Z height changing during print.

Guys, I’m having an issue where the it appears that the Z-Stop position is changing on me during printing. I have been unable to determine if its the extruder coming closer to the bed or the bed coming to the extruder. (Taz 4 frame, upgraded to hexagon hot end with the modular print bed)

Here are some example photos. I am printing ten objects one at a time, ABS plastic 270C extruder and 100C bed temp. As you can see on the right side in the middle the extruder gets so close to the bed it cannot extrude the skirt and makes a mess out of the bottom few layers on the part. The error occurs on part number 6 and it then goes on to print the other 4 pieces correctly. I don’t know if it shows it in the photo but the part directly in front of and directly behind also have some squish in the first few layers. This would lead me to think that I have a high spot on the glass.

(Sorry pics have rotated 90 degrees)

Except the plot thickens.
Yesterday I was printing another ten objects of a larger size (different gcode from above). Just as it finished the last object I paused the printer and cleaned the bed of all completed pieces and restarted the job. The printer homed the x,y and z axis and started extruding the skirt around the first object. I noticed immediately that the extruder was to close to the glass because of how the skirt was being printed. I decided to let the print finish. Now the high-spot has moved to the front row of connectors, you can see how the skirts are not even and incomplete on the front row.

For a while now every time I start a new print job I have to reset the Z height. I can set it, print a 10hr job, let the printer cool off, bring the extruder and bed back up to temp and then check the Zheight and it will be off. Normally it is always too high and I have to adjust the Zstop down. I have swapped the micro switch on the Zstop for the one on the Xaxis but that didin’t make a difference.

I have also noticed as everything comes up to temperature the distance between the extruder and bed decreases as everything warms up.

Is there a way to have pronterface display a an Absolute Z measurement so that I could see if the Z comes back to the same spot each time? I tried using an M114 command but when I home the Z axis M114 just shows 0 on Z. If I adjust the Zstop so that I have the correct clearance and home Z again M114 just shows 0 again. What I am trying to do is get the software to show me where Z stops at each time to rule out if the extruder is coming back to the same spot each time.

Sounds like the easiest place to start is with releveling your bed. You can use the TAZ 4 instructions as a guide. https://www.lulzbot.com/content/taz-bed-leveling

The directions will tell you to use a business card or a folded over piece of paper to use as a feeler gauge between the bed and the nozzle. Just be careful not to rest your hand on the bed when doing those measurements.

I don’t know if it’s relevant to the TAZ 4, but the TAZ 5 had issues with the modular bed bowing down in the center. four pads for the TAZ 6 should do the trick in fixing this problem. Place them in a square in the center of the bed.

So here is a follow up.

Got a good straight edge and started checking my bed for flatness.
When cold the glass sags in the middle of the bed.
Crank up the heat 100C put the straight edge back on the glass and guess what. Its now flat in the X direction and I have a hump in the Y direction on the Right hand side of the bed, where the parts were getting mashed.

I removed the clamps from the glass bed at the corners and I have been running it trouble free for weeks now. I think what was happening was the glass was expanding as it warmed. With the corners being held in place and supported from underneath by the Aluminum plate the glass had no were to go but up. Removing the clips allowed the glass to expand an lay flat.
I would give you a measurement of the cold and hot glass, but I don’t have calipers that big.

The thing that puzzles me is why didn’t it do it on the left hand side also?