high precision circle printing

hi, i need to print 8mm hole with the my reprap 3D printer but i can't do perfect hole.
how i can config repetier host for make really high precision.
thank in advence

This is more of a STL problem than printer config. The reason is there are no circles... Only triangles. The design software such as 123D Design asks for mesh resolution when you export. Select Fine to get better circles. This creates circles made up of many more triangles than low or medium. Other software has different methods.

If you are simply printing downloaded files you are stuck with what the author used.

Due to the process the holes will always be at least slightly undersized. Get and print a hole test chart off Thingiverse and use it to determine the size to make the holes.

Run through the holes with an 8mm drill.

If I want to print a 8mm hole I will get a 8mm hole
First the settings for creating the circle must be right, by default only 24 lines are used to create it, most programs allow to change this to 74, 144 or even more - so instead of looking like it is made by a kid the hole actually appears round.
Secondly the measurements need to be precise, for inside holes, like those holding a bearing I increase the size by a tiny percentage to allow for easy installation.
Those required for precision things like mounting holes I usually make a tiny bit smaller and drill them out, especially for tiny diameters below 5mm.
8mm should be no problem to print at all without drilling.
If in doubt do some calibration prints to get your filament and print settings right

Try to print at better resolution. .2mm
then try adjusting your flow rate down if the hole is too small.
if it really isn't working right then you absolutely need to review all the calibration parameters for motors and extruder, and make sure the printer itself is completely stable in all axis.

If you are using slic3r to generate your gcode, then checking "External perimeters first" makes hole sizes come out closer to what you asked for, but they will still be a little too small.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I always go with extra .5mm if I were to fit on any screws/bolts. Also don't forgot that flow rate plays huge impact on precision printing. I printed a case with button that cannot fit probably due to 100% flow rate