Cutting 3" thick HDU is a unique special challenge, the most typical thing is to cut standard 2" thick HDU and despite being one of the most common things in the sign industry it is very difficult to get good information regarding bits, feeds and speeds. If you notice on this nice chart provided by Electricfly above, there are no bits on the list with a cutting length of 2" or more:
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precisionboard.com
For years I cut 2" HDU on a Gerber Dimension 200 using 1/4" Gerber bits with no problem but when I got my Multicam 3000 I wanted to really up the game and come up with the best, fastest, cleanest finishing bits possible and unbelievably it's been a struggle. Being used to using 1/4" bits, wanting the least waste with the smallest kerfs and wanting the tightest radius corners, I was hoping to continue using 1/4" bits but Onsrud does not make one and I can't find another one anywhere except Gerber still makes one:
I must say however, that I haven't quite gotten the same result with my Multicam as I did with my Gerber router, there seems to be more vibration and the finish has not been as smooth. Now, if you are OK with using large expensive bits, you can find some 1/2" bits from various companies that have a cutting length of 2" or more. Recently I have been trying XEdge bits but even though they seem to be the perfect solution, I'm still trying to work out the perfect feeds and speeds and decide if these bits are the right choice. I have also used their ball nose bits with very good results but the straight end mill bits have given me some issues.
www.xedgetools.com