Change lead screw to metric?

[tornitore45]

I tried a couple of examples an found you can make some limited metric and imperial pitches but not many.

The change gear set you have seems to have several duplicated prime factors but not enough range to cover all standard.
Teeth Prime Factors
25, 5 5
30, 2 3 5
38, 2 19
40, 2 2 2 5
42, 2 3 7
50, 2 5 5
55, 5 11
60, 2 2 3 5
60, 2 2 3 5
70, 2 5 7
80, 2 2 2 2 5
84, 2 2 3 7
89, 89
100. 2 2 5 5

For example there is no way to make a 10:1 ratio in two stages but is possible in 3 stages. 100/25 50/40 and 60/30
A commonly needed ratio is 1:5 but is not possible in one stage. Two stages requires 1.25 x 4 obtainable with 100/25 and 50/40
Your limited Max and Min No of teeth limits the max ratio you can get in one stage
A 75 teeth and another 25 may be helpful to make 1:3 and 1:4 ratios
30/89 is close enough to 1:3

Metric to metric requires ratios of 2 and 5 7 and 3. You can do 7 and 3 but 5 requires 2 stages
To make a long story short you need to expand the range of gear you need to mount on the banjo.
Making a shaft reduction adapter may let you use gear smaller than 25 teeth and the picture seems to imply a larger than 100 teeth is possible if meshing with a smaller gear.

Just for fun, tell which pitch you could not accommodate and see what is needed for those.

[liveaboard]

Thanks for that; helps clarify.

It looks like a lot of my trouble is the need to reduce the feed...
a 4mm screw might make that easier.

Smaller than 25 would require a lot of effort; bigger than 100 might work sometimes but the cover might not close.

I'm going to put it on the back burner for now, until I go to Lisbon next [could be a while].
I'll be working on my shopping list though.

[liveaboard]

I was trying to do math and my brain was getting more and more kaleidoscopic...
I found a great program for gear calculations.
http://varmintal.com/alath.htm
and download gear vb61 zip file.
You can put in any leadscrew [inch or metric], your own gear set, and ask it to calculate the gears for any thread, inch or metric.
It's not clever enough to know if the selected gears will actually fit together; but it gives you several possible combinations.

So my existing gear set can apparently make just about any thread, with a 4 or a 5mm screw.

[whateg0]

Not sure about the remaining calculations, but you need to omit the transposer gear for metric. That is such a strange setup. Why did they choose to use an imperial screw? There surely were metric screws available at that time. Strange!

Dave

[liveaboard]

The lathe was sold in imperial and metric versions.
For metric, only the top + cross slide screws + dials were changed.
It seems to be quite rare. I once saw one [inches] on ebay UK, and I found a blog with pictures of one in Poland that looks like it was never used, and it has the inch dials.
Since they never used inches in Poland, and this is Polish made, I guess it's a leftover or from the factory museum or something.

But why they chose what they chose? Who knows?
Mine seems to have a 3-phase 180V motor rewound to 380V.
No one has ever heard of 3-phase 180V.

[f350ca]

I have a 16 x 80 lathe imported from Poland by Summit Machine Tools. It has inch pitch lead screws that are metric diameter and trapezoidal metric thread form.

Greg

[liveaboard]

I never actually measured the diameter of the leadscrew closely enough to know whether it's 25mm or 1"; but now I will.
I don't know how to tell the difference in the thread form.

My lathe was built sometime during the crusades.

[f350ca]

I had to make a new lead screw for the compound after a crash. I ground an 29 degree acme cutter, when I check the fit on the bent screw it didn't match. Hum. Did some research and found there was a 30 degree metric version of acme. Ground that and it matched. The new screw fit the nuts perfect.

Greg

[Magicniner]

NthreadP is a calculator which takes your driving gear size. lead screw pitch, your available change gear sizes and calculates the gear possible trains for the thread of your choice.

http://www.lathes.co.uk/latheparts/page14.html

I use it to cut metric and imperial threads on my change gear Myford Super 7 with imperial 8tpi lead screw

[tornitore45]

Calculating the gear train is the easy part. Coming up with a solution that physically fits on the banjo is the hard part.

[Magicniner]

Calculating the gear train is the easy part. Coming up with a solution that physically fits on the banjo is the hard part.

Ah, but nThreadsP gives you lots of options, it gives you all the possible options with your set of gears, it even allows you to choose to allow an acceptable error if you so wish.

[tornitore45]

The thing I hate the most about changing gear is getting all dirty, jiggling the gears to slip over the shaft key and handling all the fiddly parts involved.
I have coded the constraints about the 4 gears involved into four conditions my spreadsheet analyze and report if anyone is not met.
When I get down in the shop I already know which solution fits.
Did I mention I love spreadsheets?