Cutting a 1/2" bsp thread on Harrison L5 help

Help needed with gear choice and configuration in otder to cut a 1/2" bsp thread on my harrison - not threadcutting gearbox - L5. Charts are on the machine but have worn away. Thanks

is that it?

forgive me, i thought it was all the info needed. I need to know what changewheel no of teeth and positiion to cut 14 tpi on the L5.

thanks Ady - that is indeed the thing i needed - having a bad day!

using that c hart with the 3 speed gearbox i can get close to 14 tpi by using the closest metric equivalent but can it be made exactly - never done this before

Can you read the gears for 7, 28 or 56tpi on your worn chart, if so you probably just need to move the 3 speed lever to another position when set to that gear train

Edited By JasonB on 09/09/2021 20:21:37

there apear to be two tables. table for general screw cutting 14tpi says top-40 mid-100 bot 70 Lever-A. Another plate says for whitworth and bsf same but mid is 60. Surely tpi is the same regardless of thread profile?

mid one is probably an idler so makes no difference.

So ignore the mid gear in the 40-100-70 gear train and you get 40/70 = 0.571

Assuming an 8 tpi lead screw so one turn = 0.125" then 0.125" x 0.571 = 0.071" which is 14 tpi

Must admit when I looked at it I thought omg

edit: a nice light test cut scribe line would be a good idea, always worth checking anyway

Edited By Ady1 on 09/09/2021 22:59:09

Jason -

The leadscrew on an L5 is of 1/4 " pitch, not 1/8" . At least on mine it is.

Verify the tooth-count on the spindle pinion: on my lathe it differs from that given in the manual.

Otherwise, depending if a 40T or 20T pinion and my arithmetic not being all up the creek:

4/14 = 40/140 = 20-idler-70

or perhaps 40 - [70+30] - 60 (compound).

The leadscrew gearbox should be on the 1:1 position (I think that is "A" ).

My calculation was for a straight through 1:1 which is most likely B which now being a 4tpi screw would give a cut of 7tpi, if so A would give half the rotation so that will be 14tpi and C will give double or 3.5tpi

I assumed 40T spindle so

1:2 lever position 0.5 x 0.571 x 0.25 = 0.071 = 14tpi

1:1 lever position 1 x 0.571 x 0.25 = 0.143 = 7tpi

2:1 lever position 2 x 0.571 x 0.25 = 0.286 = 3.5tpi

If a 20T spindle

1:2 lever position 0.5 x 0.286 x 0.25 = 0.071 = 28tpi

1:1 lever position 1 x 0..286 x 0.25 = 0.143 = 14tpi

2:1 lever position 2 x 0.286 x 0.25 = 0.286 = 7tpi

Fizzy, can you post a picture of whats left of your chart just to be sure

Edited By JasonB on 10/09/2021 07:26:16

Thank you Jason -

Just dug out my copy of the manual.

What confused me was that the top shaft wheel on my L5 is of 20T and appears not removeable as the book suggests it should be, whereas the table in the book is for a 40T top shaft wheel for 14tpi.

It gives 40 - 100(idler) - 70 then gearbox position A... 2:1 reduction; but for my lathe I need 20 - idler - 70 and C (1:1).

Fizzy -

The plates on top of my lathe are all worn away too!

Though not cheap I suggest you buy one of Tony Griffith's (lathes.co.) facsimiles of the L5 manual. It is a very comprehensive operating and servicing book, and includes the change-wheel tables for both imperial (any thread-form) and metric threads. If you are going to do a lot of screw-cutting it would be worth hanging copies of the tables up by the lathe - you could laminate the copy or protect it simply with a polythene bag.

thanks all - i managed to see that it says 40-100-70, pretty sure the spindle gear is 20 so do i now need 20 - idler - 70?

**LINK**

ping

Pong

No the 20 drives a 40T, then the 100T or whatever you have to fill the gap driving a 70T on the gearbox input.

With a 4 tpi Leadscrew, for a 14 tpi thread, the reduction between Mandrel and Leadscrew needs to be 3.5 :1

(14 / 4 = 3.5 )

So, on the face of it a 20T - Idler - 70T would do the job, assuming that the intermediate settings are 1:1.

If there is a 2:1 Intermediate reduction then the 40 iDLER 70 set up will do the job..

Howard