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Old 01-31-2010, 06:02 AM   #1
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Default Multi-lead threads

I had an odd project to work on this evening. I have an older pair of binoculars, not anything very special, but ones I liked. The had gotten knocked off the shelf some time ago, and I had not realized that the focus adjusting screw had broken off. As it turns out, it was just a die-cast piece that was full of cracks, rather poorly made.

I decided that it wouldn't be too much trouble to make a new one. Further inspection revealed that this wouldn't be as easy as it looked. The adjusting screw is a 24 pitch, .250 lead. That's a six start thread, and left-hand. I turned up a new screw from 5/8" aluminum round, and got started on the head scratching.

The .250 lead isn't hard to set up, four TPI on the quick change. The problem here is getting enough clearance on the tool to avoid dragging with the high helix angle.

The six starts is a little more involved. You can manage up to four starts on a lathe like mine by using the threading dial, or by "slipping" teeth on the compound gear in the end-gear train.
The compound gear has two gears joined together, on my lathe they are 16 and 32 teeth which doesn't do me much good. If this had been a eight start, for example, I would have been able to divide the compound gear teeth by eight, disengage the gears and skip that many teeth, then re-engage the gears.

Using the lathe dog and faceplate was out, as I only have two slots in mine.

After a bit of thought, I remembered my hex collet block. By holding the hex block in the three jaw chuck and using the threading dial to cut two starts, I could then index the part one flat on the hex, do two more starts, index once more and finish the last two. Seemed reasonable.

Here's the set-up


The first pass at .005 depth of cut, just to make sure, then cut the first lead.


Checks out at .250 lead, so far so good. To get a two start thread, I had to make a mark on my threading dial. I used the numbered lines for the first start, and the red line for the second start.


Second lead started, right down the middle!


Second lead finished.


Now, I loosened the chuck, and rotated the collet block one flat in the three jaw. The next pass is back to using the numbered lines on the threading dial.

Third lead done.


Then another using the red line.

Fourth lead done.


Rotate the collet block one more flat, and back to the numbered lines on the dial.

Fifth lead done.


Final lead was cut using the red line again.

Finished thread next to the broken adjusting screw.


A little bit of work to finish, and here we are.


The threading tool in the first pic is obviously a brazed carbide tool. It didn't work out for this job, as there wasn't anywhere near enough side clearance. I switched to a HSS tool, and as you can see in the later pics, it has a chip breaker ground into it. This tool was made for a right hand thread, but I decided to try it anyway. It worked, but rolled a burr over the crest of the thread. The rather strange appearance of the thread in the next-to-last pic is a result of this burr. A quick swipe with a small three-cornered file took care of that.

I don't often need to do odd threads like this one, and a lot of the little details get overlooked. There were a few things that I should have spent more time on to have gotten a better part, but in the end, it functions just as it should.

Kevin



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Old 01-31-2010, 06:13 AM   #2
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Default Re: Multi-lead threads

Nice!
I've only done a 4 lead and have never attempted anything more.
You have done a nice job!


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Old 01-31-2010, 07:19 AM   #3
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Default Re: Multi-lead threads

Very ingenious Kevin. It is marvelous what ideas people can come up with when they need to solve a solution like that.

That must have been pushing your gear train to the limits, breakage wise.

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Old 01-31-2010, 02:00 PM   #4
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Default Re: Multi-lead threads

I wish I had a three jaw that I could rotate stock in the chuck and cut threads.
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Old 01-31-2010, 04:53 PM   #5
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Default Re: Multi-lead threads

Thanks guys.

Blogs, yes, 4 tpi is the limit I think. Actually both in the quick change and the strength of the gear train. I usually run the spindle as fast as my reflexes allow when threading, but threading this part required the slowest speed in back-gear. The end-gearing was complaining a bit.

This lathe, being an Atlas, had relatively weak gearing in it to begin with. When I was first starting into machining, I learned on this lathe. I managed to bind the leadscrew bad enough to destroy several of the end gears along with the quadrant they are mounted on. The resulting rebuild included a steel quadrant and aluminum gears instead of the dreaded Zamac parts. Those gears are about worn out now.

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Old 02-01-2010, 12:53 AM   #6
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Default Re: Multi-lead threads

Great way of solving the problem, Kevin.
4tpi kind of kept you on the edge of your seat, didn't it? The carriage must have been moving right along, even with the slowest speed on the Atlas.

Good job of it, and thanks for the show 'n tell.

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Old 02-01-2010, 02:22 PM   #7
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Default Re: Multi-lead threads

As I was reading your initial text I wondered how you were going to index the collet block and get your start in the right spot but then as I followed along I realized that you were using the center as a stop. As Dean said, the carriage was moving quite rapidly. I had to cut a 4 pitch thread this past summer and even with the lathe in back gear it was moving along to the point that you didn't want to have a lapse in concentration.
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Old 02-01-2010, 04:19 PM   #8
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Default Re: Multi-lead threads

Kevin,

It was on my Atlas 10F that I actually managed to get it set up to cut 2.5 TPI threads. I had just completed the job, then all of a sudden, two of the bolts that hold the gears shed their heads. Luckily nothing else was damaged, or so I thought, but the next time I cut some normal threads, the quadrant cracked into two pieces.
If I remember rightly, it was these multistart columns that did the dastardly deed.



Lessons were learned that day. I only wish I had the room to fit a leadscrew handle on my new lathe, as that is really the only safe way to cut really low TPI threads.


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Old 02-02-2010, 04:07 AM   #9
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Default Re: Multi-lead threads

The carriage speed is plenty fast enough to cause grief, both for me and the lathe! As it was, I bumped the center once or twice, definitely got to keep your head in it.

George, yes the center kept things where they needed to be, but I still had some worries about getting the collet block to locate correctly.

Blogs, nice looking job on the columns! I have been thinking about setting up my dividing head with some gearing to do this type of work. I have some 60 degree form cutters that should do just fine.

Kevin

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Old 02-02-2010, 04:43 AM   #10
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Default Re: Multi-lead threads

Hi Guys,

If I get the "Nervous Nellies," in such a situation, I turn off the power and rotate the chuck with the chuck key.

Stops the undies joining the army.

Best Regards
Bob



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