Thread: Suitable bit for morticing

From my limited work so far, I first tried the larger bits (19mm) and had the same problem of the bit moving around.

I then moved to a bit that was either half the width of the hole I needed or may 1-2mm on top of this.

Then I routed from both sides rather than all in one go. It does mean making sure that your setup is accurate and trying several scrap joints first until you are happy, but I did find it better.

James.

Paul,
A solid carbide spiral upcut bit is what you need for morticing; straight bits are not suitable for plunging; they are designed for flush trimming. They do not cut in the central part of their ends, whereas a spiral bit does and it clears out the waste better than a straight bit. In the article I sent you, there is a paragraph on the actual technique of routing a mortice.

The largest spiral bit available is 1/2" diameter, so if you really need to cut a 3/4" mortice, you need to rout it in two stages, i.e, with two different fence settings. However, I am not sure whether you ever need to have a mortice thicker than 1/2". Working in hardwood, I usually use either 5/16" or 3/8" mortices.

Paul

Rocker is quite correct in his recommendation of a spiral bit, as well as his statement about the size (width) of a mortice. Rarely should you need to go beyond 3/8". I tend to cut 3/8" and 1/4" mortices, either with the router or morticing chisel. 1/2" mortices are best for tables or very solid furniture. Most unit panel-and-frame doors only require 3/8", some only 1/4".

I have found it very hard to obtain the desired diameter spiral bits for routering a mortice in the past (yes I know they are available). Perhaps I used this as an excuse because they are quite expensive. So I chose to use straight bits, but these are ones that are intended for cutting mortices - they have cutters on the tip of the bit for plunge cutting. Carba-tec sell a home brand for about $20, and their quality is excellent.

The trick to using a straight bit is that you must do "ramp" cuts. This is actually very easy in practice.

1 - never cut more that 1/4" depth at a time (I can just see you using your 19mm/ 3/4" bit in one hit!! Very hard to control, demanding on the router and bit, and a sure recipe for disaster).

Rule #2 breaks rule #1 briefly - plunge cut the two ends of the mortice first (if you don't want to do this with the router, you can drill them out instead).

3 = route out the remainder of the mortice in increasing depths (which you set on your router stops beforehand.

A solid fence system is essential. Rocker's jig has such, as does mine. The essential element is the that line cannot be allowed to waver.

It becomes second nature once you have done a few this way.

Regards from Perth

Derek

Oh, one other point:

You should be making the mortice before the tenon. It is easier to fit a tenon to a mortice than the other way around.

Derek

Derek,
I agree that you can use a straight bit for morticing, if you drill out the ends of the mortice first, as you suggest, and make shallow ramping passes; but a spiral bit does the job much better, because it clears the waste, rather than trapping it between the blades of the cutter and the sides of the mortice, as happens with a straight bit. Woods that tend to burn when being routed, such as Tasmanian blackwood, really need a spiral bit; but a straight bit is OK in more easily routed hardwoods, like jarrah.

Spiral bits are a bit expensive and hard to find, but they are always available at the big woodworking shows in capital cities. If you take care of your spiral bits, cleaning them periodically with oven cleaner, they will last many years, and you can probably get away with just having two, a 1/4" and a 3/8". The CMT ones cost $41 and $109 respectively. But you may be able to get other brands a bit cheaper.

Paul,
A cheaper alternative to buying the CMT spiral bits would be to get the Onsruds spiral bits from Lee Valley tools. Their 1/4" bit is US$17.25, and their 3/8" bit is US$29.75. You would also need to get a 3/8" to 1/2" bushing adapter (for US$ 3.25) with the 3/8" bit , which has a 3/8" shank - hence its much cheaper price than the CMT , which has a 1/2" shank.

Thanks for the advice gentlemen.

I did notice that the carbi-tool spiral bits were in the vicinity of $90, but the Lee Valley option sounds good too. I guess if I want to do it , I need to spend some dollars though. As mentioned, if I look after them, I'll get years of service anyway, so the overall cost isn't too bad.

One thing I had done "right", was cutting the the mortice in stages as suggested when using the straight bit. As Rocker mentions, the problem was that the wood chips crowded around the cutter. Think I'll invest in the spiral cutters.

I'm curious about the maximum mortice width suggested. My very first project ever, was the construction of a large chunky timber post bed for myself. (Hasn't collapsed in three years so it can't be too bad ) The bed was constructed from Oregon beams and posts. The end rails which are 40mm thick were joined (via M&T) into 100mm posts. I think I had the tenons at 30mm. From what you've both suggested, I'd have been able to use a tenon around 1/2" (at a reasonable depth) for this join? Or did I do the right thing by having the thicker setup, in this case.


Again thanks for the invaluable assistance.

Paul.

Paul
The general rule of thumb for mortices is that they should be about one third of the width of the timber they are cut in, so your 30 mm mortices in 100 mm stock were fine, especially since you were using softwood. You probably could have used 1/2" mortises even here though, since the stress on the rails of a bed's headboard and footboard are unlikely to be very great.
For hardwood furniture, you would rarely, if ever, need a mortise thicker than 1/2".

I have no problems using a straight bit thats made for mortising, they are different to a normal straight bit tho. The cutters on the end are slightly deeper & the upcut hollow in the center of the bit is twice as deep, maybe even more than a standard straight bit to allow for the cut waste to escape.
I think the spiral bits would plunge better, but wouldn't the straight bits cut better when moved sideways ? I'm a newbie but seems to me when mortising your doing two different types of cut into the timber & both bits would have there good/bad points ?

Cheers
Woodchuck