4 ms·

While that's an excellent book, one of the things that I find fascinating about it is what it doesn't cover all that well: how do you make an accurate screw thr

While that's an excellent book, one of the things that I find fascinating about it is what it doesn't cover all that well: how do you make an accurate screw thread? The usual way you make a screw thread is by cutting it on a lathe. But the heart of a lathe is an accurate screw thread... How to making flat plates from scratch with the 3 plate method is relatively easy to understand. But screw threads have a much more complex geometry, with no obvious way of making one from scratch. They also have many more parameters to consider, including relative pitch accuracy, and absolute accuracy. As far as I can tell achieving accurate screw threads it something that took a huge amount of work by many different people effectively working together to get successively better and better threads, using each others' screw threads made using multiple techniques to average out errors. But I've never actually seen anyone describe from start to finish out to go from nothing to an accurate screw thread. It's not even clear to me that it's possible to do alone in a reasonable amount of time. Relevant: https://freechaptersinbooks.wordpress.com/2012/09/18/screw-thread-cutting-by-the-master-screw-method-since-1480/ https://freechaptersinbooks.wordpress.com/2012/09/18/screw-t...

jeffreyrogers 6y ago
Haha, I wondered the exact same thing when reading it. Same thing about the spindle. It talked about how you could measure how accurate a spindle was but not how one was constructed.
aaronblohowiak 6y ago
I imagine you can do this through gear reduction -- reducing the ratio of your leadscrew to the tool movement essentially reduces the inaccuracy of your leadscrew, iiuc. the percent error is the same, but you care more about absolute error than percent.
- petertodd 6y ago
  That's a good idea. But I don't think it actually works, at least with the obvious way to do it. Suppose you have a lathe whose screw thread has errors in pitch such that the position of the middle thread is incorrect. With the right gears, you can use that lathe to cut a second thread with a different pitch. But regardless of what pitch you choose, the middle of that second thread will have the exact same absolute error in that position as the original thread. So you haven't actually improved anything. The best I can think of to improve the accuracy of a thread is to measure the thread against multiple length standards, and grind/lap away material by hand to bring the actual position along the thread length as close as possible to your length standards. However, as the number of length standards you can practically measure against is limited, you'll need to already be at a point where relative pitch error is small. Averaging out errors with a long follower nut is probably one way to do that.
  - aaronblohowiak 6y ago
    Why would the error the same in absolute sense instead of relative to the gear ratio?
    - aaronblohowiak 6y ago
      I will write up my idea later today.
      - aaronblohowiak 6y ago
        After thinking about it, you’re right! I also found this: https://en.m.wikisource.org/wiki/Encyclopædia_Britannica,_Ninth_Edition/Screw https://en.m.wikisource.org/wiki/Encyclopædia_Britannica,_Ni...
        petertodd 6y ago
        Thanks! Interesting link.
mauvehaus 6y ago
With a screw origination machine, of course ;-) Henry Maudslay invented the first one that saw widespread use.
- petertodd 6y ago
  ...and notice how his screw-cutting-lathe has an accurate lead screw in it? :) https://www.ssplprints.com/image/100286/henry-maudslays-original-screw-cutting-lathe-c-1800 https://www.ssplprints.com/image/100286/henry-maudslays-orig... https://www.gracesguide.co.uk/Henry_Maudslay:_Machine_Tools https://www.gracesguide.co.uk/Henry_Maudslay:_Machine_Tools I've never actually seen a detailed explanation of how he cut that first accurate lead screw! Like I say, as far as I can tell it took an iterative variety of processes that made a variety of screw threads, with no one method alone being enough by itself.
  - mauvehaus 6y ago
    The top two pictures of the gracesguide.co.uk page show a screw originating machine, not a screw cutting lathe. And that's the machine you need. AIUI, it's used to cut a lead screw in relatively soft material by holding a cutting tool to it at an angle. Once you have that, you can generate a screw in harder material from it (carefully, one has to assume).
    - petertodd 6y ago
      Ah, I see what you mean. I was aware of those machines. But IIUC they by themselves don't generate accurate enough screws, as the blade doesn't track the material exactly, so errors build up over the length of the screw. Getting those errors out in subsequent generations of screws is the hard part.
      - yetihehe 6y ago
        You can average out errors over thread by repeately sliding a long nut over your screw while using lapping compound. Each part of nut will contact only highest (in direction of travel) parts of thread and slightly cut them. You will need to cut off about the same length as nut from both ends of your screw. Nut will have to be made from two parts so that it can be slightly tightened on rod.
jbay808 6y ago
I believe they said that their screw threads were lapped by hand while being checked by interferometer readings. An absurdly difficult process, but the result is a full-area bearing screw that never wears out.
- petertodd 6y ago
  Yes, that's how the Moore Tool Company did it. However the first screw-cutting lathe actually dates back all the way to 1800. While as far as I can tell the first use of interferometry for distance measurement was the Michelson interferometer, in 1887; it took until 1960 for the meter to finally be defined in terms of wavelengths of light.
  - jbay808 6y ago
    Oh, I see what you mean now; I misunderstood your question! Here's a screw cutting machine from Da Vinci's notebook: https://www.ssplprints.com/image/100676/leonardo-da-vinci-screw-cutting-machine-from-leonardo-da-vincis-notebook-1470-1520 https://www.ssplprints.com/image/100676/leonardo-da-vinci-sc... But as you observe, it already has a screw in it. Two screws actually, one on either side, which advance a carriage that cuts a new screw in the center from a blank. It can either duplicate the pitch, or using a gear ratio, it can cut a different pitch than the two master screws. The central screw averages the error of the two master screws, so with certain assumptions, the new can end up more accurate than either of the masters; the master screws can then be swapped out for new screws made this way, and so on. But there are limits to that approach, because common errors won't get averaged out. However you could, for example, assume that two screws made this way with the same masters are accurate duplicates of each other, then when you replace the masters with those two new screws, you can mount one of them flipped end-over-end, or with a rotational offset of eg. 90 or 180 degrees (offsetting the leadnut to compensate), and gradually average out more errors that way. Another way as you noted in a different comment is to use a long, soft leather lead-nut to average multiple threads, and use that as a master to create a new thread (assuming a low cutting force -- for example with force amplification, or for checking a thread, and so on). There are some first principles approaches you could use, like making a master cylinder and then wrapping a wire of constant-diameter around it to make a literal 'thread', and bonding it in place. It wouldn't be the most robust threadform but that's another way.
dylan604 6y ago
I learned this lesson with the difference in an all-thread rod compared to a lead screw. even though both were at a typical 1/4"-20 pitch, the all-thread had too much slop in it to make for an accurate, repeatable motion. I went down the same rabbit hole of reading about how accurately turn a screw.
- YZF 6y ago
  Slop can be taken out with preload... or always approaching a point from the same direction. Though if this is something like a milling machine then you have forces acting on your nut to deal with. But if all you need is repeatable motion you could do better than your slop.
  - dylan604 6y ago
    Lots of ways to ultimately handle slop. Stepper motor vs servo motor makes a big difference. I didn't know if I wanted speed or accuracy when I stared. Also had to make the decision of belt driven vs screw. Ultimately, accuracy was most important, so lead screw with a stepper. It's just a pain to carry around as the lead screw dictates the size of the rig.
Cerium 6y ago
One way I have heard of is starting with a lead screw composed of a rod wrapped in wire.
YZF 6y ago
One observation is that if you can measure your position accurately then you can correct your inaccurate screw to machine a more accurate screw. So an inaccurate, but repeatable lathe, can make a more accurate screw. I know that when you order a ball screw you will typically get a compensation curve with it, which lets you position yourself to better accuracy than the thread on the screw .. though presumably that's not enough to machine a better screw, otherwise they'd do that ;)
- petertodd 6y ago
  The trick there is figuring out how to actually do that compensation in the pre-CNC world. I've seen references to cams used in certain screw-cutting devices. But not a detailed description of how exactly those cams worked or how people were actually measuring the error in the first place back then. > though presumably that's not enough to machine a better screw, otherwise they'd do that ;) These days it's probably cheaper to just program the compensation curve into your CNC controller than to actually build a better screw! The really accurate machines often use separate distance encoders anyway, rather than rely on a ball screw that's being stretched and compressed by both load and temperature effects during use.

jeffreyrogers 6y ago

Haha, I wondered the exact same thing when reading it. Same thing about the spindle. It talked about how you could measure how accurate a spindle was but not how one was constructed.

aaronblohowiak 6y ago

I imagine you can do this through gear reduction -- reducing the ratio of your leadscrew to the tool movement essentially reduces the inaccuracy of your leadscrew, iiuc. the percent error is the same, but you care more about absolute error than percent.

petertodd 6y ago
That's a good idea. But I don't think it actually works, at least with the obvious way to do it. Suppose you have a lathe whose screw thread has errors in pitch such that the position of the middle thread is incorrect. With the right gears, you can use that lathe to cut a second thread with a different pitch. But regardless of what pitch you choose, the middle of that second thread will have the exact same absolute error in that position as the original thread. So you haven't actually improved anything. The best I can think of to improve the accuracy of a thread is to measure the thread against multiple length standards, and grind/lap away material by hand to bring the actual position along the thread length as close as possible to your length standards. However, as the number of length standards you can practically measure against is limited, you'll need to already be at a point where relative pitch error is small. Averaging out errors with a long follower nut is probably one way to do that.
- aaronblohowiak 6y ago
  Why would the error the same in absolute sense instead of relative to the gear ratio?
  - aaronblohowiak 6y ago
    I will write up my idea later today.
    - aaronblohowiak 6y ago
      After thinking about it, you’re right! I also found this: https://en.m.wikisource.org/wiki/Encyclopædia_Britannica,_Ninth_Edition/Screw https://en.m.wikisource.org/wiki/Encyclopædia_Britannica,_Ni...
      - petertodd 6y ago
        Thanks! Interesting link.

mauvehaus 6y ago

With a screw origination machine, of course ;-) Henry Maudslay invented the first one that saw widespread use.

petertodd 6y ago
...and notice how his screw-cutting-lathe has an accurate lead screw in it? :) https://www.ssplprints.com/image/100286/henry-maudslays-original-screw-cutting-lathe-c-1800 https://www.ssplprints.com/image/100286/henry-maudslays-orig... https://www.gracesguide.co.uk/Henry_Maudslay:_Machine_Tools https://www.gracesguide.co.uk/Henry_Maudslay:_Machine_Tools I've never actually seen a detailed explanation of how he cut that first accurate lead screw! Like I say, as far as I can tell it took an iterative variety of processes that made a variety of screw threads, with no one method alone being enough by itself.
- mauvehaus 6y ago
  The top two pictures of the gracesguide.co.uk page show a screw originating machine, not a screw cutting lathe. And that's the machine you need. AIUI, it's used to cut a lead screw in relatively soft material by holding a cutting tool to it at an angle. Once you have that, you can generate a screw in harder material from it (carefully, one has to assume).
  - petertodd 6y ago
    Ah, I see what you mean. I was aware of those machines. But IIUC they by themselves don't generate accurate enough screws, as the blade doesn't track the material exactly, so errors build up over the length of the screw. Getting those errors out in subsequent generations of screws is the hard part.
    - yetihehe 6y ago
      You can average out errors over thread by repeately sliding a long nut over your screw while using lapping compound. Each part of nut will contact only highest (in direction of travel) parts of thread and slightly cut them. You will need to cut off about the same length as nut from both ends of your screw. Nut will have to be made from two parts so that it can be slightly tightened on rod.

jbay808 6y ago

I believe they said that their screw threads were lapped by hand while being checked by interferometer readings. An absurdly difficult process, but the result is a full-area bearing screw that never wears out.

petertodd 6y ago
Yes, that's how the Moore Tool Company did it. However the first screw-cutting lathe actually dates back all the way to 1800. While as far as I can tell the first use of interferometry for distance measurement was the Michelson interferometer, in 1887; it took until 1960 for the meter to finally be defined in terms of wavelengths of light.
- jbay808 6y ago
  Oh, I see what you mean now; I misunderstood your question! Here's a screw cutting machine from Da Vinci's notebook: https://www.ssplprints.com/image/100676/leonardo-da-vinci-screw-cutting-machine-from-leonardo-da-vincis-notebook-1470-1520 https://www.ssplprints.com/image/100676/leonardo-da-vinci-sc... But as you observe, it already has a screw in it. Two screws actually, one on either side, which advance a carriage that cuts a new screw in the center from a blank. It can either duplicate the pitch, or using a gear ratio, it can cut a different pitch than the two master screws. The central screw averages the error of the two master screws, so with certain assumptions, the new can end up more accurate than either of the masters; the master screws can then be swapped out for new screws made this way, and so on. But there are limits to that approach, because common errors won't get averaged out. However you could, for example, assume that two screws made this way with the same masters are accurate duplicates of each other, then when you replace the masters with those two new screws, you can mount one of them flipped end-over-end, or with a rotational offset of eg. 90 or 180 degrees (offsetting the leadnut to compensate), and gradually average out more errors that way. Another way as you noted in a different comment is to use a long, soft leather lead-nut to average multiple threads, and use that as a master to create a new thread (assuming a low cutting force -- for example with force amplification, or for checking a thread, and so on). There are some first principles approaches you could use, like making a master cylinder and then wrapping a wire of constant-diameter around it to make a literal 'thread', and bonding it in place. It wouldn't be the most robust threadform but that's another way.

dylan604 6y ago

I learned this lesson with the difference in an all-thread rod compared to a lead screw. even though both were at a typical 1/4"-20 pitch, the all-thread had too much slop in it to make for an accurate, repeatable motion. I went down the same rabbit hole of reading about how accurately turn a screw.

YZF 6y ago
Slop can be taken out with preload... or always approaching a point from the same direction. Though if this is something like a milling machine then you have forces acting on your nut to deal with. But if all you need is repeatable motion you could do better than your slop.
- dylan604 6y ago
  Lots of ways to ultimately handle slop. Stepper motor vs servo motor makes a big difference. I didn't know if I wanted speed or accuracy when I stared. Also had to make the decision of belt driven vs screw. Ultimately, accuracy was most important, so lead screw with a stepper. It's just a pain to carry around as the lead screw dictates the size of the rig.

Cerium 6y ago

One way I have heard of is starting with a lead screw composed of a rod wrapped in wire.

YZF 6y ago

One observation is that if you can measure your position accurately then you can correct your inaccurate screw to machine a more accurate screw. So an inaccurate, but repeatable lathe, can make a more accurate screw. I know that when you order a ball screw you will typically get a compensation curve with it, which lets you position yourself to better accuracy than the thread on the screw .. though presumably that's not enough to machine a better screw, otherwise they'd do that ;)

petertodd 6y ago
The trick there is figuring out how to actually do that compensation in the pre-CNC world. I've seen references to cams used in certain screw-cutting devices. But not a detailed description of how exactly those cams worked or how people were actually measuring the error in the first place back then. > though presumably that's not enough to machine a better screw, otherwise they'd do that ;) These days it's probably cheaper to just program the compensation curve into your CNC controller than to actually build a better screw! The really accurate machines often use separate distance encoders anyway, rather than rely on a ball screw that's being stretched and compressed by both load and temperature effects during use.