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Ouch! CNC machine broke


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Looks like a safety device - a shear block - to prevent motor damage in case something jams in the drive screw.


Check the screw to make sure nothing is binding.


The block looks to be made of aluminum. They sometimes develop stress crack and eventually break overtime.


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This type of device is used to connect two shafts that may not necessarily be perfectly aligned and will also absorb some of the torque shock of stop and start for the shaft under load.

You can find things like this on eBay... Search for "Servo Motor Couplings"...


The only thing important when ordering them is to get the shaft holes the correct size... how they are made (several different methods are used that differ from the one you have shown), is a secondary consideration. They don't have to be the same type to work in your application. Let's see a photo of the whole machine please? What do you make with it?



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You are correct. The supplier of my machine (a kit I bought off of e-bay) is sending a new one post haste.


It is a 24X24X4.25 machine using a PC variable speed router. Using Mach 3 control software and VCarve Pro CAD software. Running an old Dell PC with a LED Monitor.


Here are the machine specs:


X,Y,Z 20mm rods and bearings

X = 24 inch table 30 inches

Y = 24 inch table 30 inches

Z = 4 ¼ inch

X, Y and Z 16mm Lead 5mm Ball Screws Accuracy .001

3 = Nema 23 270 oz stepper motors

3 = precision motor couplers

1 = 4 axis's controller 3.5 amp

1 = 24v, power supply




Some stuff I've done recently.













Front side of mirrors.





I have been using 2 layers of 3/4" MDF for the table but will be replacing it with this next week.






It is 80/20 3" x .75" Extrusion 15 Series 3034-Lite


Will use 1/8" ply for spoil board when cutting thru material.


Do you have a CNC?






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Joe Frickin' Friday
This type of device is used to connect two shafts that may not necessarily be perfectly aligned and will also absorb some of the torque shock of stop and start for the shaft under load.


Yes on the alignment issue, but pretty sure it's no on the shock absorption. Quite the contrary, this type of couple is designed to maintain angular phasing of the two shafts while allowing for minor misalignment/offset. I used to see them in engine test cells a lot, where we used them to couple shaft encoders to the engine's crankshaft for monitoring crank angle position. In this application the encoder had pretty low inertia, so even with the constant variation of instantaneous engine RPM, the coupler never had to transmit very much torque; breakage was pretty rare in that application unless you had/developed major misalignment.


Bud, that coupler of yours might be prone to regular breakage, given that the stepper motor probably is programmed for sudden starts/stops, which transmit pretty good torque spikes. If your new coupler breaks, you might look into an alternative, like this one.

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Thanks for the tip and the link. Yes, the stepper motors start and stop suddenly when the path changes direction.


It was operator error that caused the break. First attempt at machining aluminum. Don't ask! :dopeslap:


But broke is broke and I'm out of commission till the new coupler arrives N/C from the mfg of the kit.


I will investigate replacing all three couplers with the ones you suggested as they appear to be designed exactly for the use in my application.


Here was another suggestion from a wood working forum.


Oldham coupler



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Nice machine and nice work Bud. I do not have a CNC, but have most of the components to build one... Have done a lot of reading, but work, wife and motorcycle keep me too busy to get things started.


Just a hint on a temporary solution... I don't know how much torque is applied though this coupling, but I have seen in the past an appropriate size piece of rubber hose (like heater hose or fuel line) squeezed onto both shafts and held in place with good quality screw clamps (like for heater hose or fuel line) and a drop of super glue, that allowed the machine to work until the replacement parts arrived... The stiff hose transmits the torque and takes care of any minor alignment problem.


This solution will work, but only you know the working strength of your devices and must assume all risk to eyes, fingers, tools, and workpiece if it doens't work... :dopeslap::eek:



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Thanks John,


The shafts spin at low speed an torque.


But I think I will wait till the part comes.


Now if it was my RT on the side of the road......................... :grin:

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