4th Axis slop - solved

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4th Axis slop - solved

Post by sgiersch »

I noticed excessive axial runout after running only a couple of small projects on my new full sized 4th axis. On closer examination, I noticed I could wiggle the spindle on the 4th axis headstock with just finger pressure. Not good (see video clip)
Notice you can wiggle the cylinder with your fingers. This causes axial runout.
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. Contacted technical support and they immediately and without question sent out a new 4th axis headstock. Impressive customer service. But, upon removing the new 4th axis from the box, noticed the new one had even more free play. So much so that even the weight of the Nova chuck caused it to droop noticably. Found that if I used just the right amount of pressure from the tailstock (which I replaced with an off the shelf tailstock so I could use a standard Nova live center), I could hold it approximately on center with an acceptable runout. Too much tailstock pressure and the headstock locks up, too little and it droops. This problem worsened over the course of approximately 1 hour of use, with the headstock now showing approximately a 1/8" gap between the outer cylinder and the housing. Then I noticed additional slop whenever the direction of the A axis reverses when in use. Was going to send the whole thing back and request a refund, but I NEED a 4th axis; it and the laser is the whole reason I "upgraded" to an HD5 from my reliable BobsCNC at a total cost over $5k including laser and 4th axis. So decided to take the 4th axis headstock off and investigate. I removed the two nuts and the screws to remove the backplate. Saw that it is driven by a belt, with the two slots in the backplate allowing the belt tension to be adjusted. Decided the problem for the the A axis rotational slop was probably due to the belt being loose. So, after some trouble in getting the belt back on, I remounted the back plate and tightened the belt. Still not acceptable.
Finally decided, what the heck, I'm really going to find out how this thing ticks. Took off the backplate again, and noticed that there was a small amount of metal shavings in the bottom of the housing (not good) and that there was a gear that appeared to be mounted to the cylinder of the 4th axis. Was able, with a c-clamp, to seat the gear on the cylinder in a manner that was tight, but it still came loose with just finger pressure, so figured there has to be a set screw holding it in place. Was able , with a screwdriver, to pry the front plate completely out of the housing, and this revealed that the cylinder has 4 keyways, and the gear has 4 tiny holes with set screws (see picture). Apparently the set screws were loose enough to allow rotation within the keyways, which accounts for the metal shavings and the rotational slop as well as the 1/8" of freeplay. I saw that there was a dot of locktight on each set screw, but apparently they weren't tightened before applying the locktight. I lubricated the surface of the cylinder where it touches the housing, wondering why they wouldn't have used ball bearings. Pressed it all back together, with the intention of tightening the 4 set screws through the hole in the top of the housing. But that hole is in the wrong location to be able to access the set screws after assembly. This explains why they were not tight. So, took it apart again, drilled another access hole in the top of the housing in a location that allowed me to tighten the set screws with an allen wrench (see pictures)
. Pressed the headstock into the front plate and housing, pressed the gear onto the cylinder from the back, made sure there was no freeplay, and then used an allen wrench to tighten down those set screws. Used a drop of silicone caulk to seal that access hole in the housing, which allows me to go back in and re-tighten those set screws if I ever notice slop in the axis again. Wrestled the drive belt back onto the gear and drive motor, remounted the backplate, tightened the belt, and voila, it works like a champ.
If QA for Next Wave reads this, now you know how to fix the problem that appears to be widespread on that 4th axis assembly (I got two in a row with this defect). If a Shark owner with a 4th axis is reading this, this is how you can turn a boat anchor into a useful 4th axis.

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Re: 4th Axis slop - solved

Post by Rando »

Wow! Thanks for that detailed write-up. That's some crazy sh!t......

I suppose if we want stuff to work CORRECTLY, we have to go through the machine and make sure it's all tight and aligned....dang :o

OTOH, on the base CNC, we have (more in the past, less so now it seems) had ongoing issues of not-tight couplers between the stepper motor shaft and the leadscrews.

God have mercy on our souls....I hope this doesn't mean "Mr. NeverTight" on the assembly line wasn't promoted to the 4th-axis line! :twisted:

Regards, and thanks again for writing that up for us.

ThomR.com Creative tools and photographic art
A proud member of the Pacific Northwest CNC Club (now on Facebook)

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Re: 4th Axis slop - solved

Post by Pavel2 »

Just purchased a used 4th axis for less than 1/2 of new cost. It looked to be in good condition until I tried mounting the chuck. It's "spindle" was loose, with extreme wobble. I took it apart, discovered the belt drive pulley was not remotely engaging the spindle. There was not even a hint of lubrication inside and I was shocked at the simplicity of the unit (no bearings). I calibrated where the drive pulley should be and drilled a couple of dimples into the spindle to engage the set screws and added lock tight to the set screws when I reassembled it. Also lubed the assembly with lithium grease to test the thing out. Works fine now, but what I found interesting was, even though there is a grease zert on top, this thing was bone dry when I received it. I searched the manual that came with and have found no information on the type of lubrication to be used or a maintenance or lubrication schedule.

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