Mini-Lathe Repair

[Brian WoodParticipant @brianwood45127]

Ron,

Try drawing the angle out with the side opposite the angle at 40 mm along a base of 110 mm.

The angle subtended is then 19.98 degrees, and the geometry is so much easier to do.

Regards

Brian

[Ron LadenParticipant @ronladen17547]

Thanks Ega/John and Brian

I have just made a one off gauge and ground a 55 degree tool back to 40 degrees, so will see how it goes.

John, thats interesting finding insert tooling, I guess it stands to reason they would be available.

Ron

Edited By Ron Laden on 17/12/2018 16:49:59

[Paul KempParticipant @paulkemp46892]

Ron,

I would think your drawn and cut gauge will be fine. I was castigated recently in a thread on boiler bushes where the OP was asking if BSB (55 degree) or BSC (60 degree) threads would be used in a commercial boiler bush. I replied saying if you make your male fittings 55 degree they will fit either. Apparently this was viewed as dangerous by one authority. I did calculate the difference in width of the thread form and it amounted to only a few tenths. In your case with the belt material being compliant a degree or two either side is not likely to be the end of the world. There is the perfect world and the real world! Nothing wrong in striving for perfection where it's needed but little point in agonising over it where it isn't!

Paul.

[Iain DownsParticipant @iaindowns78295]

ON a similar lathe I broke the plastic gears by being hamfisted (an overbrutal attempt to face a piece of large dia scrap sourced from a scrapyard).

I replaced mine with metal and added some grease nozzles just above. I can't say I mind the noise and, as I did the breaking early on in my journey, I can't say that I remember it being particularly quiet before.

The other thing that happened as part of that repair was that I blew the electronics loose wire and ended up replacing the power devices with higher current ones.

when my lathe jams up (parting off mainly) nothing blows, though I do try and hit the big red button as soon as possible.

This is just my experience and not a recommendation. One thing I will say though is that I had to remove the spindle to change the gears and on my particular lathe it was pressed in SOLID. YOu need to take care in how you remove it and avoid excessive use of an engineers friend.

Iain

[Ron LadenParticipant @ronladen17547]

Thanks Paul,

You are quite right I was thinking the tool needs to be spot on, obviously good if it is but a tad over or under wont be detrimental.

I tried the tool I have made on some alu and it cuts really nicely I just have to wait for some 45mm 6082 round bar to arrive so I can produce the pulleys.

Ron

[Ron LadenParticipant @ronladen17547]

Had a practice at cutting the vee,s for the pulleys.

40 degree tool, 0.092" vee centres, 0.082" deep, skimmed the o/d to put a small flat on the crests. With a magnifier and back light the belt ribs looks to have good contact with the flanks, though you cant see that in the pic as the belt is inside out so curving away from the bar. I used a blunt knife blade wrapped with a used piece of 1200 paper to polish the flanks. However if you look at the vee,s at the top of the pic you can see that the tool needs adjusting, the 40 degrees angle on the tool is good but its off centre.

Could be a better picture.

[JasonBModerator @jasonb]

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

[Ron LadenParticipant @ronladen17547]

Posted by JasonB on 18/12/2018 09:41:37:

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.

[Neil WyattModerator @neilwyatt]

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

[JourneymanParticipant @journeyman]

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

John

[Ron LadenParticipant @ronladen17547]

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.

[Jeff DaymanParticipant @jeffdayman43397]

Pulley looks good Ron, well done!

[Ron LadenParticipant @ronladen17547]

Posted by Jeff Dayman on 18/12/2018 16:15:24:

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

[John RuddParticipant @johnrudd16576]

Posted by Ron Laden on 18/12/2018 17:03:09

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

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

[Ron LadenParticipant @ronladen17547]

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by Ron Laden on 19/12/2018 08:51:14:

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.

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

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

[JasonBModerator @jasonb]

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.

[Ron LadenParticipant @ronladen17547]

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

[JasonBModerator @jasonb]

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.

[Ron LadenParticipant @ronladen17547]

Posted by JasonB on 19/12/2018 17:03:08:

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.

[Ron LadenParticipant @ronladen17547]

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.

[Neil WyattModerator @neilwyatt]

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

[Paul KempParticipant @paulkemp46892]

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.

[Ron LadenParticipant @ronladen17547]

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

[Anonymous]

Posted by SillyOldDuffer on 19/12/2018 10:03:19:

Torque=force applied during turning. Power= force over time. Both characteristics matter.

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

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