How precision and unit work?

Most buildings and houses are few centimeters off from design specification that is acceptable error. Furniture and door / window frame are few milliliters off. How many experts in trade know how to construct the right angle without using square or straight edge? Forming the right angle that produces vertical and horizontal reference point is the base of serious precision work. My improvised work bench made out of an old fuel tank hasn’t been secured on the floor yet.

The smallest unit of most wood workings is millimeter. Anything that goes below one millimeter demands precision cutting and finish works that require decent equipment and tools. I can cut a piece or drill a hole within 0.5 mm accuracy using hand tools. I’ve done such job few times. Honestly those experiences forced me to have better tools that have great accuracy. Well-known technique is to handle this is extra size of cutting dimension or holes. One micron is equal to 0.001 millimeter.

Excerpt from Precision Modeling by Robert Burke (www.rab3d.com)

So how big should I make a blender unit?

If you were considering modelling a precision component or assembly, being

able to model down to sub micron accuracy would be a distinct advantage. So

if we consider one blender unit to equal one millimetre how accurate can we

get? Blender allows you to numerically input the distance of vertices to the

precision of 4 decimal places (0.0000). Say you extrude a vertex 0.0001mm

along the X-axis you have set its position 1/10th of a micron from the original.

Pretty darn accurate and given that the best CNC machining centres can

position to an accuracy of plus or minus 3 microns, you can model to a much

greater accuracy than you could ever manufacture to. The draw back to this is

your viewable workspace will be limited to a cube with 10 metre sides. This is

due to the clipping limit of the view camera, but unless you are designing a

gantry mill to cut the wing spars for an Air-Bus A380 this shouldn’t present a

problem.

Putting Size into Perspective

♦ The human hair is around 76 microns thick.

♦ Most general purpose machine tools in

skilled hands would be able to work within

two thousands of an inch (50 microns)

♦ The best CNC machining centres can

position to an accuracy of plus or minus 3

micron, with repeatability of plus or minus 1

micron.

♦ A 100mm-iron bar will expand 0.012mm with

a rise in temperature of 10 degrees C

Precision modeling is to design an object for building a simple part or complex machine in real life. Dedicated software for this job called CAD already exists. Since I decided to build Kossel Mini, one of Reprap’s 3D Printer models, I felt I would need to learn precision modeling.

Blender supports millimeter unit. To set millimeter as default unit, change the Scale value to 0.001 in Property View Port, Scene, Unit, Metric menu.

Properties panel also has a measurement menu that displays length, angle, and area of selected edges or faces. I made few basic objects to test the measurement menu. On the image there are one isosceles right triangle, a square, and polygonal circle. Anyone who has basic geometry knowledge can verify the numbers Blender calculated.

Toolshelf has Ruler/Protractor button that measures length and angle of temporary lines on the 3D View port in Edit mode. User guide of Ruler/Protractor is here. The instruction is straightforward. You need to practice until you get the hang of it.

I worked through till Part 4 of the Modelling a 608 Bearing tutorial by Robert Burke. I think this one is by far the best tutorial about precision modeling in Blender. As you see the below image, Blender handles one thousands of millimeter, which is enough precision of engineering applications.

I spent good hours to finish Bevel edge section due to the inaccurate Knife operation. It created one slightly off vertex which supposedly wouldn’t exist along the edge.

Busted edges after Bevel operation