Choice of ballscrew for small repetitive movements
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  1. #1
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    Default Choice of ballscrew for small repetitive movements

    Hello !

    The Z axis of our PCB milling/drilling machine mostly operates in 5mm (1/4" inch) area. Its worn and needs replacement. So, I think about following problems:

    1) If the movement is short range, do the balls in the ballscrew properly circulate and get lubricated ?

    2) Since I have to replace the ballscrew and nuts, are there some options, i.e. are some ballscrews better for ultra-repetitive short range work ?

    One PCB can have around 5000 holes and in 8 hours, the number of strokes is BIG.

    Share experience ! Thanks !

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    This might be a good application for a polymer nut and polished precision leadscrew, as it should take small motion related wear better than a ballscrew. But if you really want the best control of the stroke, a ballscrew is tough to beat.

    Have you checked the linear rails (if you're using them)? Might have the same issue.

    Another alternative is to use metal band and pulley setup, where a servo drives the pulley, the band is attached at the periphery, and is so driven when the servo rotates. No real wear, just eventual metal fatigue. Some alloys are more resistant to MF, could get bands made from them.

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    Depending on the forces and overall stroke I have seen voice coil actuators do this.
    They can be servo controlled as well.
    There are a few manufacturers but these guys have a mature product.
    SMAC Moving Coil Actuators | Electric Actuators | Voice Coil Actuators | SMAC Corporation

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    can you just install a longer ballscrew and move the ballscrew a half an inch per year? spring preloaded double ball nut perhaps?

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    You don't say what diameter screw it is, that will answer the question regarding whether the lubrication can be good or not. If that is an issue, have it do a 20 mm or whatever stroke every so many cycles.

    The most likely culprit though is the constant start/stop. This leads to ball skidding which leads to excessive wear. The suggestion regarding the band is a valid one.

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    The OEM one lasted this long. If there was a better option, wouldn't Excellon etc be using it?

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    Quote Originally Posted by Madis Reivik View Post
    Hello !

    The Z axis of our PCB milling/drilling machine mostly operates in 5mm (1/4" inch) area. Its worn and needs replacement. So, I think about following problems:

    1) If the movement is short range, do the balls in the ballscrew properly circulate and get lubricated ?

    2) Since I have to replace the ballscrew and nuts, are there some options, i.e. are some ballscrews better for ultra-repetitive short range work ?

    One PCB can have around 5000 holes and in 8 hours, the number of strokes is BIG.

    Share experience ! Thanks !
    Eliminate the wearing parts at-source. Do a sub table. Use a "voice coil" positioner. Think hard disk drives.

    The old, pre-"Winchester" (floating head) ones with FIXED head arrays had rather massive moving mass. Mine was an "ISS-80". The Invar bits a mere curio now, the frame long-since supporting a B&S surface plate in comfort.

    Repetitive movement? Yah. They do that well. Precisely, too. And FAST! It's all about the sensors and control loop, but it is also commodity technology.

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    Quote Originally Posted by thermite View Post
    Eliminate the wearing parts at-source. Do a sub table. Use a "voice coil" positioner. Think hard disk drives.
    Can't you get linear motors pretty much off the shelf nowadays ? Saw a big mill with linears a month or two ago, the first thing you notice is how quiet it is. Lots less stuff spinning and flailing and rolling and rumbling ...

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    Quote Originally Posted by EmanuelGoldstein View Post
    Can't you get linear motors pretty much off the shelf nowadays ? Saw a big mill with linears a month or two ago, the first thing you notice is how quiet it is. Lots less stuff spinning and flailing and rolling and rumbling ...
    Some from or another of them have been around for ages. ISTR the first one I saw was moving the pen for a strip-chart recorder. Back then, the position sensor was a specialized resistor - incremental, 256 steps.

    Lots of stuff out there now. All one has to do is conceptualize, then Google to see who beat you to it by several years, if not decades. Next step is to see how cheaply the Chinese are making it. Their Long March rockets are being booked all the time to hoist stuff into orbit, so there is a bit more to them than Walmart schlockery.

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    Quote Originally Posted by Madis Reivik View Post
    The Z axis of our PCB milling/drilling machine mostly operates in 5mm (1/4" inch) area. Its worn and needs replacement. So, I think about following problems:

    1) If the movement is short range, do the balls in the ballscrew properly circulate and get lubricated ?
    It will depend on the diameter/pitch of the ballscrew. With any rolling element bearing, there is a certain minimum range of motion you need to go through to ensure that the roller doesn't essentially wear a spot through the grease film.

    Quote Originally Posted by Madis Reivik View Post
    2) Since I have to replace the ballscrew and nuts, are there some options, i.e. are some ballscrews better for ultra-repetitive short range work ?
    I think a screw with a small ball would be better than a large ball. For a given diameter/pitch, you'll get more ball revolutions.

    I would speak to the ballscrew manufacturers and get their recommendations. First stop would be if you know who the manufacturer of your existing screw is. If not, I know the technical guys at Bosch (for rolled screws) and NSK and THK (for ground screws) are very good.

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    Quote Originally Posted by Madis Reivik View Post
    Hello !

    The Z axis of our PCB milling/drilling machine mostly operates in 5mm (1/4" inch) area. Its worn and needs replacement. So, I think about following problems:

    1) If the movement is short range, do the balls in the ballscrew properly circulate and get lubricated ?

    2) Since I have to replace the ballscrew and nuts, are there some options, i.e. are some ballscrews better for ultra-repetitive short range work ?

    One PCB can have around 5000 holes and in 8 hours, the number of strokes is BIG.

    Share experience ! Thanks !
    How long did it last in normal use?
    How long do you expect that machine to be in service?

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    Thanks for answers !

    Ballscrew is 25mm diam, 10mm lead.

    Of course, it has been working for so long that ballscrew is probably good enough choice. My question is more about the zillion different types of ballscrews available, are there some minor details I should consider. Good brand is obvious, THK or similar.

    For PCB drilling and milling, if the leadscrew is rough, then also Z feed is micro-sloppy and this reduces tool life and hole quality. New drilling machines use linear motors, which is ideal but I currently have 5 old "workhorses" which are built like a tank and they have probably limitless lifetime.

    If someone has problems or questions about Schmoll milling-drilling machines, tape loaders etc, I think I can help

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    Quote Originally Posted by Madis Reivik View Post
    Thanks for answers !

    Ballscrew is 25mm diam, 10mm lead.

    Of course, it has been working for so long that ballscrew is probably good enough choice. My question is more about the zillion different types of ballscrews available, are there some minor details I should consider. Good brand is obvious, THK or similar.

    For PCB drilling and milling, if the leadscrew is rough, then also Z feed is micro-sloppy and this reduces tool life and hole quality. New drilling machines use linear motors, which is ideal but I currently have 5 old "workhorses" which are built like a tank and they have probably limitless lifetime.

    If someone has problems or questions about Schmoll milling-drilling machines, tape loaders etc, I think I can help
    THK with the good seals...contact their applications people.
    If you expect to have to replace it before the machine is scrapped get two.
    Tape one in the bottom of the control cabinet.


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