Threading Gear ratios and other what nots..

[NeoTech]

yeah.. its me again.. and im stupid. *so thats done*

As you are all aware now by now is that im learning threading.. and now have gone through the usual metric and imperial i can do on my lathe with a norton gear box.
So i figured out its a metric pitch of 3mm on my leadscrew, when cutting a 1.5mm thread i do 60/120/60 gearing and some tinkering with the gearbox and voila.. threads. cool.

So i in my eternal wisdom went and looking for Aciera F3 mill.. and found out that Schaublin/Aciera is.. well evil, and made the 19.7 x 1.667 buttress thread on all the collets.
also known as W20..  so i thought well i just buy new collets.. but nooo i has to make em or i will go broke just buying one of those collets..

Heres the problem i guess.. i can make a collets in my lathe.. but how the hell do i cut the 19.7 x 1.667 buttress thread.. ive figured out a 45 x 5 form tool is needed, and it wasnt
that hard to grind one from a hss blank and thread a 1.5mm thread with it.. But now i need to figure out how i regear my lathe. I have bunch of gears with the late thats good.
But how do i do the calculation? i have googled skimmed through a bunch of books but i cant really find a practical example. And im a really practical person... Math theory isnt
on my highest list over stuff im great at.

[Pete.]

You don't strictly NEED a form tool for cutting those threads, you can set one edge of the tool to the angle of one flank and in-feed the tool at the other flank angle via the compound.

What lathe do you have?

[mhh]

If you can make the collets in the lathe...... Why not make a new drawbar aswell?

But if you do, remember to take pictures! Always nice to see what other people make!

 

[DMIOM]

Neo,

You could have a look at the Electronic Lead Screw which apparently allows free choice of thread (no connection, and I haven't needed to build one yet either!)

Dave

[BillTodd]

Your lead screw is 3mm you need a 1.66666666666666666666666 mm pitch thread,  so that's a ratio of 1.8:1 - i.e. 1.8 turns of spindle to one turn of the lead screw.   To avoid any maths just swap the ratios into teeth e.g. a  10Tgear on the lead screw would require an 18T  on the lead screw.  10T & 18T are too small, so multiply both by say 3 and you get 30T and 54T

i.e  by pass the box, if you can, and use 30T driving a 54T (or any similar combination) . If you can't bypass the box, set it to 1.5mm and use 30T driving 28T ( or 60:54 etc.)

Check pitch before cutting ;)

Bill

[NeoTech]

This is why i love this forum. ;)

Pete, i run a my Optimum D320x920 lathe, its a metric lathe - "gunsmith" lathe or what they are usually refered to.

Bill Todd, how did you do the math for that?! Im interested in being able to calculate that myself next time i find myself doing something that is waaaay over my knowledge and skill leve.. =)

 

[rhankey]

NeoTech, If your leadscrew has a 3mm pitch, and you need a 1.6666667 pitch, the ratio you need is computed simply by dividing the two pitches - e.g. 3 / 1.666666667 = 1.8.  Then all you need to do is figure out what combination of gears you have that will arrive at 1.8.  In this case a 54 / 30 = 1.8.

[BillTodd]

Bill Todd, how did you do the math for that?

No maths involved  just the ability to use a calculator

lead screw pitch = 3 
you need  1.667 (which is almost certainly a rounded fraction) hence why I said 1.6666666666......

3 divided by 1.66666...... is 1.8 

So, each turn of the spindle moves the lead-screw 1/1.8 turns, slightly more than a half-a-turn - It's always work having a rough estimate in you head as it can save errors later) 

if we multiply the pair to get nice whole numbers we get 10/18  i.e. 10T on the spindle and 18T on the lead screw (remember we needed something like half speed at the lead screw so we know the gears are the right way 'round) ,  but that's a very small gear so multiply the pair by 3 or 4 or what ever to get a sensible sized pair of gears (The Idler gear doesn't matter so just pick one big enough to fill the gap between the others)

Bill

[NeoTech]

Well, and its seems like i cant disengage the norton gearbox.. so wouldnt that need to be taken into account in that formula then.. if i set it to like 1.5 that is usually 60/120/60..

[BillTodd]

If you can't disengage the threading gearbox then just set it to do a 1.5mm pitch, then calculate the change gears as if you have a 1.5mm pitch  lead-screw

1.5 / 1.666666...  = 0.9 = just under 1, so the "lead screw" (input to the gearbox)  is going to be going faster than the spindle.

multiply by ten  to get whole numbers = 10T on the spindle 9T on the lead screw

multiply by, say, 4  = 40T on the spindle and 36T on the lead screw

Bill

[NeoTech]

Ah that was the missing part.. the gearbox would act as the lead screw pitch. cool, yeah will have to try.. Gonna go get an Aciera machine tomorrow.. and collets need to be made. =)