Mini-Lathe Repair

Ron, the idea of those little angle gauges and the reason I said to cut a rectangle with your notch in is that you hold one side of the gauge against the work and then adjust the tool in it's holder so it fits the vee, see bottom left

Oops, I forgot that bit Jason, I will have another go and get it right.

I can't remember which size, but JS used to use a particular size of BSW tap as a form tool for one size of poly-vee belt

Neil, 11tpi pipe thread apparently it's in this *** Practical Machinist thread ***

John

Well 2nd attempt but with the tool set to the gauge I had made, the flanks of the vee are now centered or as near as damn it.

Pulley looks good Ron, well done!

Thanks Jeff, I enjoyed the practice, just waiting for material to make the actual pulleys for the lathe. The belt sits nice and square in all 4 vee,s, the two left hand ribs in the picture look a bit off but that is only the belt twisting as it leaves the clamp I held it with.

By the way guys the metal gear set I fitted to the lathe in the repair have definitely got quieter, no doubt about it, I guess they must be settling in with use.

Ron

Edited By Ron Laden on 18/12/2018 17:04:16

Edited By Ron Laden on 18/12/2018 17:05:03

He's worn the teeth off them already.....

The lathe in low range has a bottom speed of 32 rpm but I thought it probably has little torque at that speed and would stall easily. This morning I put up a piece of alu bar and tried a very small cut, it was ok. I then tried a 5 thou cut and then a 10 thou and it was fine, no hesitation at all. I then changed to a free cutting steel but didnt go any heavier than 5 thou and it was also fine, I was really surprised I didnt expect that.

p.s. I dont know the first thing about thread cutting I am currently reading up on it but would it be useful for that. 

Edited By Ron Laden on 19/12/2018 08:52:07

Edited By Ron Laden on 19/12/2018 09:15:20

Your Mini-lathe has a DC motor and they have excellent low speed torque characteristics. (Torque=force applied during turning. Power= force over time. Both characteristics matter.)

DC motors with suitable electronics are much better for powering a small lathe than a single-phase AC motor as fitted to a Myford. Myford lathes use back-gear to compensate for their single-phase motor's need for speed. I expect Myford would have used DC motors had they had the choice. But before modern semiconductors, in homes wired for AC power, powerful DC motors were expensive and inconvenient. Today DC motors are a sensible option.

There is a large fly in the mini-lathe's ointment! Being inexpensive machines, their motors keep themselves cool with fan blades attached to the rotor. If the motor spins too slowly, the fan becomes ineffective and the motor is liable to overheat and burn-out. To reduce the chance of that happening the controller is set so the motor won't spin below a certain rpm.

Consequently most mini-lathes won't spin below about 150rpm at the chuck which is uncomfortably fast for the operator when threading. At 150rpm, you have very little time to disconnect the half-nuts before the saddle smashes into the chuck. (Threading as such at 150rpm isn't unreasonable provided the lathe is fitted with some automatic means of stopping. Cheaper machines rarely have such mechanisms. However, Far Eastern types will run in reverse and threading away from the chuck with the lathe in reverse solves the problem.)

On my mini-lathe almost all threading was done by manually turning the spindle with a hand-crank. It provides total control and is only tiring &/or tedious if you have a lot of threads to cut. The average bloke can easily output a few hundred watts in short bursts. Not so long ago mini-lathe sized machines were mostly powered by treadles.

Dave

Ron you don't say what diameter your bit of stock was but I suspect reasonably small. As soon as the diameter increases you will find it easier to stall the lathe. The brushed DC motors torque drops off as the revs decrease.

As an example my WM280 with a DC motor in low range 50rpm will do a 0.25" depth of cut on 1" steel, that's 1/2" off diameter. but if I can stall it with a 0.025" cut on a 9" flywheel unless I have it reving higher than most suggested cutting speeds.

Even the Brushless with their better torque characteristics can be stalled particularly as the diameter of the work or the cutter goes up when run at low speeds.

As for threading, if your machine is not slow enough or does not have the power at slow reve just cut from the back with it in reverse then you can spin it up into the motors power band.

Jason, the stock diameter was 20mm for both the alu and the steel. I was just amazed that it had the torque at such a slow speed (though it was small cuts) which made me think that if it was dialled up to say 60-70 rpm it should have more torque and the speed which is still quite slow would be ok for threading, but maybe not..?

Talking of threading I watched a video on Thread Cutting on a Mini-Lathe, there was no commentary but sub titles and it was quite easy to follow. It was cutting a M12 external thread, it was set up with the top slide set at 30 degrees to the cross slide. After each cut the cross slide was wound out to clear the work and the carriage was driven in reverse to wind the tool back to the beginning and then the cross slide reset to a pre-set zero (tool to o/d of work piece). The top slide was used to set the depth of cuts but I noticed that the carriage didnt appear to be taken back to a particular start point and the lead screw counter wasnt used. I thought for each cut there had to be a certain start point..? I,ve only just started looking at threading so probably missing something.

Ron

Edited By Ron Laden on 19/12/2018 16:53:16

Sounds like they did not disengage the half nuts, reversing the lathe to move the carage back is the clue. So if you don't disengage them you don't have the complication of re-engaging at a given point.

No he didnt disengage the half nuts so thats the answer.

I cut my first thread, quite pleased with myself but it was more a test of the lathe at low rpm. Its M8 on free cutting steel, low range 80 rpm which was not too fast so quite easy to manage. No issues with the lathe it seemed to have plenty of torque but of course 8mm is not very big.

Beware of overheating if you run too long at very low speed - this applies to all direct drive lathes, but especially older brushed motor mini lathes.

Well done with the thread!

Neil

Ron,

Good effort, well done. Might be wrong but from the picture the root of the thread looks quite wide compared to the crest, may be the radius on the tip of the tool was a bit wide? You are getting the hang of this machining lark though! Merry Christmas.

Paul.

Thanks guys,

Paul, You are probably right but I will put it down to my first attempt.. I realise there is quite a lot to good thread cutting but to be honest I was just pleased to get the nut to fit and it feels quite a good fit, probably for the wrong reasons.

Neil, I will have another go and check the motor thanks for letting me know.

Ron

Edited By Ron Laden on 20/12/2018 10:23:59

Oh dear! Power is work per unit time, not force. And work is force times the distance over which it acts.

For a motor power is torque times angular velocity, in radians per second. So if torque stays constant as speed decreases then power also decreases in proportion. The reason Myford use backgear is to maintain constant power at low rpm. So at low spindle speeds you get increased torque, and avoid the problem Jason highlights.

Andrew

Up early and had a good hours practice thread cutting, increased the speed from 80 to 120 rpm and checking the motor a couple of times it was no more than warm so that is promising.

I used a HSS 60 degree tool for most of the cutting but out of interest I tried a tipped carbide version I have and it seemed to work fine. I was surprised at that as I thought a carbide at low revs would struggle.

Edited By Ron Laden on 21/12/2018 07:50:47