myford QC gearbox

[Chris Smith 13Participant @chrissmith13]

when the gearbox is used the leadscrew turns correctly until the compound saddle is engaged, this stops the leadscrew turning thus not moving the compound. when looking into the box the selector does not engage the last gear on right, however the selector can go no further to right. I do not think these are related. The lower of the 2 gear trains looks as though it has room at either end of the shaft to move to left to or right, So 2 questions

1 whats is stopping the leadscrew from turning after engagement.

2 why wont the the selector lever engage the last gear

[Chris Smith 13Participant @chrissmith13]

gear engagement and lead screw is not driven when engaged

[Brian WoodParticipant @brianwood45127]

Hello Chris,

You don't say if this is a new development or the gearbox is a recent acquisition that you are testing.

Is the saddle itself free to move? And is this an effect in any gear ratio from the box or just when you move the selector lever to that position? It might even be as simple a problem as a missing woodruff key in the bore of the 72T wheel, there being enough grip to turn things until you add the saddle load. Are there sounds of distress during these observations? A few answers to question like these will help narrow down what the problem is caused by.

Regards Brian

[davebParticipant @daveb17630]

A leadscrew handwheel is a good thing to have with a Myford gearbox, use it to gently turn the leadscrew as you move the selectors. The top selector handle can rotate if forced. Leadscrew drive gear not secured to leadscrew. Damage or breakage or foreign body within gearbox. No oil, gearbox won't work without it!

Dave

[RobboParticipant @robbo]

Another thought is that the half-nuts are closing too tightly on the leadscrew.

Dave's point about the leadscrew handwheel is effective here. Does the saddle move when the leadscrew is turned with the gearbox in neutral.

[Brian WoodParticipant @brianwood45127]

Hello again Chris,

I can add some information now on how the gearbox works. The final gears on the right hand side of the gearbox are 32 T on the cone shaft, which is permanently meshed with a 16 T gear on the layshaft above it. That gearing provides the ratio change needed for the coarser pitches where the leadscrew is made to turn faster.

The actual ratio changes of 1 divided by 2; 1 to 1; and 1 multiplied by 2, are provided by the top selector handle and brought about by dog clutches in the gearbox. The eight cone ratios are made via the sliding selector on the face of the gearbox. So the gears you are concerned about are NOT accessible by the motion of the gearbox controls.

The more I think about your problem, the more I feel sure the woodruff key linking the drive from the outer 72 T gear to the gearbox input shaft is missing, clamping grip from the nut holding it in place is not enough to allow for carriage movement.

One final forensic question. Does the rest of the gear train from the headstock down continue to turn happily when you engage the carriage half nuts? If so, I think that confirms my diagnosis.

Regards Brian

[Chris Smith 13Participant @chrissmith13]

Hi everybody!

Thanks to those who responded to my pleas for help, I have now resolved my problem don't know exactly what the problem was I suspect a large amount of operator error was involved? once again thanks for all the help.

Regards Chris

[Ian S CParticipant @iansc]

When that happened with my big Taiwanese lathe it took a minute or two to realize what had happened, the drive on that lathe is via a shear pin, and the shaft turned until the saddle was engaged, the extra load stopped the shaft, cure, new shear pin.

Ian S C

[Brian WoodParticipant @brianwood45127]

Hello Chris,

Thank you for bringing us up to date, but it is frustrating that you still don't know what went wrong.

Regards Brian

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