Prusa quality issue - not sure of cause?

Hi guys

I've now managed to use my printer for printing several useful objects, which is great, but now it's kind of tried & tested for basic stuff, I want to try doing smaller prints where the accuracy/quality matters a bit more.

So I tried printing a small cylinder/tube as I hadn't done anything like that. There's a couple of issues I think and after looking through the wiki page on common print quality issues, I'm not sure where these fit in nor what I should be looking at to resolve.

You can see here there is a gap on part of one of the layers (or at least one, possibly more?), and it has this in a couple of places on different parts.


You can see it here, along with some layers not being quite aligned correctly (voltage?):




So there's that, but also I think there's possibly a scale thing going on but have conflicting findings.

I printed the 20mm test cube and the sides of it measure at 19.65-19.75mm which I think is not bad for a beginner.
But when I printed this cylinder I set the hole through the middle has 5mm and it comes out 4.5mm. I don't mean it just has a bit extra at the end, but it's just smaller in general than it should be, and noticeably more than the slight discrepancy on the test cube.

Appreciate any suggestions on correcting these quality issues as I couldn't kind of match a lot of it up with stuff I'd seen as common issues.

Thanks!

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Prusa Mendel i2 - Melzi V2

There are several things you should check:

Motor voltage.
Calibrate e-steps per mm.
Belt tension.

Regarding the size of the hole, this is fairly common. As your hot end lays the shell on the inside of the hole, it has a tendency to drag and reduce the circumference slightly. One technique to resolve this is to set the number of sides of the circle to no more than twice the diameter of the hole, so in your case there should be 10 sides to that circle. Having the hot end move in a series of straight lines rather than a curve can reduce drag.

NB: the hole size problem can also be caused by over-extruding, so make sure you calibrate e-steps per mm.

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[3DKarma.com] - suppliers of quality, affordable 3D printer kits and filament for the UK market.

i would try get the measurements correct 1st i had to calibrate my steps on x and y had mine at 71.11 x and y now its 72.60 on x and 74.6 on y i thought it wouldnt make much diference as y was 2mm of and x was only .5mm of but it did, i run my printer slow and external perimeters are set at 28.5 in slicer find doing a circle object slow the way to go

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Check my rubbish blog for my prusa i3

up and running
[3dimetech.blogspot.co.uk]

Sorry for not responding sooner guys. Have been pretty busy!

I looked over the printer yesterday at last and found a couple of issues as I tried to print something. The idler on the extruder had split and was thus causing erraticextruding behaviour as it wasn't applying pressure properly at all on the filament.
Also, one of the two bolts attaching the top half of the extruder carriage to the bottom half was loose! Which meant of course as well as not feeding correctly it was wobbling around.

I did re-tension the X belt also as I wasn't too happy with how it felt.

Pic here shows a new idler I tried to print while it was like this (back one) and another after it was corrected in both respects (front one):


So it's WAY better now of course.

I printed another test cube and it's come out 19.65 round the 4 sides so do I need to tweak my E steps still, or a scale setting on the X&Y somewhere else?

One other question I have is concerning support parts of a printed model, eg where there's a hole for a bolt or suchlike, and it fills the hole partly in, to then be removed once complete.
Is there a particular trick to removing these bits after the print is finished? I am finding it a nightmare on my models to remove bits like these. I presume they are more or less meant to prise or snap out but on mine they seem to be stuck firmly to the main model, and prove very difficult to then remove without badly damaging the model itself. What am I likely doing wrong?

Thanks for the help and suggestions guys

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Prusa Mendel i2 - Melzi V2

My X & Y steps are set as 78. something steps/mm, so I am increasing them to 80.14 and will try that.
I worked out that it is printing at about 95% of the intended size so this increase should get it very close I think. I'll report back once I've tried it!

Is there a setting in slicer or something for the number of sides to use on a circle? Or a ratio you can set it so it calculates itself against the size of the hole, as per 3dkarma's suggestion?

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Prusa Mendel i2 - Melzi V2

U can use the prusa calculator to fine calibration your x and y steps basically you input the distance you was expecting at the current step rate then tell it what u actually measured then give it will calculate the required steps

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Check my rubbish blog for my prusa i3

up and running
[3dimetech.blogspot.co.uk]

Thanks - I guess that would have saved me a couple of minutes on the calculator but I was pretty damn close with the above calculation.

Here's the result now:




I think 0.02mm is so close that there's no point trying closer than this as it'd be unlikely to give a consistenly better result, since I'm sure you get a bit of variation from one print to the next.
That's just under 1 thou of an inch so I'm really pleased with that!

Now to look up the circle thing....

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Prusa Mendel i2 - Melzi V2

The circle thing is something you do in your cad software. Slic3r doesn't have a setting to specify number of sides in a circle.

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[3DKarma.com] - suppliers of quality, affordable 3D printer kits and filament for the UK market.

Ahh ok thanks - will investigate.

Very pleased to get it so accurate now anyway!

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Prusa Mendel i2 - Melzi V2

Ok, I've looked in sketchup and can see under the measurement for the circle radius it has the number of segments. So far so good.

So I printed a 20mm tube (OD), to be 16mm on the inside.
It has 20 segments for a 10mm radius, but I guess I should reduce that to 10 sides if it should be based on the diameter not the radius right?

Anyway, the actual printed measurements are 19.2mm OD, and 15.4mm ID. Bit disappointing when it prints the test cube so accurately (to 0.02mm), but on the circle is 0.8mm short on the OD, and 0.6mm too tight on the ID.

So since this horrible level of inaccuracy seems to only affect circular objects rather than squares is this just down to drag as described then? Or is there something else I ought to be looking at?
The sides are fewer on this than ideally suggested so from what's described about drag on these it'd surely make it worse if I set them to the correct segments (40 segments on outer for 20mm diameter, and 32 for inside of 16mm diameter)? I'll try it and see what difference it makes then it might give me a point in the right direction.

Appreciate the input guys, thanks!

Edited 1 time(s). Last edit at 05/26/2014 06:26AM by Mr Joshua.

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Prusa Mendel i2 - Melzi V2

Update - ok, I increased the segmentation of the circles to the ratio suggested of Dx2 so 40 for the outside 20mm one and 32 for the 16mm inside circle.
The outer now measures at 19.5 instead of 19.2mm, so it's closer to the intended 20mm diameter, but the inside is now 15.25mm instead of 15.40mm so it has closer in a bit and gone further away from the intended diameter.

Not sure where to go from here really....

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Prusa Mendel i2 - Melzi V2