SoliForum - 3D Printing Community — inner diameters vs. perimeters

Re: inner diameters vs. perimeters

2014-04-03T15:44:26Z

grob wrote:

Attached is what I'm gonna try out for x/y steps-per-mm calibration. The edges between the cutouts should be 50mm, and the cutouts are there to make sure the backlash has been picked up prior to starting the edge. Should be easy enough to measure with verniers. There are probably a million other options too, up to you.
If you want to do a 100mm one for extra fun, just scale the thing up 2x in repetier object placement window. Personally I'd be worrying more about curling than relative errors in my setup, it's all a bit hit-and-miss at the moment.

I'm going to try that one it looks like a nice calerbration peice

Re: inner diameters vs. perimeters

2014-04-03T03:03:23Z

Attached is what I'm gonna try out for x/y steps-per-mm calibration. The edges between the cutouts should be 50mm, and the cutouts are there to make sure the backlash has been picked up prior to starting the edge. Should be easy enough to measure with verniers. There are probably a million other options too, up to you.

If you want to do a 100mm one for extra fun, just scale the thing up 2x in repetier object placement window. Personally I'd be worrying more about curling than relative errors in my setup, it's all a bit hit-and-miss at the moment.

Re: inner diameters vs. perimeters

2014-04-03T01:18:16Z

I change the number of perimeters routinely depending on each and every print and the parts final purpose. For decorative things two seems to work well. When it comes to functional parts with lots of bolt holes. More perimeters tend to be stronger and leaves room to drill if needed. If I am printing something with solid infill then two is more than enough and perimeters just waste time. It really just depends.

Re: inner diameters vs. perimeters

2014-04-03T01:10:43Z

grob wrote:

Stick with the external, they don't have the problem of the corners being pulled over (i.e. rounded), and they're easier to measure with calipers. Make sure it's a longish length (50-100mm), as this will reduce the relative error introduced by things like variation in wall thickness, corner bulging, blobs, etc.
I'd make you a model for a cal print that takes up backlash but I'm away from the CAD machine right now... I've been meaning to have a more detailed play with this myself anyway!
The slicer works out the offset of the perimeters around the internal holes the same way as the external walls - to meet the required final perimeter edge. In theory there should be no difference with changing perimeter count. I'd just calibrate with a print using your typical settings.

Ok. Is there any benefit to using more perimeters than the default slic3r of 3? Or less?

Re: inner diameters vs. perimeters

2014-04-03T00:53:30Z

Stick with the external, they don't have the problem of the corners being pulled over (i.e. rounded), and they're easier to measure with calipers. Make sure it's a longish length (50-100mm), as this will reduce the relative error introduced by things like variation in wall thickness, corner bulging, blobs, etc.

I'd make you a model for a cal print that takes up backlash but I'm away from the CAD machine right now... I've been meaning to have a more detailed play with this myself anyway!

The slicer works out the offset of the perimeters around the internal holes the same way as the external walls - to meet the required final perimeter edge. In theory there should be no difference with changing perimeter count. I'd just calibrate with a print using your typical settings.

Re: inner diameters vs. perimeters

2014-04-02T19:55:36Z

IanJohnson wrote:

Inner holes coming out too small is part of the nature of FDM printing. Mostly you will need to oversize the holes in the design to compensate-
http://hydraraptor.blogspot.com/2011/02/polyholes.html

Wouldn't the inner holes be determined partly by amount of perimeters (if let's say it's a smaller object and the holes aren't far away from the perimeters) .. so if so what is the perimeter setting I should be using to insure the best sized inner holes.

also when calibrating steps per MM for X and Y what should I be measuring for accuracy, the outer dimensions?

Re: inner diameters vs. perimeters

2014-03-28T23:41:23Z

Inner holes coming out too small is part of the nature of FDM printing. Mostly you will need to oversize the holes in the design to compensate-

http://hydraraptor.blogspot.com/2011/02/polyholes.html

Re: inner diameters vs. perimeters

2014-03-28T22:37:47Z

It does to a point. Let's say you want to print a line with 1 perimeter that is 1mm. The Slicer is going to draw 2 lines but your extrusion width is set at .48 so you'd get a line that is really .96mm so the .04 has to be made up with infill to get to 1mm. That's fine until you try to go the other way. For example you want a .90 line and the two lines equal .96 Slicer isn't going to tell the lines to overlap to reduce the width to .90 instead it is going to draw .96.
The problem is when you get into circles this is compounded by the fact that that .06 now becomes .12 less diameter. If your going for precision and the part is small this can become a problem. Most times for stuff like the OP is doing a good drill bit is the easiest way around the problem. Of course if you want a part that is just perimeters and no fill then they need to calculate out to an even multiple of your extrusion width. This isn't always an option so your left with what I stated above.
Then there is the obvious most of our printers just aren't calibrated with a level of precision to make any of the above that important so we figure ways to work around it.

Re: inner diameters vs. perimeters

2014-03-28T22:08:16Z

I always thought that the slicing software tried to match dims as close as possible... I would think it would print less than your "Max Perimeter" setting if that would cause the measurements to be wrong.

At least I would hope so, anyway...?

Re: inner diameters vs. perimeters

2014-03-28T20:48:56Z

About the only thing you can do is reduce the number of perimeters to something less then the wall thickness of the tube your trying to print then set infill to 1. It will zigzag in-between the inner and outer perimeters and the part is still solid. Either way there is a good chance the inner won't be perfect but it will be from the expansion of the ABS not because the nozzle didn't follow the correct dimensions. If that is a problem you may have to fudge the inner diameter of your design. The good news is you should be able to test by printing just enough to measure and then adjust until you get the desired results.

inner diameters vs. perimeters

2014-03-28T20:34:16Z

Just trying to understand how this works.

Let's say I'm trying to make a hollow box. I want the outer dimensions to be 50mm x 50mm. But I want the inner dimension to be 49mm x 49mm. Won't the inner dimensions be dependent on how many perimeters I am printing with? Let's say I have it set to 6 perimeters in slic3r, then I may end up with inner size smaller than 49x49?

So when calibrating the X/Y and measuring dimensions, I should always go by outer dimensions of the object, not "holes", right?

I am asking because I am trying to print a very small "tube" vertically that is about 6mm in diameter and 15mm high. But the inside size must be perfect and the outside size as well, because I will have something fitting inside of the tube and something that the tube fits into.

but i'm having trouble making the diameter inside be correct when the diameter outside IS correct, and vice versa.