Mini lathe transmission replacement

Gallery, Projects and General > How do I??

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modeldozer:
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:
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 :coffee: Thanks for the advice so far guys.

PekkaNF:
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:
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:
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. :dremel:

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