Change gears for 9mm lead, three-start thread?

[Cenedd]

My son picked up a couple of empty water bottles and was talking about stacking them on top of each other like an hour glass. Naturally I thought it'd be dead easy to thread short length of tube so that they'd screw together. I found a post about threads for cola bottles and identified the thread from there: https://www.bevtech.org/assets/Threa...waterchart.pdf
It seems that these are the 2nd one, the 30/25 tall. It's a three-start thread but I can't find a lot more detail than that. The pitch seems to be 3mm so I figure from that the lead is 9mm. Is this even possible on my lathe? I was thinking of something soft like acetal or some butter-soft cast aluminium tube (dimensions permitting) so nothing to strenuous. A 60° triangle is not the correct threadform but looks like it would do for such a non-critical thread.

The leadscrew is 2mm pitch and I have the following change gears: 30, 35, 40, 42, 42, 45, 49, 50, 50, 55, 56, 60, 70, 80, 100, 100, 120, 120, 127
The two 42 tooth gears must (?) go on the drive and rear of A points since the distance is not adjustable.

I've got as far as working out that I think I need a 4.5 ratio (9:2) but unless I start cutting new gears (I've been looking for an excuse but not sure this quite cuts it!) I think I'm in to compound setups and I'm not sure where I'm going from that point. Thanks in advance guys.

[Peter.]

EDIT: gotta re-think what I just wrote. It made sense at first but now I see it's wrong.

2nd edit: the 40/45 will turn your 2mm leadscrew pitch into 2.25mm and then a 30/120 will quadruple it for 9mm lead.

[Bented]

The lead multiplied by the number of thread starts, therefor a 2MM lead 3 start thread will be cut at 6MM lead starting at 120 degrees apart.

[old mart]

I would set the pitch to 9mm and cut one thread with the compound parallel with the spindle. Then the second thread having moved the compound 3mm, and the third with the compound another 3mm. Does that make sense?
Hopefully, the lathe instructions will have all the pitches available.

[Peter.]

You might want to make a winding handle for the leadscrew. You won't be able to cut such a short, high-helix thread under power.

[Cenedd]

EDIT: gotta re-think what I just wrote. It made sense at first but now I see it's wrong.

2nd edit: the 40/45 will turn your 2mm leadscrew pitch into 2.25mm and then a 30/120 will quadruple it for 9mm lead.

Thanks for that. Will have a play and see if that fits on the quadrant. Worst case it gives me an idea how I'm trying to do it.

Bented: it seems to be roughly (measured with a rule) 3mm pitch so that ought to be 9mm lead.

Old mart: that's what I figured. Meant to include that in the post so thanks for verifying that. Bonus is that my compound dials are fairly accurate - carriage dial is way off... suspect a bad conversion from an original imperial design.

Peter: I do want to make one of those but I'm fairly full on projects to make tools to be able to do the "quick and simple" thing I originally planned to do....if you know what I mean. I was hoping that with a soft enough material it would work....but I guess I'll find out one way or another easy enough!

[Corbettprime]

Thanks for that. Will have a play and see if that fits on the quadrant. Worst case it gives me an idea how I'm trying to do it.

Bented: it seems to be roughly (measured with a rule) 3mm pitch so that ought to be 9mm lead.

Old mart: that's what I figured. Meant to include that in the post so thanks for verifying that. Bonus is that my compound dials are fairly accurate - carriage dial is way off... suspect a bad conversion from an original imperial design.

Peter: I do want to make one of those but I'm fairly full on projects to make tools to be able to do the "quick and simple" thing I originally planned to do....if you know what I mean. I was hoping that with a soft enough material it would work....but I guess I'll find out one way or another easy enough!

You can buy pop bottle couplers on Amazon for less than a buck each, a dozen for 8 bucks. Build or buy?

[old mart]

If the threads are plastic, I would guess they are rounded top and bottom, and a radiused grooving tool would be better than a triangular one. Or an old type threading tool with the tip rounded. Unless you can cut a rounded profile on the inner part of your thread, a slightly bigger starting bore would be better.

[Cenedd]

You can buy pop bottle couplers on Amazon for less than a buck each, a dozen for 8 bucks. Build or buy?

Never thought someone would already be selling something for what I thought was such a weird purpose. Different thread of course - carbonated drinks seem to use a CSD thread. Took me far longer that it should to realise that stood for Carbonated Soft Drink! I've obviously got water bottles that have a "flat water" thread - has more threads and doesn't have the vents. Probably easier to change bottles and buy some....but I may now have the mental equivalent of an ear-worm. Not sure I can let it go that easily! :-D

Old Mart: they seem to be flat on the bottom and rounded on the top. I know someone else found a normal triangular tool worked well enough for the CSD thread so I was hoping it may be the case for this one too. Might well have a go and see what happens if I can get the change gears right. Worth a play for interest's sake I think - when else am I going to get to cut a 9mm lead thread?!

[old mart]

Yes, go for it, its all good fun and possibly even educational.

[BCRider]

You might want to make a winding handle for the leadscrew. You won't be able to cut such a short, high-helix thread under power.

Why not? Even if cutting in metal instead of plastic it's still just a thread with something similar to an acme form but rounded. It's simply that the advance is so large per turn.

Mind you because of this I'd want to run in one of the back gear speeds for sure.

And the helix angle is so high on something like this that it will certainly need to be a specially ground tool bit. You'll want to figure out what that angle is using a triangle diagram with the ID's circumference on the base and the 9mm pitch lead on the vertical. The angle you need is the one opposite the 9mm vertical angle. You'll want to sort of guess at the ID diameter for this or use the OD of the bottle threads as your groove diameter for the coupler.

If that is your lathe in the pictures above and not something ripped off the web I'm thinking that you'll want to mark 3 equally spaced gear teeth on the last gear that drives the lead screw and one tooth gap on the gear that drives the lead screw gear. Leave the drive engaged the whole time and instead lift the gears out of engagement and rotate the lead screw to the next mark in each case. At least I think that will work to give you the three equally spaced entries. And if that worked it would certainly ensure equally spaced starts.

[Cenedd]

Would it explain it all if I said ”Back gears?” ?!
I've got nothing to gear it down, just variable speed from a 1000W brushless DC motor.

I was thinking that advancing the compound (assuming set parallel to the ways) by the pitch and keeping the half-nut engaged (don't have a thread dial) would take care of the three starts. Ensuring any backlash taken up before the tool engages, of course.

I think the thread form is somewhat akin to a butress thread in that it's flat on one side and curved on the other but I was thinking (perhaps incorrectly) that it would probably tolerate the thread being symmetrical so I can thread all the way through a tube and be able to screw in from either side; rather than having to thread to the middle of the tube and then flip the part round to thread the other side. Whether the triangular form is a close enough approximation to the actual to fit at all, is the question but even if cut too deep so as to fit and it is enough to pull the bottle necks against a flat washer, that'd do.

[Peter.]

Why not? Even if cutting in metal instead of plastic it's still just a thread with something similar to an acme form but rounded. It's simply that the advance is so large per turn.

The bottle thread shown in the pic is only 2/3 of a turn long and the pics plainly shows no back gear on that lathe. Even now knowing it's a through-thread he's planning to make it's still a tall order.

[old mart]

I have made a winding handle for the lathe which I use which has a lowest back gear speed of 30rpm.

[Cenedd]

You might want to make a winding handle for the leadscrew. You won't be able to cut such a short, high-helix thread under power.

Well the thing I'm blessed with here is blissful ignorance: Nobody's told me it's impossible so I'll give it a try and find out!
One thing I have worked out is that it may not be about what the lathe can cut but simply how fast the carriage will move. It looks like it'll be about 15mm/second at lowest speed of 100rpm (no back gears). A through-thread won't handle this as I'd end up threading my spindle but threading away from the chuck might deal with it. Or at worst I make a hand crank.

EDIT: gotta re-think what I just wrote. It made sense at first but now I see it's wrong.

2nd edit: the 40/45 will turn your 2mm leadscrew pitch into 2.25mm and then a 30/120 will quadruple it for 9mm lead.

Had a go but those don't fit on my quadrant. Or rather they do but it's not possible to get the A gear to meet the B/C gear. Tried with a different combination - now that I see method - and that would fit but only if I cut off the end of the quadrant. I think it might technically be a banjo rather than a quadrant....but I'm not planning to attack it as a means to this particular end. I did look at the possibility of mounting an idler between the A and B/C gears but I think it may be possible to do using this combination:

A - B/C - D
60 - 90/40 - 120

I don't, of course have a 90 tooth gear but that can be purchased as a spare part for the SC6 lathe (one larger) and is the same width, bore, key and module as the SC4. I could also do 40 - 60/40 - 120 which I think would fit as well but the extra 40 isn't as easily obtainable. I'll have a measure up and see if it'll fit with the 90. Thanks for getting me started Peter. Will follow up on this post as (and if!) I make any progress.