Lathe gear problem..help please!

[Jamie StoneParticipant @jamiestone29757]

Hi there ladies a gentleman, just wondering if anyone could guide me in the right direction, I’ve just got into hobby machining about a year ago, started with a mini lathe, have just possibly downgraded to a 10×22 bench lathe, okay so here is the issue.
Change gears for threading.
Okay so I believe I’m setting the gears correctly however I don’t believe this is the case because the thread pitch isn’t correct I’m aiming for 1.25 as you can see from photos the pitch isn’t correct on the scratch pass, do the drive gears look correct? Not a clue to be honest any help/advice would be greatly appreciated. Thanks

[Jamie StoneParticipant @jamiestone29757]

[JasonBModerator @jasonb]

Janie, have a look at how to post pictures

Also does the machine have a metric or imperial lead screw and if so what pitch?

Edited By JasonB on 04/08/2021 07:19:42

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Jamie, your photos don't show, so follow JasonB's link, but this scan may help you understand a bit about Screw-cutting on the lathe.

Regards Nick.

[Howard LewisParticipant @howardlewis46836]

You need to know the pitch of the Leadscrew on your lathe.

If it is 3 mm, you need to set up the right ratio between the Chuck and the Leadscrew., so that the Leadscrew moves the saddle 1.25 mm for every turn of the chuck, such as 25T – IDLER – 60T (3 x 25 / 60 = 1.25 )

If your lathe has a 1.5 ,mm pitch Leadscrew, you will need to set 50T- IDLER – 60T, (1.5 x 50 / 60 = 1..25 )

If your lathe has an Imperial leadscrew, you would need a 127T gear in then train for an accurate metric thread, but a 63T often suffices with an error that is acceptable.

So for a 1.25 mm pitch metric thread, with a 8 tpi Leadscrew you need a ratio of 1:2.54, so that the chuck revolves 2.54 times for each turn of the Leadscrew The easy answer would be 50T – IDLER – 127T, but a 127T gear may not be available.

(0.125 x 50 /127 x 25.4 = 1.25 mm )

30T – 60T + 50T – 63T should give a similar result, (0.125 x 0.5 x 50 / 63 x 25.4 = 1.2599206 mm )

If you intend to do a lot screwcutting, it will be worth buying "Screwcutting in the Lathe" by Martin Cleeve.

It is No.3 in the Workshop Practice Series.

Howard

In all cases the calculation is DRIVERS / DRIVEN

 

 

Edited By Howard Lewis on 05/08/2021 16:34:45

[David LeyshonParticipant @davidleyshon41802]

Hi Jamie

for a metric 1.25 pitch thread I use: A.72 B.40 C.SPACER D.60 E SPACER F 50 . worth a try good luck.

Buddy.1940.

[OldironParticipant @oldiron]

Mmm no reply from the OP for 2 weeks so he must have got the info he needed.

regards

[Author]

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