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
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
