Sunday, November 3, 2013

Print quality issue [Resolved]

With a good number of hours under my belt, a print quality issue began to show up on the new printer. You can see it evident in the side walls of the battery holder pictured below.
A closer view (photo below) shows lots of gaps and a general poor finish.

I entertained a number of possible causes: Too much "retract" when executed, resulting in extrusion not resuming flow in time for printing. I even entertained ambient temperature changes being an unlikely but possible cause of the problem, as Winter was setting in, and considered increasing the hot-end temperature as one of the things to try. But before changing anything, I set another print under way, with a slow print speed (20mm/sec), and had a close look at the printing process in action. (I've made a cut-away fan-duct just for easy inspection, which I must post about.)

On observing the "gaping" happening a number of times during the test print, it was obvious that the large gear that drives the hobbed bolt was slowing down and sometimes stopping, when it should have been turning at a steady rate. Closer observation of the smaller gear revealed the issue. The extruder motor shaft was slipping within the small gear.

I knew from past experience that simply tightening the grub screw was most likely not going to be a long term solution, as had the grub screw been resting against the "flat" on the motor shaft then it wouldn't have slipped even when it had worked a little bit loose. The reversing action of the motor would have resulted in more of a clicking noise than full slippage.

On removing the small gear, it was now clear that the "flat" on the motor shaft was not long enough for the position of the grub-screw on the small gear. I should really have spotted that on first assembly. The grub screw needs to tighten down to a flat surface on a shaft to best secure it in place.
The above photo shows how the "flat" on the stepper motor shaft doesn't extend enough to align with the grub screw in the small gear.

The "flat" was easily extended using a flat needle file. The bearing of the stepper was protected from filings with some 'blue tack'. The shaft was gripped in a small vice. (See photo below.)
The new "flat" doesn't have to be perfect, just enough for the grub screw to seat against. (photo above)

The small gear was refitted, and the grub screw tightened down, ensuring it aligned with the newly filed "flat".

I then reprinted the battery holder and the difference in print quality was immediate (photo below). The plastic flow was consistent and the print finish excellent.

What is also evident, with consistent print quality is that the z-movement on the Mendel 90 is so smooth. Each layer is laid down perfectly above the other on a vertical wall, such as that seen in photo below.
Technical Note: .2mm layer height, with a Width over Height Ratio of 1.8, sliced in Skeinforge. Print speed 50mm/Sec. Now to tidy up my rechargeable batteries!
Thanks for viewing.
NumberSix

5 comments:

  1. Crumb, now I think back I'm not sure I ever did file flats on those shafts, this could explain a few things, your first picture is very familiar. Quick, to the tool box.

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  2. Amen. That grub screw is really a little devil. I've found it slipping, sticking, breaking off - at this point it is the first place I look when prints go awry.

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  3. Guess what I'm going to check on my printer tonight!
    Alan

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  4. No need to worry - my M90, built from a Nophead Dibond kit, uses what I anticipated it would - steppers with a flat going right down to where the shaft enters the body of the motor. The point is well made though as things like this can easily get overlooked by the unwary. I have checked a couple of other steppers which I bought from a RepRap parts supplier which WILL need filing down when I get around to building a second extruder either as a spare or with a different nozzle size, (or hot end type). Thank you for the blog post.
    Alan

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  5. Hi Alan,
    Thanks for follow-up. It's a detail worth keeping in mind when building.
    NumberSix

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