I've been measuring and machining adaptors to mount a new servo on my CNC mill. With very careful work, I have the servo shaft mounted and concentric with the Z axis shaft and 0.050" apart.
RATS, $%^&, now I discover I should have had the shafts the thickness of the spider in the lovejoy coupling apart. Its solid in the center. So is it any big deal to drill a hole in a lovejoy spider? how would you do it? Buna-N rubber.
Karl
I've been playing around with how brittle some things like rubber and silicone get when cold. Perhaps drilling it out with a conventional drill bit while surrounded by dry ice or something? I have liquid nitrogen to play with at work, and at that temperature, everything gets brittle!
OTOH, I don't know how long the spider will last if there's a big hole in the middle...
Nice sharp endmill
Gunner
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Are you sure that the couplers are right
Every coupler I've seen used on a cnc is a one that will not introduce any angular errors between the motor and the ball screw shaft. The lovejoy with the rubber will flex slightly. If the encoder is located on the opposite end of the shaft you might have a problem with oscillation.
John
Done that! I used a drill similar to a "brad & spur" that cuts the outside of the hole. How about a new end mill? What is the size? I'll do it for you if you like. I might have a spider the right size. Or, just drill the thing, they are only a few bucks. Are you sure that the couplers are right for the application? I thought CNC couplers were different.
I can see this could be a problem on a production machine. The forces on the coupling are tiny compared to the capacity for the unit. You might see an effect if you were working to tenths.
My X and Y have pulleys and timing belts. The Z had to have a solution where I can push the coupling together, there's no way to use the more common spiral coupling for CNCs. The machine came stock this way (LoveJoys), I'm upgrading from stepper to servo drive.
Karl
Have a look at Oldham couplings, Karl. They're stiffer than spider couplings and, in my experience, better suited to servo systems. Slop and limited stiffness that might be acceptable in a stepper drive may cause problems, or at least limit performance, in a servo system. You may be forced to compromise when tuning the servo loop, giving up responsiveness and servo stiffness to avoid instability.
Oldham couplings usually have a delrin disc that's easily bored, as long as the required hole isn't so big that it compromises the shear strength of the driving slots. McMaster carries them up to 3/4" shaft and 480 lb-in capacity. Though now that I'm looking in the catalog, it appears the hubs are not bored thru. I suppose they could be modified as well, but you can get thru-bored hubs form Huco.
If I were drilling it I would get a piece of metal tubing the size of the shaft and sharpen the edge. Then just put the coupling in a lathe with the halves clamped together and run the tubing through the existing hole in the spider.
Or do the same procedure in a drill press.
John
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