Making Your Joints Tight Fitting Using Acrylic - The Importance of Radii
You may have read about using 'nodes' with 3D objects made from wood before, but been told it may not work for acrylic due to its brittle and less forgiving nature.
However, after working with Drownspire to develop their Vambit toy into a product we could give away at Makerfaire, we have discovered that you can successfully use nodes when making with acrylic as long as you tweak the process slightly.
Firstly, the nodes need to be a bit smaller - we recommend 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 an available option.
Second, it's key to make sure you treat the slot correctly..
If you have a sharp corner, typical in a laser cut slot, you'll find that the acrylic will always fracture at that corner.
A sharp corner creates a weak point in the acrylic, which is not good when it's at a structurally important part of the design.
A small radii in that corner is magic for transferring the forces from one side of the hole or slot to the other and thus reduces the risk of corner fractures or splits.
Since the larger the radii the stronger the effect, you'll need to make an aesthetic decision on how big you want to go.
On the Vambit the radii was tiny, 0.
26mm and it was enough to make a noticeable difference.
It's a good idea to aim for 0.
5mm or larger if your design can handle it.
It's also important to put the nodes on a part of the design where you can guarantee the length; this means you don't need to bet on the thickness changing and the range of variation is a lot smaller.
This occurs when the 2 edges that are cut by the laser are the friction edges.
This 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 put on these parts.
Had the nodes been located 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 really effective - you can slightly oversize the holes to allow for oversized material and the bolt will still hold it snugly together.
If you use the radii on the corners of the cut outs you can prevent cracking or fracturing should bolts be tightened too firm.
If you want to go even further, rubber washers can also reduce the chance of over tightening and maintain tension in the blot so things don't fall apart through vibrations or similar disturbances.
Hopefully this will all help you with your next project, or help you finish off any current designs you're working on.
However, after working with Drownspire to develop their Vambit toy into a product we could give away at Makerfaire, we have discovered that you can successfully use nodes when making with acrylic as long as you tweak the process slightly.
Firstly, the nodes need to be a bit smaller - we recommend 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 an available option.
Second, it's key to make sure you treat the slot correctly..
If you have a sharp corner, typical in a laser cut slot, you'll find that the acrylic will always fracture at that corner.
A sharp corner creates a weak point in the acrylic, which is not good when it's at a structurally important part of the design.
A small radii in that corner is magic for transferring the forces from one side of the hole or slot to the other and thus reduces the risk of corner fractures or splits.
Since the larger the radii the stronger the effect, you'll need to make an aesthetic decision on how big you want to go.
On the Vambit the radii was tiny, 0.
26mm and it was enough to make a noticeable difference.
It's a good idea to aim for 0.
5mm or larger if your design can handle it.
It's also important to put the nodes on a part of the design where you can guarantee the length; this means you don't need to bet on the thickness changing and the range of variation is a lot smaller.
This occurs when the 2 edges that are cut by the laser are the friction edges.
This 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 put on these parts.
Had the nodes been located 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 really effective - you can slightly oversize the holes to allow for oversized material and the bolt will still hold it snugly together.
If you use the radii on the corners of the cut outs you can prevent cracking or fracturing should bolts be tightened too firm.
If you want to go even further, rubber washers can also reduce the chance of over tightening and maintain tension in the blot so things don't fall apart through vibrations or similar disturbances.
Hopefully this will all help you with your next project, or help you finish off any current designs you're working on.
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