So I'm going to be building a pm25 and I would like to know what type of couplers I should be using.
I've done a little bit of checking and the ones that I currently like the most are some Oldham couplers from Grainger, and the spider type couplers that I see on McMaster. Typical "beam" type or helical couplers torsional stiffness just won't cut it. I figure about 0.001" will be lost from the couplers when 'wound up'.
The Grainger couplers and the McMaster couplers I found will cost about $50-$75 per coupler. Is this about what I should be expecting to spend? I probably shouldn't be too worried as the couplers I just speced out at work for an application cost ~$450 a piece for something not too much bigger then what I am doing.
Last edited by mwood3; 08-04-2011 at 03:54 AM.
here are few links (but i am sure you know about oldham design already)
Coupling - Wikipedia, the free encyclopedia
I would not dare to buy those 60$ commercial couplars
It is better to die for something than to live for nothing.
You can get the LoveJoy's for less than $10.00 each.
CPL-Type - Jaw Couplings
I have seen the others in listings like this for cheaper than you listed but i havnt found them just yet.
Here is the oldham's for a fair price
I would rather spend the $50 a piece then not know some of their critical values such as their torsional stiffness. The beam types I believe had a torsional stiffness of like 0.09 lb/deg which translates into roughly 0.001" that is lost due to what is called "wind up" from the coupler, which is unacceptable to lose in one place that I don't have to lose it all in.
I'll be a little more formal later when I get home from work and post calculations and sources I've found.
Your steppers can put out what, 3Nm.
Stick any of these couplings in a lathe chuck and put 3Nm on them, see how much they deflect and compare that to your maximum stepper resolution.
Even rubber spider couplings are not the source of backlash that many believe they are.
My X2 CNC Brain Build: http://www.cnczone.com/forums/showthread.php?t=61345
Gecko G250 wiring errors: http://www.cnczone.com/forums/showthread.php?t=68960
yes, 570 oz*in steppers. backlash in a coupler refers to play, windup refers to how much the coupler deflects under a given load. I am currently designing and want to manage both backlash and windup as best I can.
For little X2's, spiders or helicals would be ok but for the g0704 (pm25) I went with oldhams.
There is much more mass (table and vise) to start, stop and reverse direction and the 570 motors
can do it with great velocity and acceleration.
Helicals and rubber spiders would definitely suffer windup in these circumstances and since
I'll be doing HSM along with 3D work, I'm not willing to let the tool bounce about.
The 1 inch oldhams with acetal discs from Mcmaster (#59985K3) are only $27.67 total per set
and as has been said, the hubs can be made for almost nothing.
http://www.hossmachine.info - Gosh, you've... really got some nice toys here. - Roy Batty -- http://www.g0704.com - http://www.bf20.com - http://www.g0602.com
Coupling, Oldham Coupling Disc - Motion Control Couplings - Couplings, Collars, and Adapters - 2ANU1 : Grainger Industrial Supply.
A torsional stiffness of 0.033 Deg/In-Lb. 570 oz*in = 36 in*lbs roughly. 360 degrees per revolution and estimating stock lead screws of .1" per rev. All the math translates out to be 0.00033" lost from windup. This would be acceptable to me.
From Coupling, 4 Beam, Bore 3/8x3/8 In - Motion Control Couplings - Couplings, Collars, and Adapters - 2AMB6 : Grainger Industrial Supply. The torsional stiffness of this coupler is 0.200 Deg/In-Lbs. Doing the math turns out to be .002" lost from windup. Not acceptable to me.
I am going to test with these McMaster-Carr. I've done a little reading on the lovejoy website and like the red shore material.
I find it odd that the 1" OD acetyl oldham's on McMaster can support 168 in*lbs, when the 1" OD acetyl oldham's from grainger can only support 42 in*lbs.