Missing threads

Hello from NZ.

I have a metric lathe, in all essentials a Warco BH600. This lathe has a metric lead screw, 3mm pitch , and a small chart explaining the threads that can be cut. All good, most of the time I cut imperial threads, so leave the half nuts engaged and reverse the drive.

It just so happened the other day I needed to cut a LH tread at 3.5mm pitch, and I can't set the lathe for this. I can't do 1.75 either! (LH no problem, just the pitch is missing from the chart)

I did manage to "fudge" the 3.5mm by fiddling with the change-wheels and gearbox, I think 40T-120T-46T and A-5 on the gearbox: just a shade under 3.5mm, close enough for what I needed.

Is there any fix for this, additional gear sets etc, or do I just avoid what I can't cut?

The way to think of this is you need a speed increase of 3.5/3.0 part of which might be in the QCGB. That is 7/6.
So you start multiplying top and bottom by the same number and see if anything matches your available gears.
ie 14/12, no, 21/18, no, 28/24, no, 35/30, no, etc. 56/48 maybe you have one of these two so you only need to make/buy the other but you can also use the gearbox to provide a reduction if necessary to keep the tooth count and hence cost down.

Thanks, that is what I have done.

I now have virtually a full compliment of metric pitches from 0.25 to 7.5, and imperial from 3 3/4 to 112tpi. I have "gained" 7 more threads not listed in my manual.

I clocked out the qcgb ratios , and plugged all the data + the change wheels I have into a spreadsheet. It seems strange that the charts on the lathe have so much missing data.

I thought someone on this site might have come across this already.

> I thought someone on this site might have come across this already.

They have, but they are all asleep in the day

The tables that come with lathes tend only to cover the 'preferred' sizes of thread and 3.5 9for example) is one of the second preference series.

It is usually possible to cut such an abundance of threads it isn't practical to list them all on a small chart.

Some useful data here:

en.wikipedia.org/wiki/ISO_metric_screw_thread

Neil

It is not only new lathes that do this. The Drummond and Myford charts often miss out 22tpi even though the gears supplied can do it and it is used on the lathe itself for several items.

has anyone heard of a free program called NTHREADSP?

you can get it here**LINK**

just fill in the details and it gives a list of possible combinations.

I use it because my chester comet has a chart that only works in theory, in practice half the combo.s don't fit.

Roy.

I thought possible the massive time delay might account for it...

I'm going to go through all the combinations with a DTI and map what actually works.

I have found some errors in the manual and on the charts.

My Willson slant bed goes against convention by listing 0.625mm pitch on the gearbox chart, not a common thread so far as I know. It is, of course, an imperial lathe but a useful range of metric threads is instantly available and it has true conversion via a 127/125 pair.

The chart lists some imperial oddities also, such as 4 3/4 tpi, and I think the designers just "did the maths" and put the results in what would otherwise have been blank spaces!

There are many ways to skin a TPI cat and most lathe producers only had common gears and threads to cater for

Nowadays computers can give us an answer to multi variable questions which were too much for pre-80s resources

I recently queried my computer to find a 65 tooth or less 25.4 TPI solution for my Drummond (1mm metric) with 19-65 tooth gears and got 25.399991

22 33/52 61/52 52
22 61/52 33/52 52

The problem is while the maths works, the physical gears don't fit properly unless you adapt your banjo, idlers would need to be used

The program mentioned above is very good

Edited By Ady1 on 16/08/2015 11:57:15

> I recently queried my computer to find a 65 tooth or less 25.4 TPI solution for my Drummond (1mm metric) with 19-65 tooth gears and got 25.399991

There are those who would worry about such an error