Z-offset question?

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mdivancic
Posts: 26
Joined: Wed Mar 21, 2018 9:26 am

Z-offset question?

Post by mdivancic » Tue Jun 04, 2019 3:05 pm

I’m running a Mini 1.4 with the SL tool head. I printed with ABS for the first two spools, but switched to PLA last night. I had the Z-offset dialed in very good with the ABS, nice smooth bottoms, no problems with parts separating from the print bed. I’m getting completely different results with PLA, the prints look like nozzle is to far from the bed. Should I be using different offsets with different types of resin?

Regards,
Mike
Regards,
Mike

Brent.I
Aleph Objects | LulzBot
Posts: 497
Joined: Mon Jan 20, 2014 8:21 am

Re: Z-offset question?

Post by Brent.I » Tue Jun 04, 2019 3:37 pm

mdivancic wrote:
Tue Jun 04, 2019 3:05 pm
Should I be using different offsets with different types of resin?
What you are running into is known as Die Swell. This is essentially a property of a thermoplastic to expand when forced through a small hole.

Different materials will expand at different rates, and ABS generally expands more than PLA. We recommend adjusting "initial layer flow rate" for your specific material, to avoid adjusting Z offsets when switching between materials.

mdivancic
Posts: 26
Joined: Wed Mar 21, 2018 9:26 am

Re: Z-offset question?

Post by mdivancic » Tue Jun 04, 2019 3:45 pm

Thanks for the quick response. Something to play with tonight.

Regards,
Mike
Regards,
Mike

mdivancic
Posts: 26
Joined: Wed Mar 21, 2018 9:26 am

Re: Z-offset question?

Post by mdivancic » Tue Jun 04, 2019 4:04 pm

Ok, silly question number two. The flow rate for my ABS is 100% and it’s also 100% for the PLA I’m using. Can I increase this value over 100% or should I get the Z-offset right for the PLA and reduce flow with the ABS?

Regards,
Mike
Regards,
Mike

Brent.I
Aleph Objects | LulzBot
Posts: 497
Joined: Mon Jan 20, 2014 8:21 am

Re: Z-offset question?

Post by Brent.I » Tue Jun 04, 2019 4:17 pm

mdivancic wrote:
Tue Jun 04, 2019 4:04 pm
Can I increase this value over 100%
Absolutely, you can increase over 100%. If you go too high, it will result in a "strip out" where essentially the extruder is trying to force filament through the hot end faster than it can be melted. We would recommend probably 2-5% increases at a time while dialing it in properly.

If you do run into a strip out while trying this, clean your hobbed gear after removing the filament from hot end and clip off the "groove" worn into your filament from the strip out. Re-load, and give it another try.

mdivancic
Posts: 26
Joined: Wed Mar 21, 2018 9:26 am

Re: Z-offset question?

Post by mdivancic » Fri Jun 07, 2019 6:34 am

Thanks for the help on this. Good learning experience. I ended up adjust the Z-offset for the PLA to get a good print. I reduced the flow rate for the first layer on the ABS at the new settings to get a good print. I was more comfortable with this path as the flow rate setting was getting up there on the PLA and I was worried about strip out. Back to printing now after several days of testing.

Bonus question: What's the maximum Z-offset you can apply before the nozzle comes in contact with the print bed?

Regards,
Mike
Regards,
Mike

b-morgan
Posts: 259
Joined: Fri Aug 11, 2017 8:53 am

Re: Z-offset question?

Post by b-morgan » Fri Jun 07, 2019 10:14 am

mdivancic wrote:
Fri Jun 07, 2019 6:34 am
Bonus question: What's the maximum Z-offset you can apply before the nozzle comes in contact with the print bed?
It depends on the physical characteristics of each individual printer. The printer has "switches" (physical or logical) which it uses to determine where to set X, Y, and Z to zero. Without a z-offset, if you tell the printer to go to Z0, it may not just touch the bed (above is OK, below is dangerous and possibly destructive). The z-offset is used to make the actual location of the nozzle exactly touching the bed (or some small tolerance above). Note that the z-offset could be either a negative or positive value.

While adjusting the z-offset, it would be a bit dangerous if the value puts the nozzle too low (i.e. below the bed surface). To avoid this, after the printer has been zeroed (G28), manually move the nozzle to a safe small height above the bed (for example, Z1.75, Z2.85, or Z3.0, the diameter of a piece of filament) and observe the "gap" between the nozzle and the bed as you move the printer to various XY locations. If it varies too much as you move around, you may need to level the bed.

With a decent set of feeler gauges, you can measure this gap as you lower the nozzle (the largest feeler gauge in the set may only be 0.9 to 0.8mm). If you don't have feeler gauges, a piece of 20lb paper is about .1mm, a business card about .3mm. Once you can measure the gap, you can compare the Z value with the measured value and adjust z-offset so that they match.

You are probably doing these adjustments with a cold nozzle. Keep in mind that a hot nozzle will change things so allow for that (or carefully measure it). In practice (as you have done), the z-offset is often adjusted "on the fly" as you print the first layer of a test object and you can avoid doing this intellectual exercise.

johnson
Aleph Objects | LulzBot
Posts: 13
Joined: Fri Jul 07, 2017 10:10 am

Re: Z-offset question?

Post by johnson » Sat Jun 08, 2019 4:33 pm

"With a decent set of feeler gauges, you can measure this gap as you lower the nozzle (the largest feeler gauge in the set may only be 0.9 to 0.8mm). If you don't have feeler gauges, a piece of 20lb paper is about .1mm, a business card about .3mm. Once you can measure the gap, you can compare the Z value with the measured value and adjust z-offset so that they match."

Just a heads up: If you touch the nozzle with anything metallic you run the risk of sending a short back to the board via the zero-sense wire. On the Mini you risk frying the board. You may want to try the business card or paper instead.

Probably the best way to test your Z-offset is to print off a first layer using a skirt. Your first layer should look like the attached picture.
Attachments
1st_layer_adhesion.jpg

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