Replacing drive nuts on all three axes

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4DThinker
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Replacing drive nuts on all three axes

Postby 4DThinker » Sat Mar 07, 2020 3:21 am

Our old Meteor finally started complaining about cutting perfectly and jogging quietly. After tolerating the noises I finally couldn't accept the growing precision errors when cutting hardwoods.

So with info from Len at Probotix we ordered new drive nuts for this Meteor. The X and Y nuts are all the same, but the Z axis nut has a different thread pitch. Not cheap.

Changing out the Z axis nut was the most difficult. The router mount and front face of the Z axis had to come off. Three screws to unscrew the drive nut from the aluminum block. A collar at the bottom of the threaded shaft had to be loosened. There was also a screw/washer underneath the bottom plate into the threaded shaft. The top end coupler that connects the shaft to the motor had to be loosened, and at this point I could slide the threaded shaft down enough so I could twist the old drive nut off the top of the shaft. I cleaned up all the threads, and twisted on the new drive nut. Put everything back in place and tighten all screws. Seems as though at least three different sized allen wrenches were needed.

The X and Y nuts were easier. Loosen the collars at both ends. Loosen the shaft end of the motor coupler. Unscrew the drive nut from the block it is mounted in and twist it off the end of the shaft. It helps to have each axis jogged within 8 inches or so of the stepper end before starting this process. Reverse all steps to install new drive nut.

I used a brass wire brush to thoroughly clean the threads on all axes. Years of oily sawdust had packed itself into the drive nuts and I'm sure had contributed to their failing states. I'm not the one maintaining the three fab lab probotix CNCs we have, and it is clear some 3-in-1 oil or similar was used. New rules to only use dry lubricants and weekly clean/inspect the threads on all axes will hopefully keep these CNCs running great for another decade or more.

With all new drive nuts and clean threads the Meteor jogs/sounds new. :D I was a bit intimidated by the prospect of replacing all these nuts, but in all it took me about 2 hours, greasy hands, and an oily dusty mess to clean up.

4D

4DThinker
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Joined: Wed Oct 07, 2015 10:57 am
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Re: Replacing drive nuts on all three axes

Postby 4DThinker » Sat Mar 07, 2020 8:11 pm

To make sure the new drive nuts returned the meteor to its original precision status, I made up a quick set of toolpaths to cut in a scrap of hardwood that I could then measure after they were cut. My personal meteor is a year older than the one I use at work, so I thought it might be worth checking its accuracy as well.
Accuracy test.jpg
Accuracy test.jpg (131.8 KiB) Viewed 130 times

The file uses a 1/4" 2-flute upcut endmill to cut around a 1" square, inside a 2" square, two mortises 3/8" x 2" that run parallel to the wood grain, and two that run across the grain. All cuts were 1/2" deep. Half the mortises are pocket toolpaths, and the other half are profile toolpaths.

The results didn't surprise me much. With each profile toolpath I was able to set a seperate last pass of a few 1000ths which let the bit finish each path within .001" or so. The pocketed mortises show obvious deflection in width as the bit moved down their length. Each was narrower than the programmed 3/8" width by as much as 1/32".

I've ordered new anti-backlash nuts for my Meteor as well.
I can not manually find any obvious deflection at the router mount. No slop in the acme threads. No flex of any gantry parts. My thinking is that the spring in the anti-backlash nuts has fatigued over the years and now lets the force of an endmill moving forward/backward through hardwoods push the nut a bit.

I'll run the same file on the college Meteor on monday, and report here if its new drive nuts have fixed this deflection.

4D

4DThinker
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Re: Replacing drive nuts on all three axes

Postby 4DThinker » Thu Mar 12, 2020 1:10 pm

Now that my drive nuts have arrived and been installed, I ran the same file on a piece of red oak.
Same measurable deflection on the oak as was on my original sample cut in ash. I set up a way to see if and how much my router bit could deflect under load, and found the bit could be moved about 1/32" at the tip. This was with the Z axis within 1/2" of the top of travel limits. The Z axis bearings are now my guess as where the flex is happening, and I've ordered two new bearing blocks from Amazon.

BTW, the plastic drive nuts WERE worth replacing. My Meteor sounds smoother when jogging than it has for some time. Same was true after replacing the drive nuts on my college's Meteor. I haven't cut the accuracy test on the college CNC as it is closed for spring break. I'm guessing new bearings will be worth installing on it as well.

4D

4DThinker
Posts: 265
Joined: Wed Oct 07, 2015 10:57 am
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Re: Replacing drive nuts on all three axes

Postby 4DThinker » Mon Mar 16, 2020 12:06 am

New bearings on my Z axis have reduced the amount of flex, but not completely eliminated it. Y and X axis bearing are just as old. I'm not quite sure how to trace deflection up from the bit to the Z axis assembly then the X axis assembly then the Y, but I may order new bearing for Y and X anyway.

4D


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