Screwcutting on a Myford ML7

[Suffolk BrianParticipant @suffolkbrian]

I have a Myford ML7 with a QC gearbox. I need to cut a 26TPI thread on a gunmetal casting, 5/8 in diameter. The "label" on the gearbox says to "reverse gear on stud A". If I do that the gears lock up. Also, at some time (I bought this lathe second hand) the tumbler reverse gears have been replaced with tufnol gears. But the two gears have different numbers of teeth on them. Is this correct? Can anyone tell me the correct number of teeth for a "standard" gear train? Suggestions/comments greatly appreciated. TIA.

[Suffolk BrianParticipant @suffolkbrian]

[Neil WyattModerator @neilwyatt]

Hello Brian

Welcome to the forum. I think tufnol tumbler gears are the original fitting. The size doesn't matter (as long as they engage) as they are 'idlers'.

Do you have a photo of the gears that are locking up?

Neil

[Brian WoodParticipant @brianwood45127]

Hello Suffolk Brian,

The tumbler reverse gears are usually made of Tufnol, the tooth count is immaterial as they operate as idlers

​There is a swinging latch on the gear stud just before the wide 72 tooth gear into the gearbox. Lift it and pull off the double gear and refit it with the small 19 tooth side to face inwards. Be sure to engage it with the 57 tooth half of the captive double gear on the second gear stud, jiggle it about to be sure that happens. Re-latch to keeper bar to hold it in place.

The gearbox front lever should be moved to engage in the 13 tpi slot of the gearbox ramp and the top lever is to be set to the mid position B. The gearbox is now set to produce a pitch of 26 tpi

The whole of that gear train is driven by a 24 tooth gear off the shaft upon which the tumbler reverse lever pivots
I don't understand how the gear train locks up as you describe. you should be able to pull the belts over by hand and see the saddle move accordingly once the 1/2 nuts are engaged

​Regards Brian

 

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Edited By Brian Wood on 13/07/2017 18:18:29

[RobboParticipant @robbo]

Tumbler gears – Tufnol is original, one gear is 18 teeth (left one when facing the end of the headstock), the other 20 teeth.

Everything else – what Brian says, he being the gearing expert in these parts

[Simon Williams 3Participant @simonwilliams3]

Gear train can (will) lock up if the reversible gear isn't pushed fully home on its spindle, because both the 57 and the 19 tooth gears are in mesh together. If the swinging latch will close you should be safe, but just check that you have only got one set of gear teeth in mesh, and that you are not trying to drive the final gear by two different routes with two different ratios.

Robbo – forgive me if it's a silly question – I thought the tumbler gears on an ML7 were the same as those on an S7, in which case they are 28 and 30T. Am I wrong?

Although (as Brian says, and he would know) the tooth count doesn't matter as they are idlers.

Rgds Simon

[Suffolk BrianParticipant @suffolkbrian]

Gents all; grateful thanks for your quick responses. I stripped the tumbler reverse assembly and the two 19/57 gears down, mainly to count the teeth and make sure it had been assembled properly (- by me). Having now put it all back together, it all runs sweet as a nut. Finger and thumb trouble………

Question; tufnol tumbler gears; Robbo says that the gear on the left is 18 teeth. On mine they are fitted the other way round, so that the 20 tooth gear is always in mesh, with the 18 only coming in to reverse the leadscrew direction. As these two gears have differing tooth count, surely this will impact on the headstock/leadscrew ratio. Or is it so small it makes no difference?

I have another question to do with screwcutting in the lathe, but I will open up another thread on that. Thanks again for the advice.

Regards to all,

Brian

[Rick Kirkland 1Participant @rickkirkland1]

They are nothing more than idlers. You can have ten in a row and it makes no difference whatever to the in / out ratio. Basic mechanical principle.

[Brian WoodParticipant @brianwood45127]

Hello again Brian, It is just as Rick says, idlers can be thought of as the belt joining two pulleys, it can be any length. I'm sorry about the lack of spacing but the cursor has vanished altogether although I can still type Regards Brian

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