How to make snug joints in Acrylic - 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.
However, after working with Drownspire to develop their Vambit toy into a product we could give away at Makerfaire, I discovered that you can successfully use nodes when making with acrylic. There are, however, some tricks to it.
Firstly the nodes need to be a bit smaller - less than 0.15mm/0.006in on each side. This means it won't cover the same range as in wood but it's still a good option. Second, how you treat the end of the slot is the key. If you have a sharp corner, which is typical in a laser cut slot, the acrylic will always fracture at that corner.
Basically, a sharp corner is creating a weak point in the acrylic. This is not good when it happens at a structurally important part of the design. Luckily, a small radii in that corner does wonders to transfer the forces from one face of the hole or slot to the other and can reduce the risk of the material splitting at the corner.
The larger the radii the stronger it will be, so it's up to you to decide aesthetically how big you can go. On the Vambit the radii was tiny, 0.26mm and it was enough to make a noticeable difference. We recommend you aim for 0.5mm or greater if your design will allow it.
Something else to keep in mind is placing the nodes on a part of the design where you can be sure of 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, and the technique works if you are using tabs but is not necessarily the case if you are using a slotting joint.
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 the nodes are best placed on these parts. Had the nodes been placed 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. You can slightly oversize the holes to allow for oversized material and the bolt will hold it snug together - and if you use the radii on the corners of the cut outs, you can significantly reduce the risk of cracking the acrylic by over tightening the bolt.
Taking it a step further, rubber washers can also reduce the chance of over tightening and maintain tension in the bolt so it can't come undone through vibrations etc.
Hopefully these tips will help you with your next project, or help you to finalize a design you're working on.
If you've had interesting experiences using radii we'd love to hear from you, or if you have any other advice for people wanting to make 3D designs using acrylic.
Design your own information.
However, after working with Drownspire to develop their Vambit toy into a product we could give away at Makerfaire, I discovered that you can successfully use nodes when making with acrylic. There are, however, some tricks to it.
Firstly the nodes need to be a bit smaller - less than 0.15mm/0.006in on each side. This means it won't cover the same range as in wood but it's still a good option. Second, how you treat the end of the slot is the key. If you have a sharp corner, which is typical in a laser cut slot, the acrylic will always fracture at that corner.
Basically, a sharp corner is creating a weak point in the acrylic. This is not good when it happens at a structurally important part of the design. Luckily, a small radii in that corner does wonders to transfer the forces from one face of the hole or slot to the other and can reduce the risk of the material splitting at the corner.
The larger the radii the stronger it will be, so it's up to you to decide aesthetically how big you can go. On the Vambit the radii was tiny, 0.26mm and it was enough to make a noticeable difference. We recommend you aim for 0.5mm or greater if your design will allow it.
Something else to keep in mind is placing the nodes on a part of the design where you can be sure of 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, and the technique works if you are using tabs but is not necessarily the case if you are using a slotting joint.
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 the nodes are best placed on these parts. Had the nodes been placed 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. You can slightly oversize the holes to allow for oversized material and the bolt will hold it snug together - and if you use the radii on the corners of the cut outs, you can significantly reduce the risk of cracking the acrylic by over tightening the bolt.
Taking it a step further, rubber washers can also reduce the chance of over tightening and maintain tension in the bolt so it can't come undone through vibrations etc.
Hopefully these tips will help you with your next project, or help you to finalize a design you're working on.
If you've had interesting experiences using radii we'd love to hear from you, or if you have any other advice for people wanting to make 3D designs using acrylic.
Design your own information.
