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Measuring a 3D Printer Bed’s Flatness

Once I got my printer’s z-probe and autoleveling set up I was struck with a sudden paranoia that my printer bed was saddled. The standard autoleveling in Marlin can deal with a printer bed that is not level, but it cannot fully correct for a bed that is not entirely planar*, e.g. one that is bowed or saddled. To try and set my paranoia to rest I decided to test whether my bed was truly flat.
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Alternating stripes of tape - one "sticky side up" piece of tape is held by two adjacent "sticky side down" pieces

Stripey kapton tape for glass beds

Some time ago I put a borosilicate glass bed on Squirty. I really like it because PLA will stick firmly to it when it is hot, but will pop off easily once it cools down. It is also really easy to clean. The only issue I had was how to attach it to the aluminium print bed surface. I know a lot of people use binder clips, but they have the disadvantage of slightly reducing the print area and also jut out above the print surface, risking collisions with the nozzle. Up until now my approach was to use kapton tape along the sides of the bed. It wasn’t a great solution because the glass still could slide back and forth slightly relative to the aluminium bed and it also tended to peel off over time. Today it occured to me that what I needed was double sided kapton tape. Such a thing does exist, but rather than ordering it I decided to cheat by alternating stripes of kapton tape with one “sticky side up” piece of tape stuck down by two “sticky side down” pieces. This arrangement holds the glass in place really firmly and yet I think I could still remove the bed without causing any damage.

This is a really simple idea that probably should have occurred to me years ago, but I am unreasonably pleased with it!

The bad old days. Tape around the side of the bed ineffectively holding the glass in place
The bad old days. Tape around the side of the bed ineffectively holding the glass in place

Alternating stripes of tape - one "sticky side up" piece of tape is held by two adjacent "sticky side down" pieces
Alternating stripes of tape – one “sticky side up” piece of tape is held by two adjacent “sticky side down” pieces

The solution to the collision problem - spacers that hold the Melzi board further away from the frame

Melzi holder and new wiring

In the last couple of posts I described how I put a z-probe onto Squirty the RepRap. Adding the z-probe meant new wires. These wires ended up jumbled all over the place and occasionally got caught on the z-axis threaded rods, causing all manner of chaos to break out. The z-probe also requires a lot of clearance, which means that the printer carriage spends more time up at the top of the build area than it used to. Once up there, it has a nasty habit of bumping into the Melzi control board. To try and tidy things up, I collected all the wiring for the z-probe and ducted fan components together and designed a new set of Melzi holding clips.

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Wiring for the servo and microswitch

Z-probe V2: Autoleveling

Following on from the previous post I redesigned the z-probe holder to position the microswitch closer to the nozzle. I also forked the latest version of the Marlin firmware to correct some problems I was having with the leveling behaviour. The video below shows that the autoleveling seems to be working, but I need to do some checks to understand how successfully it is correcting for any tilt in the bed. I also need to make further design changes to the z-probe itself. This post includes more details on how I designed and set up V2.

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V1 Z-Probe: the probe is far away from the nozzle

Z-probe V1: The Gom Jabbar

By far the most frustrating part of 3D printing with Squirty the RepRap has always been the strange dance you have to do to get the Z distance correct. I use a strip of paper and carefully pull it back and forth underneath the nozzle while trying to bring it close enough to the bed so that the paper just starts to rub against the nozzle. At that point you can use a command to tell the printer what the offset between the Z endstop and the bed is. It is a massive pain, and this assumes that the bed is level, which it rarely is. To level the bed you need to repeat something like this at multiple points while making fiddly adjustments to the three supporting bolts.
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Shell of a star shaped prism

Creating Shells with OpenSCAD

Antony Monjauze had a question about whether OpenSCAD could be used to create a shell from an imported stl. At first I thought this wouldn’t be possible. It seems like to create a shell you would need to know something about the geometry of the stl you were importing, and OpenSCAD can’t tell you anything about what the dimensions of an imported stl are. However Henning Meyer’s comment on this thread explains how it can be done.
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Ready for exercise

Hacking the Butler

My first hackathon was in April. It was fantastic experience. We came up with a cool product (1st Runner up!) and I also got to learn a lot, not just technical skills but also about how to approach product development and working in a technical team. I’m going to write two posts about the hackathon. This post describes what we made, the next one will talk about my learning points. Continue reading