My new friend

[Jasonb]

Dan's BA diagrams in this post, look through the rest of teh thread for more

http://www.modelenginemaker.com/index.php/topic,1020.msg13066.html#msg13066

[sco]

Thanks Jason!

[Jasonb]

I prefer to draw the gear train out like this, forget the 3 ratio gearbox for your machine

Divide each driver by the driven, then multiply the results to give the ration between spindle and leadscrew.

[steve-de24]

Simon,
The Cowells chart is a good place to start screw cutting because it doesn't use the compound gears (gears B/C and D/E in your photo).

The chart gives which gear needs to be attached to the headstock spindle (Driver 1)  and on the end of the leadscrew (Driven 2).

The idlers are any gears which connect Driver1 to Driven2.   The only thing to note is that you need an even number (2) of idler gears to cut a right hand screw thread and an odd number of idler gears (1 or possibly 3) to cut a left hand thread.

Example (1)

To cut a right hand 0.6mm thread you need 30T gear on the headstock spindle and a 50T gear on the leadscrew and any two gears as idlers.  (note 30/50=0.6)

(For non Cowells owners note the leadscrew has a 1mm pitch)

Example (2)

To cut a left hand 1.25mm thread you need a 25T gear on the headstock spindle and a 20T gear on the leadscrew and 1 large or 3 small idler gears.   (note 25/20=1.25)

Hope this helps

Steve

[sco]

Steve,

That helps a lot but I'm still missing something either brain cells or hardware - in the setup below I'm aiming for a 1mm pitch RH thread so I have 30T gears on the headstock and leadscrew and need two idlers between them but I only have one arbor plus the space available is very tight so they would have to be two low tooth count gears. 

Oh yeah does this mean you don't use the back gear for screwcutting as that adds another gear ratio?

Simon.

[Jasonb]

Looks like somebody kept the extra studs with the extra gears

[steve-de24]

Simon,

I always cut threads using the slowest back gear speed (60 RPM). Things happen quite fast enough at that speed!

Back gear affects the speed ratio between the motor and the headstock spindle. The gears we are messing with here affect the gear ratio between the headstock spindle and the leadstock so is not affected by using back gear (other than putting what's happening into slow motion).

You should have the two spindles to mount the idler gears by taking them from what mounted the coupled gears to the gear bracket when the lathe was set up for auto-traverse.  In your photo you need one more idler. Mount them on the bracket meshing the first idler with the leadscrew gear. Follow that by mounting the second idler on the bracket to mesh with the first idler. The idlers need to be of a diameter such that when the bracket is swung upwards the second idler meshes with the headstock spindle gear.

I go for a fairly loose meshing between the gears.

Steve

[sco]

Thanks Steve - I'll keep fiddling / experimenting and hopefully get there!

In the meantime how about telling us what you make with your Cowells 

Simon.

[Twizseven]

As another Cowells owner, I will be watching with interest how you progress.  I have had a collet chuck and collets off Jo, which I have already put to use.  At the moment I am making a pair of Elmer's No 19 (Standby) engines.  They are almost complete, just have the pistons to make and fit to conrod, and the soldering of the air pipe to the bearing and a pretty base.  I am just using the Cowells lathe and Cowells Mill.  I have them set up in my office rather than go down into the workshop.  I have not tried screw-cutting as yet.

On the topic of gear meshing, my dad always cut a strip of newspaper and ran that between the gears, before locking them in place.  I've used the same method on the ML7.

Regards,

Colin

[steve-de24]

Simon,
I've just set up my Cowells to cut a 1mm pitch RH thread. 30T gears on headstock and leadscrew as you said (30/30=1).
I used the 35T and 40T gears as the idler gears but this is not critical as long as there are 2 of them for a RH thread.

Hi Colin.

Steve

[sco]

ok just had a moment of inspiration!

Earlier I tried a single EXTRA idler between the two gears on the bracket in the picture - as I turned the chuck the leadscrew handle turned clockwise and I immediately thought 'wrong way' I must need TWO extra idler gears.  This is because on my other lathe the saddle is moved towards the chuck by turning the leadscrew anti-clockwise - it's going to take a while to get used to this when switching between the two lathes.

It would be nice if the bolts holding the gears on the bracket had an anti rotation feature so you didn't have to counter hold them - I might see if I can achieve this by modding a SHCS.

Simon.

[sco]

Right the issue I've got is that the screwcutting gears have a bore of 3/8ths and the compound gears have a bore of 8mm so I need to make a boss for the screwcutting gears to idle on then I should be in screwcutting business....

Simon.

[steve-de24]

Simon,
Yes, tightening the bolts holding the idler gears to the bracket is a bit fiddly as is remounting the coupled gears when you restore the lathe to auto traverse mode.  For this reason I made a spare bracket with sufficient spacers, sleeves, nuts, bolts and washers so that I don't need to touch the coupled gears when I set up the lathe to cut a thread (and the idler gears can stay attached to the spare bracket which is very convenient if I want to cut the same pitch thread as last time). I made my spare bracket from a piece of 3.3/4 x5/16 x 1 inch aluminium which seems to do the job as well as the steel one.
Steve

[steve-de24]

Simon,
There should be sleeves, spacers etc to do the job, but don't worry they are simple to make.
Steve

[steve-de24]

Simon,
Just found my notes from when I made my sleeves.

Sleeves
overall length = 17mm
12mm dia x 2.5mm thick flange on one end.
Remainder is 9.5mm dia x 14.5mm long
5.1mm dia hole right through (clearance for bolt)

Spacers
12mm outer diameter 
9.6mm inner diameter
8mm long