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RapMan Z wobble fixed

Posted by ablainey 
RapMan Z wobble fixed
July 30, 2011 03:33PM
I just worked out how to fix the Z wobble banding in RapMan prints with no new hardware or software tweaks. See Rapman Forum and scroll down to the pics (or read it all if you want to know the how and why). This may also be applicable for other repraps with banding opr quality issues caused by threaded rods.


Edited 2 time(s). Last edit at 07/30/2011 03:35PM by ablainey.
Re: RapMan Z wobble fixed
July 30, 2011 04:12PM
That's great! Does the RapMan use a Z-axis like the cupcake cnc? 4 threaded rods? I wonder if I can improve things with it too via a similar technique.
Re: RapMan Z wobble fixed
July 30, 2011 04:22PM
Yes I think they are pretty similar (I havn't seen inside a cupcake), teh RapMan is 4 threaded rods in teh corners, pulleys at the bottom of each with a single belt that runs round the outside. If you can loosen the nuts etc in the cupcake so you can manually turn each threaded rod then it should work.
the idea is to take the periodic thread error of the rods which translates as a cyclic vertical error in the print (creating bands) and you adjust the rods to make a 4 phase ballenced system with an bed movement error as close to zero as you can get it.
If that makes any sense.
Or in laymans terms, loosen and turn the rods a bit, retighten and see if it is better, repeat till happy.
i hope it works for you.
Re: RapMan Z wobble fixed
August 02, 2011 03:24PM
I gave this a go with my cupcake and it may have reduced the banding, but I also could just be imagining things. Here is the procedure.

Printed a few layers.
Loosened the pulley grub screw on a rod and spun the rod about 10 degrees.
Print a few more layers to see if the banding got better or worse.
If better then 10 more degrees. If worse 5 less degrees.
I started with the front left rod and got things as good as possible and then went to the back left rod.
Next up was the front right rod. My extruder is only supported by 3 of the nuts so from there I just ping ponged around.

Maybe I was doing it wrong or maybe I have other issues, but I did not get anywhere near the quality achieved in that picture you posted. The question I have is that when you spin the rods for the adjustment on your rapman does it have the side-effect of modifying the extruder height for that corner? It seems like it would, but I don't know enough about the rapman to know. Maybe my machine just has too many other mechanical inaccuracies after printing almost 10 pounds of plastic. I dunno.

I think your theory has merit and I'd really like to reproduce it and I'd love any suggestions to the process I used above.

btw. Do you have anymore photos showing off the increase in build quality? That cube print really is quite amazing.
Re: RapMan Z wobble fixed
August 03, 2011 06:02AM
Billy I have been working on writing a proper procedure for a few days. I am now suggesting 45deg for each initial turn as it shows any change much better, then turning less for fine tuning. 10 degrees is very acurateand good for fine tuning but any change may be hard to see as you have noticed.
I'll have to look at the cupcake design to get an idea of how it is different to the rapman. The principle should be the same but there may be some other factors perculiar to the cupcake.
Yes the rod adjusting effects the extruder height. If you are turning the rods clockwise it will raise the bed slightly in that corner. Anti-clk will lower it. This will change the thickness of a few print layers imediatley after adjusting and that's why I let the print continue for 5mm. So it settles down. Also remember that one full turn of the rod will change the bed height by around 1mm ! so you need to level the bed before starting a new print and watch the nozzle so it doesn't crash into the bed!

One additional thing to note is that when you get down to really fine tuning it. adjusting one side can make adjacent sides slightly worse quality. So it then becomes a ballencing act to get the best quality. As you said 'ping ponging' from one to another giving them fine tweaks.

yes I have some more photos. I have taken some for the instructions document I am putting together. I will try to get some more showing the quality change through the layers during the process. Starting with really bad banding at the bottom 5mm, then the effect of each adjustment. This should give people a much better understanding of what is happening and what is achievable.
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