Beaver Milling Machine Gearbox Repair

[Darren]

Having stripped the x feed gearbox a couple of days ago to tighten a loose grubscrew I did notice that the motor shaft worm drive threads where a bit worn. The trouble is this motor is wired up to run continuously with no separate power switch. After 60 odd years I guess the wear was inevitable and it's finally given up.

Nothing particularly difficult to re-make, in fact a doddle, except for the short 6tpi worm thread. On a bigger machine this wouldn't present a problem. But it may just be too much for a 7x12 mini lathe.

I have a feeling this is already a replacement part, but not certain?

The large worm wheel itself is fine and it looks as if the motor spindle part is designed to wear first. Good idea as it's by far the easier bit to make from scratch.

I know, I know, ....

Later

[andyf]

Darren, just a thought: I think mini-lathes have a 16tpi (1.5mm if metric) leadscrew, so a 6tpi thread would involve gearing up by 1 to 2.7. That might be a tall order on a small lathe, even with a spindle handcrank.
But I wonder if it would be possible to turn things round and use a handcrank on the leadscrew to drive the spindle via the gears. You would then be gearing down at 2.7 to 1, which might make life easier.

Andy

[Darren]

Hi Andy,

Yes I was thinking that I might have to make a hand crank ... something to delay things ...  

I have turned a 5/8 (22.25mm) 12tpi thread quite successfully on the 7x12. But this 3/4 (19mm) 6tpi I think it going to be too much to ask.

I will have a go, even if I have to make a hand crank ...  

On the other end of the bed is a rapid transverse motor, and it's a bit quick to mill with for rough cuts. In fact it's quick to transverse with and mackes the whole machine bounce a bit. I have been thinking of seeing if I could fit a treadmill DC motor an vary the speed. Now that would be very useful.

But for now I need to fix the low speed side and get this worm fixed as it gives a very nice finish that I can't match by hand cranking.

[Darren]

Here's a vid of a Harrison lathe making a 6tpi double lead acme thread, effectively two 3tpi threads......

Oh how much I'd like such a lathe right now .....  

What is that thing on the toolpost, some sort of auto index for tool changes .... 

[bogstandard]

Darren,

With that being a CNC lathe, that lump on the top is the same as a normal swing round toolpost, but it will be under NC for tool changes.

If you are having trouble will the threading, don't be afraid to ask for assistance.


John

[Darren]

Thanks John, very much appreciated, You know I will have a go first as always a glutton for punishment 

But I just may have to give you a knock, we'll see how it goes ..... 

[kvom]

If you take a lot of small cuts, the 6TPI shouldn't be a problem IMO.  You'll certainly find out fairly quickly.  Slow speed and back gear (assuming you have one) is good.

[bogstandard]

Chris and I were discussing this when I did a little work on his lathe.

The mini lathe just hasn't got the strength in the gear train setup to attempt to cut that sized thread using the spindle as the drive. It is almost guaranteed that you will end up with smashing something. If my old beefy Atlas could do it when cutting 3 TPI, the mini lathe doesn't stand a chance.
 
My error cost the purchase of a new cast iron gear quadrant, and the making of a few 3/8" gear holding bolts. That was caused by putting 30%+ more loading on than the lathe was designed to do (max on the Atlas was 4TPI), you would be overloading it by at least 100%. It snapped the quadrant and gear holding bolts like they were twigs.

As previously mentioned, the only way to do it, is to transfer the gear drive to the leadscrew by putting a handle on the end, and that would be the very easy and safest thing to do, also, you will have total control of the speed of the cut. If under power, at 6TPI the distance is covered in the blink of an eye, doing it by hand, as slow as you want to go.

John

[kvom]

I misread the part about the mini-lathe.  John has the right idea (as usual).

[Darren]

I prepared the blank ready to thread. It took me six different variations of gear sets and swapping about to find one that would physically fit on the 7x12 to turn a 6tpi thread. I almost gave up but the last try worked.

Well there is no way this little lathe is going to do this under power. Sadly not with a hand crank for the headstock either. I tried turning the chuck by hand an the change gears are under so much strain all they want to do is jump out of mesh. I have to physically hold them in place with the leaver and it takes some effort. I quickly came to the conclusion that this was going to bust something if I didn't give up quickly. So I've left it at that.

Feel rather defeated now .... 

I sorta worked out that for one inch the headstock needs to turn 6 times but the leadscrew would have to turn 17 times at 1.5mm pitch. That's some gearing up going on there at 1:2.83

But the other way around if I cranked the leadscrew by hand as Andy suggested I would be turning it 17 times per inch to the headstocks 6 turns. That's a nice thought, but will it make it easier on the gear train?

Another alternative is to try the Union lathe. I've not done anything with it as yet and it's currently in bits. I'm waiting to finish building the inside of my garage before setting it up.
The thread chart only goes down to 8tpi but I do have the gears for 6tpi.

The leadscrew is 8tpi, so the gear ratio is quite small 1:1.33
Would that make life easier for the gear train which is a whole lot beefier than the 7x12 to start with.


I'm assuming a lot here, I've never had the Union lathe running, let alone cut a thread with it.
Maybe I should just admit defeat

[Darren]

I've just answered my own question. Popped back in the workshop and tried turning the leadscrew with an allen key that holds the gear on.

Turns easily with no strain on anything. I'll make a hand wheel tomorrow. I think this is going to work.....

Thanks for the suggestions chaps .... 

[John Stevenson]

Look Ma - no gears !!

Wot about finding a bolt with 6 tpi on it 1-3/8" or 1-1/2" UNC or Whit, cutting a bit off and stitching it on the end of you blank then using a spring loaded follower to follow it and it will push your screw cutting tool ahead of it.
No gears needed.

John S.

[John Hill]

I've just answered my own question. Popped back in the workshop and tried turning the leadscrew with an allen key that holds the gear on.

Turns easily with no strain on anything. I'll make a hand wheel tomorrow. I think this is going to work.....

Thanks for the suggestions chaps .... 

If you put the handle on an intermediate gear that would send some strain one way and some the other?

[Darren]

Mr Stevenson, I do take it you are extracting the wee wee again this evening, unless you happen to have a box of 6tpi screws under your workbench....

John Hill, honestly, turning the leadscrew there doesn't seem to be any strain, quite the opposite to turning the main spindle.

here's some pic's for the illiterate

The new blank compared to the worn part

I used my new parting tool to turn all of this, worked a treat cutting left and right......



Turning the thread by powering the spindle by either motor or hand power made a right mess as the strain was just too much and the tumblers kept jumping out.



Then I tried turning the last allen screw with an allen key, this was easy with no perceptible strain on anything inc my fingers




So, tomorrow I'll make a hand crank for the leadscrew ....  

[andyf]

Glad the idea seems like it might work, Darren. It was a bit tentative, because I've never tried it. It's always good policy not to put a theory    into practice oneself, but to let others find out if it actually works......
You probably don't need one, but there is a recipe for a bells and whistles leadscrew crank for a Seig mini-lathe at   http://www.toolsandmods.com/library/ralph_patterson_leadscrew_handwheel.pdf
If yours is the "Real Bull" version of the mini-lathe, things will be easier, because they have a bit of leadscrew sticking out beyond the RH pillow block, threaded for an endfloat adjusting nut (which may have a grubscrew inside, to lock it in place).
Even if you don't put a dial on it right now, it's well worth making a provision for one to be added later, in case you ever work up the enthusiasm for marking the divisions.

Andy.

[Darren]

Well I knocked up a hand wheel for the leadscrew.....

Turned up a couple of bits to press fit together



Hammer like bu**ery more like it



Once together faced the other side



Drilled and threaded for a handle



I was going to make a bush for the handle, but my lathe belt snapped ...... again ..... something else to fix now 

It fits on here with two grubscrews to clamp on the flats







Trouble is it needs to be bigger, MUCH BIGGER, whilst I can shave metal off at the start of the thread there os no way you have the strength when material really starts coming off. It's just not going to work.
On a thread this size we need power, buckets more than a 7x12 has to offer.

Oh hum.........

[andyf]

Maybe fasten a bar across its diameter, with a couple of bolts through the disc? The bar projects at one side, with a handle on the end, affording both greater leverage and an opportunity to graze your knuckles on the bench top 

It shouldn't need that much effort, should it, if you only take light cuts? I'm thinking of the comparison with cutting (say) an 8mm thread by hand with a tap or die, where you take the whole thread groove out in one pass.

Andy

[Darren]

The trouble is Andy that this is a large thread for such a small lathe. At the start I agree that not much effort is needed. But as you cut deeper, In my case only very slightly deeper, it becomes apparent that the force required is increasing rapidly and I have a long way to go yet. The little lathe is just not going to be able to take it.

I managed 12tpi under power, but 6 tpi is way out of it's depth.

I am currently thinking to set up the Union lathe I have, but it will be a while as I need some bits for it and I need to do some work to the garage before putting it in there to set up. It's threading chart gives the gear train for 8tpi so I'm hoping it can handle 6tpi as it's close. Coupled with the fact that it has a back gear should help enormously in transmitting the motor power and slowing the actual threading speed down to hopefully something more reasonable. The Union lathe is quite a bit sturdier than the 7x12, so fingers crossed. Also the lead screw is 8tpi so won't need to spin nearly as much as the mini lathe. That should also help, I think 

I wonder if I should put my spare 1HP motor on it.......

Failing that I'll have to go cap in hand.......

[bogstandard]

Darren,

If you are trying to do it with a straight plunge cut using the crosslide, that might be your problem. That is why the compound setover method is recommended. It uses a lot less power to cut the thread, but is more complicated to do.


John

[Darren]

Yes, but I've only dipped my toe in, or the tip if you prefer, and the whole lathe is straining.

Really I've only scratched the surface and I'm struggling to hold the tumbler lever in place. making a right mess at the same time. If I continue something is going to give.

[andyf]

Bogs' advice is as sage as ever. And if you are going straight in with the cross-slide, a chip curls off each side of the tool, so at its tip they both try and occupy the same space in the V which can jam things up.

Still, after all the work Bogs did to straighten your lathe out, I'm sure neither you nor he wants it bending into a corkscrew shape.

Mission creep strikes again - you start doing smething simple like stripping down a milling machine gearbox, find that a repair is needed, have two abortive stabs at it under power and by hand (the latter involving making a handwheel), and now you are thinking about fixing up another lathe and putting a bigger motor on it.

Perhaps cap in hand might save a lot of grief....

Andy

[sbwhart]

Darren

You may be asking too much of your little lathe.  One reason I made the key way cutter was to stop punishing my lathe, have a think to see if you've got any other options to cut the thread it may be quicker in the long run.

Stew

[Darren]

Ah but, I really do want to set the other lathe up, if only to see if it's any good. So now is maybe the push I need.

After all it's been a long time coming and just may surprise me, one way or the other .... 

I still have a "get out of Jail" card to play if all else fails .... 

Back to the mini lathe, Bogs spent so much time an effort on it that it's now finely balanced and a sweet little thing.
I feel I really should not be abusing it this way, and if I bust it I would be gutted. It has it's place in my little empire and long may that continue.

[Darren]

Not given up yet, been beavering away getting the Union lathe running..... getting closer .. 

BTW, I discovered I broke a tooth on the 7x12 in my attempts, 

[ozzie46]

BTW, I discovered I broke a tooth on the 7x12 in my attempts, 
[/quote]


    Quit biting your lathe and see a dentist.   

  Ron