Cutting deep mortise question

[Bob Cooper]

I'm continuing my first bench build and first experience w/cutting Mortises and Tenons. So these questions are likely pretty basic.

So last night i cut a mortise in the leg for a lower stretcher. The size is 1.25 x 3 x 2.25 deep. I outlined it w/pencil, used a chisel to mark it -- really didn't have a marking knife handy -- used forstner bits on the drill press to remove most of the waste and then used a mortising chisel and flat chisel to clean up the sides. So far it looks ok but i haven't build the stretcher yet. In hindsight i should have built the stretcher first so that i could fit it as i trimmed the waste. That said i expect that i did not remove too much waste so i'll be OK.

So...here's my dumb question. One approach occurred to me but i'm thinking it may be dangerous. Would it be dangerous to cut these w/a plunge router? i could either create a template or i could hand cut the first 1/2" and then plunge down from there? Either way i'd still remove most of the waste w/the drill press. What worries me a little is...is there any kind of kickback that can occur when you hit those inside corners? It's a 5" wide leg so i have plenty of support.

Thoughts?

[Prashun Patel]

I've done this using a hybrid approach:

Use a forstner to drill out most of the waste.
Use a chisel to pare and perfect the walls of the top 1/4" of the mortise.
Use a top-mounted bearing guided trim bit to clean up the walls.

A long bit will go deep, but will require you to clean up a large wall at once. If you move the router in the correct direction, it will work fine. The bearing will largely save you from digging in IF the bit skips.

But I prefer to do it in a safer way: use a shorter pattern bit so that you can clean up 1/4" or 1/2" increments in successive passes. Lower the bit on each pass. Once you are close to the bottom, you can switch to a longer bit if you have not achieved the depth you require.

[John TenEyck]

A plunge router with spiral upcut bit and a collet following a cutout pattern is a well accepted method of cutting mortises. You can cut out some of the waste first with a Forstner bit, but you don't need to. The router bit actually cuts smoother when milling wood equally on all sides. In any case, get a 1/2" upcut spiral router bit, or end mill, make a template, put a collet on your plunge router and go to it. You will be very pleased with how easy it is to make straight and smooth walled mortises of the exact size you wanted.

John

[Doug Hepler]

Bob,
I cut the mortises for my workbench using a handheld router, and frequently use that method on smaller mortises as well. As I recall they were 1/2" or 3/4" wide. There are no safety issues that are specific to cutting mortises, just the general ones about router use. For mortises that size, I would use a guide bushing in a template on top of a "Tage Frid" mortise box. I will explain. A Tage Frid Mortise jig is just a trough in which the sides are parallel to the bottom. Astonishingly simple. Just clamp the workpiece in place. See this link and this one to the original article. When I made mine, I made it to fit the sizes of workpieces I was using: 4 x 4's and 2x4's.

Frid used small stop blocks on the tops of the sides to control the length. I prefer using a guide bushing and a template. To make the template you cut a square hole in a piece of 1/2" plywood that is basically the size of the mortise you want plus twice the thickness of the wall of the guide bushing, minus the diameter of the router bit.

For mortises this size, I would use a spiral upcut router bit and cut only 1/4 - 1/2" at a time. You can get a 4" long bit that might do the trick for you, or you could finish the last 1/2" or so of depth with a Forstner bit and chisel. I think hand cut mortises are grand, but when you have a lot of deep ones to do, power tool methods become more attractive.

BTW I always cut mortises first and then mark the tenons to fit them. I think that's what you are doing.

Doug

[Bob Cooper]

guys...great and very helpful. this is actually a really good project to learn on since everything is big and beefy and if it's not 100% perfect...it's a workbench and i'm probably the only one that would notice. Doug...thanks for the links...and btw go gators (85/87 EE)

[Stan Calow]

From John: "The router bit actually cuts smoother when milling wood equally on all sides." That's a great insight. It explains a lot. Thanks.

[Bob Cooper]

let me ask this...i'm looking at a whiteside upcut bit...are the walls of the shaft of this bit wider than what the flutes cut? or asked another way...can this bit only cut 3/4" deep in total or 3/4" at a time? whiteside RU2075

i try to buy most all 1/2" shank bits but here's one area where 1/4" seems to be better suited.

[Doug Hepler]

Bob

This is a case where you really need a 1/2" shank. A 1/4" shank may flex under forces involved here. There are 4" bits available for example on Amazon I found. CMT 191.507.11 Solid Carbide Upcut Spiral Bit, 1/2-Inch Diameter by 4-Inch Length, 1/2-Inch Shank

Router bits do vary by cutting length as well as overall length. Most 4" overall have 2" cutting length. You do need the walls of the mortise to be smooth and uniform enough to make a good glue joint. As a rule I don't ask people to change their specs to make their question easier to answer, but in this case you might consider having a 2" deep mortise with a 2" long cutting length on your bit. With the jig setup I suggested you should get acceptable mortises.

Doug

[John TenEyck]

As Doug said, you need a 1/2" diameter bit for this. As an aside, it's always best to buy a 1/4" bit with a 1/4" shank, a 1/2" bit with a 1/2" shank, etc. That way there is no stress riser from a change in diameter between the shank and bit. OK, back to the case at hand. For a 2-1/4" mortise you will likely need a bit about 4" long. It doesn't matter if the bit only has, say, 1-1/2" cutting length. You plunge in maybe 1/4", mill out the waste, plunge in another 1/4", and repeat until you get down to 2-1/4". Only that last 1/4" of the bit is doing any real work, and the flutes are pulling the waste up out of the mortise. If those flutes are a little short it doesn't really matter. Just stop occasionally if the hole doesn't stay clean and vacuum or blow out the chips, then go back at it.

The only time you would use the full cutting length of a router bit is for cleaning up the side of a mortise, or edge of a piece of work, where you are taking off 1/16 - 1/8" of material. Never plunge full depth and try to route sideways. That's a sure way to break a small diameter bit and overheat any bit.

John

[michael langman]

Hi Bob, I just did mortices in my first bench top last week using a 1/2"- 2 fluted endmill with a 2" length of cut, and normal helixes for cutting edge.
I saw a youtube video where someone made a jig to locate the mortice the right distance from the edge of the bench top, using 4 pieces of plywood glued together on their edges to produce the correct square opening in the middle of the 4 pieces, for the router to run up against with a guide bushing.
I tried to find the video online to put here but could not find it.
My mortices were 1 5/8" x 2 1/2" x 1 7/8" deep.
I drilled out most of the material on the drill press in my benchtops. Left about 1/8" on the sides for the router but drilled to full depth.
If you go slow and take 1/4" depth of cuts you should be fine.
I'll try to find that video as it was a really good way to make a template.

[Bob Cooper]

They get pricey quick don’t they. But I like CMT and Whiteside bits and it’s a bit that will get used all the time I think.

So Doug let me ask this since you comment that it would be better to have me final depth at 2” yet per John the cutting depth doesn’t really matter since the bit’s width is the same as the shaft. Is there a reason to have the depth equal to the cutting length? I’m just curious as I agree that 2” is plenty of surface.

[michael langman]

I found this picture to give an idea of what I was talking about.

http://www.1910craftsman.com/wp-cont...mortiseJig.jpg

[Bob Cooper]

Mike...thanks. Sure wish a bunch of you guys were physically closer sometimes.

[Bob Cooper]

just read the article...looks like a good jig and very handy. Last time i had to build a new sled to get the tenons done and now i'm going to need to build this nice mortise jig...two steps back

[Doug Hepler]

Bob,

John TenEyk and I see this very much the same, with slightly different emphases. If you have a bit that you want with a 1.5" length of cut, go for it. (If I already had a bit with a shorter length of cut, I would not buy another one.) He mentions trimming the walls and clearing the chips, I'm thinking it would be good to have a length of cut that does that. If you use a template as I described earlier or some equivalent setup and are careful to keep the router base from rocking, you should be able to get smooth walls with a shorter length of cut, and you can keep the mortise clean by blowing into it through a drinking straw (close your eyes first). Hope this helps.

Doug