Cutting a 45 degree bevel on Acrylic sheet.

[Luke Floden]

Is there any equipment at the space capable of cutting a 45 degree bevel on a piece of an Optix acrylic sheet?

I am trying to make a pyramid out of acrylic and need the bevels to create clean edges. Attached is an exploded view of the pyramid pieces.

[Daniel Tyler]

A table saw with a thin kerf blade and a cross cut guide would be my tool of choice for this type of cut.

On Thu, Mar 31, 2016 at 8:21 AM, Luke Floden <robo...@gmail.com> wrote:

Is there any equipment at the space capable of cutting a 45 degree bevel on a piece of an Optix acrylic sheet?

I am trying to make a pyramid out of acrylic and need the bevels to create clean edges. Attached is an exploded view of the pyramid pieces.

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[Pat McCarthy]

Can a router cut cleanly? If mostly so, then with the proper bit, that would be one way

 

Either way, tool must be for plastic… and you may want to “finish” on a plate sander (swapping out the 220 grit for 800++ followed by a run at 1600+++)

[Tim VanSant]

Given those dimensions, none of the bevels will be 45 degrees.

[Pat McCarthy]

What do the dimensions have to do with it?

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[Pat McCarthy]

The angles under discussion are these:

 

Not the apex angles

(Unless MY Geometry from 40 years ago is off, and the dimensions of 2 dimensions has an effect on the third one….)

 

 

From: lv...@googlegroups.com [mailto:lv...@googlegroups.com] On Behalf Of Tim VanSant

Sent: Thursday, March 31, 2016 4:50 PM
To: LVL1 - Louisville's Hackerspace <lv...@googlegroups.com>

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[John Howard]

It's a compound angle. Each will be 45° if measured parallel to the base. However for the cut you will want to measure perpendicular to the surface of each side. That angle will depend on the ratio of the height and width of the pyramid.

I haven't checked it though this page has a calculator that could solve this for you:

www.slyman.org/m_projects_mathematics_pyramids_polyhedra_intersurface_angle.php

[Pat McCarthy]

Well well well, I learn something new every day!

 

Working from the diagram, I came up with:

 

Looks like a chamfer of 50.77°  Cool!

[Brian Wagner]

Discussions like this make LVL1 great!

[galenpowers]

Back to the cutting question now that the angle issue is resolved: 

This is harder than it sounds at first glance. These parts are not very big, this could get scary for ones fingers.... make some test parts first. Acrylic doesn't cut well with standard tools. Anyway, with that said:

If you want a really good finish, a jointer might be worth considering. If you are you going to solvent bond the joints and want good cosmetics at the joints, a better finish will help. You could just block sand the cuts also. If you want to sand, you could cut some angled sanding blocks while you cut the parts.

You will want to give some thought to 50+/- degree thing because it may cause you to run these parts standing up on the table saw which could prove challenging and dangerous. If you do these parts on the table saw with the parts on edge, clamp the parts to a piece of straight square lumber and consider making a zero clearance throat plate. Acrylic is pretty easy to shatter using general purpose cutting tools. As Everseeker said, shops that do a lot of acrylic use blades and special drills that are ground with not much rake to help reduce part breakage. 

One approach would be to cut the 4 triangles with square edges first, then put the angles on with a jointer, shaper or uniplane. Care will have to be taken to keep all four the same size. It doesn't look like there is a bottom in this but if there is, one approach would be to glue the 4 pieces together and scribe around them to size the bottom piece, cut it with square edges them bevel the bottom to fit. 

Back to the triangle thing, if you want to take the easy way out using a cad tool, draw the pyramid, cut half of it away with a negative cube and query the relationship between the remaining faces. This was a slightly different pyramid:

[Paul Gossen]

I'd think that any cutting tool would have the problem of the acrylic shattering, especially for your project because of the clean knife edge you are trying to make. Maybe a planer, jointer, or shaper could work if you take care to sharpen the blades and take off very little per pass. 

Tightly sandwiching your acrylic between 2 pieces of wood to support the edges during the cutting may prevent the edges from shattering ... or at least keep the shattered edges from flying around. 

Sanding is a good option. To do that with any productivity you need a good vacuum sander with high quality open grit paper to keep the sanding surface cool and take away the acrylic powder. Otherwise, the sandpaper heats up, softens the acrylic, and then quickly plugs the sandpaper grit. I have the tools and sandpaper for this (I use them to sand epoxy for wood restoration work). Connect with me a I'll bring my vac sander to LVL1 for a test. 

I second Galen's order of operation: cut 4 triangle blanks first with square edges, then put on the beveled edges. Cut the triangle blanks out on the laser cutter to guarantee that they are exactly the same. Cut them tall (long in the perpendicular distance from the base to the apex) because you will shorten them a bit when you put on the bevel. 

No matter what tool you use, you'll need to build a jig to get the bevel angle perfect on all sides, otherwise the edges won't match when you assemble your pyramid. I'm not expert at jigs, but my guess is that you'll get the best results if you could stack all 4 pieces together in a jig, and then put a bevel on one side of 4 pieces in one operation. Do the sides first, then stack the 4 sides and put the bevel on the base to set your finished height (this is why you cut the triangle blanks a bit tall).   

[Luke Floden]

Thank you all for all of your help. I had no idea about my error with the bevel's angle or even how to achieve it. Seeing as I am not quite 18 years old yet I would either have to find a chaperon or I could come down when someone willing to chaperon for me is there. If anyone is willing to help me with this please message me and we can figure something out.

Also for those interested I attached a picture of my full sculpture (I intend to add independent RGB lights to each level/pyramid).

[Rick Paul]

Solution: cut pieces slightly oversize. Build a jig to sand the correct angle on disk sander.