gadget_lover
Flashaholic
Why would you want a multi-start thread?
I can think of several examples. You would want one when you need to move a nut a long way with each turn. The head of your mini-mag light is one such example. Another use for multi-start threads is when you might want to be able to quickly screw something together, like the cap on a water bottle or a fountain pen.
In the first example (long travel with each turn) you might ask "Why not just use a coarser thread?". The answer is simple. A coarse thread is a deeper thread. A 4 TPI thread is about 1/5 inch deep but a 16 TPI thread is just 1/20th of an inch deep. This is a major concern if you are working with a flashlight tube that has walls that are only 1/8 inch thick.
Can you even use a 4TPI thread on a 3/8 inch shaft? After all, the thread is .2165 deep. Let's do the math!
.375 shaft diameter minus .4330 (.216 off each side) and you end up with a negative number. Try the same with a 4 start lead 16 pitch and you have .375 minus .125 ( .0625 from each side) so there is still 1/4 inch of shaft left for your screw.
In the second example (quickly screwing something on) , having 3 threads ensures that wherever you start from, the threads will engage with a twist of the wrist. This is important because the wrist only rotates about 120 degrees, and fingertips add another 80 or so degrees of rotation. A standard thread will take up to one revolution to find the start of the thread, then at least one more to screw it on.
You can see an example of 'quick screw' in action on your water bottle, jelly jars, milk cartons and some flashlights.
In theory, it's complex. In reality, theory is often wrong.
The theory of a multi-start thread is fairly straightforward. Two or more parallel threads are cut with equal spacing. Because the threads are equally spaced the start of each thread will be evenly distributed too. A two start thread will have two threads that start on opposite sides of the work. A three start thread will have one starting every 120 degrees.
This is a good point to introduce a couple of terms. The LEAD is the number of turns per inch of a single thread, expressed in TPI. A nut will move 1/LEAD inches per turn.
Then there is PITCH, also expressed in TPI. The pitch is based on the distance from the center of one thread to the next. The depth of the thread is determined by the pitch.
A 2 start thread with a lead of 4 ( two 4 TPI threads that are interleaved ) will have a pitch of 8 TPI and the thread depth that is the same as a standard 8 TPI thread.
Your basic mini-mag uses a multistart thread to allow you to quickly focus it with only one hand.
The thread for the mini-mag head is 4 START, 20 TPI pitch, and 5 TPI lead. There is a new thread every 90 degrees, and the head moves 1/5th of an inch with each revolution. The threads are only .6 inches long, so 3 turns will unscrew the head all the way. It goes from totally focused to totally unfocused in less than a turn.
How do you make these magic things?
So how do you make these things? That's a good question. Most often, you single point thread them on a lathe. You can get multi start taps too, but we'll talk about using the lathe.
Obviously you will need to set up your change gears to cut the LEAD, and not the pitch. The major and minor diameters are based on the PITCH since the thread depth will be based on the pitch too. The puzzling part is how to cut the second thread (and subsequent threads) exactly between the path of the first thread.
When I first pondered the multi-start thread, the obvious solution was to use only the cross slide to provide the infeed while threading. That leaves the compound (set parallel to the work) so that I can move the tool over the exact distance of the pitch to pick up the second and third thread. The process is to cut one thread (multiple passes), then move the tool over exactly one pitch using the compound, cut second thread, repeat as necessary. It works, but many machinists hate losing that 29.5 degree infeed using the compound.
Wisdom of the ages
I found an old Popular Mechanics article on Google that talks about multi-start. See end of post for link. The 1947 article shows several popular methods.
The first method is to cut your first thread, then rotate the work 180 degrees in the chuck (or the dog if turning between centers). Then you can cut the second thread. This works but you can lose concentricity as the work has been remounted.
The second method is quite ingenious. It uses a property of the threading dial that did not occur to me before. The threading dial not only shows where to close the half-nuts so that the thread will start at the same spot every time, it also shows indirectly where the thread will start 90 and 180 degrees off.
Here's an example using the Threading Dial table for my 7x10 lathe to cut a 2 start, 12 TPI lead, 24 pitch thread.
First I set the change gears for 24 TPI. Then I check the threading chart for good starting points for a 12 TPI thread. It says 1.3.5.7 which means I can use any of 1, 3 , 5 or 7 and it will start at the same spot every time. I cut the thread as deep as needed for a 24 TPI thread using 1 ( it can be any odd number) on the thread dial because it's easy to remember.
Now it's time for the second thread. What to do?
Since I used odd numbers for the first one, I will use the thread dial number 2 because it is half way between two valid numbers for the first thread. This causes the new thread to be exactly 1/2 revolution off from the first one.
If I were doing a 4 start thread, I'd use the half numbers between the odds for the second, third and fourth threads. My leadscrew is 16 TPI, and while the threading dial has 8 positions marked the half-nuts will close at 16 spots.
Thread: dial
1 : 1
2 : 1.5
3 : 2
4 : 2.5
Another method mentioned in the article is the non intuitive use of taps to cut multi-start threads. The trick is to use it as a multi point tool. You again set the change gears for your desired LEAD, but you use a tap with the desired PITCH. The the tap is mounted parallel to the work and the cutting edge of the tap is centered at the normal tool height. You then cut the threads like you were using a single point tool. You use the threading dial for the LEAD in this use, not the pitch.
Example, an external 2 start 24 TPI pitch with 12 TPI lead: Use a 24 tpi tap (any diameter) with change gears set for 12 TPI. Use the threding dial settings of 1, 3 , 5 or 7 (on my lathe) and the depth for a 24 TPI thread.
A good write-up from 1947 is here:
http://books.google.com/books?id=z94DAAAAMBAJ&pg=PA222&dq=multiple+threads&as_pt=MAGAZINES
Daniel
I can think of several examples. You would want one when you need to move a nut a long way with each turn. The head of your mini-mag light is one such example. Another use for multi-start threads is when you might want to be able to quickly screw something together, like the cap on a water bottle or a fountain pen.
In the first example (long travel with each turn) you might ask "Why not just use a coarser thread?". The answer is simple. A coarse thread is a deeper thread. A 4 TPI thread is about 1/5 inch deep but a 16 TPI thread is just 1/20th of an inch deep. This is a major concern if you are working with a flashlight tube that has walls that are only 1/8 inch thick.
Can you even use a 4TPI thread on a 3/8 inch shaft? After all, the thread is .2165 deep. Let's do the math!
.375 shaft diameter minus .4330 (.216 off each side) and you end up with a negative number. Try the same with a 4 start lead 16 pitch and you have .375 minus .125 ( .0625 from each side) so there is still 1/4 inch of shaft left for your screw.
In the second example (quickly screwing something on) , having 3 threads ensures that wherever you start from, the threads will engage with a twist of the wrist. This is important because the wrist only rotates about 120 degrees, and fingertips add another 80 or so degrees of rotation. A standard thread will take up to one revolution to find the start of the thread, then at least one more to screw it on.
You can see an example of 'quick screw' in action on your water bottle, jelly jars, milk cartons and some flashlights.
In theory, it's complex. In reality, theory is often wrong.
The theory of a multi-start thread is fairly straightforward. Two or more parallel threads are cut with equal spacing. Because the threads are equally spaced the start of each thread will be evenly distributed too. A two start thread will have two threads that start on opposite sides of the work. A three start thread will have one starting every 120 degrees.
This is a good point to introduce a couple of terms. The LEAD is the number of turns per inch of a single thread, expressed in TPI. A nut will move 1/LEAD inches per turn.
Then there is PITCH, also expressed in TPI. The pitch is based on the distance from the center of one thread to the next. The depth of the thread is determined by the pitch.
A 2 start thread with a lead of 4 ( two 4 TPI threads that are interleaved ) will have a pitch of 8 TPI and the thread depth that is the same as a standard 8 TPI thread.
Your basic mini-mag uses a multistart thread to allow you to quickly focus it with only one hand.
The thread for the mini-mag head is 4 START, 20 TPI pitch, and 5 TPI lead. There is a new thread every 90 degrees, and the head moves 1/5th of an inch with each revolution. The threads are only .6 inches long, so 3 turns will unscrew the head all the way. It goes from totally focused to totally unfocused in less than a turn.
How do you make these magic things?
So how do you make these things? That's a good question. Most often, you single point thread them on a lathe. You can get multi start taps too, but we'll talk about using the lathe.
Obviously you will need to set up your change gears to cut the LEAD, and not the pitch. The major and minor diameters are based on the PITCH since the thread depth will be based on the pitch too. The puzzling part is how to cut the second thread (and subsequent threads) exactly between the path of the first thread.
When I first pondered the multi-start thread, the obvious solution was to use only the cross slide to provide the infeed while threading. That leaves the compound (set parallel to the work) so that I can move the tool over the exact distance of the pitch to pick up the second and third thread. The process is to cut one thread (multiple passes), then move the tool over exactly one pitch using the compound, cut second thread, repeat as necessary. It works, but many machinists hate losing that 29.5 degree infeed using the compound.
Wisdom of the ages
I found an old Popular Mechanics article on Google that talks about multi-start. See end of post for link. The 1947 article shows several popular methods.
The first method is to cut your first thread, then rotate the work 180 degrees in the chuck (or the dog if turning between centers). Then you can cut the second thread. This works but you can lose concentricity as the work has been remounted.
The second method is quite ingenious. It uses a property of the threading dial that did not occur to me before. The threading dial not only shows where to close the half-nuts so that the thread will start at the same spot every time, it also shows indirectly where the thread will start 90 and 180 degrees off.
Here's an example using the Threading Dial table for my 7x10 lathe to cut a 2 start, 12 TPI lead, 24 pitch thread.
First I set the change gears for 24 TPI. Then I check the threading chart for good starting points for a 12 TPI thread. It says 1.3.5.7 which means I can use any of 1, 3 , 5 or 7 and it will start at the same spot every time. I cut the thread as deep as needed for a 24 TPI thread using 1 ( it can be any odd number) on the thread dial because it's easy to remember.
Now it's time for the second thread. What to do?
Since I used odd numbers for the first one, I will use the thread dial number 2 because it is half way between two valid numbers for the first thread. This causes the new thread to be exactly 1/2 revolution off from the first one.
If I were doing a 4 start thread, I'd use the half numbers between the odds for the second, third and fourth threads. My leadscrew is 16 TPI, and while the threading dial has 8 positions marked the half-nuts will close at 16 spots.
Thread: dial
1 : 1
2 : 1.5
3 : 2
4 : 2.5
Another method mentioned in the article is the non intuitive use of taps to cut multi-start threads. The trick is to use it as a multi point tool. You again set the change gears for your desired LEAD, but you use a tap with the desired PITCH. The the tap is mounted parallel to the work and the cutting edge of the tap is centered at the normal tool height. You then cut the threads like you were using a single point tool. You use the threading dial for the LEAD in this use, not the pitch.
Example, an external 2 start 24 TPI pitch with 12 TPI lead: Use a 24 tpi tap (any diameter) with change gears set for 12 TPI. Use the threding dial settings of 1, 3 , 5 or 7 (on my lathe) and the depth for a 24 TPI thread.
A good write-up from 1947 is here:
http://books.google.com/books?id=z94DAAAAMBAJ&pg=PA222&dq=multiple+threads&as_pt=MAGAZINES
Daniel
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