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Screw cutting gears

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Andy Freeman 115/10/2011 13:09:14
14 forum posts
I Have a Myford Super 7 with a gearbox. As I have never tried screw cutting before I could do with a little help.
I want to cut some metric threads so bought a set of 33 and 34 tooth gears. The notes said you just have to replace the 24 tooth i/p gear on the tumbler reverse with one of these gears - depending on what pitch you want.
My question is, on the tumbler reverse, there are two gears. Am I correct in thinking that I just replace the gear that is engaged when its running forward.
or is it the gear that these two tumbler gears mesh with I should be replacing?
Thanks for your help
NJH15/10/2011 14:58:26
1579 forum posts
120 photos
Hi Andy
I too have bought 33 & 34 tooth gears but have not yet used them .
I understood that they replace the gear shown - but maybe some knowledgeable gent will confirm this.
( Looks as if I should do a bit of cleaning up!)

colin hawes15/10/2011 16:47:08
235 forum posts
7 photos
I don't have a Myford lathe but I would like to advise that on most imperial lathes you need to keep the leadscrew engaged and reverse the motor to return to your thread start when cutting metric threads as it won't pick up the thread again if you try to use the thread indicator.
Nicholas Farr15/10/2011 20:13:00
1030 forum posts
377 photos
Hi Andy, I believe Norman's yellow arrow is pointing to the correct gear to replace, the other three you can see are never normally removed on most lathes with this type of arrangement.
Regards Nick.
ady15/10/2011 22:47:37
612 forum posts
50 photos
I think it might be better if you actually understand whats going on before you replace anything.
Part of the skills set, so you know exactly what you're doing.
Nobby15/10/2011 23:28:21
500 forum posts
105 photos
That is the correct gear to change.
On the Myford chart for metric pitches The 21 gear tooth gear is a good one to get.
For 1, 1.25 , 1..5 & 2 mm combined with standard gears.
ie for lets say 1.5 driver 21 1st stud 50-45 2nd stud idle35 wheel leadscrew 40 set up 2
Take Colins advice and leave half nut closed etc thereby not getting split threads
Regards Nobby
John Stevenson15/10/2011 23:30:06
2918 forum posts
2 photos
Yes it is the one Norman posted.
You need to select either 33 or 34 depending on the pitch, the chart gives which one and what lever in the gearbox to select.
Also as Colin says you need to keep the halfnuts engaged and wind back, either by hand or under power.
Ignore the screwcutting dial when doing metric.
John S.
Nobby17/10/2011 17:50:46
500 forum posts
105 photos
Whoops talking Myford S7 with a gearbox

Andy Freeman 130/10/2011 20:47:43
14 forum posts
Just a quick posting to say thanks to everyone that helped me out here.
I fitted the 'new' 33T gear and set the gearbox to 28 TPI (I think) as per the instuctions.
Trying to cut a thread resulted in just a parallel turned bar! The feed seemed way to slow.
Having just aquired this Super 7 with gearbox I assumed that everything was 'in its correct place' - if there can be such a place.
This was the mistake as I found that the 2nd stud gear had been reversed on its shaft. I turned this around and got a lot faster feed rate on the lead screw.
Trying not to forget the sequence of events, set to zero on the cross slide, advance the top slide, stop the motor, reverse the direction etc. etc., I ended up with my first M8 x 1.25mm thread. It was only about 10mm long but I was pleased - and so was my Dad and its his lathe!
John Stevenson30/10/2011 21:16:28
2918 forum posts
2 photos
Second stud gear as you call it is made to easily reverse, one way is fine feed [ as you found out <g> ] and the other way is screw cutting.
John S.
NJH30/10/2011 21:22:38
1579 forum posts
120 photos
Hi Andy
Yes this gear is reversible - one way round for fine feeds the other way for screw threads. You will see from the chart on top of the gearbox there is a box corresponding to the positions of the two gearbox " levers" There are two numbers in the box - one refers to the TPI with the composite gear on stud 'A' in one position, the other to the feed rate( fine feed) with the composite gear reversed.
eg . Top L on chart either 8 TPI or 0.0139 in per revolution feed depending on the gear position on stud 'A'.
I suggest that you get hold of the Myford Quick change Gearbox manual if you can also a copy of the Myford series 7 Manual by Ian Bradley would be of use.

Edited By NJH on 30/10/2011 21:23:21

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