Need Help!- How to choose stepper lead screw coupler

[bobster23]

Hi all,
I am building a cnc with which i'm hoping to make 2 layer pcbs (among other things). I want the cnc to be very exact since i want to be able to make boards with small mouted devices (smd).

I got the stepper motor working but am stuck with coupling it lead screw. I got lost with all the different kinds of couplers. Where can I get in depth information on the different kinds of couplers and what are the different parameters I have to take note of. Wikipedia's explanation is a bit shallow for my needs and didn't manage to google anything that is clear.

My stepper pretty exact (1.8 deg per step) and strong (270 oz-inch) so I want to get the most from it (in my price range)

I want to be able to understand this part of the build so I can make an informed decision (price vs. functionality).

Thank you very much
bobster

[Drools]

I have heard good things about the lovejoy couplers. I used some less expensive rubberized units that I purchase used on the bay.
I'm not sure how far you have gotten into your build but I'm doing much the same as you for the same reason. My steppers are Tormach 270oz with 1/2-10 single start screws. I may upgrade the screws in the future.
If you have not looked at a spindle yet I was impressed with this one.
http://www.cnczone.com/forums/showthread.php?t=41190
On the bay auction he has some links to youtube video where he cuts some very fine SMD pads and some very fine traces.

[eSilviu]

Originally Posted by bobster23 Hi all, I am building a cnc with which i'm hoping to make 2 layer pcbs (among other things). I want the cnc to be very exact since i want to be able to make boards with small mouted devices (smd). I got the stepper motor working but am stuck with coupling it lead screw. I got lost with all the different kinds of couplers. Where can I get in depth information on the different kinds of couplers and what are the different parameters I have to take note of. Wikipedia's explanation is a bit shallow for my needs and didn't manage to google anything that is clear. My stepper pretty exact (1.8 deg per step) and strong (270 oz-inch) so I want to get the most from it (in my price range) I want to be able to understand this part of the build so I can make an informed decision (price vs. functionality). Thank you very much bobster

I have a play of 5-10 degree with my couplings, I use M8 thread, and I can mill pcb with good results. SOIC package is not a trouble at all.
In my case, backlash in motor couplings is helping me: motor start with no load, and after 10 degree thread is "coupled" to the motor...
So I can achieve faster acceleration on motors., and backlash is under 0.1mm.

[bobster23]

Dear eSilviu,
It sounds like you managed to do exactly what I'm trying to do. What type of coupler did you choose?

Thanks
Bobster

[eSilviu]

Originally Posted by bobster23 Dear eSilviu, It sounds like you managed to do exactly what I'm trying to do. What type of coupler did you choose? Thanks Bobster

I think "Oldham Couplings" are exactly what you need. You can sand them a little, to get the desired play (a few degree, just not to load the motor from the start). and are cheap too.
I... i've just cut the motor's shaft in 2, the acme thread too, and inserted a sheet of metal in there. covered with a plastic tube, and it's working for 2 years by now.

[wildwestpat]

Hi Bobster

I get your drift about needing accuracy for surface mount pads etc on pcbs. However the angular error introduced by the oldham / lovejoy type of coupling is to all intents and purposes zero if the alignment of the motor and screw shafts is OK. These couplings are expensive and are used amongst other things to ease assembly. The multi beam coupling has an elastic cushioning effect which might be exceeded by the stepper trying to jerk the table along against its inertial. Suggest you stick with a solid coupling or use a lovejoy / oldham with a stiff insert (delrin not the soft ones that are available).

Have you thought about the other errors because the error introduced by a properly aglined lovejoy / oldham can be ignored compared to the errors inherent in in the stepper motor. The quoted 1.8 degrees given as the basic step has a tolerance and varies throughout the 360 degree rotation of the shaft. The use on microstepping whilst giving a smoother drive unfortunately also introduces errors. Before you get as besotted with accuracy as I am establish what accuracy you actually need both as a tolerance over a short distance (thinking of a SMD's row of pads as well as an absolute positional accuracy. There is room for a bit of cunning in always indexing from the same corner of the board for both sides. The lead screw errors are small even for the C7 grade compared to the stepper error in dividing the 360 degree into 200 equal parts.

I am currently trying to design an X2 mini mill cnc conversion that has maximum ridgidity. I have selected oldham couplings with delrin inserts. I have checked these for backlash under static load and cannot detect any. I have also cleared the ball screw and will be using thrust races with a separate axial ball race as this will permit me to run with a high radial compression on the bearings which should be reasonably wear resistant. The machinng is a little less critical as it is only the bore of the axial ball race and the stepper NEMA mounting spigot that have to be concentric easing the final install. This arrangement could have used solid couplings but assembly would be more difficult as I want to investigate the use of different stepper motors.

I found some usefull design limits on the Huco web site which indicate that aglinement accuracy of 0.5 degree with a safe load of 1.7 Nm for the 19 mm size and 4Nm for the 25 mm size couplings stiffness appears to be more than adequate at 115 or 205 Nm/rad

Hope this helps