Drilling acrylic

[Pbruce]

I’m installing the skylight. I just researched the BH archives and YouTube. Acrylic bits are expensive and hard to source. Youtube sources seemed to strongly advocate using a step bit. Has anyone tried this? I also saw a guy take a normal HSS twist bit, chuck it in a drill, and spin it backwards against a grinding wheel. He shaped it like a pencil (resembling a proper acrylic bit) and then used it to good effect. Is this a viable option? Do others recommend screws through the acrylic skylight into the steel frame at the sides as well as in the middle, or some kind of captive strip? Thanks

[spinningwrench]

I used a technique my local A&P has successfully used for years. Take a twist drill slightly larger than the screw size. Heat the butt end red hot and “cauterize” the hole through the plexiglass. The melting action eliminates the typical stress risers of drilling which leads to cracking. You will need to lightly deburr the slight melted rigid you develop on the back side. He has installed many windshields and has never had one crack. You can do several holes before needing to reheat the drill bit.

[Nev]

Yep I used a step drill, no issues. I used screws through the acrylic, but not sure if that's a recommendation. So far it's holding up well.

[DBeaulieu]

I've used acrylic bits and step drill on my windscreen and window install on another project. Both work well, though the step drill seemed like the easiest to use in practice. Slightly oversized holes with room for silicon tubing cut to thickness of material to go into the drilled hole...allows for expansion/contraction/movement of the material around the fastener.

[marcusofcotton]

I've always had success using a step drill on acrylic. A nice feature of the step drills I've used is the radius to the next larger step, makes a nice finished edge on the top side of whatever material you're drilling through - bring it around to the backside as well if accessible.

[Bcone1381]

I've only ever used Acrylic Drill Bits. My step drill fist step is 3/16" dia....that is kind of large.

Acrylic drill bit at Spruce.

https://www.aircraftspruce.com/catalog/mepages/plexidrill.php?clickkey=9022

Acrylic Sheet Fabircation Manual
Acrylic Sheet Fabrication Manual--Plexiglas.pdf
I would follow industry best parctices for drilling Acrylic. Figure 8B on P.15 shows an acrylic drill bit that Spruce sells for about $7-9. P.16 starts with "Drilling Techniques. How to polish to remove stress risors, etc. using wax when drilling. how to avoid fracturing the sheet, benefits of backingup the hole your drilling. etc.

The money you spent on your acrylic is significant. I believe the acrylic bit will reduce damage risk for small holes and prove to be a good value. We never will understand the benefit of finishing off the inner hole surface to remove stress risers until we skip that part and see cracks later on.

[Pbruce]

Thanks. That’s a reasonable price. Funny how hard it is to find things sometimes. I searched “plastic bits, acrylic bits, acrylic drills”, just about anything I could think of on ACS and came up blank.

[spinningwrench]

McMaster will have plastic bits if that is the route you go.