Waviness or not...That is the question

#2183

 

Ok, I just saw the latest video on youtube by robo 3d. If you want to know one reason why there is waviness in their printer just watch the video and keep a close eye on the screws for the z axis.

 

What part of the video are you getting at?

 

You can't miss the z axis screws wobbling big time. More than once throughout the video. First time is about 20 seconds into the video. Another time is about 6:45 into the video. That is really bad screw wobble on the printer in this video.

 

That seems normal to me

 

Will if that is normal then that is why they have a problem. I had wobble about that bad too. Once I fixed it my prints came out good. I don't want to get into a pissing match but that shows bent screws and or poorly made couplings. Had both on my machine.

Mike I have seen some of the nice things you have done to upgrade your robo. I can't believe you would think screws running out that much would have no effect on the print quality. Before I fixed the runout in my machine z axis screws I could see the x/z carriage wobble when the screws rotated.

 

As long as the smooth rods are stable it should not be a big problem. I tried stabilizing my z-axis threaded rods and it didn't make any difference because my smooth rods were always stable. Switching to metric rods fixed everything!

 

Kadows is correct, the smooth rod is what stabilizes the movement from the threaded rod. If not fully stable it could cause ribbing.

 

The thin mounts on the z smooth rods are to thin to stabilize anything. That is why there are a few designs for lower mounts and upper rod mounts.

 

The smooth rod inserts .25" into the acrylic base. That's where the majority of the support comes from.

 

I guess I can't convince you that when things wobble they transfer that motion to other parts and cause poor print outs. I know this was what was wrong with my machine. A 12 inch long steel rod pressed 1/4" into plastic is not going to support anything. Supporting it at the top and bottom is the only way to begin to control the motion. Remember you also have a heavy motor mounted on the X axis that changes direction at speeds of 60mm per second or more. Even at a slower 30mm a second that is a lot of inertia to control.

Another thing about that press fit into the plastic. The rods what they were putting on these machines were 5/16" diameter. I changed my smooth rods to the proper size 8mm. The 8mm ones pressed easy into the acrylic base by hand. They also pushed nice and easy by hand into the x axis end mounts when I changed the x axis rods. The difference between the 5/16" rods and the 8mm ones was 4 thousandths of an inch. The 8mm rods were bigger by 4 thousandths of an inch. If that hole in the bottom plate was such a great fit for the 5/16" rods I'd never be able to just push 8mm rods into those holes by hand. I've been a toolmaker for many years and know all about press fits and what it takes to make things. I do this everyday for a living.

One more thing. Who puts .004 undersize rods in linear bearings? I wonder if their china manufacturer just tried to save a little money and did that without telling the guys at robo 3d? 5/16" rods go in 5/16" bearings. 8mm rods go in 8mm bearings. You can see the slop by twisting the carriage on the x axis with 5/16" rods. That causes the head to flop around translating into loss of motion as the x axis tries to change direction. This creates small lines on the sides of the prints. I know this because I spent a lot of time making changes one at a time. Changing the rods made my prints just a little better than they were with the old rods. I betting the guys at robo 3d designed this machine for 8mm rods because 8mm rods pressed really nicely into the x and z axis. This could be where a lot of their problems are coming from. Wonder if any of the first machines has 8mm rods in them?

 

The improvement is small changing over to 8mm rods but it does help. Nothing like the big ribbing some people got and I had before I fixed my z axis couplings and threaded rods.

 

I have never had ribbing, still running stock rods (for now) and the rods wobble just as much as in the video.
When I got my Robo, the first thing I did was go through and make sure every screw was tight, every cable/plug securely fastened and the belts were properly tensioned.

The only issue I had initially was some x-axis slip due to a warped z-axis carriage where the x-axis motor was mounted. Once I sorted that out no more slip.

 

Stock printer and i am getting some great looking prints.

I haven't added the z axis stabilizers either cause it's working just fine, and my z axis wobbles. I think Mike pointed out that it's suppose to do that.




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Do you want me to work out the free body diagram for you? The rotational inertia must be greater than the support resistance for it to move. This is also making the assumption the threaded rods are moving during a layer, which they're previously haven't been. The X movement is a function of the deceleration, which I've never seen that be an issue.

A 1/4" hole can provide a LOT of support. Your biggest enemy is the moment being put on the threaded rod, which is marginal. You'll have this moment whether you support it 1/4" or 12". That's simple mechanics.

I get you're searching for answers but try and make sure you're basing these not on feeling but logical analysis

With all this said, there is good reason to reinforce the top smooth rods, robo 3d will be doing this in future revisions supposedly.

I haven't measured my smooth rods, but I have full reason to believe they are actually 8mm, and not 5/16". Were they 5/16" you would see a .0625mm deviation. A .004mm is within the standard deviation of smooth rods. Typically they're rated as 8mm +0mm -.005mm

 

My rods were .004 of an inch under size. Not .004mm under. That's way off from what it should be in an 8mm bearing. And I am basing everything I say on my machine. I only want to help if I can. If no one is interested then that's ok. My machine works good now that I made the changes I talked about. I did the one at a time and saw improvements with each one. Seems like you and some other guys got machines that were a lot better than my machine. That said maybe you can not see what I had to do to get my machine as needed improvements. I'm sure you would feel different if you unlucky enough to have got my machine.

 

I'll measure my rods when I get home. Misread earlier.

I appreciate you trying to find answers to this, we all are. We just have to be sure we don't miss the mountain exploring a mole hill.

Several variables can play a part. Slop in the nut, how centered the threaded rod is in the coupling, how straight the threaded rod is, how rigid the z smooth rods are, if they're undersized, how much flex do these see, etc. It could be any and all of these, but we need to explore them one by one in a careful controlled manner.

My current plan is to extend the smooth and threaded rods to the top of the case and secure them with a printed plastic component that fits a bearing for the threaded. I believe this should greatly increase the rigidity of the printer. Though I've never seen much ribbing and only slight waviness.

 

That's a great idea Mike! I have been thinking of extending them through the top of the case and making blocks that support them. This way the support blocks would be on the outside on top of the case. Of course this requires making clearance holes for the rods in the case.

 

I just measured my rods. Using the sharp edge of my caliper I was measuring around 7.93mm. This was the same measurement I was getting from rods I knew were 8mm. With the flat section they measured around 8mm

I'd say with almost absolute certainty all smooth rods on the robo are 8mm