Your lathe is old enough to be from before metric was invented but just measure the number of threads (peaks) over as many inches as you can be bothered to, Say you hold a ruler up to the leadscrew and count over 5 inches and get 40 then it is 40/5 or 8tpi.

Remember it is going to be a round figure in imperial like 4 or 8.

You want to do it over as long a distance as possible because a 3mm pitch is close to 1/8 in so you need some care to be accurate enough to tell the difference – don't lose count.!

To make it easier you can mark the bed with felt tip and hold a piece of card on the saddle against the leadscrew so that it goes click on each thread as you move it along between the marks.

Edited By Bazyle on 28/12/2016 09:33:50

Good idea to mark each crest with a thin felt marker as you count, [red is good ] so as not to lose count. The finer the pitch the easier it is to lose count. As Bazyle mentioned best over a long distance, 5 / 6 " or so.

Looking at the image of the Fortis gears on Lathes.co.uk there appears to be a small short banjo hidden behind the gear train. There are probably only a few change gears as the Fortis has a full quick change gear box.

It's a bit difficult to see and appears very rusty in the shot. There needs to be some sort of adjustment available I would have thought to change from metric to imperial pitches, just guessing!

Cheers

John

Edited By Journeyman on 28/12/2016 10:11:11

Metric or imperial leadscrew makes no odds at all as to the lathe'ability to cut threads of either pitch range.

Just remember to count the first pitch (at zero distance) as zero, not one. Might be easier to count on in tens, marking each tenth thread. Then carefully measure the total distance before ding any maths. Checking different 'sets' of tens is a better metod as it might well show up if a counting mistake has occurred.

Confirmation is likely by checking the gear tooth counts, along with diameters, and checking for DP or Mod gear tooth form. Most sensible manufacturers would not mix Imperial gears with metric lead screw and vice versa.

'46 threads over 6"' – Do remember the standard minimum error on those two is plus or minus 1. So 46 could be 45 or 4, and 6" could be 5" or 7". 6.0" indicates aminimum error of plus or minus 0.1". Same goes for metric, oc course.

Yet another is to compare the threads cut with the gears employed, if really unable to sort it out any other way, but I doubt this would be of any use if the enquirer could not sort it out by simple accurate measurement.

The 'banjo' could be a simple one with no slot an just a fixed position and angle so only one set of gears could be used to drive the gearbox. Then the improved version might have introduced a better arrangement. The thread plate on the gearbox mentions a sliding gear which appears to provide an 8:1 ratio change. No indication what that is. We need to see just what Sean has then we can suggest improvements.

To return to the OP's question, "how do I find out if the leadscrew is metric or imperial" the answer is "it's whichever comes out to a whole number answer"

Doing the arithmetic:

46 threads in 6 inches is a pitch of 46/6 pitches per inch, which comes out to 7.67 ppi (for which we do have insurance).

Try a bit of metrication. 6 inches is 152.4 mm, so the pitch is now 152.4/46 = 3.31mm per thread

Something wrong here, as neither come out to a "convenient" answer.

Go back to imperial, and measure 47 pitches in 5.9 inches would be darn close to 8TPI.

Playing with the metric numbers, you have to go quite a long way from the original measurement/thread count to get a believable answer, even allowing for the possibility of a half mm pitch number.

It seems from the flavour of the above that we're all expecting the answer to be an imperial lead screw of 8TPI. Of course you can indeed cut either inch or metric threads from any lead screw, given an appropriate selection of change wheels. But that doesn't seem to be a real possibility from the OP.

So to identify a definitive answer to the original question I guess we need a re-measure. If the OP would like to revisit the figures given, checking that he measures the distance from a point on one thread to the exactly similar point on a later one, and counts the number of threads excluding the "zeroth" (yes it is a word) one, then we can help some more.

Seasons wotsits to all

Simon

Of course, if it is 8TPI and you hold a steel rule against the lead screw, the "teeth" of the thread will line up neatly against the 1/8 inch divisions on the ruler, just as they would against an 8 TPI thread gauge.

Edited By Simon Williams 3 on 28/12/2016 20:28:52

That is very interesting and I haven't seen a system like that before. And it is quite a big change looking like 2:1 reduction when out and 4: 1 speed up when in.

I think if you want to cut metric you have to bite the bullet and make a new banjo and a number of new gears – do you know the DP? 14, 18, and 20 are quite common. (number of teeth over diameter). We can help you get the right tooth count if you go this route.

Alternatively there is this system look for the post by Jacques Maurel.

Edited By Bazyle on 28/12/2016 21:30:43

Might be best if you post a pic of a ruler laid alongside the leadscrew. The numbers you posted don't match anything standard.

Best to post the pics to Photobucket or similar and then use the Image icon in the top of the post window here to copy and paste the Direct link onto your post.