Upgrade motors for 200lb gantry and spindle on diy build?


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Thread: Upgrade motors for 200lb gantry and spindle on diy build?

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    Default Upgrade motors for 200lb gantry and spindle on diy build?

    Based on the advice I got here, I bought a Gecko G540 with 4 381oz low inductance Nema 23 motors for my desktop CNC build. At the time I was intending to build a lighter machine than what I actually ended up building so I wanted to ask if you guys think I need to upgrade some or all of the motors?

    The main difference is going to be the spindle. Instead of starting with a relatively light 2.2kw router spindle, I have decided to invest a little more upfront to buy a Tormach 10,000 rpm R8 milling head. The stated weight on these units is 150lb so, with the gantry and VFD etc, the motors could easily be moving 220lb or more. Is this likely to be an issue for the 381oz Nema 23's?

    My machine as currently set up has 36" x 18" x 8" of travel so it's not a large machine. I'm just wondering if I need to replace the z-axis motor with someone bigger. My plan is to add an additional high speed spindle for finishing passes in the near future so it's not going to be getting any lighter either.

    Is there some formulae that you guys use for working out how much motor you need for a specific machine weight / size ? Or is it all trial and error?

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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    I have a feeling your motors are fine, and just what you want for a router build, but I'd rethink the idea of putting that mill spindle on it. Unless you're really building a mill for metalworking, in which case you're likely going to have to rework the whole design to be much stouter, it just seems out of scale for your machine. Wood cutting is best done at higher RPMs, so the low range of that spindle won't do you much good. And it's way too heavy, as you're starting to realize. If you want a more heavy-duty spindle than the 2.2kw Chinese ones, think about using a Colombo, HSD or Perske instead. These are beefier, but still have ER spindles that are easier to change tools in than R8, and don't weigh more than about 35 lbs. This will still require some counter-weighting to keep it from dropping to the table when you power down (I like gas struts for this) but not nearly what it would take to deal with that mill spindle.

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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    It really depends on how you are driving it, and how fast you want to go.

    I'm curious to see what a 150lb spindle looks like on a 70 lb gantry??

    Gerry

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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    I think by going with this heavy spindle you are negating all the benefits of the lightweight frame. It's possible to move this around with the steppers you have but I think you'll need some form of gear reduction. Which probably would reduce your chances of doing any "high speed finishing..." unless you go with servos. Lots of motor sizing calculators online.



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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    Quote Originally Posted by ger21 View Post
    It really depends on how you are driving it, and how fast you want to go.

    I'm curious to see what a 150lb spindle looks like on a 70 lb gantry??
    My gantry beam is lighter than average because it's made with carbon fiber (mostly) with epoxy granite and embedded steel plates to hold screw threads (for the rails). It's also fairly short as it's a benchtop machine. It's more than strong enough and stiff enough to hold a 150lb spindle. I tested it with more than 3 times that weight and there was no visible flex. I won't really know how well it holds up under load until I test it though. Carbon fiber doesn't really bend or flex much before it breaks.


    I am not worried about the strength or stiffness of the frame or gantry beam. I just wasn't sure about the size of motor required to lift a 150lb milling head on the Z or if I need more to move a 250lb gantry / spindle / VFD etc on the x axis.

    As with most other diy CNC componants, sellers tend to just list all the options without any guidence to show customers why they should choose one over the others for their purpose. As far as I can tell, this 381oz holding torque number has no relavence to how fast the motor will turn or how much weight it will move.

    I can see that larger motors with 1200 to 2000oz of holding torque are recommended for "larger Bridgeport type mills" so I am assuming that once you get give a certain weight you need larger motors but you guys have direct relavent experience with this stuff. If you tell me that a 381oz Nema 23 has enough whatever to move 230 or 250lb and to lift 150lb then I'll stick with them.

    Most of the Bridgeport type CNC mills I have seen use a moving table for the x and y. It's just the Z that has a super heavy load to move. Could the extra torque on the X and Y axis be because it's needed to hold the table steady when milling hard metals like steel and titanium as lower speeds?



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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    Quote Originally Posted by awerby View Post
    I have a feeling your motors are fine, and just what you want for a router build, but I'd rethink the idea of putting that mill spindle on it. Unless you're really building a mill for metalworking, in which case you're likely going to have to rework the whole design to be much stouter, it just seems out of scale for your machine. Wood cutting is best done at higher RPMs, so the low range of that spindle won't do you much good. And it's way too heavy, as you're starting to realize. If you want a more heavy-duty spindle than the 2.2kw Chinese ones, think about using a Colombo, HSD or Perske instead. These are beefier, but still have ER spindles that are easier to change tools in than R8, and don't weigh more than about 35 lbs. This will still require some counter-weighting to keep it from dropping to the table when you power down (I like gas struts for this) but not nearly what it would take to deal with that mill spindle.
    The machine is primarily going to be used for milling aluminum molds. I may use it for some wood in the future but I have a high speed router I can use for that if I need the extra speed. This is a metal milling machine above all though.

    I have been looking for a used Colombo, HSD and other brands and haven't seen any good deals at all. If I am going to spend the extra, I just want something that is guaranteed to cover my needs. After watching the Tormach in action, I am 100% convinced it will do what I want. The R8 spindle offers more flexibility too, including the ability to use any of the ER collets without being limited to small diamater tools. With a speeder, you get the best of both worlds too (apparently).

    If there is a better spindle option for my needs for the same or less money than the Tormach PCNC 700 spindle head, I haven't found it yet. It seems to offer a lot for the price and I like the entrepreneurial spirit of the company.

    I intend to keep an eye out for a deal on a suitable Colombo until the last minute but I am losing hope of finding one I can afford in good condition. The few affordable ones I have seen looked like they had been in a car accident, or they were the 380v variety that is of no use to me.


    As I am building the machine myself, there is no such thing as "too heavy" for any of the components. If I need to upgrade the motors to accommodate a heavier component then that it what I will do. With this being a learning experience for me, I kinda expected to be switching out some parts as the design evolves.



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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    Quote Originally Posted by louieatienza View Post
    I think by going with this heavy spindle you are negating all the benefits of the lightweight frame. It's possible to move this around with the steppers you have but I think you'll need some form of gear reduction. Which probably would reduce your chances of doing any "high speed finishing..." unless you go with servos. Lots of motor sizing calculators online.
    I didn't know they had sizing calculators for the stepper motors. It didn't even occur to me to look. That's exactly what I need. Thank you!


    BTW, I don't have a lightweight frame for the machine I am building. I guess it's all relative but mine will end up at over 400lb when i'm done. My use of carbon fiber does reduce the weight over a similar sized steel design but it wasn't what I am going for. It's mainly about strength and vibration reduction.

    My only concern with weight (apart from the motors) is that I need to be able to move it at some point. if I have to move the machine, I would take the spindle off and move it seperately.

    Are you talking about the potential issue of the machine cutting slower as it struggles to move a heavier spindle around? The advice I got here was that I shouldn't worry about speed on a small benchtop machine. Have I hit the weight limit where I should start to worry about it?



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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    You do realize that you can vary the motor and lead screw sizes per axis, right.

    In this forum it is pretty common for people to implement the same motor on every axis. In industry it is less common, rather you see servos or steppers matched to the needs of the axis. So if for example you have performance issues on the Z axis you are free increase motor size or adjust lead screw pitch. You can even add counter balance weights to the Z.



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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    If you use a 5mm pitch ballscrew for the Z axis, you'll probably be fine. You can also use Springs or pneumatic cylinders to carry some of the weight.

    Gerry

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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    Quote Originally Posted by Goemon View Post
    I didn't know they had sizing calculators for the stepper motors. It didn't even occur to me to look. That's exactly what I need. Thank you!


    BTW, I don't have a lightweight frame for the machine I am building. I guess it's all relative but mine will end up at over 400lb when i'm done. My use of carbon fiber does reduce the weight over a similar sized steel design but it wasn't what I am going for. It's mainly about strength and vibration reduction.

    My only concern with weight (apart from the motors) is that I need to be able to move it at some point. if I have to move the machine, I would take the spindle off and move it seperately.

    Are you talking about the potential issue of the machine cutting slower as it struggles to move a heavier spindle around? The advice I got here was that I shouldn't worry about speed on a small benchtop machine. Have I hit the weight limit where I should start to worry about it?
    While, yes, you shouldn't have to worry too much about top end speed, because likely by the time you reach it you have to start decelerating. But if I remember correctly, you wanted high speed finishing, which would mean high acceleration really. Which I thought was a main goal of the lightweight frame. You should take a look at the Tormach and see what kind of stepper is running that Z axis and how they have it counter-balanced.



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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    Quote Originally Posted by louieatienza View Post
    While, yes, you shouldn't have to worry too much about top end speed, because likely by the time you reach it you have to start decelerating. But if I remember correctly, you wanted high speed finishing, which would mean high acceleration really. Which I thought was a main goal of the lightweight frame. You should take a look at the Tormach and see what kind of stepper is running that Z axis and how they have it counter-balanced.

    It wasn't me that was looking for high speed. It's never been a priority for me as the machine will only be used to make molds for my composites business. It's not a production machine. Any high speed finishing mentioned was referring to spindle speed. I don't mind if it takes an hour or a week to make each mold.

    My frame is not lightweight. It's a fairly chunky steel, epoxy granite and carbon fiber build. My aim has been to make it as strong and stiff as I am able (given my lack of welding or steel machining capability) because the advice I got here was that this is one of the most important requirements for milling metals well.

    With all that said though, I want to make sure my motors are sufficient to make the machine work. I've put in too much effort to allow the motors to be the weakest link.

    The only replacement axis motors sold on the Tormach site under the spare parts stab are 640oz. I am going to assume they use this one on all axis as they don't specify which axis it is for. This doesn't really tell me if my 381oz motors would work though. The Tormach is a moving table design made to accommodate 400lb work pieces.



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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    Quote Originally Posted by wizard View Post
    You do realize that you can vary the motor and lead screw sizes per axis, right.

    In this forum it is pretty common for people to implement the same motor on every axis. In industry it is less common, rather you see servos or steppers matched to the needs of the axis. So if for example you have performance issues on the Z axis you are free increase motor size or adjust lead screw pitch. You can even add counter balance weights to the Z.

    I did know that as I have seen kits with 960oz motors for the Z with 381oz for everything else. My thought on upgrade options (should I need it) was to switch to a 2 motor / 2 lead screw X axis and then buy a 960oz stepper for the Z.

    I am probably going to stick with the 1/2" 2 start lead screws I already have because switching them out means changing a whole lot of other things and redoing so work that I can't face doing again. My lead screws came from a large industrial mill so I hope they are good as they are.

    I was thinking of using some of the gas springs I have on my shelf to counter the Z axis but I had assumed that the purpose of these springs was related to backlash. I didn't realize that people used them to compensate for small motor / heavy spindle combos. It would be an easy and cheap fix as I already have a bunch of them.

    - - - Updated - - -

    Quote Originally Posted by wizard View Post
    You do realize that you can vary the motor and lead screw sizes per axis, right.

    In this forum it is pretty common for people to implement the same motor on every axis. In industry it is less common, rather you see servos or steppers matched to the needs of the axis. So if for example you have performance issues on the Z axis you are free increase motor size or adjust lead screw pitch. You can even add counter balance weights to the Z.

    I did know that as I have seen kits with 960oz motors for the Z with 381oz for everything else. My thought on upgrade options (should I need it) was to switch to a 2 motor / 2 lead screw X axis and then buy a 960oz stepper for the Z.

    I am probably going to stick with the 1/2" 2 start lead screws I already have because switching them out means changing a whole lot of other things and redoing so work that I can't face doing again. My lead screws came from a large industrial mill so I hope they are good as they are.

    I was thinking of using some of the gas springs I have on my shelf to counter the Z axis but I had assumed that the purpose of these springs was related to backlash. I didn't realize that people used them to compensate for small motor / heavy spindle combos. It would be an easy and cheap fix as I already have a bunch of them.



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    Default Re: Upgrade motors for 200lb gantry and spindle on diy build?

    Quote Originally Posted by Goemon View Post
    It wasn't me that was looking for high speed. It's never been a priority for me as the machine will only be used to make molds for my composites business. It's not a production machine. Any high speed finishing mentioned was referring to spindle speed. I don't mind if it takes an hour or a week to make each mold.

    My frame is not lightweight. It's a fairly chunky steel, epoxy granite and carbon fiber build. My aim has been to make it as strong and stiff as I am able (given my lack of welding or steel machining capability) because the advice I got here was that this is one of the most important requirements for milling metals well.

    With all that said though, I want to make sure my motors are sufficient to make the machine work. I've put in too much effort to allow the motors to be the weakest link.

    The only replacement axis motors sold on the Tormach site under the spare parts stab are 640oz. I am going to assume they use this one on all axis as they don't specify which axis it is for. This doesn't really tell me if my 381oz motors would work though. The Tormach is a moving table design made to accommodate 400lb work pieces.
    For finish passes of contoured surfaces, using higher feed speeds can give you better surface finish, because it decreases contact time between tool edge and work, which prevents the aluminum from building up on the edge of the tool. But you'll be using very small stepovers as well to keep the surface finish consistent. That said, the advice you were given was based on the notion that you were using a 2.2kW-3kW spindle, not a milling head.

    If I'm not mistaken, the Tormach uses ballscrews with a 5mm lead. Even though the table can accommodate 400lb that doesn't mean it takes 400lb of force to turn that screw and move the table. A manual mill has something like a 10TPI ACME screw. It would be extremely difficult to back-drive that screw, that is, make the screw turn by pushing on the table. This of course prevents nasty kick-backs. Ballscrews can easily be back-driven, and thus the 'holding' forces are taken up by the motors. So with the tool sizes associated with the aforementioned electric spindles, the 381oz-in steppers would likely have been fine. With a milling head, you're moving a bit slower, taking larger cuts with larger tools, so I have my reservations about you having enough 'holding' force at the motors - at least with enough 'headroom'. Maybe if we knew exactly what kind of tools or endmills you plan to use, their size, how deep/fast you plan to cut, it would help.



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Upgrade motors for 200lb gantry and spindle on diy build?

Upgrade motors for 200lb gantry and spindle on diy build?