Hollowing Stuff and Holes

Oopsie ..
pillowblocksI’ve been working wtih Russ Green of New England Brownstone on some bearings / pillow blocks. The other day I received a set from an order that Russ made .. and darn it .. problems. What else is new?

The problem is that nine of them are VERY hollow .. the shaft-hole opens completely into the hollow body. Three of them have the shaft-hole completely closed.

So. The question is .. why?

sectionI was puzzled at first what happened. A little thought .. and a sectional view shows what happened. I hollowed out the pillow-blocks and then ‘punched’ a hole through the body. The problem is that .. well .. I hollowed the darn thing. Look close at the hole .. which is a tube .. and if you can wrap your head around it .. without any wall thickness. THIS .. was the problem .. and it confused the heck out of the software so we got the wierd printings.
The Why of it ..
scene1Let’s take a cube. I want to hollow it out and end with a hole poked through it. Let’s start with the blue cube and stick a circle on the side. This is the starting point.
scene2From this point on I’m going to take a section through the cube vertically at the center.
scene3Ha. This is what we would have if a solid cube was cut in half. This isn’t how the computer sees it .. the software doesn’t know anything about solids .. it knows planes and surfaces .. but this is what you would see if all we did was print the cube and slice it in half. The problem with this to start is simply we don’t need a solid cube .. and it really REALLY kicks teh price per part up.
scene4To save on cost one of easiest way is to hollow out the object. This is done using MeshLab .. by creating a copy of the mesh off-set by the desired wall thickness.
scene5When having an object printed at Shapeways simply hollowing out the object doesn’t do anything re: the cost. Without an escape hole, the suppport material is trapped inside and as far as the software – and Shapeways is concerned .. it is a solid block. With the appropriate escape hole (for FUD it is .4mm 1 hole or .2mm for two holes).
scene6So. At this point I punch a hole through the body of the cube. Here is the problem. Pushing a circle through the body creates a tube .. and it doesn’t have a wall thickness. It is just a tube in computer. If the cube were solid there would be no problem .. but because the cube has been hollowed it won’t print .. or at least .. it won’t print correctly.
scene7Soooo. The answer is to create a tube .. WITH .. a wall thickness. This should fix all the problems and we get a good print.

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