The importance of radii
We've written about using 'nodes' with 3D objects made from wood before, but suggested it may not work for acrylic because it is more brittle and less forgiving.
There are, however, some tricks to it.
Firstly the nodes need to be a bit smaller <0.15mm/0.006" on each side. This means it wont cover the same range as in wood but still a good option.
Second, how you treat the end of the slot is the key. If you have a sharp corner, typical in a laser cut slot, the acrylic will always fracture at that corner. See this example.
Effectively a sharp corner is creating a weak point in the acrylic. Not good when this is structurally an important part of the design. A small radii in that corner does wonders to transfer the forces from one face of the hole or slot to the other and reduces the risk of the material splitting at the corner.
The larger the radii the stronger it will be so you will need to make an aesthetic decision on how big you can go. On the Vambit the radii was tiny, 0.26mm and it was enough to make a noticeable difference. I would aim for 0.5mm and greater if your design will allow it.
Another trick to keep in mind is putting the nodes on a part of the design where you can guarantee the length. That way you don't need to bet on the thickness changing and the range of variation is a lot smaller. This occurs when you have 2 edges that are cut by the laser that are the friction edges. This works if you are using tabs but is not necessarily the case if you are using a slotting joint.
For example, in the design of this spinning top I put the nodes on the tab as opposed to on the slot.
The tabs on the triangle parts fit into the slots on the circle part. Dimension X and Y will be the same each time as cut by the laser therefore I put the nodes on these parts. Had I put the nodes on the slot for the handle (as in diagram below), the friction points would be against the surface of the material, a part that can vary if the thickness varies.
An alternative joint is the t-slot joint which is popular with people who make more engineering type products. This joint uses tabs to locate pieces then a t shaped slot with a captive nut. This type of joint is great. You can slightly oversize the holes to allow for oversized material and the bolt will hold it snug together. If you use the radii on the corners of the cut outs you greatly reduce the risk of cracking the acrylic by over tightening the bolt.
If you want to go another step, rubber washers can also reduce the chance of over tightening and maintain tension in the blot so it wont come undone through vibrations etc.
I hope these tips will help you with your next project, or perhaps to finalize a design you're working on.
We'll be interested to hear you're experiences using radii too, and any other advice you might have for people wanting to make 3D designs using acrylic. Let us know below!
Advice from Dan Emery - originally posted to the Ponoko Blog.