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
Originally Posted by 8iggles
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.
Originally Posted by 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
A couple of pics
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]
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:
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.
Originally Posted by 8iggles
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...
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 ??