over to the half angle does overlap threads, technically, but in a purposeful way.

[lordedmond]

the only time I have heard of the overlaying threads I am not sure if it was on here or not

was that the user had set the lathe up to curt threads in the foreign language to the lead screw and used the thread dial that did mess up the thread , he did own up to having the gear train set up wrong

I will try and find the thread

Stuart

[Woodsman22]

Vtsream; my apologies, I mistook this comment:

"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."

   by John to be a reference to Jerry Kieffer's criticism of the 7 X 12 lathe that I had quoted in a comment above and I thought that you were agreeing with him. That is my error. 

    Also, thank you Dave (DMIOM) indeed I never did say that the 7 X 12 was a bad lathe, I merely wish to improve what shortcomings that it does have. Also, Joules yes I am quite sure that there is a wealth of knowledge to be gained from members in this forum- that is why I joined and asked the questions that I did in my first post. As to the Sherline. it would not meet my needs as stated in subsequent posts.