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  1. #11
    Quote Originally Posted by Tenson View Post
    Hmm.. I think I'll go for unsupported rails on the Y axis to keep the gantry weight down, plus build simplicity.
    BIG BIG BIG MISTAKE if you do this unsupported rails are a complete waste of money.

    Don't be affraid of weight, Mass really helps when it comes to cutting, It absorbs resonance and gives far better finish.
    If your trying to cut weight to gain speed then you are sizing components wrong. The feeds you require for the machines intended purpose should be got from choosing components to allow it.
    Ballscrew pitch and motor selection along with drives and correct voltage all play a part combined with build quality and component quality. Get anyone wrong and you comprimise the machines abilty in some way.

    In your case unsupported rails with a light weight gantry of any resonable width would be a disaster for cutting any material denser than softwood, and even then with lower DOC.

    DONT DO IT. . . . FORGET UNSUPPORTED RAILS.!!!!!!

  2. #12
    Ooookay, I will use supported :)

    What is the best way to drive the 150cm long X axis? Should I use a central ball-screw or two ball-screws, one on each side? If the latter, what is the best way to power them, i.e. 2 motors connected to 1 controller?
    Last edited by Tenson; 25-03-2012 at 03:03 AM.

  3. #13
    2 ball screws means that both sides are controlled and this will stop the gantry squewing. I have a motor on each side. Mach 3 can link the two together. A single motor linked by belts to the two screws may have been a better choice as the screws are guaranteed to rotate equally.

    you will find quite a few threads about this.

    Bruce

    OO look at me showing off and giving advice
    The more I know, I know, I know the less. (John Owen)

  4. #14
    Quote Originally Posted by motoxy View Post
    OO look at me showing off and giving advice
    Good advice :)

    It's possible to put two motors on one driver if they're mechanically linked, but I wouldn't advise it. Either put one motor on each screw, or one motor and a long timing belts to link the two screws. The former should go a bit faster, since you have twice the input power with two motors, but in practice I doubt it will go that much faster as you have to guarantee neither motor stalls. Clearly if one does then the gantry skews, which depending on the magnitude could damage things. Once the machine is tuned properly either is fine as the motors will never stall.

    I used two motors and two drivers since it would require a very long belt and several idlers to link the rotating ball-nuts.

  5. #15
    Quote Originally Posted by Tenson View Post
    Ooookay, I will use supported :)

    What is the best way to drive the 150cm long X axis? Should I use a central ball-screw or two ball-screws, one on each side? If the latter, what is the best way to power them, i.e. 2 motors connected to 1 controller?
    Eh Eh sorry if I shouted a bit load but can't stress enough what a bad idea unsupported rails are.!!

    RE Single or twin screws on X. If Y axis much wider than say 600mm then personally I'd go for twin screws.

    Powering them you can either have 2 motors each with it's own drive, 2 motors on 1 drive will work but not correct way to do it and could give problems. . Another don't do it.!!
    You could use 1 slightly larger motor and connect the screws together with a timing belt, this is my personal favorite and how i've built my machine. It works perfect so don't be put off by timing belts.

    Both work ok but you have to be aware of the downsides to each and choose which you prefer to live with.!
    Using 2 motors works ok but comes with the issue if one motor stalls the other keeps going which can do major damage to the machine if happens at high feeds.
    It can also be a problem keeping the screws in sync if both motors are not tuned perfectly in sync with each other and one runs ahead or behind the other, this will result in slowly twisting the gantry thou can be corrected by regular homing and squaring, but still wrong and not wanted.
    The way to avoid these issues is to not stress the motors so you effectively need to run them below there full potential because you can't afford any missed steps or threat of stalling from lack of torque at high feed rates or rapid moves. The quality of build regards ballscrew aliagnment as to be spot on, which it should be on any machine, but even more so with twin screws/motors as no sticking or binding can be tolerated for fear of stalling motors at high feeds where torque is lower.
    Mostly this is not a problem if your aware of it before building because you can choose higher lead ballscrews or build in gearing again via timing belts to the motors to achieve the speeds & feeds you require and keep the motors operating well below the point they start running out of torque.
    The sync problem is just a slow setup and tuning job which can either take 2mins or 2 weeks depending how fussy you are.!! Warn you thou it can lead to premature baldness.!!. . I'm down to my last few strands. . :naughty:

    The 1 motor pritty much eliminates all this hassle with perfect sync and also means you can run the motors closer to there max. It's also very easy to change pulleys to gear the system up or down.
    The only downside is it's not as easy to build because you have build in the belt system and it's tensioning/tracking system and also have the ballscrews machined different from standard machining to add extra bearing support for the pulleys which will put greater side ways force on the ballscrews effectively trying to bend them when the belts are tensioned.

    In my experience this is all worth the effort because when done and settled down it never loses position or sync and ounce setup thats it forget about it and never needs doing again with absolutly no fear of racking the gantry unless the belt snaps. (which I've only snapped 1 in over 3 yrs and that was because I left the pulley lose)
    If good quality belts are used then belt stretch is minimal and with correct tension and good tensioning system never an issue and very very accurate.
    It's no cheaper(well slightly) than 2 motors because you have the belts and extra bearing supports to consider.

    Like I say both work ok if your aware of there limits and tendency's it's just I personnaly favor belt system.

    Oh and at 150cm your in a funny area regards pitch and screw dia.???? I'm pritty sure you'll want 10mm pitch which often (well from china anyway's) only comes in either 16mm or 25mm dia they don't do 20mm dia only in 5mm pitch.
    At 150cm 16mm is on the very edge of being to thin and will give big potential for whip at high speeds. 25mm would be better but comes with the cost that it requires more torque and power to over come the extra mass and inertia of the screw/nut when accellerating and deaccellerating. This means bigger motors which require larger power supply's and drives that can handle the power so it basicly cost's more money.
    You can over come this by either taking the route like jonathan did and use rotating ballnut on 16mm screws, but again this will come with expense and i'm sure jonathan will tell you exactly how much and all it's bennifits. . . OR . . bite the bullet and pay for the larger screws and use with nema 34's and correct drives, psu etc. . . . OR . . .use 20mm 5mm pitch and gear 1:2 to give an effective pitch of 10mm at the cost of slightly less torque. ( My machine use's this system thou with nema 34 motors) This gives a slight advantage as it's easy to gear back down and double the resolution of the machine without comprimising the area where the motor works good. It can be done the other way round and gear 10mm pitch to give 5mm resolution but the motors work in an area which is not ideal.!
    It's horse's for course's really so take your pick.!!!!!

    Hope this helps.!
    Last edited by JAZZCNC; 25-03-2012 at 11:09 AM.

  6. #16
    Thanks for the detailed reply.

    So the problem with a single central ballscrew is that the gantry can twist a little back and forth? While the problem with using dual ballscrews + motors is that it can twist should one motor stall? I don't think my budget with allow the custom machining of ballscrew to fit a timing belt. Where did you get yours from JazzCNC?

    The X axis will be 80cm wide. How about running a single 2505 ballscrew on the Y axis with a bigger motor? There is a nema 34 12Nm motor on Zapp Automation. However it needs 6.2A per phase, so what is a suitable driver for it? Is it best to use 2x 6A drivers, one per phase?


    What is the problem with 5mm pitch on a 150cm axis? I'd have thought it best to use the same screw pitch on both X and Y axis so they have the same speed, provided the motors have enough torque.

    Oh, and finally how can I build an auto tool change? I'd really like to have that at least to choose between 2 bits. Hmm.. I just searched that and it looks like I'd need a special chuck with gearing to transfer power from the motor, plus a system to open and close the chuck. PITA.
    Last edited by Tenson; 25-03-2012 at 06:42 PM.

  7. #17
    Ok the screws come from Chai at linearmotionbearings which you'll find on ebay. He's a great guy and very trust worthy. It's best to let Chia do the machining as he doesn't charge very much and will machine to your dimensions if asked. I do this with him all the time.

    Yes single screw can twist the gantry when cutting at the outer edges, the wider the gantry the worse it will be. At 80cm your on the edge IMO.
    If you do proceed with single then make the distance between the bearings as wide as possible to help combat this and have a strong wide cross brace undernieth.

    The 25mm screw would work but you don't need 12nm motor 6nm would be ok.
    I think you miss understand the phase/drive relationship you only need a drive which can handle the motors rated amp's, so in the case you gave 6.5A. you don't need 1 drive per phase.
    The thing you have to be aware of is the differnce between nema 23/4 and 34 motors. Larger 34 motors tend to spin at lower speeds than 23/4 motors. typicly a 34 will runout of puff around 1000-1100rpm where as 23/4's will spin 1500rpm and above. The difference comes from how they produce and carry the torque up the rev range, 34's tend to hold higher torque further up the rev range then drop off really quick as it peaks. 23/4's will carry torque further up the rev range all be it a lower rate and gradually drop off to nothing.
    Also 34's need really high voltage to get best performance. Voltage is key to speed and torque in both 23/4's and 34's but 34's require much higher to get best performance.

    Unless you have a very well equiped workshop or lots of money then forget autotool changer far too expensive.!! . . . . better alternative is to buy Gerry's Mach screen it's about as close to autotool changing as you'll get without the expense, find it here and it will be the best $20 you have ever spent. http://home.comcast.net/~cncwoodworker/2010.html

  8. #18
    Hi Jazz,

    On the datasheet for the motor is said 6.2A (phase) which I take to mean each winding will draw 6.2A. So if I wire in parallel it will draw 12.4A. In series will will need about 3A but twice the voltage. Won't it?

    Each side column of the gantry rides directly on the centre point of the linear bearings, and I was going to use solid 12mm alu sheet as a cross-brace underneath. In that case I guess maybe a single central ball-screw with a powerful motor would be okay. However you point that a nema 34 spins slower is worrying as I want all axis to be capable of the same speed. What do you think, is it okay to use a nema 34 only on the Y axis?

    Why is running two motors from one controller not a good idea? Provided it can output the required current and voltage it should be okay, won't it?

  9. #19
    Quote Originally Posted by Tenson View Post
    On the datasheet for the motor is said 6.2A (phase) which I take to mean each winding will draw 6.2A. So if I wire in parallel it will draw 12.4A. In series will will need about 3A but twice the voltage. Won't it?
    No... in series it's 6.2A and in parallel 12.4A. Until I finish my own drivers you're not going to find drivers that will deliver 12.4A, so have to use 6.2A. As you say, that means a higher voltage is required ... so the only driver worth buying for that completely excessive and uncalled for Nema 34 motor is mains voltage:

    http://www.zappautomation.co.uk/2m22...?cPath=9_3_132

    Quote Originally Posted by Tenson View Post
    Each side column of the gantry rides directly on the centre point of the linear bearings, and I was going to use solid 12mm alu sheet as a cross-brace underneath. In that case I guess maybe a single central ball-screw with a powerful motor would be okay.
    Not at all - how strong you make the gantry with a single screw is largely irrelevant with regards to racking (although in general you clearly want to make it as strong as you can afford) since you're stopping the gantry rotating about the central ballnut, not stopping it bending. To rotate (rack) the rails do not have to deflect much at all, so the only way to do it would be to have a very large spacing on the X-axis linear bearings, however doing that you're just compensating for bad design in the first place. Use two ballscrews - I guarantee you'll regret one.

    This happens with almost every build log - starts off thinking one ballscrew will be fine, then by about page 3 we manage to persuade that it's not.

    Quote Originally Posted by Tenson View Post
    However you point that a nema 34 spins slower is worrying as I want all axis to be capable of the same speed. What do you think, is it okay to use a nema 34 only on the Y axis?
    You won't need a Nema 34 motor.

    The only way to get a Nema 34 to have decent torque at higher speeds is to use the mains voltage drivers, so lets add up the costs:

    Driver, 176
    8.6nm motor, 108
    Two RM2510 ballscrews from eBay, roughly 190
    4 * 30T HTD pulleys, 22
    Long (to link screws) and short timing belts, 27
    Ballscrew end bearings & mounts, 50 (bit of a guess but shouldn't be far out, maybe more due to the long belt?).

    Total 563

    The other option is, as Jazz suggested, to use rotating ballnuts. See here for my experience with this setup:

    http://www.mycncuk.com/forums/showth...-design-ideas/

    (On the last page you'll find pictures of what I'm suggesting below, but the rest is worth reading for a more in depth explanation of why they work so well.)

    To get the same (feederate and acceleration) as the above, probably better and certainly more than you'll ever need from a rotating ballnut setup you'll need:

    2* m752, 80 (eBay)
    PSU for 2 motors is shared with Y/Z motors so about 15 extra.
    2* 3nm motor, 50 (eBay)
    Two RM1610 ballscrews from eBay, 150
    4 * 30T HTD pulleys, 22
    2 * short timing belts, 10
    4 * 7206 angular contact bearings, 40
    Ballscrew end bearings & mounts, DIY for 10.

    Total 376

    So if I wanted to I could charge 187 to machine two rotating ballnut mounts and you'd end up with a better system for the same price. Realistically I'd charge less so it would be cheaper overall.* When I've worked it out before the price difference has been greater since I worked it out with two nema34 motors, not one, but either way it's a significant saving.

    Or for about the same price (376) you could just use the RM1610 and spin the screws as normal. I'm confident that will be adequate and a better option than RM2005 since the critical speed (speed at which the screw starts whipping, so the limiting speed) for a RM2005 screw is well under twice the critical speed of a 1500mm RM1610 screw so it wont actually get a better feedrate. Plus it clearly requires much more torque to accelerate a 20mm ballscrew compared to 16mm, so you'd need a bigger motor anyway.

    In your situation I'd go for RM1610 and either rotate the screw or the nut. I went for rotating nuts since my machine has 2000mm ballscrews so rotating a 16mm screw wasn't an option...and equally because I wanted to try something new(ish). I've left my machine on 10m/min with 1m/s^2 acceleration on X and 1:1 ratio. It will do about 15m/min, even more if I change the pulleys, but 10m/min is already plenty so no point stressing the components. That's with the same 3nm motors and drivers, so you can expect better.

    To give you an idea of how much more stable a system rotating ballnuts are; I recently didn't notice that one of my ballscrews had worked loose. So one end wasn't fixed at all and the other wasn't tight either. I only noticed the problem when after a few hours cutting aluminium I leant over/on the ballscrew to remove the tool and it moved... i.e. it made no difference! Try that with a rotating screw and you'd probably have ended up with a bent ballscrew.

    Quote Originally Posted by Tenson View Post
    Why is running two motors from one controller not a good idea? Provided it can output the required current and voltage it should be okay, won't it?
    Think what happens if one motor has a slightly different load.. then stalls.


    *Other vendors are available.
    Last edited by Jonathan; 25-03-2012 at 11:54 PM.

  10. #20
    Quote Originally Posted by Jonathan View Post
    No... in series it's 6.2A and in parallel 12.4A.
    Hmm, surely the parallel connection will draw twice the current of a single winding, not twice that of the series connections. Similarly the series connection will draw half the current of a single winding, not half that of a parallel set-up.

    Not at all - how strong you make the gantry with a single screw is largely irrelevant with regards to racking....
    To rotate (rack), the rails do not have to deflect much at all...
    This happens with almost every build log - starts off thinking one ballscrew will be fine, then by about page 3 we manage to persuade that it's not.
    Lol, okay I get it :tup: I'll go for two ball-screws.

    I still wonder though; why isn't it better to use the same screw pitch for all axis, so that providing the motors don't run out of torque, each axis will have the same speed and resolution?

    I really don't want to go the rotating ball-nut route, nor linking the ball-screws with a belt. Going with dual ball-screw I'd like to use two motors. I'll bear the tedious tuning procedure when I first get it up and running. At least I know to expect that now thanks to you guys!

    Think what happens if one motor has a slightly different load.. then stalls.


    My question was really about why it is better to power each motor and ball-screw from a separate drive circuit, rather than just using both motors from one circuit. The motors can surely still individually stall, even with two driving circuits.

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