Author Topic: Mini lathe transmission replacement (Read 32040 times)

loply · « **on:** June 05, 2011, 11:18:47 AM »

Afternoon folks,

I got my mini lathe all tuned up (jibs sorted, etc) a few months ago, and about 4 weeks ago started working in earnest on my first real part - ball bearing handle for the compound.

Why take 4 weeks to make such a part?

Well, first the headstock gear broke, so I had to stripnit and replace... Decided to put tapered bearings in whilst I was at it, and to increase motor pulley ratio for better torque.

A day later.... BANG! Electrics blown, motor runs at full speed only.

New electric board fitted, I'm almost finished my rather swanky new compound handle when.... CRUNCH! Headstock gear completely gone again... Was only taking a light pass in alu.

I think the whole gear has shattered as both high and low are completely gone.

I'm pretty sick of this but determined to fix it properly.

I had already been thinking of making a whole new transmission which provided 3/4/5 speeds and bypassed the headstock gears all together, and was also thinking of bypassing the electronics and using just full speed motor (if I have 4 or 5 ratios...)

Of course to do this I will need a lathe, which I no longer have, so I will doubtless have to repair using another gear made from recycled plastic bags in order to make the new transmission.

Anyway! I'm looking for ideas or advice on how to do this.

I have a pair of V belt step pulleys with 2/3/4/5" pulleys on each one, which could potentially give me a speed range of 150 to 1000-odd RPM without the need for the toy electronics.

Looking at the lathe I'm thinking to mount one of the stepped pulleys directly onto the back of the spindle, and not use the layshaft at all. I will then reposition the motor and create a system for tensioning and un-tensioning the belt so that speed changes can be made.

This will require some kind of spindle extension to be made so that it's long enough to take the pulley, which is about 70mm through it's bore.

I can then remove the whole gadget box on the front and build a much simpler on/off system, and put a tool holder there instead!

Does this sound like a plan? Anybody have any better ideas or done anything similar?

Thanks,
Rich

loply · « **Reply #1 on:** June 05, 2011, 12:07:25 PM »

Well, head off, here's the result...

Sure enough the smaller of the two gears has completely blown itself to pieces!

Luckily the larger gear, which is harder to replace, is more or less intact... The low speed end has lost one tooth, the high speed end is okay.

I may not have to pull the taper bearings off the shaft therefore, if I can make my new transmission using only high speed ratio anyway

John Swift · « **Reply #2 on:** June 05, 2011, 12:19:22 PM »

Hi Rich ,

during the last 12 months I've fitted new brass gibs , tapered bearings and metal headstock gears to a clarke CL300M lathe
all from arc euro trade :-
http://www.arceurotrade.co.uk/Catalogue/Machine-Spares/C3-Mini-Lathe-Spares

note - the metal spindle gears are in 3 parts and you need to check the key will drive all of them !!

I can see how changing the pulleys to increase the torque will break the plasitic gears more easily ,
a mod i've thought about doing , but not done yet

John

PS look here at one version of what you are thinking about :- http://tool20895.homestead.com/treadmill.html

andyf · « **Reply #3 on:** June 05, 2011, 12:44:44 PM »

Sounds like a plan, Rich, but you will only have 4 speeds with two 4-step pulleys unless you can arrange for one pulley to slide along its shaft into a position where (for example) the belt can run from one 5" pulley to the other 5" pulley, and a tensioner which can take up a lot of slack (when running from one 2" to the other 2"). If you could perhaps remount the motor so you could move it closer to or further from the spindle to take up the slack, you would get 12 ratios (2" to 2", 3" to 3", 4" to 4" and 5" to 5" all being effectively 1:1, so you wouldn't actually need 5" to 5").

Without the gears in the headstock changing the direction of rotation, the spindle will run in reverse when the switch says forward and vice versa. Some of these little DC permanent magnet motors only run at half speed in reverse (which with your pulleys would have the spindle going forwards) so it might be beneficial to transpose the wires to the motor.

Hope this makes sense!

Andy

picclock · « **Reply #4 on:** June 05, 2011, 02:19:57 PM »

FWIW replace both of the headstock gears with metal ones (ArcEuro). If the lathe stalls or jams the motor just buzzes or the belt slips. I have never had mine blow the fuse yet. I've geared mine down to allow very large diameter turning of 5" plus steel with no problems and lots of use.

Some info at

http://s917.photobucket.com/albums/ad19/picclock/

If you need any other help post or PM me. The 7x12 forum is very good for stuff on these lathes.

http://groups.yahoo.com/group/7x12minilathe/messages

Best Regards

picclock

modeldozer · « **Reply #5 on:** June 05, 2011, 02:29:04 PM »

hi rich,

If I may add my 2c´s worth. Just finished making a new shift fork for mine as appart from poorly made the pivot pin on the fork actualy needs to be offset to work properly (see last page of my post).

With the increased torque from different pulleys I think both the headgears needs to be metal and the "fuse" moved to for example the motor pulley. Either a soft alu/plastic key or just a grubscrew that can slip if things go wrong.

Another mod I have seen that makes sense is to add an ammeter in series with the motor wires so one can monitor the power being used.

Abraham

loply · « **Reply #6 on:** June 05, 2011, 02:39:17 PM »

Well I've just been and reassembled the lathe, it's fairly obvious that torque in the HIGH gear will be insufficient to machine the 50mm steel bar I would need to make the adapter to allow mounting of the step pulley to the rear of the spindle.

This means I will need to replace the broken LOW gear too.

The options then become:

1) Simply fit metal headstock gears and be done with it
2) Temporarily fit plastic headstock gear (I have a spare...) then make the new transmission

As far as I can see the advantages of making the new transmission would be:

1) Allow higher torque at very low speeds (100rpm or so) where it still stalls despite the reduced speed at present
2) Remove the dependence upon the sensitive electronics
3) Repositioning motor has cooling advantages reducing odds of burning out motor
4) Possibly some efficiency gains since the layshaft bearings and gears have been eliminated

The big problem is going to be that the big step pulley needs to be about 25mm away from the end of the existing spindle in order to clear the screw cutting gears, so I would machine a chunky extension to the spindle to which the pulley bolts, but ensuring this is concentric will be essential to smooth running, and that would depend upon whether or not the screw that is cut on the end of the spindle is concentric...

Hmmmmm

Thanks for the advice so far guys.

PekkaNF · « **Reply #7 on:** June 06, 2011, 04:00:22 AM »

Wish I could contribute...That picture where tha lathe vomitted out those plastic gears made me sick. I'm wondering if it is build with plastic gears and such - is the shaft distance correct for the metal gears? Maybe thinkking was - we'll get away with bigger tolerances with those plastic gears.

I would have a look on automotive (used) belts and wheels. I might even use only one timing belt wheel on the spindle to avoid all the hasssle on the head stock and rather have a layshaft loaded for more wheels and bigger reduction - more space and latitude there to put levers for belt tightening (and change).

It might be easy to fit a cup form pressed steel timing belt (or multi v-rib) wheel on the end of spidle and strip all that gear contraption clear of it.

Can you beg set of timing belt (new or used) and a waterpump (timing gear wheel) and some redundant wheels from garage you frequent? I would look multirib v-belt or timing belt drive. They get changed routinely and on many car makes it's not only belts, but also waterpump and some other auxiliars. Some gear wheels are aluminium, some are pressed from steel (easy to fit on a boss, fix with screws even on pilar drill and then turn to correct boss. On valve timing transmission you are generally limitted to 1:2 ratio, but some waterpump etc. gears might digger. Multi v-ribs gives you normaly a bigger ratio.

Can you rig you lathe temporary to work with one stage belt drive to machine parts?

Pekka

Bogstandard · « **Reply #8 on:** June 06, 2011, 06:55:23 AM »

I have been reading many posts over the last few years about the Mini Lathe, and have come to the conclusion that many people think that because it is a 7" lathe, it will turn a 7" billet perfectly OK, and so they overload them, major problems occur, then they start to think about beefing them up even more, and even bigger problems raise their ugly heads.

I did some major work on a mini lathe, and I will tell you now, even though it had taper bearings and steel gears, for the spindle diameter (the critical part), I wouldn't use it for anything more than say 3" diameter, and even then, would be very careful on depth of cuts, purely because the lathe wasn't designed to take it.
If the gears break, then it is a sure sign that you have overloaded it to beyond the designed tolerances. People seem to think they are better than the original machine designers by fitting stronger third party parts. There is a reason for nylon gears, as a safety valve to prevent further damage.

I'm not saying you can't machine a 7" part at all, as long as the bit you are trying to machine on it isn't any larger than say 3".

When will people start to realise, if you want to start machining larger pieces, then you need to go for a larger lathe. It is just false economy to try to beef one up that isn't designed to do the job in the first place, all you will end up doing is struggling, causing yourself heartache and emptying your pockets of cash.

Small machine, small bits, large machine, large bits (and small bits), no arguments on that score, it has been that way since the beginning. I'm very sorry if you can't afford a larger lathe, but that is a fact of modern day life, if you can't afford to buy something larger, then you will have to stick to making smaller things.

DO NOT OVERLOAD SMALL MACHINES

Bogs

websterz · « **Reply #9 on:** June 06, 2011, 09:12:28 AM »

I have to agree with John. I have tried to do some stupid things with my 7x12 and it always let me know when I was pushing too hard. I just finished turning some 3 3/4" stock down to half that diameter on one spot and I won't do it again until I have a bigger lathe. The surface speed on material that large is just too much. To get enough oomph to prevent stalls you end up overrunning the proper tool feeds and either getting horrible finishes or burning up tools (or both). Until I got the material down to 3" or so it was a wavy chattered mess. In the end the job got done, but then again my machine has been modded out the wazoo! Tapered roller bearings, metal headstock and change gears, QCTP on a solid steel block (the compound is just too flimsy), tapered gibs, and more that I am forgetting I am sure.

PekkaNF · « **Reply #10 on:** June 06, 2011, 09:18:27 AM »

Bogs,

I haven't use this particular lathe, I have one 60kg french made cheapie and then some heavier ones.

While I agree on big picture ("if you want to start machining larger pieces, then you need to go for a larger lathe" et.al.) with you I disagree slightly on 3" category.

* Any tool should be able to tolerate reasonable use and some misuse. This reasonable should be defined by manufacturer.
* If machine has serious not so obvious limitations that should be stated on the spesification.

If you can chuck 7" billet on 7" lathe and there is no indication (maybe there is on the user manual?) that you can only turn e.g. 2" of free maching steel, 3" aluminium with 1 mm2 swarf then what we are left with?

I don't think plastic gearing is very elegant fuse against shock loads. I would think plastic shear pin might be more apropriate.

I'm no way expert in judging what size of work is apropriate for this size of lathe, but for me it is: If the tool tip is towards the centre from the bed way, then it should be fine.

There seems to some trouble on gearing of smaller lathes and mills, I'm having hard time beliving that it is all end users fault. I have no information on quality of the design of these, but I would imagine that very little money is used design to keep costs down.

Half of the fun of this hobby is to see if our tools can be modified and improved, sometimes they come out satisfacory, sometimes it turns to a learing process, which is't too bad either.

I'm considering hacking one taiwanese Myford ML10 (or something) copy. It's missing all moving parts on back gear, belt drive is shot, missing some gear to cut screws and a chuck is clunker. Probably could salvage it with some timing belt, AC-motor and a VFD. I'm not looking to make it beefier just more special, maybe to turn parts between centres or to take ER25 collet chuck.

Pekka

Bogstandard · « **Reply #11 on:** June 06, 2011, 10:16:43 AM »

Pekka,

I will not argue over this issue, if you want to ignore many years of design and machining experience, and if you think you are better than the original machine designer, you just go ahead and do whatever you want.

There is a great difference between doing minor mods to a machine to get it to work better, and what you are trying to do, which is to take it beyond it's already designed limitations.

Bogs

lordedmond · « **Reply #12 on:** June 06, 2011, 10:21:13 AM »

I to agree with John ( Bogs)

with the smaller typer of lathe a good guide is what can you turn above the cross slide not the bed thats the more sensible cutting dia. IMHO

yes my myford S7 big bore can swing 9 inches in the gap but and its a big but I have back gear down to 25 rpm and a 2 hp inverter fed 3 phase motor on the back so it don't stall

another point if I went to a Hardinge or Monarch My lathe would feel punny

Above all enjoy your machines and listen to them they will tell you a lot

Stuart

jim · « **Reply #13 on:** June 06, 2011, 11:28:10 AM »

i've got the plastic "sacrificial" gear in all my machines. the lathes have never given any trouble, on the mill it has done one plastic gear.

the gear on the mill went after an hour and a half of fly cutting some lathe tools to size.

with the benefit of hindsight, it was abuse that bust that gear.

i think that the reason the lathe plastic gears have never gone wrong, is down to being a skilled turner!

websterz · « **Reply #14 on:** June 06, 2011, 12:29:45 PM »

Quote from: Bogstandard on June 06, 2011, 10:16:43 AM

Pekka,

I will not argue over this issue, if you want to ignore many years of design and machining experience, and if you think you are better than the original machine designer, you just go ahead and do whatever you want.

There is a great difference between doing minor mods to a machine to get it to work better, and what you are trying to do, which is to take it beyond it's already designed limitations.

Bogs

Don't hold back Bogs...tell us what you really think.

spuddevans · « **Reply #15 on:** June 06, 2011, 12:50:49 PM »

I think that it is not in keeping with the general atmosphere of this forum to discourage anyone from posting about their experiences and tools, even when they might have caused the problem themselves by ignoring advice given.

That being said, Pekka, it might be wise to listen to the advice being offered from many experienced engineers, the last thing we want to see is a picture of you, or anyone for that matter, having recieved injuries because a machine was pushed beyond its capabillities and let go.

We all want to be the one who pushes the "envelope" further, to "boldly go where no-one's been before", but let's do it safely.

Anyway, that's taken the post off-topic enough, let's get this thread back on track to help the original poster with his questions.

Tim

Rob.Wilson · « **Reply #16 on:** June 06, 2011, 04:48:55 PM »

Maybe we should take a look at the forum Rules
http://madmodder.net/index.php?topic=4.0
Welcome to the MadModder.

This is a place for those who like to build, craft or modify anything and everything can gather. We are going to take our cues from the famous HMEM model engine site. I want this to be a place that fosters ideas and learning... plus a good place to show off your talents and skills.

1. Flaming. Yes we all have different opinions. They can be said without abusing anybody else.

We reserve the right to edit or delete posts as we see fit in order to keep the family friendly atmosphere of this site. Remember that although this is a public forum, it is privately owned. We don't like to censor... don't make us do it.

Rob

doubleboost · « **Reply #17 on:** June 06, 2011, 05:04:07 PM »

Very well put Rob
We need to keep the forum a nice plesant place .
I personally have gained a wealth of usefull tips and information ,far more than i have put in.
So please lads keep the moderators happy

loply · « **Reply #18 on:** June 06, 2011, 07:26:39 PM »

Evening folks,

Midnight! Just finished popping the spindle out of the lathe, couldn't remember how I did it last time so it took a while

I have decided just to fit metal headstock gears and may add a two-speed intermediate shaft later.

Bogs, I appreciate your input and acknowledge all of your points, however... I was turning a 60mm aluminium part and taking very light cuts when the gear broke, and when it broke it completely shattered as you can see. Whether or not I have overloaded it previously to weaken the gear is unknown, but, it would seem that these gears breaking is a continual problem and the positioning of the "fuse" in a place takes hours to reach is one of MANY bad design choices on 7x lathes. When you make reference to users "knowing better" than the manufacturer... Well, it's not a case of knowing better, it's a case of the manufacturer using plastic because it's cheaper. If they were trying to make a better lathe for more £££ I'm sure the gears would become metal pretty quick...

Also, in this instance close inspection leads me to think the gear shift fork has engaged with a tooth on the plastic gear (the fork is wobbly as hell and at the right angle can stick its prongs into the teeth), if I'm correct then the thing would have exploded even if no cut was being made, and the solution is to fix the bad shifter design. I'll pop metal gears in there whilst I'm at it, in order to move the 'fuse' to an easier location.

The lathe once upgraded is drastically better than stock, so the manufacturer made many bad choices - plastic headstock gears which explode randomly and take hours to replace, saddle jibs which are impossible to set, electronics which are completely vulnerable to the ingress of swarf, etc.

In this case, I'm pretty certain I do kow better than the manufacturer. Or rather, I care enough to undo some of his economic compromises!

Having said all that, I acknowledge your point and don't intend to turn 7" pieces on this thing, but I do disagree with your implication that replacing cheap plastic gears with metal ones is somehow a bad idea or that I was expecting too much to try to turn 60mm aluminium.

loply · « **Reply #19 on:** June 06, 2011, 07:46:10 PM »

One other small point - I can only speak for myself, but about the only thing I have to make on the lathe is new parts for the lathe!!!

I bought the thing for fun and most impotently to LEARN - a few months ago I had never made a metal object in my life, now I have made things with bearings that look like they came from a factory, and have a real appreciation of what works and doesn't, what's critical and isn't, etc.

Sure... I could have gone out and put a £3,000 CNC lathe on my MasterCard, but what the hell would I do with it?

The hours spent fettling with this lathe are great fun and great for learning and I wouldn't change it for the world. Unless I was trying to manufacture something, in which case I'd get that MasterCard out...

No point telling a fisherman to go to the supermarket for his fish...

ketanswali · « **Reply #20 on:** June 07, 2011, 05:00:55 AM »

loply,
You might want to check if your control board has overload protection. The nylon gears are indeed there as a fail safe. Yes they are economical. Usually, the idea is that they break instead of blowing your control board, but sometimes it is a toss-up between which will go - the nylon gear, or the board, or both at the same time. The design is such that it should break the gears first. The control board to be found in C2 or C2 type mini-lathes do not have overload protection.

So if you put in metal gears in mini-lathes which do not have overload protection, you have to be more careful when using the machine, because the nylon gear fail safe is now gone, so probability of blowing the board has now increased. Over torquing taper roller bearings on such machines (without overload protection), can also have a small risk of blowing the control board if the motor needs to draw more power to allow the spindle to turn.

The probablility of blowing the board are reduced if you have a C3 / C3 type mini-lathe which have overload protection, however it does not mean that the risk is totally removed.

mini-lathes are built to a price, a price at which it makes it possible for more people to enter the hobby, so yes, there are compromises made too.

Davo J · « **Reply #21 on:** June 07, 2011, 05:02:02 AM »

Hi Rich,
I thought this forum was called Mad Modder for a reason. Or should it be change it to Cautious Modder,

I have been reading your posts and as you say it's the journey and the learning thats fun, thats what this hobby is all about and keeps us coming back for more.
There are a lot of us in your shoes, where we have to work with what we have and cant afford anything bigger, or don't have the room.
Keep posting your mods, as some of us are interested and are reading it, even if we don't reply sometimes.

Dave

PekkaNF · « **Reply #22 on:** June 07, 2011, 06:08:14 AM »

Quote from: Davo J on June 07, 2011, 05:02:02 AM

Hi Rich,
I thought this forum was called Mad Modder for a reason. Or should it be change it to Cautious Modder,

Dave I had long time a go a physics teacher and an avid leisure pilot who said: "There are only two kind of pilots: young-an-brave; and old-an-cautious" Then he always made a very sad face.

I apreciate Ketans input here, I'm damn sure he knows what he is talking about.

Pekka the flack magnet

John Stevenson · « **Reply #23 on:** June 07, 2011, 06:46:56 AM »

I fully agree not to overload the original drives or motor and controls.

However if you do need the *extra* bit of power and drive for the odd job there are ways round this.

This setup allows me to use the whole of the 7.5 Hp from the big TOS lathe onto the C0 without any fear of blowing a driver board.

John S.

picclock · « **Reply #24 on:** June 07, 2011, 07:29:29 AM »

OK, there are a few points that should be mentioned here, which seem to be being ignored.

Assuming a constant tool cutting speed the work that the lathe doesn't vary (much) with the diameter of the work. So whether your turning a 1" piece of steel or a 5" piece of steel at 50fpm, assuming the depth of cut is the same the work done is the same. So the power required to do the cutting is the same.

But, obviously the 5" diameter piece of steel is being turned at 1/5th of the speed. Consequently the spindle and final drive gears are subjected to 5x the torque of the 1" piece. The bending force on the cross slide / compound is increased but the turning force on the lathe bed remains the same.

So provided the spindle, gear train and cross slide are up to the increased forces then there really isn't a problem. On the 7x12 minilathe forum there are many members who have modded there lathe in this way and have good experiences. I am one of them.

In order to increase the torque from the std motor to allow turning larger diameters most people use a layshaft with additional gears.

@ Loply
Don't be discouraged. The plastic gears can disintegrate for lots of reasons. It may be that the lever that engages them is indexed incorrectly and only partly meshed the teeth sometimes. I've never had a problem with 60mm Aluminium (apart from the price

), even before my machine was modded.

@ John Stevenson
Laughed my socks off - your lucky to have such a large lathe to work with. A lot of people, myself included, simply don't have the room or finance for such a luxury. But that's when the creativity (hopefully) kicks in.

Best Regards

picclock

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Author Topic: Mini lathe transmission replacement (Read 32040 times)

lordedmond

Rob.Wilson