screwcutting 13tpi on an ML7

[Mo deller]

I need to make a new draw bar for my mill and it is 1/2x13 unc. I would like to screwcut one on the lathe and found a table in L H Spareys book that shows for an 8tpi leadscrew I should use a 40 tooth driving a 65 tooth gear.
In the Myford book and inside the gear cover there is no mention of being able to cut 13tpi.
As I understand it Mr Sparey says the driver and the driven can be joined by any one gear and the ratio will not be changed. At first I thought that was why myford didnt list it as there appeared no gear would fit between the two. Then I noticed that the twin slot arrangement for intermediate gears was able to move round a bit. After fiddling I have a 40 tooth on the headstock driving a 45 that is driving the 65 on the leadscrew.
To my limited understanding that should work,or am I missing something?

Peter ???

 

[mklotz]

You can put any number of idlers in a gear train without affecting the input to output ratio. That ratio is determined solely by the first and last gear. You can prove that to yourself with a simple bit of algebra.

 

[tel]

Yes, that will work, as Marv says, the idler, or any amount of 'em, do not affect the ratio of the driving/driven pair.

your 40/65 combo comes out at 4/6.5 or, which is more to the point 8/13 - exactly what you want.

 

[Mo deller]

Yes, that will work, as Marv says, the idler, or any amount of 'em, do not affect the ratio of the driving/driven pair.

your 40/65 combo comes out at 4/6.5 or, which is more to the point 8/13 - exactly what you want.

Thanks for that both of you.
I would like to know why 13tpi is one of the ones missed out on the myford table. That made me think it just wasn't possible.

Thanks again
Peter

 

[Mo deller]

Started this today and it was a case of third time lucky.
First I thought I could use the tumbler reverse gears instead of having to switch off, scrap piece 1.
Start again and after a few cuts it dawns on me that something aint quite right with the shape. I had set the topslide over to what I thought was the correct angle but wrongggg.
scrap piece 2.
Couldnt work out how to get the right angle so just used the cross slide and now have a new drawbar wa hey!
Must try and get this angle business sorted in my head but I have had enough for today.

Peter.

 

[John S]

I would like to know why 13tpi is one of the ones missed out on the myford table. That made me think it just wasn't possible.

Thanks again
Peter

When Myfords compiled the screw cutting charts for there new series of lathe, America and hence the furrin 13 tpi UNC thread range hadn't been discovered or invented.

Don't let it worry you, many of the home grown American lathes can't thread this either which is probably a great source of amusement to Joseph Whitworth.

John S.

 

[Mo deller]

Thanks John, that would explain it then.

Peter.