Everyday, in every Navy Yard and Commercial Yard, LH double lead threads are cut on valve stems. When I served my apprenticeship we did these threads on manual and NC lathes. FANUC CNC's have specific cycles for doing multi start threads. Some old American lathes had the capability to do multistart threads with the thread chasing dial. The math is easy; your lathe has to be able to generate leads that are multiples of the total TPI. In the case of a 7/8-9 2 start, the lathe needs to be capable of 4.5 tpi threads 180 degrees out of phase.
Thanks, now you have got me looking for multistart threads. My toothpaste brand has one start, my OJ has two, a tub of working hands lotion has three and a jar of mayonnaise has four.yay! I found them all
I did enjoy this video. You did good figuring on this one. Not much is more satisfying than understanding, after a serious struggle. AFTER I saw your compound was parallel, I knew how you were going to get to the midway point.
Lee, you can still use the standard thread cutting procedure (as normal) by adding a .0625" shim on the right side of the threading tool in the tool holder. No need to set the compound slide parallel to the Z axis, leave it at 29.5° as normal.
@@simpleman283 What are you talking about? The thread pitch of an 8 TPI thread is .125", the second thread start is half of that (.0625") for a resulting 16 TPI thread. (1" divided by 8 = .125" and 1" divided by 16 = .0625") How can the square root be applied to this calculation? The angle of the compound has nothing to do with shifting the threading tool to the left by 1/2 of the thread pitch of 8TPI.
@@AerialPhotogGuy What are you talking about? Well, that is a good question. I've been watching these kinds of videos to learn as much as I can, before getting my first machinist tools. I had temporarily forgot the tool stays perpendicular even when the compound is set to an angle. The part about the sq. root was me just thinking about the 30/60/90 triangle. Mr. Lee would soon figure other ways. Like your suggestion & another commenter about choosing a different mark on the half-nut dial, because he already did the hard part by thinking about this problem till he understood. I am a carpenter by trade, but I kind of felt stupid when I realised my mistake here. That is how my learning has always been burned in, by making mistakes. So I count this as a win even though I stuck my foot in my mouth.
@@simpleman283 No problem at all! We all learn by making mistakes. I remember my mistakes better than remember my successes, LOL Machining is fun and I, too was a career carpenter before building my semi-retirement machine shop. Best of luck to you and sorry for the delayed response! 🙂
Several comments allude to a method of cutting multi-start threads without indexing the work or change gears using just the thread dial. A 1947 article from Mechanix Illustrated (online in google books) explains in detail how to do it. As Mr. Lee notes, about the only time you might ever have to cut a multi-start thread would be making a valve stem. Multi-start threads are also useful where you need rapid movement with limited rotation in some mechanism.
I too noticed the threads on a pen and on a glue bottle and their importance in early school years! The cap of a Bick pen could make a nice tight alignment taper or could have turned into a cone clutch on machines in my imagination! YOU and I both must have been BORED in class!! Thanks for the lesson on using the cross slide to find different thread locations for multi threads! I've never thought about using anything but the threading dial and engaging at different positions. Another multi start thread of my early school days was the multi-start thread on the inside of a bicycle hub!
We are in the future, or there is someone out there, video. This has been a life time recurring haunting since ~1966, I always assumed it was designed for super lazy consumers and drives me crazy, like the mold maker or the edges of ends, say in plastic, are not spaced properly so when encountering one it is an ambiguous algebra problem. In other words one has to go into blind man mode to count within a degree or two of the unknown reverse drop downs before being certain that all ready to engage. I've got enough on my mind and get side tracked over a peanut-butter container. I'm a long time electromechanical troubleshooter, even did a left/right lead patent pro se. In an idealistic world they would all drop down at the same position.
Interesting stuff, though I'm having a hard time figuring out where I'd use something like this. I have a vague recolletoin of multi-start being used on lead scerws, or perhaps it was ballscrews, or something like that...
You can also leave the compound at 29 degrees so you're not form-cutting the threads so much, and change where you engage on the threading dial for the second start. If you engage on any number for the first start, then you can start at any half a number for the second start. Or if the first start engages on 1 4 7 or 10 for start 1, then start 2 would start at 2.5, 5.5, 8.5, or 11.5 - if your threading dial has enough graduations on it.
My thought on why on why this style of thread would be used on consumer products is that it is easier and quicker to open and close the container and might equate to more sales because of the convenience factor. 😊
Yeah, I was going to say the same thing - the more starts on the thread, the less you have to "fumble" with the cap to get it to thread on. It really has nothing to do with how far you turn it to tighten the cap - that has to do with the pitch of the thread. You could have the same pitch single start thread, and it would still only take 1/3 of a turn to close the top on the OJ container, but it would be a lot harder to find the start of the threads for the human using it. The machine wouldn't care.
Here is my revised answer: NO, it does not have to be multi start, but it could only engage on half the threads. You could make a nut w/8 TPI & it would go on. A 16 TPI single start nut would not thread on.
