Perimeter flow?

Is there a way to adjust the perimeter flow in Slic3r? I have noticed when I print things with holes and mating parts (like the hex socket for the bolt-head on a Wade's big gear, or the spool holder halves for my Mendel90), the holes come out a bit too small, and pegs/dowels come out a bit too large, and according to the troubleshooting guide that means the flow rate for perimeters is a bit too much. Is there a way to fix that in Slic3r? It's the only slicing software I have had any kind of luck with, so I'd prefer to stay with it...

Anybody? Nobody?

I didn't see your first post for some reason.

All slicing applications will presently cause circles to print too small, the smaller the diameter the greater the discrepancy. IIUC one reason is geometry - the slicing software makes the nozzle trace a path along the exact middle of the extruded line, assuming that the width is equal on both sides of the nozzle. Thus if it is printing a hole with a 5mm radius and an extrusion width of 0.5mm, the head will describe a radius of 5.25mm, assuming that there will be 0.25mm of plastic extruded on each side of the nozzle. However the inside of a circle has a smaller circumference than the outside and so requires less plastic. Which results in the inside being over-extruded and the outside under-extruded, which in turn causes the plastic to be pushed toward the inside of the circle, making the hole smaller. To correct it, the slicing application will have to know that it is printing a circle and what size the circle is - but STL files model circles as a series of short straight lines, so detecting that the shape is a circle of a particular radius is not as easy as it sounds.

Any slack or backlash in the printer mechanics will also work in such a way as to make holes smaller (think about it), and so add to the geometrical error - in many cases this will be the main source of error. Printing more slowly can improve that particular effect by reducing overshoot and undershoot (due to e.g. elasticity in the drive belts). You can set Slic3r to have a threshold value for "small perimeters" and print those perimeters very slowly, which usually improves both the size and shape of holes that are smaller than the set threshold. If your holes are printing oval-shaped instead of circular, it could be due to your nozzle being slightly off vertical.

The third reason is shrinkage - all prints shrink a bit as the plastic cools. So we have three sources of error that will always tend to act in the same direction - making a hole smaller.

If you use polygons with a low number of sides instead of circles (which are really polygons with a high number of sides) you can usually get a better fit. Use triangles instead of small circular holes, for example. Otherwise just experiment to find what oversize is needed in the design to result in the correct size hole in the print - it can change a bit depending on type of filament, hotend temperature, extrusion factor and the slop in your particular printer mechanics, so it is not possible to make a definitive list, but once you find the oversize necessary for a particular size it will work consistently for the same material and settings on your printer, so you could make a test design of a bar with various size holes and then measure the results to get a list of corrections to apply to any future design for your particular setup.

Dave

Problem is, right now I do not know how to use ANY CAD software to modify drawings. I try to print things like gear sets for my extruder, and end up with holes that are too small for my motor shaft and bolt. Best I can do in that situation is ream out the holes, and clean out the nut traps with a knife, but that really is not a satisfactory solution because I can't guarantee concentricity.

Print Settings -> Advanced -> XY Size Compensation

Allows you to inset the perimeters slightly to account for over-extrusion.

Thanks!