Difficulty with gears for some thread pitches. (mini lathe)

I needed a fine but unspecific thread pitch and it appears to be impossible to achieve some combinations of gears because the swinging plate will not allow it. It is annoying me, just after having sussed out the system; the sums are surprisingly straightforward but real life gets in the way.

This must have been a problem for many mini lathe users. Of course there are combinations that are impossible because the larger idler gear can prevent the smaller gear meshing but the swinging idler plate can only swing so far and the straight slot is too short. etc. etc. That prevents otherwise legal combinations.

I guess I could design a replacement to deal with this but my drafting skills are limited and I can see I would end up doing a lot of bodging and filing. (Surely not!!!!)

What's the experience of other mini lathe owners?

Andrew, what do you call fine?

Straight out the box it will do down to 0.4mm pitch which is 63.5tpi which most of us here would class as quite fine.

Have a look here. I made this a while ago, makes life easier.

**LINK**.

Here is another option:

http://mikesworkshop.weebly.com/new-threading-banjo.html

The thread was for the inside of a telescope extension tube. They put fine grooves in the tubes to disperse any light that hits the sides of the tube. So any pitch and depth would be ok, afaik.

Both those alternatives seem attractive but they are only one point fixing. How is rotational strength? I suppose the answer is 'sufficient'

Just in case you haven't spotted it, but the mini-lathe banjo has two clamp nuts that can be adjusted to accommodate the gears.

My mini-lathes double action banjo was one of the features I didn't care for much - it's fiddly to set-up compared with other types, and the second clamp nut is hard to get at from the front. I guess the double clamp arrangement is used on a mini-lathe because it fits neatly into the limited space available, not because it makes life easy!

Aside from that, I don't recall having any difficulty setting up my mini-lathe to cut fine threads, but it is true that the banjo can't take all the combinations of gears that are theoretically possible. This is true of all banjos, not a particular limitation of the mini-lathe.

Dave

Dave

Hello Andrew,

Here are two gear set ups for 0.2 mm pitch threads [127 tpi] which would surely be fine enough with the standard issue gears.

Change wheels A = 30; B = 60; C = 20; D = 80 ---- Pitch equals 0.198 mm with 16 tpi leadscrew

OR A = 20; B = 50; C = 20; D = 60 ---- Pitch equals 0.200 mm with 1.5 mm leadscrew

Regards

Brian

Yeah - I can do the maths but the banjo (far from being a banjo shape) actually gets in the way of many combinations of wheels. The suggested alternatives wouldn't suffer from this, I think.
I'm glad I found this problem at a non critical time as I can think ahead for when I really need one of those impossible pitches.

Edit: So I may even need more than one banjo if I want to do all possible mathematical combinations.

Edited By andrew lyner on 20/01/2019 10:42:55

Hello Andrew,

I don't actually own one of these lathes so I am not aware of all the niceties of the limitations arising from the shape of the change wheel banjo, so I have not actually tested those combinations and offered them as calculated arrangements which you may not have tried.

You may now have found other ways round that

Regards

Brian

Assuming that we are talking about a C3 mini lathe.

If the pitch does not matter, for purely a light baffle, to minimise reflections, just set up as fine as possible.

Presumably, 20:80/20:80 which will give 0.09375mm pitch with a 1.5mm Leadscrew.

If you make two 100T gears, (1 Mod ) and modify the bottom Banjo clamping arrangements slightly, you can get a pitch of 0.06 mm. But the gear cover needs to be modified (hacked about to make clearance for the primary 100T, or removed temporarily )

Howard

Hi Brian

One of those links has a picture of the standard banjo. You can see it takes up a lot of room and it bumps into other bits. There is a protective casting around one of the upstream gears which I think must be to prevent you graunching it when messing about with wheel combinations.
There will be more than one solution for some pitches and, of course, it could require a vast array of wheels to achieve. I must say, I wouldn't have encountered the problem if I had been after a common (M) thread. it just happened that I was looking at the table (an extended one and not the one in the Warco book for the lathe) and I tried a couple that just wouldn't fit. It got me thinking. The 'second' adjusting screw can be a real devil, even for possible combinations.
However, on the whole, I am pretty impressed by just what my mini lathe can do for me. It's the only way I would be getting that facility with the money I want to spend on my DIY stuff. I'd get very frustrated if I was ever working to a schedule, though.