Thread: Hi all

  1. #1
    Hi all

    Been at the thinking stage for a while now :idea: and finally got started this autumn - If i said budget and making it up as I go along, you get the picture.

    Well been reading and getting ideas and I do have some plans, but they just keep changing along the way:confused:.

    I've been saving for ages and finally ready to get the electronics - I want to mill and cut foam / balsa / Ply / plastic etc for RC modelling. The X & Y I'm doing with toothed belts and Z with a leadscrew.

    I want the X&Y to be able to traverse faster than the Z as it will be mainly thin sheet so will I need bigger motors for X&Y ??

    I've been looking on Ebay at 3 axis kits but would rather get from here in UK if possible - Has anyone got any links please??

    I will try and get some pics for you

    Thanks in Advance
    Shaun

  2. Quote Originally Posted by 8iggles View Post
    Hi all

    Been at the thinking stage for a while now :idea: and finally got started this autumn - If i said budget and making it up as I go along, you get the picture.

    Well been reading and getting ideas and I do have some plans, but they just keep changing along the way:confused:.

    I've been saving for ages and finally ready to get the electronics - I want to mill and cut foam / balsa / Ply / plastic etc for RC modelling. The X & Y I'm doing with toothed belts and Z with a leadscrew.

    I want the X&Y to be able to traverse faster than the Z as it will be mainly thin sheet so will I need bigger motors for X&Y ??

    I've been looking on Ebay at 3 axis kits but would rather get from here in UK if possible - Has anyone got any links please??

    I will try and get some pics for you

    Thanks in Advance
    Shaun
    Shaun,

    Welcome!

    Whats your planned work area?

    How do you plan to use the toothed belt? The tricky bit is to keep it tensioned, which usually means some form of sprung jockey pully system. How are you assessing the motor size? I did a tutorial on motor selection for designs using leadscrew, will probably update it for belt/chain driven once I've figured out how to do the calculations.

  3. #3
    Quote Originally Posted by irving2008 View Post
    Shaun,

    Welcome!

    Whats your planned work area?

    How do you plan to use the toothed belt? The tricky bit is to keep it tensioned, which usually means some form of sprung jockey pully system. How are you assessing the motor size? I did a tutorial on motor selection for designs using leadscrew, will probably update it for belt/chain driven once I've figured out how to do the calculations.
    Hi irving2008

    My work area will be 800mm x 600mm

    I'm going to have an open belt fixed at each end, with a gear that works along it, like a rack and pinion
    Where can I find your Tutorial? I haven't had a look round the site yet

  4. #4
    A couple of pics


  5. Interesting bit of construction.. I take it thats your machine minus its drive trains.

    re belt drive, I can't help but feel there's going to be some difficulty maintaining accurate postioning with an open belt. What pitch are you thinking of?

    The tutorial is [ame="http://www.mycncuk.com/forums/showpost.php?p=10046"]here[/ame]

  6. #6
    Quote Originally Posted by irving2008 View Post
    Interesting bit of construction.. I take it thats your machine minus its drive trains.

    re belt drive, I can't help but feel there's going to be some difficulty maintaining accurate postioning with an open belt. What pitch are you thinking of?

    The tutorial is here
    Yes no drive trains yet, as I said in 1st post budget..the lack of spare cash - I've used threaded rod for cost and adjustment

    The belts I have are 5mm pitch and pinion is 10 teeth then reduction before stepper motor, I think the change in direction will be the largest force on the motor.
    Its still a blank canvas so I can go for some decent ballscrews if all else fails, but again it all comes back to cost and failiure is not an option

    I'll try and get some pics of where i'm at with the drive

  7. #7
    The tutorial is [ame="http://www.mycncuk.com/forums/showpost.php?p=10046"]here[/ame][/QUOTE]

    Thanks I Have printed it off for digesting, it may take a while though as I got a "D" in physics some 30 yrs ago - wow that long :surprised:

  8. Quote Originally Posted by 8iggles View Post
    Yes no drive trains yet, as I said in 1st post budget..the lack of spare cash - I've used threaded rod for cost and adjustment

    The belts I have are 5mm pitch and pinion is 10 teeth then reduction before stepper motor, I think the change in direction will be the largest force on the motor.
    Its still a blank canvas so I can go for some decent ballscrews if all else fails, but again it all comes back to cost and failiure is not an option

    I'll try and get some pics of where i'm at with the drive
    I have done some thinking and reading around timing pulleys and belts. It all comes down to the shear force on the teeth. The load has three components - static force, cutting force and inertia.

    The static force is given by F = m * u where m is mass and u is friction coeff.. for ball races say 0.01, and lets say gantry is 30kg so F = 0.3N.
    Cutting force for most woods would be 5N. Inertia is given by F = m * a. we want to accelerate to maximum speed in 0.02sec (for a medium spec machine). maximum speed for cutting is, say, 1200 mm/min = 0.02m/s. The acceleration therefore is .02m/s divided by 0.02 sec = 1m/sec^2 so F = 30 * 1 = 30N

    so our pulley has to push on the belt with a force of 0.3 + 5 + 30 = 35N (ignore the 0.3, its noise!).

    At 1200mm/min the pulley revs, for a 10 tooth pulley on 5mm pitch will be 1200/(5 * 10) = 24rpm = 0.4rps (2.5rads/sec)

    The diameter of a 10tooth pulley on a 5mm HTD is 15mm so the radius is 7.5mm and the torque therefore is 35N @ 7.5mm radius = 0.26Nm

    Power = Torque * rads/sec = 0.26 * 2.5 = 0.65W

    From ther HPCGears catalog pages 787-790 the required HTD belt can be calculated as follows. Choose service factor c0 as 2.0, calculate c2 = c0 + c3 + c6 = 2.0, taking C3 and C6 both = 0. Pb = Power x C2 =.65 x 2 = 1.3W, which means from Graph 1 (page 788) aN 8M belt is needed.
    there is one tooth in mesh so C1 factor is 0.1 and C7 taken as 0.8, therefore Pu > Pb and Pu = Pn * c1 * c7 so Pn > pb/(c1 * c7) = 1.3/(.1 * .8) = 16.25W which suggests a 15mm wide belt minimum. Note however that the torque load of a 15mm dia pully is a bit high.

    A better choice would be a 20 tooth pulley, with a OD of 32mm, radius 16mm, torque 35N @ 16mm = 0.56Nm and Pb = 0.56 * 1.25rads/sec^2 = 0.67W so Pn>16W as before. This also needs a 15mm wide belt but is more in the sweet spot in the belt tables.

    Now, 1 rev of pulley = 100mm movement, and assuming we want 0.05mm resolution we need 1 rev of motor = 10mm so we need 10:1 gearing, so motor speed at 1200mm/min = 2 revs/sec or 400 steps/sec.

    So motor needs 10tooth, driving 100tooth/20tooth compound gear driving against the belt. Mechanically could be tricky to arrange.

    On this basis only a tiny motor would be required due to the gearing... It doesn't look quite right to me, I need to think it through more...

  9. #9
    More pics...







    Wow more homework, thanks so much for the info...I'm using 2 rollers so theres at least 5 teeth in mesh and as luck would have it I got 15mm wide belt for the X axis and 10mm for the Y axis...I was worried about the inertia when changing speed / direction of the gantry - it runs freely but is quite heavy

    The small router on the first pics is only a temp thing just to let off some of the excitement that's building and demo it to my son - I'm hoping to make a bearing block and end up with a keyless chuck driven down the centre of the square tube from a motor on top...I thought it would be better in the centre of the bearings rather than overhung to one side ??

    Thanks again
    Shaun

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •