I’ve had problems with my Z-axis losing steps on retraction when cutting hardwoods, ever since I’ve had my machine. It’s been a frustrating problem, to say the least, and I have scrapped some pieces from it when I don’t catch it quick enough. If you do a forum search, for Z-axis issues, others have had the same problem. A full solution was not found, or maybe just not posted. The typical answer is to check mechanics. This is not the wrong answer, but the problem may something else.
The issue only comes up when plowing through hardwood, I don’t have any trouble at all with baltic birch. That is a clue…
When the issue comes up, I can power down the machine, and feel a bind in the Z when moving it manually. Moving it up and down a few times seems to free it up. Depending on my frustration level, I’ll either do this manual up and down a few times until it feels smooth, or I’ll take it apart and clean it.
I’ve been doing most of my work in Baltic birch lately, so this has not been a problem at all. Well, Saturday I started working on a plaque in pecan. I started decking it down with a 1" Whiteside, only 1/16" passes with 80% stepover, and after 2 passes the Z loses steps on retract. I toothbrush the wheels and rails, manually move Z until it feels smooth, and go back to it. Again, and again, and again. Frustration mounts. I take it apart and go through it. I noticed the Z rails were not parallel, I adjusted them perpendicular to the wasteboard, and got them parallel within 0.001". I thought I found my problem. I went ahead and replaced the wheels and belt while I had it apart. I put it back together and got it all nicely adjusted. First pass with the newly adjusted rails, new wheels, and belt went perfectly. Then the second pass…lost steps on retract. I’m ready to scream. I may have. I did the usual toothbrush clean and manual up and down routine and went back to it. I watched it like a hawk. That’s when I saw it… A chip came up and perfectly rested between the right wheel and the rail. It was a nice chip, the kind of chip you are proud of making. The kind of chip you get when cutting hardwood, and not Baltic birch. When this pass finished up, this beautiful chip was pulled in between the wheel and rail causing a bind and lo and behold the Z-axis lost steps.
So, I think I have the problem figured out, now, how do I fix it. I tried a dust boot in the past, and honestly, I would rather just vacuum the chips out of my enclosure periodically. I’m thinking about making a chip shield. I would attach it similarly like a suck-it.
This morning I was scanning the forum and found @Julien posted this. I was thinking about something similar. Does anyone have a shield that they have made they would like to share. I’ll probably work on my idea this weekend.
Sorry for the long post. This has been a struggle, and I hope this may help others diagnose this issue.
TL;DR - If your Z-axis loses steps on retract, it may be chips getting between the wheels and rail, causing a bind.
Wow, nice detective work, and thank you for sharing, it does seem like a likely root cause.
Hopefully someone has a chipguard on hand that is available and compatible with the stock Z axis.
I still think you should reconsider dust collection though 
(with a fixed-Z height design, it’s night and day, just my two cents)
As Julien says, extraction would be my first shot at the problem.
After that, linear rails all use a wiper of some sort on the rail, some rubber wipers around your V-Wheels to clear the rail as they move up and down? I’ve seen plenty of crud get attached to the X rail on mine even with extraction.
It would also be really nice to see Carbide move over to lost step detecting drivers on the control board as Prusa does on their printers, the Trinamics are quieter, faster and do stall detection, this is relatively cheap and doesn’t require closed loop and encoders.
James - I’ve seen a couple of avenues to go down, and while there is always room for improvement, take a look at these options…
I’ve also heard of people installing brushes - similar to how bandsaws have stationary brushes that wipe the wheels as they go around.
Seems like there is another detective around. V wheels jamming due to chips
I may look at extraction again. It’s just so much easier to set the bit, close the door and hit go, and not have to deal with a dust boot. I can’t find a Z-wheel cover. I saw the Y and X, just not a Z. I love the bellows @Vince.Fab made for his Z. I was thinking that bellows would be the best way to deal with the problem. I would want them on top and bottom.
I do get stuff stuck on my X and Y rails and clean that off pretty often. I could see where brushes may help with that, but I don’t think brushes or wipers would help with what I found, chips falling in the top of the wheel at the rail.
These Z covers may do the trick. I haven’t seen any pictures with anyone with them on a stock Z though.
I made a quick “cover” for the top part of the Z- axis. It’s definitely not the final solution but it did help enough for me to get through this plaque without anymore problems.
I put a couple of mounting slots on it, just in case I want to add on to it.
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