Concept

Resin prints shrink. How much depends on the specific resin used along with other factors such as the shape of the piece being printed. I was talking with Walt Gillespie of Rusty Stumps about this .. specifically small holes being different from what the CAD drawing specified. Since I was having the same problem I decided to make a test printing to address this. I am going to create a print that has holes that match AWG from 12 ga down to 30 ga. I will make holes in a block of solid resin and then various tubes with walls ranging from .15 mm to 1 mm. Once this is complete I can use an assortment of wires in various sizes which I can us a micrometer to get exact diameter of and test them in each of the holes therefore using the wires as a gauge to find the exact hole size after printing and shrinkage.

I put everything into an Excel spreadsheet to get all the numbers I would need and later to record the results.

CAD Hole Size

On the left we have AWG sizes from 12 gauge – an example is 20 amp yellow sheathed Romex using for home wiring, down to 12 gauge – an example being 15 amp white sheathed Romex used for home wiring all the way down to 30 gauge an example being wire wrapping.

The plan being to print these holes in a solid block and then with both thin and thicker walls to see if there is a difference.

Setup

To start I set up a grid to position the largest holes, the 12 ga with from left to right, AWG size then .15, .3, .6 and 1 mm wall thicknesses.

This is the end result. The openings run from 12 ga on the left to 20 ga on the right. Vertically from a solid block up to 1mm tall ‘tubes’ – 1.5, .3, .6 and 1mm thick walls. The holes go all the way though the block.

The final results were ..there was no difference between a hole in a solid block and one with even thin walls. I suspect that precision measurement would come up with a different result but I am simply trying to side wire of various sizes through the holes to find the smallest it will pass through. This is crude but fine for what I want .. to find what size holes I need to create in my CAD drawing for a certain size wire to pass through. That means I am going to ignore the wall size thing and just show the results for solid block, thin wall etc.

12 – 30 AWG Test Print

Results

On the left we have the results.

  • The first block of three columns simply show the AWG (American Wire Gauge) Size in the first column followed by the size in inches and mm. I have labeled the last two s “Wire CAD Dia” – meanging this is the size of a hole in the CAD file. In a perfect world we might expect the hole in the print would be the same as the one in the drawing. Oh well. Not happening as there is shrinkage and the hole always is smaller in the print than in the drawing.
  • The next block shows the Dia as printed. This was crudely sized by seeing the smallest hole that wires of various Dia would fit. Note I only had assorted wires to check fit 13, 15-16, 18-23 AWG “CAD Dia” holes. We have three columns showing the dia of the printed hole in inches, mm and the last column simply showing the percentage of shrinkage. Useful mostly to amuse one I think.
  • The last block explains how to know what size hole that needs to be drawn to get the needed hole when printed. The first two columns in inches and mm show the size hole needed for the desired hole after printing. I have a pin – commonly called a ‘Map pin” or as found in the sewing section a “Quilting pin” that is 0.023″ Dia. If we look down column 4 we find that an attempt to print a 20 AWG 0.032″ hole gave us 0.023″ hole. Ok. That is the size we actually want for our pin. Moving to the right on column 7 we see that for that we need a 0.040″/1.024 mm hole. That is the hole Dia that we need to put in our CAD drawing to get the .023″ printed hole.
  • The only date I could directly get is for 18-23 AWG. The holes smaller than AWG 23 had small indents but appeared to have close up during printing. For AWG 12-17 I simply laced the data (wires). To fill in the missing numbers I averaged the percent change from CAD hole size to printed hole size in the center of the table which was 130.57%. I could then take that and calculate the missing numbers which is shown in Pink.
  • If there is any interest I will share the STL file so you can test your own printer and help you print models with holes the size you actually want.

Example

  • I had created this O scale 12″ handwheel A. It has a CAD hole 0.7 mm / 0.028″ – which I “assumed” would give a 0.005″ clearance over the 0.023″ pin I was going to use as a valve stem. The hub has .6 mm walls. That the hole was too small was part of the reason I made this hole test.
  • From the test I found that my CAD needs the hole to be 1.024 mm/0.024″ diameter for a 0.023″ pin.
  • Handwheel B is just A that I enlarged the hole. This reduced the hub wall thickness to 0.42 mm / 0.017″. If ind that .4 mm is just about the minimum wall thickness that works.
  • Handwheel C  has the hub diameter increased to 2.342 mm / 0.082″. This increases the hub wall thickness to 0.66 mm / 0.026″. I have found that a .6 mm wall thickness is about the smallest ‘safe’ minimum thickness.

This hopefully helps take some of the guesswork out of printing small holes

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