# Deep Mortises with a Plunge Router



## FreeTime (Dec 2, 2012)

I’m building a stand for a refurbished joiner and am using up some ash stock I was gifted. I’m working with 6/4 cross pieces into 8/4 legs and was hoping to get at least a 2.5” deep mortise. Another question I had was … is there a common ratio (mortise:finished stock thickness) to use for mortise width? Currently I’m adjusted for 5/8” on the 8/4 (1.75” finished) stock.

Is there any way to obtain a deep mortise with a Router? I have a DW-816 router that has a 2” plunge depth and 2.675” full up collet to base distance. (I’m getting a 1.6” cut depth with a 2” CL 3/8” dia. up-cut bit using a fixture designed to support guide bushing use.) I knew I would lose ¼” depth to the template but I gain in mortise width flexibility.

From what I can tell from generally bit availability, I’m assuming the 2” plunge depth must be about standard for routers. (The specs I could find on the Bosch routers did not specify a plunge depth). Even if I could find a longer bit to perform the cut in 2 setups – what usable length would be pushing the bounds of safety?

There is no budget for (and not enough use to warrant) a dedicated mortise machine (and the mortise width would be limited to standard tooling availability). The only other option I could think of was to finish out the mortise with a top bearing mounted bit on a fixed base. That would get me to full CL of the bit but not much more if I wanted to keep the bit safely in the collet and I wouldn’t have any good way of really controlling it starting the significant final cut.


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## Cherryville Chuck (Sep 28, 2010)

I usually aim for a tenon of around 40% of the thickness of the piece with the mortice, 50% max. Since you are going 5/8" into a 2" piece that isn't an issue. You can get bits with 2 1/2" cut length and I think I've seen references here to one with 3" cut length but you would have to have 1/2" or more exposed below the base before you start your cut and that is a little risky. I use a mortising attachment for my drill press that I think I only paid about $70 for at a woodworking show. It will go that deep without any real problems and isn't that expensive. You have the options of 1/4, 5/216, 3/8, and 1/2 or you can make multiple cuts for other sizes but I only do that when I need larger than the 1/2. The others are close enough to my target sizes.


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## Al B Thayer (Jun 2, 2014)

*Maybe not with a router.*

Mark

You might want to set them up on a drill press and hog out the slot with a drill bit. Then clean them up. Stopping and starting a router cut like that would be very risky.

For me something like your building. I would cut through mortises and use a wedge tenon. It will be more than twice as strong.

Al


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## AndyL (Jun 3, 2011)

It is possible to get deep morticing bits up to 6" long, this is a UK supplier but I'm sure something similar will be available in the US:
Wealden Tool Company Limited Deep Pocket
Obviously you have to work in stages as the longest ones will extend through the base of the router even when fully up, so you need to have cut the mortice at least that deep before you start with those bits. The way you use these safely is like a drill - start by making repeated plunges along the mortice, and only move the cutter side to side to clean it up after most of the material has been removed.


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## Santé (Jan 14, 2010)

I have a long bit but I must first make a 50mm mortise and after use the long bit to deepen the mortise.
With my jig, I do not lose a mm deep because the router is direcly on the wood to be mortised. The jig is located at: http://www.lescopeaux.asso.fr/Techniques/Docs/Sante_Mortaise_a_la_defonceuse.pdf


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## Phil P (Jul 25, 2010)

FreeTime said:


> Is there any way to obtain a deep mortise with a Router? I have a DW-816 router that has a 2” plunge depth and 2.675” full up collet to base distance. (I’m getting a 1.6” cut depth with a 2” CL 3/8” dia. up-cut bit using a fixture designed to support guide bushing use.) I knew I would lose ¼” depth to the template but I gain in mortise width flexibility.


Hi Mark

As Andy L points out you can get specialised cutters up to 120mm depth of cut, but they are going to be too much for a DW618 IMHO. I actually use this type of cutter in a Trend lock jig quite regularly and I'd say you need the maximum depth of plunge you can get, together with a lot of power and some mass. I use a deWalt DW625 or an Elu MOF177e (they are near enough the same) for this task and even they can get pretty hot in laminboard core doors. With a DW625 on a Trend jig and a 12mm diameter x 100mm cutter I can easily get your 2-1/2in (63mm) depth of cut within the plunge range of the DW625 (75mm or 3in). Ideally you want to start your cuts with a shorter cutter which can retract into the base of the router before finishing with the "big yin" - that's because the biggest cutters always protrude beyond the base and you need to be ultra careful starting them up to avoid an accident

Out of interest this is the sort of cutter I use (as linked-to by Andy):-










The one on the left is 16mm diameter x 120mm depth, the other two are 12mm diameter x 90mm depth of cut (although a jig will cost you 10 to 15mm of that). These cutters have to be used with great care, advancing only 10mm (3/8in) or so in depth for each clean-up pass and vacuuming out between passes. The biggest I've cut to date were around 22mm (7/8in) wide x 240mm (9-1/2in) long x 100mm (4in) deep. They took about 6 or 7 minutes to cut including setting-up the jig

The equivalent Bosch model would be a GOF2000CE (or in the USA a 1619EVS?) which has the required plunge depth. It should be able to make these cuts

The jig I use is the Trend lock jig - a demo of this in use, here. I'm not suggesting that you buy one, rather that you make something similar in plywood - before I bought one I did just the same. Just don't think about using one of those self-centring base things for this task - they just aren't man enough

Regards

Phil


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## FreeTime (Dec 2, 2012)

Guys,

Thanks for all the great input. I thought my current router might be a little under powered for this type of work but it didn't seemed too stressed with the 1.6" deep test cut (I took it real slow)... maybe an upgrade to more hp in the future. If I can minize the lateral force on the bit with pre-drilling or plunging to remove (both as suggested) - I think I'll be ok. 

I like the idea of wedged M&T - I didn't know about it until now (thankks) but did a little research and think I'm going to attempt it. (I was thinking about a through M&T until I ran into the perceived depth limitation.). I'm not looking forward to manually cleaning up the corners but I agree - it will probably be worth it

I will need to acquire a longer bit than I have (Whiteside seems to have viable selection) but will clean up the last 30% of the cut from the far side with a flush trim bit against the cleaned up first cut.

Thanks again
Mark


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## Cherryville Chuck (Sep 28, 2010)

You are going to spend some pretty good coin for the right bit. If you plan on doing very many you might want to consider the drill press attachment. You get 4 sizes with it and it makes square holes which eliminates one problem. In the long run it is a more cost effective choice.

If you use the router you will have round corners as you point out. You can either square the corners of the mortice with a chisel -OR- round the corners of the tenon to match the mortice. I think that rounding the tenons is easier. If you mortice both pieces you can use floating tenons which can be rounded with a bull nose router bit.


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## FreeTime (Dec 2, 2012)

I only have a bench top drill press - it does't look like I'll have enough vertical? I couldn't find any guidance on this ... do you know if it would work?


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## Phil P (Jul 25, 2010)

Hi Mark

In the absence of any other way why not start the mortises off with a router and go as deep as you can, then finish to depth with an auger bit, chisel and mallet? No need to go to the complexity of squaring the shoulders or cutting the splayed verticals that a foxed (wedged) tenon requires, although (draw-bore) pinning the joint would add a lot of extra strength and with 2in of depth you have just about enough material to do that with

Regards

Phil


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## Cherryville Chuck (Sep 28, 2010)

FreeTime said:


> I only have a bench top drill press - it does't look like I'll have enough vertical? I couldn't find any guidance on this ... do you know if it would work?


I went out to my shop and measured to be sure. The attachment adds 6" of length to the quill on the drill press. The one I have is made by Delta and the manual for the kit lists the drill presses it fits down to their 10" model so there is a good chance one will fit yours. If you are mortising a long leg to something the benchtop model would be better than my freestanding one. I have to have a tall stand to support the free end where you can block it up from your work bench.

You can see from the picture that there are several split ring bushings (11, 12, and 13) to fit the holder (part #10) to your quill. You would need to make sure that one of these fits yours but I think the sizes are fairly standard as the 16.5" Delta I have is Taiwanese made as are (almost?) all the other hobbyist quality drill presses.


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