MasterSpoon
Plastic
- Joined
- Jan 12, 2014
- Location
- Queensland, Australia
Hello
I wish to start by saying that I am not asking if there is an off the shelf item I can buy. I am presenting a design idea. This post will also be quite long..... so sorry about that.
About me and the problem
So I recently purchased a lathe with the intention to learn how to use one, I am not a machinist but love building stuff and making things. I work mostly in Metric so it is important that I can cut metric threads. The lathe that I purchased has a 8 TPI imperial lead screw. Youtube was my friend to teach me how to cut threads on a lathe, problem was that I did as all the video's said and engaged the 1/2 nuts at the same spot on the threading dial each time but it wasn't working. I wondered if I had overlooked something and went back to research. I quickly found that what all the tutorials I had watched failed to mention was that the thread chasing dial doesn't work for metric threads. The general consensus was that to cut metric threads with an imperial lead screw that you just leave the 1/2 nuts engaged, stop the lathe, back the tool out, reverse the lathe, stop the lathe, put the tool back in, run the lathe forward, rinse and repeat. The big problem with this is if your threading to a shoulder, which lets face it, is likely to be most of the time, then you have to hope that the lathe stops in time before the tool crashes into the work. I also found that the finish on the thread was better when running say 250 rpm rather then 50 rpm. I didn't like my chances of the lathe stopping before crashing, so this won't work.
I did find someone talk about a work around / another method, very similar to the leave the half nuts engaged, but is engage it on a number on the threading dial, at the end of the thread disengage the half nuts and stop the lathe straight away. Back the tool out, put the lathe in reverse, get the same number on the threading dial when the lathe is running backwards. Then the rest is the same as the method of keeping the half nuts engaged. This removes the risk of crashing the tool and provided that you stop the lathe straight away so the dial doesn't go around a few times should be good at keeping your spot. While this sounds great I still much prefer the idea of a thread chasing dial.....
In my searches I found references to Metradial metradial
There is a tutorial on this site and a couple of threads about using them. This looks great but also kinda scary, it looks expensive (well too expensive for the home user anyway) and only works with certain lathes. This had me thinking there must be a better way. So I went to the drawing board. The first thing that I needed to understand fully is exactly what a thread chasing dial does and why it doesn't work with metric.
The Math & theory behind a thread chasing dial
My lathe is a China / Import lathe, it had a 8 TPI lead screw. All the math / explanation below will be based on my lathe however will transfer to other lathes as the theory is all the same.
The point of a thread chasing dial is to make sure that the threading tool is in sync with the existing thread that you are making further cuts on. So basically at a set distance the angle of the headstock is correct. It is in effect a counter, on my lathe we know that when 8 threads have gone past a set point that it has traveled 1 inch.
The big difference between a metric thread and an imperial thread is how the thread is defined. An imperial thread is designated by how many turns it has over a 1 inch length where as a metric thread is designated by how long a single thread is. Eg a 3/8-24 imperial thread will have 24 turns over a 1 inch length vs a M10 x 1 will have a thread length of 1mm so 25.4 turns to an inch. On a lathe with an imperial lead screw and thread chasing dial they normally have a gear that has 2 x lead screw TPI. For my 8TPI lead screw that means that my thread chasing dial has 16 teeth on its gear.
How it works for imperial threads is fairly simple. For 1 full rotation of the thread chasing dial (16 teeth) the lead screw has traveled 2 inch, a 1/2 turn (8 teeth) = 1 inch. As imperial threads are defined by how many threads per inch this means that provided the thread being cut is a whole number not a fractional thread then every 1 inch of travel the thread will line up. Remember regardless of what thread pitch you are cutting the thread dial will always tell you the exact same information, eg 1/2 rotation = I have traveled 1 inch, full rotation = I have traveled 2 inch, 1/4 rotation = I have traveled 1/2 inch. The half nuts can be engaged the same amount of times per inch of travel as the lead screw TPI. So for my 8 TPI lead screw it can be engaged 8 times.
Understanding the above about how it works with imperial you start to realize that there can be several points in a rotation of the threading dial to engage it. There is no formula to work out minimum its just brute force calculations. For my 8TPI lead screw / 16 tooth dial gear If it had enough lines I could choose between 2 inch (full turn), 1 inch (half turn), 1/2 inch (quarter turn), 1/4 inch (one eighths turn), 1/8th inch (one sixteenth turn). To work out if you can use that distance is simple, can the thread you are cutting be divided by the distance traveled and come out with a whole number . So lets take 20 TPI for example, 2 inch = 40 teeth, 1 inch = 20, 1/2 inch = 10 teeth, 1/4 inch = 5 teeth, 1/8 inch = 2.5 teeth. This means you would be able to use all by the 1/8th inch / one sixteenth turns of the threading dial. Lets try 24 TPI, 2 inch = 48, 1 inch = 24, 1/2 inch = 12, 1/4 inch = 6, 1/8th inch = 3, this actually means that with a 8 TPI lead screw cutting 24 TPI (or any number that can be divided by 8 and be a whole number) you don't actually need the threading dial, any engagement point will work. Ok one more lets do 9 1/2 TPI, 2 inch = 19, 1 inch = 9.5, 1/2 inch = 4.75, 1/4 inch = 2.375, 1/8 inch = 1.1875, the only whole number is on 2 inch which means cutting this thread you would need to pick a spot and stick to it.
So now that we understand how it works with imperial lets look at metric and why a conventional one will not work. As we know 1 inch = 25.4mm, remember the thread chasing dial will not change what it shows regardless of thread. It is still showing you what distance has passed / how many teeth have gone past on the lead screw. We can establish that for each tooth the distance traveled is 1 inch / TPI = 25.4mm / 8 TPI in my case = 3.175mm per tooth. We can only engage the half nuts on each thread turn, which means we can only engage the 1/2 nuts every 3.175mm.
As the thread chasing dial will only ever tell us the same thing we then need to work out how many rotations of the headstock will get everything lined up again. So for the moment don't think of calculating lead screw rotations, we are calculating headstock rotations. For say a 1mm thread for each tooth / full rotation of the lead screw the head stock will spin 3.175 times. We need a whole number of turns before we can re-engage the 1/2 nuts again to make sure we are picking up the correct threads. In my searches I kept coming across the number 127. Not sure exactly how this came into play I made a quick excel spread sheet, first cell of travel per rotation in mm (3.175) in this case, used excel to populate down that column and add a rotation for each new row. In the next column I had excel divide the distance by the thread pitch as this would tell us total head stock rotations. I noticed that regardless of what thread pitch I put in the first whole number was always 127. I also found that the smaller the thread pitch the fewer turns of the lead screw required. I came up with the formula of
Number of teeth to count on leadscrew = (127 x metric thread pitch) / distance per rotation of lead screw in mm
So lets do a quick example, nice and simple 1mm pitch
number of teeth on leadscrew = (127 x 1mm) / 3.175 = 127 / 3.175 = 40
This means that the headstock has turned 127 times, this has turned the leadscrew 40 times, so 40 x distance per rotation = 127mm. 127 can be divided by 1
What about say 1.25mm
number of teeth = (127 x 1.25mm ) / 3.175 = 158.75 / 3.175 = 50
So headstock turned 127 times, lead screw turned 50 times, 50 x distance per rotation = 158.75 which can be divided by 1.25
Using the above we can see that while a conventional thread chasing dial will not work for metric as its hard to count 40 rotations on a dial wheel that is 16 teeth a rotation, however the theory of counting rotations is still valid. We just need to take a different approach. If your only going to cut 1mm pitch then go nuts and cut a 40 tooth gear, use it 1:1 to a dial and engage on the same number / spot every time and your sweet. But why not get one that will work with ANY thread metric or imperial that you want to cut ?
The Design
If you have been reading this from the start you should have a good idea as to what counting the teeth / rotations of the lead screw can get you and that if you can accurately count teeth then you can chase ANY thread metric or imperial.
This leaves us with what actually ends up being a very cheap and easy thread chasing dial to make. Parts needed are
1 x 10 tooth gear
1 x 10 count / revolution rotary counter, best to get one that will go backwards (anti clockwise results in numbers going down) also and has a reset (*I have not checked if these go backwards* Eg RL Series (small size rotary counter) or RL-219 (large size 5 digit rotary counter) )
and a way to connect the two, something like a flexi shaft would be ideal, maybe a drill or dremel flexi shaft ? If you use a flex shaft then mount the gear on the end, have the gear in mesh with the lead screw, other end connected to the rotary counter and you can mount it somewhere nice and convenient, near the tool post maybe ?
How its used
Pretty simple, set all your gears, do the math using the formula above to calculate how many turns of the lead screw you are counting, as the gear is a 10 tooth gear and the rotary counter is 10 count / rotation it means that the rotary counter will be showing you numerically how many teeth have passed.
Engage the 1/2 nut, hit the reset on the rotary counter to get you back to zero
cut the first pass on your thread then disengage the 1/2 nut
retract your tool and move back to the start of the thread, set your tool back in ready
when the number comes up that you calculated earlier re-engage the 1/2 nut, hit the zero again, cut the second pass
rinse and repeat
If you forget to hit the zero on follow up passes its ok, you just have to do more math, the number on the counter will need to be able to be divided by the number of teeth you calculated earlier. eg 1mm thread can use 40, 80, 120, 160, 200, ect ect on the dial
I know this has been a massive post and thank you for anyone that takes the time to read it. I am in the process of making a digital version using a 600 pulse / rotation rotary encoder and Arduino but is still a while off and had the idea for the mechanical one and thought that I would put all the information up for everyone else as information seemed quite limited and someone else may want to make one.
Thanks
Wayne
I wish to start by saying that I am not asking if there is an off the shelf item I can buy. I am presenting a design idea. This post will also be quite long..... so sorry about that.
About me and the problem
So I recently purchased a lathe with the intention to learn how to use one, I am not a machinist but love building stuff and making things. I work mostly in Metric so it is important that I can cut metric threads. The lathe that I purchased has a 8 TPI imperial lead screw. Youtube was my friend to teach me how to cut threads on a lathe, problem was that I did as all the video's said and engaged the 1/2 nuts at the same spot on the threading dial each time but it wasn't working. I wondered if I had overlooked something and went back to research. I quickly found that what all the tutorials I had watched failed to mention was that the thread chasing dial doesn't work for metric threads. The general consensus was that to cut metric threads with an imperial lead screw that you just leave the 1/2 nuts engaged, stop the lathe, back the tool out, reverse the lathe, stop the lathe, put the tool back in, run the lathe forward, rinse and repeat. The big problem with this is if your threading to a shoulder, which lets face it, is likely to be most of the time, then you have to hope that the lathe stops in time before the tool crashes into the work. I also found that the finish on the thread was better when running say 250 rpm rather then 50 rpm. I didn't like my chances of the lathe stopping before crashing, so this won't work.
I did find someone talk about a work around / another method, very similar to the leave the half nuts engaged, but is engage it on a number on the threading dial, at the end of the thread disengage the half nuts and stop the lathe straight away. Back the tool out, put the lathe in reverse, get the same number on the threading dial when the lathe is running backwards. Then the rest is the same as the method of keeping the half nuts engaged. This removes the risk of crashing the tool and provided that you stop the lathe straight away so the dial doesn't go around a few times should be good at keeping your spot. While this sounds great I still much prefer the idea of a thread chasing dial.....
In my searches I found references to Metradial metradial
There is a tutorial on this site and a couple of threads about using them. This looks great but also kinda scary, it looks expensive (well too expensive for the home user anyway) and only works with certain lathes. This had me thinking there must be a better way. So I went to the drawing board. The first thing that I needed to understand fully is exactly what a thread chasing dial does and why it doesn't work with metric.
The Math & theory behind a thread chasing dial
My lathe is a China / Import lathe, it had a 8 TPI lead screw. All the math / explanation below will be based on my lathe however will transfer to other lathes as the theory is all the same.
The point of a thread chasing dial is to make sure that the threading tool is in sync with the existing thread that you are making further cuts on. So basically at a set distance the angle of the headstock is correct. It is in effect a counter, on my lathe we know that when 8 threads have gone past a set point that it has traveled 1 inch.
The big difference between a metric thread and an imperial thread is how the thread is defined. An imperial thread is designated by how many turns it has over a 1 inch length where as a metric thread is designated by how long a single thread is. Eg a 3/8-24 imperial thread will have 24 turns over a 1 inch length vs a M10 x 1 will have a thread length of 1mm so 25.4 turns to an inch. On a lathe with an imperial lead screw and thread chasing dial they normally have a gear that has 2 x lead screw TPI. For my 8TPI lead screw that means that my thread chasing dial has 16 teeth on its gear.
How it works for imperial threads is fairly simple. For 1 full rotation of the thread chasing dial (16 teeth) the lead screw has traveled 2 inch, a 1/2 turn (8 teeth) = 1 inch. As imperial threads are defined by how many threads per inch this means that provided the thread being cut is a whole number not a fractional thread then every 1 inch of travel the thread will line up. Remember regardless of what thread pitch you are cutting the thread dial will always tell you the exact same information, eg 1/2 rotation = I have traveled 1 inch, full rotation = I have traveled 2 inch, 1/4 rotation = I have traveled 1/2 inch. The half nuts can be engaged the same amount of times per inch of travel as the lead screw TPI. So for my 8 TPI lead screw it can be engaged 8 times.
Understanding the above about how it works with imperial you start to realize that there can be several points in a rotation of the threading dial to engage it. There is no formula to work out minimum its just brute force calculations. For my 8TPI lead screw / 16 tooth dial gear If it had enough lines I could choose between 2 inch (full turn), 1 inch (half turn), 1/2 inch (quarter turn), 1/4 inch (one eighths turn), 1/8th inch (one sixteenth turn). To work out if you can use that distance is simple, can the thread you are cutting be divided by the distance traveled and come out with a whole number . So lets take 20 TPI for example, 2 inch = 40 teeth, 1 inch = 20, 1/2 inch = 10 teeth, 1/4 inch = 5 teeth, 1/8 inch = 2.5 teeth. This means you would be able to use all by the 1/8th inch / one sixteenth turns of the threading dial. Lets try 24 TPI, 2 inch = 48, 1 inch = 24, 1/2 inch = 12, 1/4 inch = 6, 1/8th inch = 3, this actually means that with a 8 TPI lead screw cutting 24 TPI (or any number that can be divided by 8 and be a whole number) you don't actually need the threading dial, any engagement point will work. Ok one more lets do 9 1/2 TPI, 2 inch = 19, 1 inch = 9.5, 1/2 inch = 4.75, 1/4 inch = 2.375, 1/8 inch = 1.1875, the only whole number is on 2 inch which means cutting this thread you would need to pick a spot and stick to it.
So now that we understand how it works with imperial lets look at metric and why a conventional one will not work. As we know 1 inch = 25.4mm, remember the thread chasing dial will not change what it shows regardless of thread. It is still showing you what distance has passed / how many teeth have gone past on the lead screw. We can establish that for each tooth the distance traveled is 1 inch / TPI = 25.4mm / 8 TPI in my case = 3.175mm per tooth. We can only engage the half nuts on each thread turn, which means we can only engage the 1/2 nuts every 3.175mm.
As the thread chasing dial will only ever tell us the same thing we then need to work out how many rotations of the headstock will get everything lined up again. So for the moment don't think of calculating lead screw rotations, we are calculating headstock rotations. For say a 1mm thread for each tooth / full rotation of the lead screw the head stock will spin 3.175 times. We need a whole number of turns before we can re-engage the 1/2 nuts again to make sure we are picking up the correct threads. In my searches I kept coming across the number 127. Not sure exactly how this came into play I made a quick excel spread sheet, first cell of travel per rotation in mm (3.175) in this case, used excel to populate down that column and add a rotation for each new row. In the next column I had excel divide the distance by the thread pitch as this would tell us total head stock rotations. I noticed that regardless of what thread pitch I put in the first whole number was always 127. I also found that the smaller the thread pitch the fewer turns of the lead screw required. I came up with the formula of
Number of teeth to count on leadscrew = (127 x metric thread pitch) / distance per rotation of lead screw in mm
So lets do a quick example, nice and simple 1mm pitch
number of teeth on leadscrew = (127 x 1mm) / 3.175 = 127 / 3.175 = 40
This means that the headstock has turned 127 times, this has turned the leadscrew 40 times, so 40 x distance per rotation = 127mm. 127 can be divided by 1
What about say 1.25mm
number of teeth = (127 x 1.25mm ) / 3.175 = 158.75 / 3.175 = 50
So headstock turned 127 times, lead screw turned 50 times, 50 x distance per rotation = 158.75 which can be divided by 1.25
Using the above we can see that while a conventional thread chasing dial will not work for metric as its hard to count 40 rotations on a dial wheel that is 16 teeth a rotation, however the theory of counting rotations is still valid. We just need to take a different approach. If your only going to cut 1mm pitch then go nuts and cut a 40 tooth gear, use it 1:1 to a dial and engage on the same number / spot every time and your sweet. But why not get one that will work with ANY thread metric or imperial that you want to cut ?
The Design
If you have been reading this from the start you should have a good idea as to what counting the teeth / rotations of the lead screw can get you and that if you can accurately count teeth then you can chase ANY thread metric or imperial.
This leaves us with what actually ends up being a very cheap and easy thread chasing dial to make. Parts needed are
1 x 10 tooth gear
1 x 10 count / revolution rotary counter, best to get one that will go backwards (anti clockwise results in numbers going down) also and has a reset (*I have not checked if these go backwards* Eg RL Series (small size rotary counter) or RL-219 (large size 5 digit rotary counter) )
and a way to connect the two, something like a flexi shaft would be ideal, maybe a drill or dremel flexi shaft ? If you use a flex shaft then mount the gear on the end, have the gear in mesh with the lead screw, other end connected to the rotary counter and you can mount it somewhere nice and convenient, near the tool post maybe ?
How its used
Pretty simple, set all your gears, do the math using the formula above to calculate how many turns of the lead screw you are counting, as the gear is a 10 tooth gear and the rotary counter is 10 count / rotation it means that the rotary counter will be showing you numerically how many teeth have passed.
Engage the 1/2 nut, hit the reset on the rotary counter to get you back to zero
cut the first pass on your thread then disengage the 1/2 nut
retract your tool and move back to the start of the thread, set your tool back in ready
when the number comes up that you calculated earlier re-engage the 1/2 nut, hit the zero again, cut the second pass
rinse and repeat
If you forget to hit the zero on follow up passes its ok, you just have to do more math, the number on the counter will need to be able to be divided by the number of teeth you calculated earlier. eg 1mm thread can use 40, 80, 120, 160, 200, ect ect on the dial
I know this has been a massive post and thank you for anyone that takes the time to read it. I am in the process of making a digital version using a 600 pulse / rotation rotary encoder and Arduino but is still a while off and had the idea for the mechanical one and thought that I would put all the information up for everyone else as information seemed quite limited and someone else may want to make one.
Thanks
Wayne
