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Turned my first pen....

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raynerd:
Just to clarify something with multiple start feeds, take this scenario:

Two pens are ready to thread the barrel for the barrel/cap thread - one is threaded 36TPI quad lead and the other is threaded 9TPI standard single lead. When I come to screw the cap on the pens, in my mind the cap will go up the thread the same distance per turn (revolution) on both pens, however the quad lead will catch the start of the thread quicker - right? A pen maker I spoke to said that a multi lead thread allows the cap to be turned up the pen further (i.e tightened or released quicker) with less revolutions of the cap. This to me makes no sense as they are both 9TPI but with the quad lead, you just have 4 of them running parallel. Clearly there is something I am not getting. Do the threads on a multi lead thread cross?

The pen maker also said a multi lead thread is "stronger" but I don`t know what he means by that.

Thanks for the posts so far, really informative.

Chris






 

John Hill:

--- Quote from: sbwhart on April 24, 2009, 07:15:31 PM ---That is spot on John  :thumbup:

That is what I was on about using a chaser for I probably didn't explain it well   :scratch:

Cheers

Stew

--- End quote ---

UM, sorry Stew, :bow:  I didnt read your posts carefully enough to realise you were talking about the thing I had in mind.

sbwhart:
No worries John  I was just pleased someone else was on the same wave length as me.

Stew

bogstandard:
Just to put your minds at rest, here is a little info for you to chew over, it might put things straight.

What you are making is a 9tpi thread multiplied by four but at the same depth as a 36 tpi one, all running parallel to each other. So the multistart will travel forwards four times faster than a single cut 36 tpi thread.

This pic explains it a lot better than trying to tell you, it is based on a standard four lead screwthread. See how far forwards it moves for each revolution.



It is for that reason, chasers shown above cannot be used for cutting multistart threads. They will only cut or dress a single thread.

Darren, it can be an absolute pig to do.

The main problem, the smaller the TPI, the faster the cut over a given length is done. Also, this lathe is designed for a min TPI of 12, by making it do 9TPI would grossly overload the gear train when driven by the spindle, and so has a tendency to smash the gears or holding bolts (I have had it happen to me), so you have to modify the lathe so that the drive comes from the leadscrew, usually by fitting a handle onto the end of the leadscrew and turning it by hand. This solves two problems, the first being the stress on the gear train, and the second being, you don't have to have the reactions of a s**thouse rat to to disengage the half nuts before the cutter hits the chuck when doing it under power.

These are columns I made for one of my engines, they are based on a 2.5 TPI, two start thread, using a specially ground cutting tool. Doing these on my large lathe under power, smashed the heads off two 3/8" diameter gear train holding bolts like they were lollipop sticks. So it does happen, if anyone doesn't believe what I am telling them. They are only just over 1/4" diameter, so it wasn't the cutting itself that caused the breakage, it was the stress in the geartrain trying to move the saddle across at high speed to do the cutting.





Bogs

raynerd:
Cheers Bogs - understood. As always, your post is incredibly informative and I now both have a better understanding of multi-lead threads and the capabilities of my lathe. As you have already told me, I am going to have to modify my lathe so that the gears can be cut my hand to reduce stress on the gear train and this shouldn`t be a problem because any threads I cut with low TPI will be on soft materials anyway and will be perfectly OK cutting by hand turning.

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