Monday, July 23, 2012

Matching Holes & Posts Test

Getting to know our Cube printer (or any 3D printer) is very important.  Discovering how it behaves in various situations will help us become more productive with less need for multiple re-design & print iterations, which saves filament.

One of the most important things we can know is the relationship between the dimensions of printed posts vs. the dimensions of printed holes.  Matching posts and holes allows up to make parts of different colors by printing two parts of different colors that snap together using a post on one part that goes into a hole in the other.

Fortunately, this relationship is easy to test.  I've created a sample set of objects that will be available as an STL on the Cubify.Com store.  As usual for my designs, it will be sold at the lowest  permissible cost.

Posts and Holes Test STL

Each hole includes raised lettering indicating the designed size.  Each post has a small handle that doubles as a size label.  The holes and posts range from 3mm to 22mm.  Both post and hole has been chamfered for easier fit.  Below is the finished print still on the Cube printing bed.

Printed Post and Hole Test Objects

Theoretically,  each post should fit into its corresponding hole, as indicated in this image.

Post & Hole Design Match

But, due to the nature of extrusion printing, that is not what we see.  We find that we will have to make adjustments in order to make things fit by making holes with designed dimensions that are larger than the post meant to go into them.  Here is the actual match in the case of my print.

Posts in Actual Matching Holes

Notice that the largest post, 22mm, cannot be fitted into any hole.  And, interestingly, posts 12mm to 20mm fit nicely into the holes that are 2mm larger in diameter.  The 20mm post fits into a 22mm hole.  That's nice and neat and probably holds true as posts and holes get larger.

It's a bit messier in the 3mm to 11mm range.  Because we increased each hole by a full 1mm, there are posts that do not neatly fit into any of the holes.  The 3mm post, for instance is too big for the 5mm hole; but, too small for the 6mm hole.  The same kind of things is true for the 4mm, 10mm and 11mm posts. To really get a reasonably good match we would have had to increment the holes by .5mm.

Here is the mapping overlaid on the original design.

Diagram Matching Posts and Holes

The fact that we have to design matching posts and holes in different sizes is not a big deal once we learn that this is what we have to do.  Having a tool like the above helps us be accurate the first time when we design.  It's well worth the material costs to create it because it will save a LOT of material later. 

STL Download


  1. Wow -- what a great test!

    There's lots of useful info in that one test print.

    I appreciate your taking the effort to chamfer both the posts and holes to avoid end effects. Nice touch!

    That'll save me time for sure (and lots of other folks too). Thanks for posting those results.

    Very nice!

  2. You are very welcome. I only wish that the minimum cost for an STL was $1 rather than $3 because I think tools like this can help every single Cube or other 3D printer user.