metric screw cutting Myford Super 7

[c wastellParticipant @cwastell25217]

I'm trying to cut a 1.25mm thread on a 46mm dia. brass plug using a Myford Super 7 with quick change gearbox and the metric conversion kit. I have set it as per the book: 50T 63T 45T 60T wheels but the test thread on a piece of aluminium, is coming out at 1.3mm by the thread gauge. Can someone please tell me what I'm doing wrong?

Many thanks in advance.

[c wastellParticipant @cwastell25217]

1.25mm thread comes out 1.3

[LambtonParticipant @lambton]

Looking at the chart in the instruction book to my gearbox (repeated on the plate inside the gear box end cover) it looks as if you have not set thinks up quite correctly. Don't forget to set the gear box levers into their correct positions as shown on the chart.

Eric

[Nick HughesParticipant @nickhughes97026]

A bit more information (and alternatives) in a previous threat here:-

**LINK**

Nick.

[Brian WoodParticipant @brianwood45127]

Since you already have the metric conversion kit for the lathe, to get a 1.25 mm pitch screw thread you do need to complete all the steps correctly in the process. I think you missed the compound gearing out to get the result you report, in other words 50—63—-45—-60 where the 63 and 45 gears are operating as idlers. Set that way you would get a thread of 1.3229….mm pitch

The tumbler gear should be 50 T and gear X on the shaft into the gearbox should be 60 T The gearbox should be set for 16 tpi and if you have all the gears in the metric quadrant arranged at per the Myford illustration there should not be a problem.

The Link posted by Nick Hughes is helpful in pointing out some alternatives that you can use to avoid using the fiddly metric conversion

Regards

Brian

[c wastellParticipant @cwastell25217]

Thank you Brian. I think you have spotted the error but I'm afraid I cannot grasp the solution. The tumbler gear is driven from the tufnol wheels then the other three are horizontal along the conversion mounting and 60 tooth is the final gear. What is the compound gearing that I have missed out? Sorry to be so dense, I really am groping in the dark.

[Brian WoodParticipant @brianwood45127]

I'll try to take you through it step by step; it will help if you have the Myford gearbox booklet open at page 6

On the stub shaft driven from the final Tufnol gear fit the 50 T gear. In the table that is shown as the Tumbler Stud.

This 50 T gear meshes with a 60 T gear below it that also has a 63 T gear on the same shaft, they are keyed together in the bore on the short linking piece with a male key down one side.

To the right, on the next short shaft is the combination 45 T and 50 T gearing, built up in a similar way. The 45 T gear is on the inside and that then drives the final 60 T gear on the gearbox input shaft. This is gear X

There should be a plain collar fitted to keep the 60 T gear in position and engaged with the 45 T gear.

All these gears should be meshed with a little clearance so that they run together as a set without binding; the easy way to do that is to put a slip of paper between meshing gear and secure them with the paper in place

The final gear train will be as follows:

50 drives 60 [linked to 63] with 63 driving 50 [linked to 45] the 45 gear then drives the final 60 T gear on the gearbox

Mathematically, the driver gears are 50, 63, 45 and the driven gears are 60, 50 and 60.

I hope that helps

Brian

It might be instructive for you to follow the logic and set up a gear train for a 2.00 mm pitch thread and 'cut' a pencil mark on a bar to see if you got it right

[LambtonParticipant @lambton]

Brian has given a very helpful and lucid explanation as to the correct setting up of the gear train to produce the required 1.25 mm pitch. May I respectfully add that it is also necessary to set the gear box selectors to position 1 and

and position B i.e. to 16 tpi setting – see the gear box chart on page 3 of the handbook and on top of the gearbox.

Thank you Brian.

Eric

[c wastellParticipant @cwastell25217]

Thank you so much Brian and Lambton for the info. It is now crystal clear and I have it all in place. I would never have worked it out so I am very grateful

C.

[Brian WoodParticipant @brianwood45127]

I'm glad to help, gearing seems to be a special problem that many struggle with.

Brian

[c wastellParticipant @cwastell25217]

test piece completed, perfect. Thanks again.

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