Glass Print Bed for Printrbot Simple Metal

Posted 11/08/2015

I’ve been having some problems lately getting good prints on my Printrbot Simple Metal 3D printer, and after a lot of inet research I concluded the problem was a warped print bed.  The bed that comes with the Simple is pretty nice, but just not thick enough IMHO to avoid some warping. And once the bed becomes even a little bit non-planar, then the very nice ‘auto-leveling’ (more accurately ‘bed tilt compensation’) ceases to be very useful.

There was some discussion about putting a glass plate down over the original bed, which takes care of the non-planar issue, but then the Hall-effect metal bed sensor can’t ‘see’ the metal print bed through the glass plate, unless the glass plate is too thin to do any good.  What to do?

I’m a long-time Electrical Engineer and antenna researcher, and so it occurred to me that I might be able to fake out the bed sensor by applying some adhesive-backed metal tape to the top of a glass plate, thereby establishing a new ‘zero’ reference at the top surface of the glass plate.  I ran some simple experiments, and found that the bed sensor worked fine with even very thin strips of just about any metal, including copper.  Since I knew I could get copper tape in various thicknesses and widths, I thought this idea might be worth a try.

The glass plate:

I remembered seeing a post somewhere that someone had found that a popular picture frame size worked just great for the Printrbot Simple Metal print bed, so I headed over to my local JoAnne’s fabric to see what was available.  I found lots of cheap picture frames in various sizes, but nothing that even remotely approached a good size for my print bed – bummer.  So, I decided to go the custom route, and, after carefully measuring my bed, got a piece of 3/32″ (~2.3mm) glass cut to 9.5 x 6.5″.  I brought the piece home and carefully smoothed the edges with my belt sander (a sander drum attachment for a drill would work also).  Unfortunately, when I laid the piece on my print bed, I discovered I had screwed up – the 9.5″ dimension was slightly too large, and the pieced didn’t quite fit between the two sets of mounting screws.  I was really bummed out, until I realized I might be able to recover from this disaster by simply grinding cutouts for the screwheads, thereby converting a ‘bug’ into a feature! ;-).  Indeed I was able to do this with a Dremel tool and a small grinding bit, and when I was finished I had a nice, self-registering glass plate for my Printrbot!

Screw head cutouts ground with Dremel tool and small grinding bit

Screw head cutouts ground with Dremel tool and small grinding bit

The Sensor Tape:

I didn’t have any adhesive backed copper tape handy (I used this stuff by the roll in my prior life as a research scientist, but didn’t think to take any with me into retirement), so I tolled the net for a while for sources.  I finally wound up ordering a 10′ roll of 1/4″ adhesive-backed copper tape from eBay

CopperTape

Assembling the new Print Bed:

I used heavy-duty document clips to hold the glass plate down on the original print bed, and then placed copper tape patches at the X/Y home location and the two ‘slope compensation’ sensing points, and then covered the entire thing with blue painter’s tape to provide the same first-layer adhesion as before.

151108_PrintBed2 151108_PrintBed4 151108_PrintBed5

151108_PrintBed1

Testing:

I used a 20mm hollow cal cube as my test object, starting at a point near the X/Y home position.  As expected, I had to adjust the Z offset some, and wound up with a Z offset of about -0.8 to get a decent print.

151108_20mmCube1 151108_20mmCube2

Then I tried moving the cube around on the print bed, and found that I had to keep moving the Z offset further negative as I moved the print position away from the X/Y home position.  To get the cube to print properly, I would up with a Z offset of -2.0mm, way more than I had expected, and then when I tried to move the print back to near the X/Y origin with the same Z offset, I got the extrusion drag marking shown – bummer!

151108_PrintBedSlopeProb1

After seeing the above problem occur, I tried to figure out what was going wrong, and having a heck of a time with it.  Here’s what I know:

  • The Z offset required to get decent first-layer adhesion gets markedly more negative the further away the print position is from the X/Y home position
  • Trying to print near the X/Y home position with the Z offset required for a position far away from X/Y home causes extrusion head dragging.
  • However, in a somewhat contradictory finding, if I manually move the extrusion head position arm (the Y axis, I think) toward the outer edge of the print bed, it starts to drag about halfway across for X values near home, and this condition becomes more marked when the bed is manually moved in the positive X direction (to the left looking at the front of the printer).  In other words, when moving the extrusion head in manual mode, it appears the print bed has a marked upward slope as X & Y increase, causing significant head drag.  However, when printing, the opposite effect seems to occur, where the print bed appears to have a  marked downward slope as X/Y increase.  This makes no sense at all!

So, at the moment I’m officially baffled. In manual mode, the printrbot behaves as if the bed is sloped upward as X & Y increase, but in printing mode (where I assume the ’tilt compensation’ is in effect), the printrbot behaves as if the bed is sloped in the opposite direction.  What gives!?

More to come (I hope)

 

 

 

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2 thoughts on “Glass Print Bed for Printrbot Simple Metal

  1. Sean

    I have the same setup and the exact same problem. I find that on one print the extruder is way too close to the bed towards the back of the bed and on the next print its less so.

    Reply

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