Increasing small negative space accuracy

[Kitecraft]

Hello.
I've been learning how to fine tune my Rostock and have been seeing good success.
Using the Make 3DP models, I've made significant improvements to accuracy. Temps, extruder multiplyer, speed,....
But, it's still not accurate enough for small holes.
I'm trying to get a 3.4 mm hole, but it comes out to be 2.8 - 2.9 on the printer.

The rest of the part:
Y axis - designed 54mm, result = 53.8 mm
X axis - designed 24mm, result = 24.1 mm
Z axis - designed 13.1mm, result = 13.2mm

I'm thinking that maybe one of the settings on the Rambo might be a key to this. Just a small change to a number some where maybe?

Overall the prints look very nice and tidy and clean and shiny.
Thanks for any advice!
Cheers

[Mac The Knife]

That is something you have to compensate for in your design,,,, if its really critical, you clean it up with a drill bit. Somewhere on this forum, someone posted a link as to why the holes never turn out right. The first think to wrap your head around is that the hole is not round. It is made up of small segments of straight lines. Second, you're dragging a piece of plastic around in a circle, and it wants to cut across the open area.

[bot]

I believe this to be a result of several factors described in this article:

http://hydraraptor.blogspot.ca/2011_02_01_archive.html

When Reprap machines print holes they tend to come out undersized, even if the linear dimensions of an object are spot on. There are several effects that all make holes smaller than they should be:

Faceting error
When CAD systems convert cylinders to triangles they produce a polygonal prism, so holes represented in an STL file are polygons with their vertices on the circumference of the original circle. That means the sides of the polygon are inside the circle, shrinking it by cos(π / n).

You need 10 vertices to reduce the error to 5% and 22 for 1%. So this error quickly becomes small as n increases but that creates another error:-

Segment pausing
When a circle is broken into a lot of little segments the start up time for a segment becomes significant. Reprap in the past has suffered from this really badly and I am unsure what the current status is. Slow serial comms and complex floating point firmware add pauses where extra filament can ooze from the nozzle.

I have never suffered from pausing because I use a 100Mbit Ethernet connection, which has a very low latency, and the data is transmitted in binary and in the units my firmware works in. This means that no further processing is required other than calculating which of the three axes has to go the furthest. However, I use trapezoidal acceleration on each segment, so for very short segments the average speed will be a little lower.

Arc shrinkage
When a flat strip of filament is bent into an arc there is too much plastic on the inside of the curve and too little on the outside. That makes both the inside and outside edges a smaller diameter than they should be. Adrian calculated a formula for it here: http://reprap.org/wiki/ArcCompensation. The formula comes out with a figure that is too small though. I think there is a secondary effect:

Corner cutting
When filament is dragged round a corner it likes to take a short-cut. This depends on how elastic the filament is and how much it is being stretched. I think when the nozzle moves in a circle the filament is continually trying to cut the corner and ends up forming a smaller diameter circle. I think this is the dominant effect on my machines.

...[read more]

Also, another useful video about calibration:

[youtube]http://www.youtube.com/watch?v=Mbn1ckR86Z8[/youtube]

[bot]

Mac beat me to the punch!

[Kitecraft]

That's excellent info. I didn't find it in my searches so thank you for bringing that up.
Arc shrinkage I can deal with. i get that.
The faceting error amount is the bit I need to figure out and that should help a lot as well.

It's not that I really don't want to have to bore the holes out (I really don't), but I do like to understand the underlying causes.
Again, thanks. It's very helpful.

Cheers,