End mills vs. router bits for mortising
My choice: end mills. Here’s why.
I do most mortising with my trusty Elu 3338 plunge router, currently available as the DeWalt 625, with various jigs and methods, most involving a template guide riding in a slot. I formerly used solid carbide upcut spiral router bits with generally good results, though I often encountered two problems.
First, 1/4″ and 3/8″ spiral bits are usually sold with cutting lengths of 1″ and 1 1/4″, respectively. I often want to make mortises deeper than that. Second, 1/4″ and 5/16″ diameter bits, especially some long HSS versions, will sometimes vibrate in the cut and produce steps on the mortise walls. Even a 3/8″ bit may be made with a surprisingly thin web at the core of the spiral which can cause the bit to flutter when cutting dense woods.
My preference now is a standard, four-flute, center-cutting, single end mill in uncoated solid carbide with a plain shank and a 30-degree helix. These are available in longer overall lengths with longer cutting lengths than router bits, thus allowing deeper mortising. The core of this type of end mill is more substantial than that of the same diameter spiral router bit. I find the cutting action of these four-flute end mills has less vibration and is smoother and more balanced than that of router bits. This results in cleaner mortises. Furthermore, these end mills are generally less expensive than comparable router bits.
I usually run the end mills at 18-20,000 rpm. Some cautions that I employ are:
- chuck at least 1″ of the 3/8″ bit into a clean, top quality collet such in the DW625
- cut the mortise in small depth increments (e.g., 1/8″ in a dense wood)
- use a vacuum to frequently clear chips that may build up in the jig apparatus
- do not mortise deeper than the flute length
- use a steady, reliable jig setup
I always listen and feel for signs of strain from the router or bit and adjust the depth of cut and feed rate accordingly. I have read of mostly speculative concerns regarding burning and chip congestion in the mortise, but I have not had either problem.
The disadvantage of an end mill is that the cutting diameter equals the shank diameter. Therefore, I usually mortise with a 1/4″ or 3/8″ end mill using a router collet of the same size for each. A 5/16″ end mill can be used with a shank adapter though I prefer to avoid these adapters.
This is what works for me in my shop using these specific end mill specifications with the mortising tools and techniques described.
Sources for end mills are industrial supply houses such as MSC, Enco, McMaster-Carr, and Grizzly. It will take a while to go through their catalog algorithms or directly study their catalog pages but these are good ways to learn about this type of tooling.
From left to right, 1/4" and 3/8" upcut solid carbide router bits, and 1/4" and 3/8" end mills. The 3/8" end mill is 4" long with a cutting length of 1 3/4".
The mortise is 3/8" wide, 2 3/8" long, 1 1/2" deep and was cut in bubinga, a dense wood, with the end mill pictured. The walls are very clean and true.