Figuring our the QC gearbox

I've added a picture of the QC gearbox chart on the Fortis lathe i have . Could some of you kind gents explain it to me . I understand on the bottom of the chart bottom row of figures read thous ,have i at least got that right .

This lathe is suppose to be able to cut 72 metric threads and 48 english threads . Thats a lot of variants in the thread world ,all i want is to cut an M10 1.25 thread .

Much appreciate everyone input since I've joined this forum it's been great . (i can't help that I'm an information chunky )

Sean

Hello Sean,

​The gearbox is a perfectly standard imperial screw cutting gearbox covering the whole range of 4 tpi to 224 tpi and it works as follows.

​The selector A B C is the handle on top, the dimples will be stamped accordingly, the sliding ramp on the face of the box picks of the internal gearing to vary the pitch the gearbox will deliver. The sliding gear referred to halves the input shaft speed to the gearbox when engaged to give the range of finer pitches. there will be 48 imperial pitches possible

​The Metric series are possible only from changes to the external change wheels, there should be a chart for the various outcomes inside the change wheel cover. There should also be a combination wheel of 100/127 teeth in the set of change wheels to bring about the necessary translation of imperial to metric threading

I hope that helps somewhat
Brian

Sean, i was wanting to cut some metric threads on my southbend . The 127/100 compound gear costs anything up to over £100 for boxford /southbends .

A kind American sent me a metric calculator spreadsheet which shows what effect changing the various change gears has. Also shows the pitch errors. I found i could cut 0.5mm, 1.0mm, 2.0mm and a few others ,just by changing my 56t drive gear(one attached to the gearbox ) with a 79T change gear. Was only £20 compared to the costly 127/100 gear.

Im a novice myself to screw cutting and found it relatively easy to follow.

I dont know how or if there is a similar spreadsheet that can be used for the fortis but other may know, and maybe this one will work.

Hope this helps

Edited By mark smith 20 on 03/12/2016 11:34:46

Looking at the fortis gears photos on lathes.co.uk website it doesnt have a banjo so im not sure how you could do this on yours ,others maybe know more.

Depending on the application, ie only a handful of turns for a fixing screw you may be able to get away with a 20tpi thread as 'nearly' the same.
Have you tried any imperial threads yet? You should start by setting up for say 16 tpi and using a felt tip in the toolpost to draw a line on a bar as if cutting. Then measure over a couple of inches and see if it is right. That will show if you have the ABC selector in the right position and most importantly the changewheels between the spindle and gearbox right.

If you get that sorted then photo the changewheels and list the gears you have we may be able to come up with a better approximation.

Hi Mark ,i believe my fortis does have a banjo,i'll try and find a photo of it .

Sean