Y Axis Vibration Issues at Slow Speed Only!?

First off I have a homebuilt machine. We just added new stepper drives from Amazon. They move amazing compared to the old hand me downs I had before. Motors were used before and were working fine, we just wanted to increase speed and fix some homing issues etc, which is why we did the upgrade of drivers. Anyways, I have rapids set to 3000mm/min. When moving rapid it vibrates a bit as it speeds up, then gets up to speed and is smooth as butter. If I set it to 1000mm/min it just constantly vibrates. Same with other slow speeds, such as when homing around 600mm/min. All vibration during Y axis movement only (diagonals too, but nothing in X or Z)

The machine itself is not binding. I can slide the gantry by hand quite easily. So I’m certain it’s not mechanical. I have tried doubling, and doubling again, my steps per mm to see if that smoothed it out. Nope, no change. I haven’t gone less but suspect it will be the same. My next step tonight is to try changing how many Amps I send to the motor once I know how much it can handle. I’m just wondering if anyone has run into an issue like this in their CNC travels, of it only binding when moving slow. Belts are evenly tight to move the Y axis as well, and as straight as I can get them.

I know it’s not a longmill, I almost wish I had just gone that route! But any help is much appreciated. Thanks everyone.

@Andrew I would look into the acceleration settings (Grbl $$ settings), as a large percentage acceleration can make steppers stall or judder, and at slower speeds a high acceleration setting would result in a large percentage change - and less so at high movement speeds.
Not sure it is a fantastic match to your described fault, but it is ‘close-ish’.

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That could be it. I had it around 80mm/sec originally on old drives I believe. Since everything was moving great I did crank it up to 150 for fun. I didnt notice if the problem was there before. But I’ll go down that route tonight and check if that introduced the problem.

So I changed my acceleration down to below the previous old drivers numbers. I think I went to 60, then 30mm/sec2. No change whatsoever. The vibration seems to stay from 0mm up to 1500mm/min for speeds regardless how much acceleration before it goes away. I have ordered a new power supply to get it up to 48V hoping this might cure it. Currently running 24V. My Y motor is quite large and more demanding, so this could be the source. Should be here tomorrow.

Such a strange issue, normally my thought would be the motors struggle at high speed, rather than slow speed when they should have more torque and the ability to move smooth. Will update again after the power supply unless anyone else has thoughts.

Ok, I wanted to respond so that anyone else reading can find this solution. Short story it WAS mechanically binding, so please try anything you can to make sure it moves smooth!

My issue was still odd. I have 1" steel shafts with large bushings that the Y gantry ride on. When dry I can usually push it very easily. Of course weather changing seems to have caused things to expand and contract at different rates. Anyways, I ran into the machine being a bit too tight, so I had to spread the main gantries apart a hair. But that’s not all. Because as I said, it moved fine at higher speeds, but not lower speeds. Just adjusting this didn’t fix the problem. I tried spraying a dry silicon spray lube. This actually made it WORSE! Still completely perplexed at this, but it is what it is. We tried a thin layer of grease that they use at the factory, and voila, all binding gone! Of course, not without more issues. I decided to coat the whole shaft with a light amount of this grease, which again, actually caused it to bind again! :face_with_symbols_over_mouth:

Wiped all the excess grease out, and used regular old WD40 to kind of wash some of the grease away, and lighten up the lube. This seems to be the magic spot. I ran 2 cuts for about an hour of cut time without a single bind. (though the wood started on fire, so that’s a whole other story…)

To sum it up for others reading in the future, my guess is the silicon spray isn’t slippery enough. The grease is, however it was an NLGI 2, which may be a touch thick, which if I had to guess caused suction/adhesion more than it did lube. After “watering” it down with WD40, it worked perfect. I suspect going forward the answer will be NLGI 1 (thinner) grease. But what a frustrating ride. I don’t think anyone with the longmill will have this issue, as you guys use V wheels on a track, as opposed to a bushing on a shaft. But this might help someone else in the future, so it’s worth posting. Thanks everyone for the help.

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