working out screwcutting gears.

now i have sorted hand crank for lathe and proved it works, i need to work out how to cut 8tpi.

lowest listed for lathe is 11 tpi.

i want to make adaptor so i can use southbend bits on it.

i assume i need some sort of idiot proof calculator as not good with computer stuff.

saw thread on here with link to calculator but can't find it.

can cut various multiples of 8 from 16 to 40 if that helps.

cheers

If you look at the gears for 16 tpi, then you are half way there, you just need to half the number of teeth on a driving gear or double the teeth on a driven gear. The choice will depend on the space available. I am assuming that a gearbox is not being used.

Doug

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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Doug sorry if being dense here.

gears listed for 16 tpi are 60,48,50,27.

only gear i have that is half is a 30 if i swap that for the 60 i should get 8tpi?

am i right?

no gearbox.

John will look at them in min thanks.

Doug sorry if being dense here.

gears listed for 16 tpi are 60,48,50,27.

only gear i have that is half is a 30 if i swap that for the 60 i should get 8tpi?

am i right?

no gearbox.

John will look at them in min thanks.

dunno what im doing wrong but tried combination given and just end up with ultra fine threads.

sure computers hate me.

in fact when i put in to cut 20tpi doesn't match what machine lists.

This another way of looking at the problem but they give the example in metric. No matter the number used has to produce 2 gears you have.

Problem with 11 tpi is that the pitch is 0.0909090909090909....................... so to get a whole number it has to multiplied by some factor of 11. Say 44

1/11 * 44 = 4

assuming you have an 8 tpi lead screw

1/8*44=5.5

So you need gears that 4 and 5.5 divide into. So multiply some number that gets rid of the 1/2 tooth and gives a tooth count to match what you have or need to buy. Any even number will get rid of the half so say 6

4*6=24

5.5*6=33

Might have problems with 33 so multiply both by 2 so 48t and 66t. Or multiply by any number that results in whole number.

John

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Edited By John W1 on 17/10/2015 19:37:58

Edited By John W1 on 17/10/2015 19:41:50

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

Naughtyboy,

The ability to screw cut on your type of lathe is a very useful feature indeed. I strongly recommend that you take the time and trouble to fully understand the principles of screw cutting. It is not hard to understand really once you have a go.

I recommend that you get a copy of Using the Small Lathe by L C Mason and refer to chapter 10 which gives as clear an explanation of the principles of screw cutting as you can get.

This is the online changegear calculator I use:

**LINK**

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

Edited By naughtyboy on 18/10/2015 09:23:45

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

What pitch is the lead screw. If you don't know measure it. Or maybe some one will know if you mention the make of the lathe and if it's metric or imperial.

John

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

if i have counted it right it's 13tpi.

also marking out 1" in bar it takes 13 full turns of lead screw to go one end to the other.

its a clarke cl430

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

i have e-mailed clarke to find out for certain but be at least tomorrow before i get answer.

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

if it is same manual that came with it, it is useless and clarke know.

shows pulley set up for completely different type of lathe and as you discovered gears for 20tpi.

and a couple of other errors.

will try that gearing tomorrow though thanks.