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- Jul 8, 2009
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I've been using Simplify3D as my slicer for a number of years now, mostly with good results - until recently.... I started having issues with internal bores and pockets not being dimensionally accurate - they all wind up too small.
When I first started 3D printing, 8-10 years ago (Or more?), I was using a kit-built printer and learning as I went - by trial and error. (Mostly error.) I started out printing junk off Thingiverse and very quickly discovered that I couldn't find EXACTLY what I wanted. At this time I was using Slic3r for my slicing software - the price was right. But I NEEDED to learn 3D CAD. Talk about going down one rabbit hole only to find you need to go down another even deeper rabbit hole to find your way out. I tried Fusion 360, back when is really was free AND powerful - and absolutely hated it. I just could not wrap my head around the parametric modelling. Then I found Designspark3D with its' direct modelling. With enough poking and prodding I could get it to follow my commands/wishes. What was I designing? Upgrade parts for my 3D printer of course. In the course of these upgrades I decided that I could no longer tolerate the rat's nest of a wiring job that I had done in my haste to see my printer move under it's own power. One of these upgrades was an enclosure for all the electronics, gotta keep the angry pixies confined ya know.
In the process of printing the enclosure, let's just say things went - awry? Part of the enclosure was an opening for a 40mm cooling fan. To say the opening was a little wonky is an under-statement. It was visually an ellipse. NOTHING matched the model's dimensions. Not knowing any better I assumed that my cheap Chinese clone of an I3's steps/mm settings were wrong. I found a way that I could, by brute-force, change the steps/mm settings through the g-code. I starting printing 20mm test cubes - LOTS of 20mm cubes. I eventually got to the point where I could print a 20mm cube that was dimensionally accurate to within a few hundredths of a mm. HUZZAAH!
Fast-forward a couple of years and I was watching a video by James, of Clough42 fame, about issues he was having with 3D prints not being dimensionally accurate. He remarked that this was caused by the thermal expansion. This is the same reason that pattern makers in foundries have used rulers that are scaled over-sized for many-many years, to compensate for the shrinkage. Talk about a DOH moment, what I did with my brute-force method was doing the exact same thing - compensating for shrinkage. He was also using Simplify3D and showed how to use the scaling in S3D to scale the model for printing a dimensionally accurate part. For several years that has served me fairly well. I had problems with hole sizes, but I always wrote that off as part of the inaccuracies of FDM printing.
I'm working on parts for RC truck models now and I'm trying to print bearing pockets - so I NEED an accurate fit. I scaled the model to 103% in the slicer and the external dimensions was about as spot-on as you are going to get with an FDM printer. BUT, all of the internal bores and pockets that I could reasonably measure were about 97% of the model's size. I think what S3D does when it scales the STL is to offset to the outside of the outer perimeter by that scaling factor. That would explain why the external dimensions were OK, but the internal dimensions were all undersized. This leads me to the conclusion that I can't use S3D to scale model. I don't know about Slic3r, or the Prusa Slic3r, or Cura, but I would guess that they all behave similarly.
So I tried to scale a part in my 3D CAD, just to see how it behaved. I just whipped up a simple cube with a couple of holes going through it, and scaled it to 103%. Everything worked as expected, what was initially a 10mm bore measured 10.3mm, and the 20mm cube measured 20.6mm.
The moral of this rather long-winded story? For a dimensionally accurate 3D FDM printed part... scale your model in CAD, not your slicer.
Don
When I first started 3D printing, 8-10 years ago (Or more?), I was using a kit-built printer and learning as I went - by trial and error. (Mostly error.) I started out printing junk off Thingiverse and very quickly discovered that I couldn't find EXACTLY what I wanted. At this time I was using Slic3r for my slicing software - the price was right. But I NEEDED to learn 3D CAD. Talk about going down one rabbit hole only to find you need to go down another even deeper rabbit hole to find your way out. I tried Fusion 360, back when is really was free AND powerful - and absolutely hated it. I just could not wrap my head around the parametric modelling. Then I found Designspark3D with its' direct modelling. With enough poking and prodding I could get it to follow my commands/wishes. What was I designing? Upgrade parts for my 3D printer of course. In the course of these upgrades I decided that I could no longer tolerate the rat's nest of a wiring job that I had done in my haste to see my printer move under it's own power. One of these upgrades was an enclosure for all the electronics, gotta keep the angry pixies confined ya know.
In the process of printing the enclosure, let's just say things went - awry? Part of the enclosure was an opening for a 40mm cooling fan. To say the opening was a little wonky is an under-statement. It was visually an ellipse. NOTHING matched the model's dimensions. Not knowing any better I assumed that my cheap Chinese clone of an I3's steps/mm settings were wrong. I found a way that I could, by brute-force, change the steps/mm settings through the g-code. I starting printing 20mm test cubes - LOTS of 20mm cubes. I eventually got to the point where I could print a 20mm cube that was dimensionally accurate to within a few hundredths of a mm. HUZZAAH!
Fast-forward a couple of years and I was watching a video by James, of Clough42 fame, about issues he was having with 3D prints not being dimensionally accurate. He remarked that this was caused by the thermal expansion. This is the same reason that pattern makers in foundries have used rulers that are scaled over-sized for many-many years, to compensate for the shrinkage. Talk about a DOH moment, what I did with my brute-force method was doing the exact same thing - compensating for shrinkage. He was also using Simplify3D and showed how to use the scaling in S3D to scale the model for printing a dimensionally accurate part. For several years that has served me fairly well. I had problems with hole sizes, but I always wrote that off as part of the inaccuracies of FDM printing.
I'm working on parts for RC truck models now and I'm trying to print bearing pockets - so I NEED an accurate fit. I scaled the model to 103% in the slicer and the external dimensions was about as spot-on as you are going to get with an FDM printer. BUT, all of the internal bores and pockets that I could reasonably measure were about 97% of the model's size. I think what S3D does when it scales the STL is to offset to the outside of the outer perimeter by that scaling factor. That would explain why the external dimensions were OK, but the internal dimensions were all undersized. This leads me to the conclusion that I can't use S3D to scale model. I don't know about Slic3r, or the Prusa Slic3r, or Cura, but I would guess that they all behave similarly.
So I tried to scale a part in my 3D CAD, just to see how it behaved. I just whipped up a simple cube with a couple of holes going through it, and scaled it to 103%. Everything worked as expected, what was initially a 10mm bore measured 10.3mm, and the 20mm cube measured 20.6mm.
The moral of this rather long-winded story? For a dimensionally accurate 3D FDM printed part... scale your model in CAD, not your slicer.
Don
