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How to home / zero the z-axis and 1st layer

Posted by sxt173 
How to home / zero the z-axis and 1st layer
June 02, 2011 02:23PM
Hi all, I'm a newbiew and just started printing my 1st (fugly) parts a few days ago; very exciting!

I am having a lot of trouble with getting the z-axis to an acceptable start point. I searched but did not find a comprehensive solution in the forums.

My setup below:
  • Mendel from BotMill
  • Techzonecom Gen 3 electronics
  • Running Repsnapper v353 - (no adjustments made)
  • Printing w/ PLA
  • Non-heated aluminum bed with blue masking tape

My many problems:
  1. I have tried playing with the z-axis metal flag and where it meets the opto-endstop, but this is very crude and I have not gotten consistent results.
  2. Should I 1st let the device home itself and then adjust the springs on the build platform to level at different points? This also seems very imprecise to me.
  3. Can I level the platform where the hot tip is touching at all points and then insert G-code to raise the tip by a certain height before the print begins? I tried G1 Z1, but I'm not sure what the whole absolute/relative positioning thing is.. How high off the platform should the tip be?
  4. I am having mixed results getting the 1st layer to stick (printing @ 250C). Either it sticks fine, but the 2nd layer is too low and hits the 1st layer (probably because the 1st layer started too low), or I get what I like to call "crazy extruder worm syndrome" where nothing sticks.
  5. Even if I eyeball-level the platform, it seems that the aluminum is not 100% flat. What's the best solution? Replace with thicker sheet? I saw that some guys use glass w/ Kapton tape on top.
  6. I preheat the tip in repsnapper to 250C. While it is heating, the tip starts oozing a lot. Right now, I am manually clearing the ooze as it starts printing. How do I avoid the ooze?
  7. Finally, would a heated platform solve my not-sticking issue? I'm thinking glass with Kapton tape heated to 70 degrees.

Thanks much in advance!
Re: How to home / zero the z-axis and 1st layer
June 02, 2011 03:05PM
I levelled my main frame then levelled the bed to match, then spent a few hours moving the tip around the bed and relevelled the bed to match the tip. Once that was done I set the tip to the correct height by winding the opto up or down a 1/4 turn of the screw at a time until I could just slide a piece of paper between the tip and the bed.

This produced consistent results however I had to do it all over again when I put a piece of glass on top of my bed

Experimenting in 3D in New Zealand
Re: How to home / zero the z-axis and 1st layer
June 02, 2011 03:16PM
But I'm assuming you don't start prints with the tip height at the thickness of a sheet of paper? That would just "squish" the plastic that's extruded.. Once the tip is level at all points, how do you set the correct start height?

Any ideas on the other questions/issues?
Re: How to home / zero the z-axis and 1st layer
June 02, 2011 03:33PM
Fugly parts are always going to happen. Much less as time goes by, but things clog, nuts come loose, things melt and slip. We've all been there, I assure you.

I mount the opto on the vertical smooth rod on the left. This is where the x-axis motor is, so it's less likely to lag behind when the axis is coming down for a home. The flag is mounted to the bottom of the X-motor mount, pointing down.

Leveling the bed is straight forward, as long as you follow the steps in order.
0. If your aluminum/acrylic/wood print bed is not level, you must replace it, or limit your self to printing only around the high spot. If the high spots are on the edges (sagging), then you need to flip your bed. Aw, just get something flat!!!

1. Tighten the nuts under the print bed, so the screws are absolutely secure and perpendicular to the bed surface.
2. Use springs. You can get a 10-Pack of pens from the dollar store for, well, a dollar.
3. Tighten the nuts under the y-axis platform so that there is just enough clearance (2-3mm?) from the bottom of the print bed to the top of the fender washers. This will give the springs good tension, and prevent the bed from collapsing under it's own weight.
4. Starting with the corner nearest to the front of the machine on the side where you have your opt-limit switch, and the opposite diagonal corner, bring the z-axis to home as close as possible to the front corner, and adjust the nut until the bed just touches the nozzle. If you re-home z-axis, it should touch, but not push the bed down at all.
5. Jog the nozzle to the opposite corner and make a similar adjustment to the nut there. Do not make any additional adjustment to your z-axis homing flag or switch. If you do, you need to go back to step four (4), and begin again.
6. Once the two diagonal corners are level, tighten each with a second nut. If you are using nylon-backed nuts, this is not required. You can also use glue, paint, or locktite, but these need to dry before you proceed, and should be done after all four corners are leveled.
7. In a similar fashion, make the leveling adjustment to the other two corners (diagonal to each other). Check all four corners, and make adjustments as required, but always work in diagonal pairs.
8. My prints always start with the nozzle tip touching the bed or 0.1mm up from the bed, so layer 1 and 2 are compressed (my layers are usually 0.4mm). Sometimes, I will manually raise the bed with a 1/4 turn to the z-axis at the begining. If I get a worm, I restart the print, otherwise I let it go.

If you are using Skeinforge, there is a place where you can specify how much you want to come up from z=0 at the start of the print. I believe its in the "Bottom" tab.

If you are using cold aluminum you will always have sticking trouble The reason for this is that the aluminum conducts the heat out of the filaments instantly, so it has very little time to bond to the surface. If you want to reduce the effect, use additional layers of blue tape. These will effectively insulate the extrudate from the highly conductive surface of the bed. Acrylic covered in Kapton is a better choice for cold printing, but you will need to flatten the acrylic. I've used a heat gun, and two heavy granite tiles to do this, and the results were very good.

Ooozing is normal, but 250 is too hot if you are using PLA. Are you using PLA? If so, bring the temperature down to no higher than 200c.

At the start of the print I always run the extruder to make sure there is oozing. This insures that the tip is extruding onto the bed as soon as it hits the edge. This anchors the extrudate, and prevents the "worms".

My current setup is an 1/8" 8x8 Boro-silicate glass pane (aka Pyrex) on top of a printed PCB heated bed. The bed sits on standoffs at the corners where it attaches to the acrylic print bed below, which I no longer use. There is no Kapton on the glass, but I do use it in the corners to hold the glass onto the PCB.

Nophead and others have commented that they were not able to stick to bare glass. This could be due to lower temperature settings on the heated bed. I run mine between 100c and 120c, and the ABS sticks well. The really cool thing is that below 90c, it sticks really bad, so it's easy to remove parts without cooling too much.

Hope this helps!
Re: How to home / zero the z-axis and 1st layer
June 02, 2011 03:55PM
@jcabrer - Outstanding. Copy and paste it in the wiki somewhere?
Re: How to home / zero the z-axis and 1st layer
June 02, 2011 04:14PM
droftarts Wrote:
> @jcabrer - Outstanding. Copy and paste it in the
> wiki somewhere?

@jcabrer: Agreed, this should become a wiki!

I will be trying out your recommendations when I get home and will also upgrade to a (flat) glass heated surface.
Re: How to home / zero the z-axis and 1st layer
June 03, 2011 03:39PM
I am currently looking for a new way to heat my bed as at the moment I can only get to 55C which is not hot enough for glass

Experimenting in 3D in New Zealand
Re: How to home / zero the z-axis and 1st layer
June 03, 2011 04:57PM
Anybody try using the Makerbot Heater Board?

I'm thinking about Kapton taping this to the bottom of a pyrex glass plate. Would it be a plug&play as far as connections to for example techzone gen3 remix boards?
Re: How to home / zero the z-axis and 1st layer
June 03, 2011 07:32PM
I have found different colors of ABS have quite different adhesion properties. Black will stick to anything, but fluorescent green is like Teflon...
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