Multi-Start Threading With A Single-Start Tap: Theory and some practice.

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AtticusLudwig

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Oct 2, 2017
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Illinois
Hi everyone, lurker for the last few months, and now I'm finally posting a theory I've been working on since before I got my little metal lathe, or built my wood lathe. Also this is just a text post for tonight, I'm going to try to do some drawings to illustrate some ideas and principles I had, since my turning skills are not so great yet. The process works, but the results aren't pretty yet.

***I have looked around, but haven't seen anyone doing this sort of thing so correct me if I'm wrong. It takes time, and it's failure prone, but it produces real working multi-lead results from a single lead tap.***

So after I first got into fountain pens, I happened upon the Hakase Pen's video, and I was blown away by the lathe, the front facing nature of it, and the way that he can cut threads. I ended up building myself a little front facing wood lathe, and then I decided to tackle the threading.

I heard that with those lathes, the craftsmen can cut triple or even quad start threads when in threading mode. I also looked into buying multi-start threads which I deemed prohibitively expensive for myself for right now, and making your own multi-lead taps which I want to try, but I was certain I could do it the way that the Hakase craftsman does it.

The theory is this: If I have a tool, which has several teeth, spaced with a pitch of 1mm like you see in the video. If I hold the tool still, and the pen/chuck rotates and moves via a leadscrew and engaged half nut. I can cut double-start threads by using a leadscrew with a pitch of 2mm, triple-start threads with a leadscrew of 3mm, and quad-start threads with a 4mm lead screw.

I did the math, with a shaft that has a 2mm, 3mm, and 4mm acme threads cut into it (and corresponding half nuts) You can cut a myriad of threads at several different leads, starts, and pitches. This includes quad-start threads of .5mm, .75mm, and 1mm pitches.

I don't have a shaft that looks like that, and I've only owned my metal lathe for a total of 5 days, so I'm not quite about ready to machine myself one. But I figured out a different way. If I mount my a regular tap (in my case a M10x1.0) into the tool holder, with one of the flutes presented towards the work (the same orientation as If I were going to tap a hole, but offset so that the piece runs into one side of the tap), and set up the thread cutting gears on my mini lathe as if I were going to cut a 3mm thread. I can cut myself a triple-start 1mm thread.


An example of how I set up the lathe:


The reality:

This really works! I was amazed when it actually worked the way I wanted it too. I'm sitting here right now with a "cap" and "section" which are threaded to tripple start M12x1.0. There's a couple of caveats though, I spent a whole day doing this and I broke more than I produced. I am new though, so that could be a lot of it. I also had to turn my pen blanks down from square (ugh.) which took a lot of time. The acrylic blanks I was using wanted to deflect the tap, and then grab it violently, snapping the acrylic where I had turned it down to diameter.

If you feed too fast you risk butting the tap right into the work piece and clogging up the feed and gears, and if you feed too slow the work wants to grab and snap. I found somewhere between 125-150RPM seemed to work best, that was with my little Grizzly 7x14" so setting feed rate was anything but consistent.

I had the most luck doing interior threading, and I realized that the tap seemed to want feed in on the approach and cut when I reversed out. This seemed counter intuitive, but I can confirm that it worked better for me than cutting on the approach and sliding past on the reverse. For interior threading you do have to use a tap that is smaller than the hole you're threading. I used a M10 tap in a 1/2" hole (Roughly 12.2mm dia. I hate mixing imperial and metric, but I didn't have a big enough metric drill bit, or a small enough metric tap.)

When cutting male threads, the piece in the chuck seemed to want to deflect, until it would catch and try to roll over the tap. If you twisted the tap slightly, so that the cutting face presented to the lathe was the "bottom" of a tap flute instead of the "top" the tap would cut when the lathe was reversed, and simply couldn't roll over the tap that way. This also seemed to be a problem with my machine, the tool post that came with the lathe is problematic at best in terms of holding a tap (which It wasn't quite designed to do) . If I could make a custom holder I would. Or simply some sort of square block, drilled and tapped for a set screw, and bored out to the diameter of the tap.

Also I had issues with backlash on my cross slide, and I have no DRO, so if you have those issues figured out, pulling the tap away from the piece, backing out and then re-setting depth might work a lot better.

It's a bit of a crazy setup, but It does allow me to cut multi-lead taps with just my little Grizzly, and a $9 tap I ordered from McMaster. Tomorrow I'm going to try to complete a kitless pen with the stock I have, and hopefully be able to photograph it and show you in the next couple days.

Best,
Atticus
 
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magpens

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Feb 2, 2011
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Hi Atticus !! . Welcome to IAP !!!

Make a short post on the "Introductions" forum here, and you could win the monthly draw for newcomers !

I find your discussion above VERY interesting. . However it is VERY late here, and my comprehension is VERY slow. . So I will read more thoroughly tomorrow.

I will be following your progress !

Why not make your first kitless pen using standard single-start threads ? .... Just so you can say you "made this pen".

I know you can buy M10 x 1 and M12 x 1.25 which, though not the real McCoy for pens, would at least get you started. Or maybe you are already experienced so some extent with pens.
 
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bmachin

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Jul 28, 2013
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Owensboro, KY
Atticus,

Welcome to the addiction.

On to your theory. You got off to a great start by setting up your change gears for a 3mm thread to get a 3 start 1.0mm thread. At that point I think things went awry. What you need is 3x3.0mm threads starting 120 degrees apart. Using a 1.0mm tap as a thread chaser is not going to get you that. What you really need is a single point threading tool. Skiprat has an excellent tutorial in the library on triple start threading on the metal lathe here:

http://content.penturners.org/library/techniques/skiprats_triple_start_threading.pdf

I'm willing to stand corrected, but I just don't see how what I think you are doing gets you to what you think you are getting.

Anyway, An exterior threading tool is quite easy to make with a little bit of fussiness (assuming you have a grinder). An Interior tool is a little more difficult and may be worth buying.

Get some pvc pipe for practice material. It's cheap and it's big enough that you can see what's happening when you're turning female threads.

Good luck,

Bill
 

bmachin

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Owensboro, KY
On further thought I see how it works. I don’t know why you would want to do it this way given the fixturing and alignment problems involved.

The other practical problem With you method is that in pen making we seldom cut more than five or six threads at a time so 150 rpm isn’t in the cards. As a matter of fact most threading is done by turning the lathe spindle by hand.

FWIW

Bill
Nice theorizing though. Had me fooled for a while.
 

PatrickR

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Apr 8, 2017
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Zionsville, IN
A little like re-creating the wheel. You can cut them with your lathe. I don’t and don’t care to learn how, so I bought taps and dies. Thanks for the video. I enjoy watching craftsmen work. I hope he has an apprentice or two. One of the most impressive things is how these guys don’t measure anything. All by look, feel and matching parts.
 

AtticusLudwig

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Oct 2, 2017
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Location
Illinois
Bill, Thanks for your input. You're correct about feed speeds, I was too fast, and hand feeding it in might be a better idea. I'm going to make a video about the process soon hopefully, because I think my text write-up is not a perfect (or even clear) description of my process.

As to the why: I know there are better, more efficient ways to cut multi-lead threads. This was a thought experiment that turned into a real experiment. I thought the results were definitely worth sharing because it opens up another technique for cutting multi-start threads, and I am always in favor of offering more ways to tackle the same problem, even if those different ways aren't better.

Also, (again I am new to this but) in my experience cutting multi lead threads with a single point cutting tool, I had problems with depth of cut, backlash on my cross slide even after adjusting, and the nagging issue of 1mm = 39.37 thou. Not to mention repeating all of that on an interior thread.

I ended up with wonky threads, cutting them single point. It's totally fair to say that's due to my inexperience, unfamiliarity with workflow, order of operations, and to some extent my machine. However, I found the holding and alignment issues with the tap were easier to fix than the other issues I was having with single point multi-threads.

Thanks for all of your input so far, I'll keep you updated.
-Atticus
 

anthonyd

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Oct 9, 2011
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Ancaster ON Canada
Hi Atticus,

Here is an article from 1947 that covers what you are trying to do as well as many other methods, that is right up your alley. I have an old Logan lathe and it has been fun to try the threading methods in the article.

Tony
 

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Paul in OKC

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Jul 26, 2004
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Oklahoma City, OK, USA.
I like your thinking. As a machinist I have looked at several ways to thread for pens, just haven't chased too many. If you can hold the tap in a tool holder instead of the drill chuck, you could do similar to single point threading by advancing the compound to split the thread. Great idea! Keep it up!
 
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