For what it’s worth-- I’ve been printing a drill index and found that CAD drawn holes print small. I think everyone knows that. So I thought I’d see if I could nail down some kind of a factor to add. I would’ve expected a percentage. But what worked for me was as follows:
for 1/6" hole, add .254mm, for 1/2" hole, add .254mm. However, for 1/8" holes, I had to add .270mm, so not consistent in my test. Anyhow, I guess it’s helpful to add about .26mm and then adjust from there. Just thought I’d share.
That’s great except the size adjustment will most likely depend on the type of filament and the size of the hole. Adding a fixed value probably won’t work as you found out.
I wouldn’t bother though. I bought a drill index complete with M42-cobalt 3-step drills from 1.5mm to 6.5mm for little more than a $20 bill. These are the most common sizes that might be used in 3D printing. And I wouldn’t hesitate to use these drills in my machine shop. In fact, I do. I have the 1-10mm set in the shop.
If you really want to test yours as well as your machine’s skills, print a tube insert with a 4x1mm flange that’s 10mm long, 2.5mm OD and a 1.95mm ID.
With a brim, it looks like this.
I’ll give that a try.
I wasn’t printing the drill index to save money. I was doing it for the experience. And… I bought a Dewalt drill index that’s lousy. I’m sure the drills are fine but the container is not what I was expecting from Dewalt. So, other than experience, I actually have a practical need, a better container made by me. I can’t see missing a chance to print something.
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What kind of instrument do you have for measuring a 1.9mm ID?
The thing is when you draw something in parasolid CAD software, like Fusion, circle and every other shapes are made of vectors. But stl are composed of triangles. So when you make a cross section, a layer, you get a circles composed of segments. And this also true if you import a step file into the slicer since it’s converted to an SLT when you import it.
So the first factor that can influence the tolerance on small holes is the STL mesh density. More mesh resolution more accurate holes size.
Second is the resolutions set inside the slicer. That will influence how precisely the toolpath will follow the mesh path.
Arc fitting, and arc resolution is another factor. This setting allows the slicer to generate arc gcode command moves (G3) over classical linear moves gcode commands (G1).
Those also add a new step of imprecisions since we are converting back segments to arcs.
Another thing that can influence that size of the little holes is the seam, especially if pressure advance (k factor) is not super well tuned. Scarf seams should help to reduce the effect, but I didn’t test it yet.
So they are many factors the can influence the tolerance of the holes, especially the small ones.
That’s why a lot of slicers have a setting to add and extra tolerance on small holes.
Gees! That gives me a lot to study up on. Thanks, I appreciate it.
It’s my pleasure, I like 3d printer related nerdy talk 
Machinist Pin Gauges, of course (.075 is close enough).
I was making a Bordon tube insert to keep the PTFE tubes from collapsing when inserted into the connectors or hot end. I’ve had trouble with the tubes at the point where the connectors clamp and compress the PTFE tube. After a while, the tube starts moving back and forth on filament push and pulls and eventually causes a load problem.
I’ve since gone with brass inserts since I couldn’t keep the filament from hanging on the step at the 1.9mm opening in the 2.5mm tube. And I couldn’t taper the ID near enough to 2.5mm without post processing.
That’s the short answer to your next question. 
I think I actually understand that. Sounds like you were making a metallic liner for the PTFE tube.
I was printing a part in white Flashforge PLA Pro as a test because I had extra, I got the tolerances in a good range. I then printed it in black Flashforge PLA Pro and it was too tight so I had to loosen the tolerance.