There is one on lathes.co.uk. 2 in fact. One for lathes without a gearbox and another for ones with. There is only one other I am aware of but it's for metric threads.

They might not suite your needs though. They both work on the basis of entering the gears you have about you and then the thread pitch that you want to cut and they will calculate the nearest you can get. I vaguely remember some quirks that were fixed by playing with the accuracy needed entry but might be thinking about some other software,

There is a book on the archive that goes through how gears for a specific thread are calculated. It's here and you can download a pdf, or djvu if you click on show all formats.

**LINK**

John

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If your lead screw is 8 tpi and you want to cut 8 tpi

1/8 x 8 = 1

1/8 x 8 = 1

so any 2 gears with the same tooth count will do it.

I'd better add that the idler in between can have any number of teeth.

John

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Edited By John W1 on 17/10/2015 19:49:51

This is the online changegear calculator I use:

**LINK**

It's the best and simplest I have found. Works for either metric or imperial.

problem i am having is cutting an 8tpi thread as can't get gearing right.

This is exactly why I suggest that you gain a basic understanding of setting up a lathe for screw cutting – it is not rocket science

The best book by far is the Workshop practice book on screw cutting by Martin Cleeve

I doubt if it actually is 13 tpi – more likely to be a metric lead screw with 2mm pitch

There's a manual for this lathe on the web:

I'm afraid I've been playing with my spreadsheets again!

The 16 tpi figures given above agree with this table. That rather suggests that the leadscrew is 3mm pitch. If so, the actual threads work out as follows:

The top half is metric and the bottom half imperial. The pink columns are the threads you're aiming for and the green columns shows the actual threads cut. There is clearly a typo in the 40 tpi row. The closest you can get to 8tpi with the gears you have is shown in the last row of my spreadsheet (if you can get the gears to mesh). Given the errors in the other threads you'll have to decide if this is close enough for you.

As an aside, if I bought a metric lathe I would accept that the imperial threads might be an approximation (or vice versa) but I think I would be a bit disappointed that neither were exact (just for the sake of a few extra plastic gearwheels). Or have I made a grave error in my calculations? .

HTH

Rod

It looks like you have a setting for 16 tpi A,B,C & D are 60,48,50,27 so if you change the 48 to a 24 you will get 8 tpi. Or some approximation as I think this one has always been metric.

Looking at the manual the gear behind A isn't the same size as the one on the end of the spindle. Makes thing tough to work out.

If you post a photo of the screw cutting chart that came with the lathe, add any other gears that come with it and count the teeth on gear on the end of the spindle and the one that meshes with it behind A it should be possible to work out what to put in the screw cutting calculations. A photo showing the gear set up that meshes with the spindle might help too in case it's very different to the manual.

The problem might be that on what I call real lathes the tooth count on the spindle is usually the same as the one that meshes with it, often via tumbler reverse that is running as an idler. If it's arranged as shown in the manual the lead screw pitch needs to be multiplied by a factor otherwise the sums wont work out.

John

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