Metric threading on imperial lathe

[colinbParticipant @colinb]

I know this has been covered a thousand times, but I still haven’t managed to get my head around it, so I’m afraid I have to ask for your help. Mainly, it’s the involvement of a gearbox that’s throwing me.

So, the lathe is a Fortis (=~ Clausing 100 MK3), 8tpi lead screw, with a gearbox like this:

https://www.lathes.co.uk/fortis/img8.jpg

Perhaps you could help me work through how I would set this lathe up to cut a 2mm pitch thread?

I found this table for a different lathe, and understand that “gear box position” means I should set the gearbox lever and tumbler to cut 16tpi and add a translation 127/100 pair in the train. I can’t even work out from the plate above how I would set the gearbox to cut 16tpi!

[bernard towersParticipant @bernardtowers37738]

is it top lever to C and lower lever to the leftmost position and threading lever on the apron engaged thats if you have two levers on the apron then again I could be talking rubbish!!!

[bernard towersParticipant @bernardtowers37738]

By the looks of the lathes.co.uk site you have two levers one for sliding and one for threading!

[colinbParticipant @colinb]

What is sliding when it’s at home?!

[Martin ConnellyParticipant @martinconnelly55370]

I think the sliding gear is the lowermost gear of the gear train under the cover. On the lathes.co.uk website final picture it looks  like it is shown in the “out” position, the larger final gear being driven by the smaller gear on the compound above it. In the “in” position the smaller gear that is just visible through the spokes will be the final gear driven by the larger gear on the compound. It looks like the ratio between in and out positions is 8 so having the sliding gear in the wrong position will give you 8 x 16 = 128tpi.

I think the stud gear will be the one driving the compound gear and the screw gear the one driven by the compound.

Martin C

[colinbParticipant @colinb]

Ok, I’ve worked that one out – the sliding gear is a single.gear in the gear train that can be slid in or out, changing the ratios and thus you that’s accessible.

If 28tpi or lower is desired, it needs to be in. If 32 tpi or higher, it needs to be out.

So if I have a standard gear train, and sliding gear in, then selecting B1 would give me 8tpi. Adding a 127/100 compound transposing gear into the mix would give me a 2.5mm pitch – I think. Yes?

[colinbParticipant @colinb]

Thank Martin, you beat me to it!

[Brian WoodParticipant @brianwood45127]

The gearbox is very likely to have ratios through it that are identical to those used in Myford boxes.

We need a little help from you to check that fact and a list of all the change wheels you have available to set up the outside gearing feeding the gearbox.

Please describe the change wheel set up to cut an 8 pitch thread. I need also to check the internal gearing for the gearbox, and the gearbox controls to achieve that. I think it will be position B for the three way lever on the top and the front slider should be dogged into position 1, but I need that confirmed.

Regards   Brian

[Pete RimmerParticipant @peterimmer30576]

That gearbox is similar in layout to the South Bend lathe which also has the sliding gear but in a different location. I think that you are restricted to only being able to swap out stud gears for your metric translation, because I don’t think that it has a slotted banjo.

How many teeth does your stud gear have as standard? The stud gear is the top one of the pair at the pivot point of the reversing lever.

[Clive FosterParticipant @clivefoster55965]

It would be useful to know how the gearing on the Fortis EM version able to cut both English and Metric threads is arranged.

At first sight the sliding gear arrangement make sit impossible to fit a 127/100 conversion gear in the usual manner because that would normally need to be fitted where the “double idler” that the sliding gear pair engages goes.

Clive

[colinbParticipant @colinb]

Thanks everyone, those have been the pushes I need to get it done

For posterity, how it made sense to me was – ignore actual gear train initially, just work with lead screw tpi and a 127/100 compound pair. I.e.

• Convert tpi to pitch (between teeth) in decimal inches
• Divide by ratio of translation gear (127/100)
• Multiply by 25.4

= metric pitch given by that translation

Here, with an 8tpi lead screw, we get:

((1/8)/(127/100))*25.4 = 2.5mm pitch

So with sliding gear in, lever to B, front slider dogged to 1, and a 127/100 compound gear in the train, the lathe will cut a 2.5mm thread

Turning all this around, you have the equation that lets you see what tpi (lever/dog combination) you need to set the lathe to for it to cut a given metric pitch.

Worked through for a 0.7mm pitch:

1/((0.7/25.4)*(127/100)) = tpi needed =28.5 tpi

C8 is the nearest setting available (=0.714mm)

If I want to get closer to 0.7mm, I need to mess with the gear train – finding one that changes the gear ratio coming into the gearbox such that one of the gearbox ratios spits out “28.5tpi”

 

[colinbParticipant @colinb]

Thanks Clive. Now I’ve got my head a bit better around the theory, I can start to get into the execution!

The lathe is currently dismantled and relatively inaccessible, so that exploration will have to wait I’m afraid!

Thanks again to you all

[colinb]

On 8 June 2024 at 13:49 colinb Said:

So, the lathe is a Fortis (=~ Clausing 100 MK3)

There is some infomration on metric thread cutting with a Fortis here:

https://www.mig-welding.co.uk/forum/threads/metric-threads-imperial-lathe.133152/

[Clive FosterParticipant @clivefoster55965]

DC31k

What a splendid find. Super clear diagrams of how the gearbox is laid out clearly showing why the normal 127/100 compound gear can’t be fitted on the standard drop train banjo. Which is pretty much fixed position anyway with only sufficient shift to set gear mesh.

A possible way of doing things would be to use a monster two arm banjo with a lower arm sticking more or less straight towards the back of the lathe carrying the twin gear that meshes with the sliding gear. A second arm sticking up in conventional angled back fashion carries the 127/100 compound gear so the 127 meshes with the large driver for the sliding gear and the 100 meshes with the stud gear. Everything still turns in the right direction. Change the stud gear as required to get metric threads out of the box. One stud gear will give you several metric conversions.

The major potential problem with that arrangement is getting the gear alignments correct. The position of the driver for the sliding gear is fixed to align with the stud gear in its standard position. But now the 100 section of the 127/100 conversion gear needs to align with the sliding gear driver so the stud gear needs to be a gears width further out to mesh with the 127. Longer stud and double width gears would work but is clunky.

I’m unsure as to whether you’d need a proper slotted banjo. Quite likely that a simple plate with holes correctly positioned for the 127/100 and sliding gear driver support studs would work fine.

Clive

[colinbParticipant @colinb]

DC31k, fantastic find, a thousand thank yous!

I’m surprised I hadn’t stumbled across that thread actually, since this lathe in another mig-welding thread started by GraemeVW is in fact the one now in my possession!

Apparently it was bought in the post war years by a chap who was an engineer by trade, and used by him until basically 2020, when his health took a COVID driven turn, and it came to the gentleman from whom I bought it.

I’m delighted to have it – not just as it’s in very good nick, but because it also came with a ridiculous amount of accessories – gears, two faceplates, three chucks, fixed steady, drive dogs, two milling attachments, inside/outside jaws, live centre, drill chucks, and an Archer No.2 tapping head.

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