Horizontal Mortiser Question

[Dick Sylvan]

I am building some large carriage style doors for the entrance to my shop. I am planning on using loose tenon joinery with the tenons being approximately 3/4" thick (rails are 2-1/4" thick). I will be cutting the mortises on my Hammer C3-31 with a bit that is close to 1/2" in diameter. Since this is the first time I have used the mortiser, I am concerned about one aspect of cutting the mortises. I will have to make two separate passes to get the right width for the mortise. I am concerned about deflection of the bit on the second pass because only one side of the bit will be "supported" by the previously cut pass. Is this a legitimate worry or is the bit rigid enough not to worry?

[Mike Henderson]

I use a solid carbide bit when I use a horizontal slot mortiser and they're pretty ridgid. But unless you're limited by the mortiser for some reason, what I do is make a full depth plunge cut on each side of the mortise, then clean out the middle.

But I've never worried about bit deflection.

I usually do regular M&T joints instead of loose tenon joints, and if you're a bit off with the mortise (you shouldn't be with a slot mortiser but just in case), you can adjust the tenon to compensate. I guess you could trim a loose tenon the same way.

For making mortises, a slot mortiser is just magic (in my opinion).

Mike

[Steve Rozmiarek]

Dick, I agree with Mike. Just curious how deep the mortises will be? If you use a small bit, like 1/4, it may deflect when moving laterally so the bit is half on wood, and half in space. They don't deflect when plunging though, so if you do get deflection with the 1/2 bit, just plunge to clean the waste, then lateral to clean up. I doubt your bit will deflect though, if you are not extreemely deep.

[Frank Drew]

For making mortises, a slot mortiser is just magic (in my opinion).

Mike

I agree with Mike; however, if your bit is fairly long it's possible to get a bit of chatter with too aggressive a cut, so listen to your bit, it will tell you when to ease up a little.

I also agree with Mike in preferring integral tenons, and for a big, heavy door it's hard to beat a through, wedged tenon.

[Dick Sylvan]

Thanks guys. One other question. After the two plunge cuts are done at the ends of the mortise, do I do a series of plunge cuts to clean out or just lateral cuts?

[Jeff Duncan]

A couple other thoughts/suggestions. When doing loose tenon construction make your tenons after you cut the slots. There is always some amount of play in the bit, your 1/2" bit for instance will leave a slot slightly oversized. My slot mortiser has about .005 runout at the chuck which is about average. The longer the cutter the more the runout is magnified. I have all my mortise stock pre-cut, once my slots are cut I run the stock through the drum sander to fine tune it.
I don't think you'll have too much trouble cutting the slots the way you want to. Slot mortisers are fairly sturdy machines and I agree with the others that you won't get too much deflection form a 1/2" bit. Another alternative though that I use on interior doors all the time is to do a double mortise joint. Instead of trying to hog a large 3/4" slot I route 2 3/8" slots which gives you more gluing area than a single larger mortise.
Lastly I've seen the method of boring out both ends then cleaning up the waste. I just run my cutter back and forth and gradually increase the depth. Not sure if there's an advantage to the other way or not?
good luck,
JeffD

[Mike Henderson]

Thanks guys. One other question. After the two plunge cuts are done at the ends of the mortise, do I do a series of plunge cuts to clean out or just lateral cuts?
You can do either, but I usually do a bunch of plunge cuts, almost like drilling out the mortise, then go side to side. That way, the bit is just cleaning out the mortise. (note: the reason I use this technique is that the slot mortiser I was using had a problem with the chuck. It would tend to loosen when a lot of lateral force was used. Doing plunge cuts with cleanup minimized the lateral forces.)

But another very valid way to do it is to take small bites, side to side (as Jeff describes above). Don't try to go side to side full depth because you'll be putting too much lateral force on the bit.

Once you try it, you'll figure it out quickly.

Just as a comment, the only time I broke a bit when using the slot mortiser was when I dropped the bit taking it out of the tool. It hit the concrete floor and broke in two. Never broke one in use.

Mike

[Frank Drew]

I like the plungecut-the-ends-then-sweep-the-middle method, taking maybe 1/4" cuts with each side to side pass. Also, I found that it helps to have an air gun to keep the chips cleared out as you cut the mortise; even upcut bits don't keep the mortise completely clear and you don't want to be continually recuttting the same wood.

As Mike said, these machine are pretty close to magic after you've tried all the other methods of making mortises.

[Frank Drew]

Another alternative though that I use on interior doors all the time is to do a double mortise joint. Instead of trying to hog a large 3/4" slot I route 2 3/8" slots which gives you more gluing area than a single larger mortise.


This makes an excellent joint; it's easier to do with loose tenons than with integral tenons, though.

Again in praise of a slot mortiser -- if you like loose tenons, cutting mortises in the ends of rails is just as easy as cutting mortises in the edges of stiles.

[Dick Sylvan]

I like that idea, too. With a large door (4' x 7' x2-1/4") would the double tenons be OK? Pluses and minuses? Thanks.

[Peter Quinn]

I work in a door shop and while I haven't made any 2 1/4" think entries myself, I have seen a number of them produced by the lead door man. Also seen a set of monster arch top carriage house doors made. All of our M&T doors beyond 1 3/4" thickness get double tenons rather than increasing to 3/4" thickness. 1 3/4" thick doors get a single 5/8" tennon, I think the 2 1/4" is either double 1/2" or double 3/8" tenons, not sure.

I would worry about deflection on the slot mortiser with tenons that deep made in two passes. I have never tried it but my instincts tell me its worth a few good test pieces in both edge and end grain if you go that route. I'm using Garniga solid carbide cutters for long slots and I think the 1/2" would give some problems, the 5/8" maybe less so. At work I cut them on a bridgeport with 4 flute roughing end mills, but they have them up to 3/4". I have seen them deflect even at that thickness if you push the feed rate too hard (it has a power driven table).

[Frank Drew]

I like that idea, too. With a large door (4' x 7' x2-1/4") would the double tenons be OK? Pluses and minuses? Thanks.

That's a whopping big door, Dick; double tenons are more work but would be very appropriate, although there are a quite a number of older doors that have survived well with single tenons; a lot of those that I've seen, both here and abroad, have through tenons.

On your wider bottom rail, though, don't make continuous mortises the full width of the rail; you get a stronger joint if you interrupt the mortise for an inch or two somewhere around midway with either just a shallow groove (say, 1/2" deep) or nothing at all.

[Charlie Plesums]

The discussion on plunge vs. side cuts needs to address the type of bit. The ones I use in my slot mortiser are birds mouth bits which do a great job going sideways, but basically don't plunge.

Some people use an end mill, which does a better job with plunge cuts, but the examples I have seen aren't as clean with the sideways clean-out as I get with the birdsmouth bits.

YMMV

[Cliff Rohrabacher]

If you nibble it out, you should be fine. If on the other hand you bury the bit and wail away, you can expect the slot to be strangely shaped.

[Dick Sylvan]

Well, I'm not in a hurry, so I will take the slow, patient route. Thanks for all the suggestions.

[Jeff Duncan]

I haven't tried the solid carbide yet as they're a bit spendy. I use HSS end mills from MSC that are very inexpensive and do a good job.

I can't see any downside to using 2 tenons except for the extra time involved. The doors I make are a little smaller, 1-3/4" thick and generally between 27"-36"wide by 7-8' tall. Before I got the slot mortiser I did use mortise and tenon construction and cut the dual tenons. It is a bit trickier to get it just right, but once you get the setup good you run your whole batch at once. Using loose tenons is much faster and easier. Nothing's worse than cutting a batch of rails only to remember at the end you forgot to add on for the tenons:eek:
good luck,
JeffD