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Message from discussion interesting note about the belts on the Ultimaker
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Kevin Reilly  
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 More options Sep 20 2012, 11:42 am
From: Kevin Reilly <kevin.t.rei...@gmail.com>
Date: Thu, 20 Sep 2012 08:42:26 -0700
Local: Thurs, Sep 20 2012 11:42 am
Subject: Re: interesting note about the belts on the Ultimaker
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@ Dave and Bill: So just a quick update on my previous post, after
repositioning the bearings and adding a second tensioner to the long x
belts, my backlash in the x went from over 1mm to less than 0.2 mm. All the
test objects I printed were less than 0.1 mm from spec. I discovered one of
my bearing on the x rods wasn't 100% perpendicular to the rod and was
rubbing a bit on the bearing cover, which was adding some resistance in the
x axis. Adding the second belt tensioner help TONS. This one belt IS
stretching. When I first assembled it was tight.'after adding the first
tensioner it was tight. Now I need two? Crazy...

Back on topic: I think we should be tackling backlash from both angles.
With my CNC mill I spent tons of time getting the backlash down by tuning
the mechanics of the machine (I think I got it to 0.15 mm or so), then
honed it in using software and got zero backlash (I was machining micro phi
if channels and needed the precision). I understand the issues with trying
to do this with fast moving machines but software correction did work for
me when I was running it at 100 - 150 mm/s (cutting thin carbonate and
kapton sheets).

How hard would it be to have an experimental version of Marlin where some
of us can experiment with this?

On Thursday, September 20, 2012, Boman33 <Boma...@vinland.com> wrote:
> Although it is easy to add or subtract steps on a static system, but once

you have advanced motion control software it adds lots of problems for the
trajectory planning software when it is moving quickly.

> The steps cannot be instantaneously added so there are issues of

acceleration and deacceleration for adding those steps and that interferes
with the planned movement of the other axis.  There have been long
discussions in the CNC groups about the problems with backlash compensation
and fast moving machines.

> On top of that, I think most of the backlash is dynamic and varying with

print speed.  Even if the X-Y moves correctly, the head might wobble
because of acceleration forces.  So in summary, fix the hardware.

> Bertho

> From: catohagen  Sent: Thursday, September 20, 2012 04:08

> I work with cnc machines, and usally backlash is adjusted with parameters

in the control, maybe this could be fixed in the UM firmware ?

> if using a dial gauge and your X-axis have 0.3mm backlash (gcode moves

the axis 40mm but the hardware moves 39,7mm) the firmware could handle this
with driving the stepper the desired extra steps to compensate for backlash.

> --

> --
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--
Kevin Reilly

PhD Candidate
Department of Chemical and Biological Engineering
University of British Columbia
P: 604.827.4123    C: 604.313.9149


 
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