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Thread cutting - Apron does not engage with the lead screw

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Lathe-argic26/05/2020 21:35:44
3 forum posts
3 photos

As a precursor I am a complete beginner. Apologies in advance for the use of incorrect terminology. I recently inherited a Myford Super 7 lathe with what appears to be a problem. When I engage the cam lever on the apron to automatically travel the length of the lead screw it doesn't move. I can move it manually using the handwheel at the back. The lead screw itself is turning as it should and the lathe is set at a low speed. I have tried adjusting the spring loaded screw to raise and lower the half nut on the underside of the apron but that made no difference. I can't really see well enough ito check inside to see if the thread is okay or not. Any thoughts or advice as to what I should check first and where to go from now? Thanks in advance

Bazyle28/05/2020 00:01:22
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5235 forum posts
201 photos

With the lathe not running obviously feel whether the half nuts are moving at all, There are two one above and one below the leadscrew. Or perhaps you mean the lever does not move? I don't have a Myford so does it have a lever or button on the apron to select between screwcutting and power feed? If so there is an interlock between the two features to stop them being used at the same time.

John Baron28/05/2020 06:55:03
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301 forum posts
122 photos

Does the leadscrew turn at all on its own when the chuck is turning ? If not have you got the tumbler gear lever in the down position ? The tumbler gear lever is the one on the left of the headstock.

David George 128/05/2020 07:03:12
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1229 forum posts
415 photos

Hi lathe-argic welcome to the forum. Where are you based it helps as perhaps someone near could give a hand or a local to you supplier can supply materials etc. Even with the virus problem it is possible to do a video connect to see what the problems are. What are you interested in making.

David

Lathe-argic28/05/2020 14:51:33
3 forum posts
3 photos

Thanks to all for your responses.

David, I'm based near Sudbury in Suffolk. Ideally I want to turn a screw for a workbench but more importantly I'm interested in learning the skill.

Bazyle, It has a lever. When I press the lever down I can see the half nuts closing down on the lead screw but the apron's not moving when engaged.

John, The lead screw is turning when the chuck is turning. I'm thinking that maybe a gear has slipped or maybe broken teeth on the gears inside the apron assembly.

Many thanks

 

Edited By Lathe-argic on 28/05/2020 14:51:52

Tony Pratt 128/05/2020 15:32:26
1130 forum posts
5 photos

hi Lathe-argic, if the half nuts go fully onto the lead screw & it is turning the apron will have to move as the half nuts are part of the apron assy, are the nuts fully engaging with the lead screw?

Tony

SillyOldDuffer28/05/2020 16:09:15
5798 forum posts
1235 photos

Does the lead-screw stop turning when the half-nuts are engaged? If so, likely a sheer pin has broken. Sheer pins are designed to break before anything else should the saddle crash into the chuck. When they break, not unusual for there to be enough friction to spin the lead-screw off load, but slip immediately it's asked to do work.

If that's the problem, fix is to punch the remains of the broken pin out and replace it. A new pin can be made on the lathe!

Dave

Jon Cameron28/05/2020 16:30:33
336 forum posts
90 photos

I have the Myford ML4 but I think the suggestion from Dave could be the likely issue.

Although I am having to replace the screws in the back of the half nuts on my lathe as one of them has broken and only one half nuts was moving.

Ive broken the sheer pin twice now, not through a crash but trying to use the handle to traverse the saddle without first disengaging the change gears.

Jon

old mart28/05/2020 17:36:08
1795 forum posts
138 photos

To do a very basic check of the leadscrew nut engagement, without the lathe switched on, you can move the saddle slowly right or left, and at the same time try to engage the nuts with the apron lever. The saddle will be locked as soon as the nuts engage if that is working correctly.

If that is ok, then you should be able to move the saddle right and left by turning the leadscrew handle.

The engagement of the leadscrew nuts means that much more torque is needed to turn the leadscrew which is now having to move the entire saddle weight. This is why the leadscrew may turn on its own but not turn when the nuts are engaged. A safety frangible link is used in the leadscrew mechanism on most lathes to lessen the damage caused by pile ups. and this may be slipping. 

There is plenty of info on Myford lathes on "lathes UK".

Edited By old mart on 28/05/2020 17:38:43

Lathe-argic29/05/2020 08:50:25
3 forum posts
3 photos

Thanks for all your help. I managed to engage the apron to the lead screw by unwinding the spring loaded screw (as far as it would go) for the half nut adjustment. This seemed counter intuitive and I feel a little stupid for not trying it earlier. I would say that the bond feels decidedly weak between the half nut and lead screw. The half nuts probably need a good clean or even replacement.

Bizarrely though another problem surfaced. When I ran up the lathe the lead screw remained motionless. I fiddled around with lead screw handwheel and it seem to locate on a gear in the gear box and started working again.

ega29/05/2020 09:03:54
1716 forum posts
150 photos

Lathe-argic:

I suggest you invest in a copies of the Myford manuals for lathe and gearbox.

KWIL29/05/2020 09:13:17
3251 forum posts
63 photos

Now you tell us you have a gearbox, get that Manual for a start.

JohnF29/05/2020 10:34:42
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986 forum posts
142 photos

Here is a link to a Super 7 Manual PDF file **LINK**

https://groups.io/g/myfordlathes/files/Super%207/Super%207%20manual.pdf

John

ega29/05/2020 10:44:13
1716 forum posts
150 photos

PS The ball lever on top of the gearbox has a neutral position in which the leadscrew is not connected - apologies if you knew that!

Mick B129/05/2020 12:04:31
1580 forum posts
84 photos
Posted by Lathe-argic on 29/05/2020 08:50:25:

Thanks for all your help. I managed to engage the apron to the lead screw by unwinding the spring loaded screw (as far as it would go) for the half nut adjustment. This seemed counter intuitive and I feel a little stupid for not trying it earlier. I would say that the bond feels decidedly weak between the half nut and lead screw. The half nuts probably need a good clean or even replacement.

Bizarrely though another problem surfaced. When I ran up the lathe the lead screw remained motionless. I fiddled around with lead screw handwheel and it seem to locate on a gear in the gear box and started working again.

Halfnuts can get really choked with heavily-impacted swarf and still be nice and pristine underneath when you poke the crud out with something hard and pointy.

This may be off-the-wall, but when something a bit similar to your second para happened on my Warco, it was one of the keyed bushes that link gears on the same shaft had broken in the middle, so the gears were rotating nearly independently apart from the friction in the break.

KWIL29/05/2020 15:53:34
3251 forum posts
63 photos

Lathe-argic,

Quote "I managed to engage the apron to the lead screw by unwinding the spring loaded screw (as far as it would go) for the half nut adjustment. This seemed counter intuitive and I feel a little stupid for not trying it earlier "

The correct setting is once you have fully engaged the half nuts, as it appears you have now done, whilst still engaged, rotate that screw until it stops, the loosening the half nuts, give it a 1/4->1/2 turn tighter. This stops you fully tightening the half nuts arounsd the leadscrew.

Best done when you know the half nuts are clean.

old mart29/05/2020 16:12:39
1795 forum posts
138 photos

It would be a good idea to dismantle the apron and thoroughly clean and re lubricate everything.

JohnF29/05/2020 19:18:33
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986 forum posts
142 photos

Lathe-argic, It would be worthwhile adding some details about your Super 7 these machines have been around since the 1950's and there are several marks with mods done on various parts of the machines.

I appears your has a gearbox, does it have power cross feed ? probably posting a photo or two in your albums would help member identify the most likely area of age or you could look on the Myford site and with you machine number you can tell when it was made.

The method of lead screw removal varies depending on the model and you need to do this to strip down the saddle/apron.

John

Howard Lewis30/05/2020 06:54:26
3288 forum posts
2 photos

Since the Saddle moves when the handwheel is rotated, the half nuts would seem to be engaging with the Leadscrew. So it sounds as if the power is not being transmitted from the Mandrel to the Leadscrew.

I would suggest starting at the Mandrel, and checking, one stage at a time, through the tumbler reverse, the gear train into the gearbox, through the gearbox and then to the Leadscrew, in each ratio of the gearbox.

Does the problem exist in all gearbox ratios?

Somewhere along the line, you should find a part that is not transmitting enough torque for the Leadscrew to drive the saddle along the bed.

It may be a gear that lacks one or more teeth, not secured to the shaft, (Sheared key or loose /missing grubscrew ), gear not engaging, or a failed shear pin.

IF the problem only exists when one gearbox ratio is engaged then the problem will lie within the gearbox and the gears that provide that ratio. If that is the case then you need to investigate the gearbox.

Howard

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