External Perimeters first

Hi,

After some reading I have determined that the best advice people are giving to get hole sizes and general dimensions accurate are:

- Make sure the printer is mechanically calibrated
- Make sure the printer firmware and Slic3r are properly calibrated
- Print external perimeters first so that excess gets squeezed into the inside of the print rather than making holes smaller.
- Design holes to be bigger knowing they will shrink (not ideal, but given the relative simplicity a pragmatic solution).

With all this in mind I set Slic3r (v1.0.0) to print external perimeters first and ran a print. On analysing the gcode I noticed that whilst it prints 'external' perimeters first it does not seem to consider the outer faces of holes to be 'external', therefore the holes come out undersized and the benefits of printing external perimeters first are lost.

I managed to force the behaviour I was after by setting Slic3r to a single perimeter and 'extra perimeters when needed'. This seemed to do the job and gave me two perimeters (guess thats my one plus one 'extra perimeter when needed'). On a simple test piece this did pretty well. However on a more complex test piece with overhangs it did not - the outer perimeter did not print well because it was overhanging and starting with very little support beneath or to the side of it.

This leads to two questions:

Is Slic3r supposed to consider the outer faces of holes to be 'external' perimeters? If so then what setting might I have incorrectly selected to mean it did not for my object?

How will the use of 'start perimeters at non-overhang points' affect a print with only a single perimeter?

Thanks

Andy

Interesting questions which puzzle me too! As far as holes are concerned, that subject rears its ugly head very often. It seems to me that when pulling plastic around a small circumference, it tends to be "pulled" inward, hence reducing the hole size. At the other extreme, if you had a hole 150mm diameter it would probably be quite accurate because the extruder is going in a 'bent' line rather than a circle.
Interestingly, I noticed on a rather old STL of the Wade's extruder large gear that I printed, that the 13mm hex nut cavity was indeed 14.1mm in the cad drawing but,in fact, printed perfect size - the nut fitted like a glove. Having said that, I printed an updated X end (jonaskuehling) where he had reduced the hex socket to 13.6mm - this time the nut was a fairly easy press fit. Of course, different owners will have different results depending on their setups.

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Waitaki 3D Printer

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waitaki
Interesting questions which puzzle me too! As far as holes are concerned, that subject rears its ugly head very often. It seems to me that when pulling plastic around a small circumference, it tends to be "pulled" inward, hence reducing the hole size. At the other extreme, if you had a hole 150mm diameter it would probably be quite accurate because the extruder is going in a 'bent' line rather than a circle.

Good conclusion. Have a look at nophead's polylines blog post for a detailed analysis of this.

Craig

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[blogger.kritzinger.net]

Hi,

Thanks for the replies - although not really answers to the questions I asked. I am well aware of why the holes come out undersized - the list I presented are the proposed solutions to this, including designing the part with the shrinkage in mind (i.e. using nopheads polylines concept). I was actually interested to know if others had had success with Slic3rs 'print external perimeters first' option as this is often also cited as a way to improve things.

Whilst on my first attempt it appeared not to work as advertised (it continued to print from the inside out) for some reason it is now working correctly (perhaps a setting didn't quite get pulled through my toolchain on the first try).

I had hoped that giving some background would help give some context to my quesiotns however it seems the mention of undersized holes made people concentrate on the wrong thing so I suggest this topic is now closed and I will open another with just my basic question.