Myford super 7 with gearbox - leadscrew stopped turning

I have been using my Myford Super 7 with gearbox for about a year. I finally decided to try out the thread cutting facility. I made a couple of cuts, everything worked well. I selected forward and reverse for the leadscrew and it turned fine. The half nuts engaged and released as required. I went away for a cup of coffee, came back and the leadscrew will not turn under power. It is free to turn using the wheel at the door.

I can’t blame the coffee so any idea what is going on? I am thinking it’s a shear pin somewhere but can’t seem to find anything on the internet about it. I can see from the manual that there is a woodruff key at the gearbox end which I suppose might have fallen out, but I would not have expected it to shear.

any help appreciated.

Check that the tumbler reverse hasn't become disengaged (lever in mid position)?

Start at the spindle turning it by hand. Look at the back of the spindle at the gears (open the cover) see if they are turning all teh way down to the input to the gearbox. If that is turning try different settings of the levers of the gearbox. All this by turning the chuck by hand not under power in case there is something damaged.

I am not really familiar with the Myford gearbox as my lathe does not have one but I can't think its drive mechanism is nothing but straightforward engagements. If you have not had a disaster like crashing the saddle into the chuck it is very unlikely that a sheer-pin would have snapped or dropped out for no reason, even if the Myford box is equipped with such a thing. You say it was all working fine until you left it for a few minutes; was the motor left running with the clutch dis-engaged? If so maybe vibration has caused something to be dis-engaged?

Without being in your workshop with you and observing the problem first hand it is very difficult to suggest a course of action except to say that the whole design of a Myford is quite 'primitive' (I don't mean that pejoratively) with mostly everything being very easy to dismantle and re-assemble with a bit of common sense, so all you can do is work through the gear-train until you see where the drive has become dis-engaged. I have a feeling it will be something obvious when you stumble upon it and nothing very serious. Intriguing nonetheless, let us know the answer when you find it.

Edited By Chris Crew on 24/08/2021 09:03:36

Have you broken stripped a tooth or two on the two fibre gears by the tumbler reverse. ? I hope not.. John

Nick,

Myford screwcutting gearboxes are not equipped with shear pins, so this observation must have some other case.

Try looking at the tightness of the bolting holding the banjo up in engagement with the 24 tooth drive pinion. It grips around the boss into the gearbox and has a cap head screw there to pinch the closure. Below and above the swarf tray is a bolt through an adjustment slot in the base of the banjo. If that is not secure the whole banjo can slip, especially under load, and disengage the drive.

Regards

Brian

Chris mentions the shear pin, and that would be my first suspect. On my Chinese lathe I crashed the saddle and all seemed well, except it wasn't! The pin had sheared but there was still enough grip to turn the lead-screw apparently normally. A few months later the remains gave way and the lead-screw stopped turning. The clue is everything works apart from the lead-screw.

Work back from the stopped lead-screw to find the point where the rest of the lathe is still turning.

Dave

I know its after the event, but I was taught never leave any machine alone on "auto" feed...

I suspect that the carriage has crashed at some point and broken something. I have a Myford S7 with gearbox, I will have a look at the drawing and booklet to see if there is any mention of a shear pin.

Regards

Derek

Look guys, the headstock mandrel drives the gearbox through a tumbler reverse arrangement which has two settings with a "neutral" in between. If it wasn't firmly engaged in one of the drive positions it could easily slide back into "neutral". From the OP's description it was working fine before going away and drinking the coffee, unlikely to have had a crash when he was drinking it!

Edited By John Haine on 24/08/2021 09:56:44

Hi John, ok I maybe read it wrong, but I read it as "it worked ok, I left it under power/screwcutting, while making coffee came back and now not working..."

As you suggest it could be the tumbler lever or it could be either gear selector lever on the gearbox not fully engaged.

Either way I may have got hold of the wrong end of the stick....and not for the first time!

Regards

Derek

Or if there is a missing tooth somewhere in the drive train it could have been stopped in that position before coffee and hence won't start after.

But agree something not fully engaged may have just dropped completely out of engagement.

Until Nick comes back and describes what moves and what does not we are all speculating.

But, I knew the Myford gearbox would give lots of 'false neutrals' (motorcycle term) until you settle the gear selector well into its pin. My shock a few months ago was that I got a 'lockup' when I would guess that gear teeth were pin-on-pin and not seated, and when I engaged the clutch it promptly stripped a few teeth from a fibre wheel.

I now always rotate the leadscrew hand wheel to check that gears have properly engaged. Wonderful thing experience from a lesson learnt.

We need more details for a diagnosis.

Tony

Check the Grub screw item 174 in Myford gearbox manual, this secures a 26 Tooth gear which is the primary drive into the gearbox.

As P S7 advices, start checking from the start of the drive, and work your way through until you find the part that no longer rotates. The problem lies with the previous part of the drive train., probably a missing tooth, loose grubscrew or sheared key.

Ensuring that gears have the correct backllsash is important. Too little = noise and wear; Too much wear, inaccuracy and damaged teeth.

howard.

Also check that the high/low gear selector on the top of the gear box is either in high or low, the mid position will give you another neutral. Normally I keep mine in this mid position as it allows me to turn the leadscrew with it's handwheel without all the added friction of turning the gears in the gearbox. Phil

As Philip . R suggests, I'm guessing the three position ( A, B or C ) lever on the top of the gearbox is between positions ie not engaged The Myford manual says this can be engaged (gently) with the lathe running. Or it could also be that the sliding selector lever on the front of the gearbox is not fully latched down (lathe should be stopped before moving this lever)

Hi everyone. Thank you so much for all the ideas. Before stripping anything down I was keen to follow up on the possibility suggested by some of you of the gearbox being in some false neutral position. And so it was. After much mucking about with the levers it has burst into life. I’m not exactly sure what combo works and what doesn’t but suffice to say the leadscrew is now turning in both directions as designed. Now I know of the issue I will spend some time, on a wet afternoon, trying all the combos to see what works and what doesn’t. Thanks again.

Hello Nick,

Perhaps in an effort to save you a lot of wasted work, I have found, and use from time to time, the two "neutrals" either side of the central range choice lever on top of the gearbox.

There may be others but I don't think there is room enough sideways between the gears on the selection cone [operated by the ramped slider on the front of the gearbox] to reliably work as a neutral.

Once again, just to confirm matters, these gearboxes do not incorporate a shear pin but they can disengage the banjo under heavy loading as I described in my first reply.

Glad to hear you are operational again

Regards

Brian

Many thanks Brian. There is more to the gearbox than I first thought. Exciting to explore.

regards

Nick