I cut mortises by drilling circular holes and then chisel out the remainder. I have been using spade bits but was wondering if there was an advantage using forstner bits for the circular holes? The forstner bits are much more expensive…
Regards,
Buzzsaw
Replies
Fostner bits are a real plus in this kind of an application. They run true and the holes can overlap, which can't really be done with a spade bit. I think if you try a fostner bit you'll never go back to a spade bit again.
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HEY! BUZZ, Forstner bits are very accurate tools used for practically flat bottomed holes
Properly used in a drill press, you can't beat them for accuracy.
Spade bits,( IRWIN being the best around,) can bore reasonably straight holes ,
But, if it hits a knot or soft spot ,(Void) it will wander; (Even bend,)
They can't produce a flat bottom hole.
I used to bore out locksets in wood doors containing a full pane of glass (Open light door) Occasionally , the lock rail would be just deep enough for the lock body
To avoid striking (And breaking) the glass with the point ,I used a spade for most of the bore THEN bottomed out with Forstner bit.
Spade bits are easy to sharpen (File) Except the newer ones with spurs
I get long life from my spade bits (even file them down to bore a slightly undersized hole when needed)
Only use my forstners for exacting work and buy them piecemeal as when I need a different size for a job. Forstners can be filed, but go easy and protect them from banging around in a drawer. Steinmetz.
Edited 8/25/2004 2:21 pm ET by steinmetz
Thanks very much Mangler and Steinmetz. I bought a 7/8" forstner bit yesterday for $12 which was about 3X the spade bit. I'll give it a try!Regards,
Buzzsaw
Buzz,
I use forstner bits extensively, and they sharpen easily with an auger bit file. Stay off off of the outside; sharpen only from the inside. As long as you don't overheat them and draw the temper, a forstner lasts along time, with only occassional touchups with the file. That holds true for multispur bits as well.Alan
http://www.alanturnerfurnituremaker.com
Thanks Alan. Great info...Regards,
Buzzsaw
Buzz, a litle known secret for anyone interested in power boring accurate holes in wood is: Auger bits made for a brace and bit
Cut off the tapered square head, then observe the tapered screw at the point.
IF you file off half of the whole conical point to create a 'flat half cone, you can use them in your electric drill to bore, and it won't pull the #$%#&^* bit into the wood
Just apply gentle pressure and see what you're missing.
When you look for good used auger bits in the flea markets, check the spurs.
They should not be filed dowm to a nub or broken off or split, but have plenty of usable 'Meat' left to dress with a small file.
If you're lucky, you'll find some of these gems with two spurs They are the best especially for harder wood. Steinmetz.
Steinmetz,
Help me out. I was a little confused with your reply. Brace and bit is entirely foreign to me but when I looked it up on the net it showed one of those old hand-crank drills with a piece of wood braced into a vise. I guess this is what you call a brace and bit. Now that I read again what you wrote it makes sense. Thanks.Regards,
Buzzsaw
Forstners for all blind mortises; spade bits for through-mortises.
Only reason for using the spade bit is the point will show through the other side long before the hole reaches bottom, so I can flip the piece and drill through from the back side so there's no tearout. Forstners require more down-pressure and can tearout something fierce if you go all the way through.
"Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
Thanks AlbionWood. The first few mortises that I cut I used spade bits but went all the way through. This caused a lot of tearout. I now go half-way through then flip it over and drill half-way through. I just end up marking my cuts on both sides of the wood for through mortises (all I've done so far). Is it assumed that through mortises are stronger than blind mortises? Also, what is the rule of thumb for depth of a blind mortise? Would 2/3 of the width of the wood make sense? 1/2 way? Thanks...Regards,
Buzzsaw
For me it's easier to just mark one side, drill until the point of the spade bit just pokes out the other side (easy to set a drill press to do this), then flip the piece and use the tiny hole to center the bit and drill through. This almost always give a nearly perfect hole. If I try to mark both sides, I often introduce a slight error and the mortises get wider...
You can do this with a Forstner bit as well, if you carefully set the drill press depth stop and if the wood is friendly. With Red oak or similar, though, I get tearout before the point of the Forstner comes through.
Now you ask hard questions:
Is it assumed that through mortises are stronger than blind mortises?
No, I don't think so. Through mortises can weaken the mortised piece and make it susceptible to splitting if there's any torsional stress. Most of the time this won't matter, but I only use through mortises if there's a reason for it, and strength isn't one of those reasons.
Also, what is the rule of thumb for depth of a blind mortise?
I don't know of one; maybe someone else does. Too many variables, I think: thickness of wood, width of tenon, etc... Are you gluing the joints, or pinning them? If pinned, then there are some rules of thumb to allow enough room for the pin and prevent relish failure. But if gluing, all you need is surface area for strength. How much strength you want will depend on what the joint is doing, i.e. what kind of stresses are expected to act on it.
For example, if the joint is in compression, then you only need a little stub-tenon to hold the piece in place. (The bases of posts in a timber frame are sometimes done this way.) At the other extreme, a leg-to-apron joint (on a chair or table) might be subjected to racking stress, which needs a very strong joint. The trick here is to make the joint strong without weakening the leg too much. Again, the rules are different if it is glued vs. pinned."Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
Albionwood,
Thanks for the reponse. Unfortunately I don't have a drill press but that will be my next purchase. A dust collection system is also high on my list. These last mortises I cut with marking on both sides came out ok but I like your idea as well. It's just that it's a little more difficult to tell when you have gone through the wood when you don't have a drill press. I may try a blind mortise next as I have only done through mortises. Thanks for your info.Regards,
Buzzsaw
Forstner bits are generally not recommended for use in hand drills. (Same is true for larger-diameter spade bits, for that matter.) They tend to grab the walls of the hole and try to jerk the drill out of your hand.
I like Steinmetz's idea with the auger bits. For that matter, absent a drill press, a brace-and-bit is safer and only a little slower for larger holes (3/4" and up).
Another option is to simply chop the mortises with a mortising chisel. Up to about 1/2" mortises are faster and easier to do this way than the drill-and-pare method, IMHO. Tage Frid's book shows how to do it."Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
These mortises I'm cutting are through mortises and 3 1/2 inches deep. Chiseling that much would take forever. I guess I should seriously consider getting a drill press. Do you think a bench top model could be sufficient? If so, any particular manufacturer?Regards,
Buzzsaw
Buz, Those auger bits can be used in a good electric drill (where you are in command using the on/off switch)
Also, the holes can be laid out to slightly overlap, which makes the 2nd /3rd/nextholes etc etc go easier.
es, the proper name is 'Handbrace' Stanley made millions of them.
(The ratchet type being the most versitile) Steinmetz.
Yes, a BT drill press is sufficient for most work - that's what I have. Occasionally I wish for a floor model, but not that often. More often I wish for better quality - mine is a cheapo Taiwanese knockoff, a genuine POS. Don't have any recommendations for you, as almost anything would be better than the one I have... OTOH, I've been using it for years and have certainly gotten my $75 worth.
You might be surprised how fast you can chisel a mortise once you get the hang of it. In my experience, paring the sides after drilling out the waste takes almost forever, and it's really hard to get good straight parallel sides. Using a good mortise chisel I can chop one out in about the same time it takes to drill and pare. It is definitely more work, though, especially for those deep mortises; and for through-mortises drilling and paring seems to work better.
If you're going to be doing a lot of these, a BT chisel mortiser can be worth the $250. Really speeds things up. I'm at the point now of seriously considering upgrading to a slot mortiser. "Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
You know, another alternative is to make mortises with a plunge router & spiral upcut bit. That eliminates chisel work (unless you want squared ends, like I do). Once you set it up, you can crank out things like leg mortises pretty quickly & accurately. There are about a gazillion jigs for doing this, varying in complexity from an edge guide to shop made horizontal mortising machines. Maybe the only other things that you can do so many different ways are tenons.
Yes, I do that a lot, and it works great. But you're limited to about 1 inch deep with a router. "Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
That is interesting about the drill press. I would've thought you would have a floor model. I should just bite the bullet and get a decent bench top model. I've seen some Deltas that look sufficient. I was planning on doing through mortises that stretchers will fit into for a kitchen island I'm working on but may switch to blind mortises. I've never done a blind mortise before but I think it may be worth trying. I also won't have as much to chisel. I think you or someone on this thread said to try a forstner bit if I'm doing blind moritses. I was thinking of a 1" wide X 2" length stub tenon (in a 3 1/2" X 3 1/2" post) that will sit in a blind mortise cut into the same post. I was thinking of making the mostise 2" deep. I think that covers all of the rules of thumb. Any other tips/tricks on blind mortises? This is a first for me...Regards,
Buzzsaw
The only reason I can think of to do through mortises on a post that big would be if you wanted to wedge the tenons from the outside rather than peg or glue them in place. One really cool joint for table stretchers is a vertically-wedged tusk-tenon, but it requires even more chiseling. Attached are a couple of examples. These are incredibly strong and rigid joints.
Blind mortises are easier than through-mortises, in general.
How wide is the stretcher piece? A 1" wide tenon sounds kind of narrow to me. Are you planning to glue the joint, or pin it? Glue joint strength is mostly a function of surface area. A 1" x 2" tenon gives 4 sq. in. of surface (both sides); a 2" x2" tenon has 8 sq. in.
If you are pinning, you just need to make sure the tenon is wide enough so there is plenty of wood left on either side of the pin. 1" leaves enough room for a 1/4" pin, but if you wanted a fatter pin I'd go with a wider tenon. For stretcher tables, I'd go with a double-pinned joint, so you'd want a tenon at least 2" wide. Attached is a not-very-good example.
There is plenty of information about M&T joinery in various books. I recommend Tage Frid's book on Joinery, and also Roy Underhill's books (The Woodwright's Shop series). These are often available cheap at used bookstores; try an online search. Taunton's recent "Complete Illustrated Guide" (by Rogowski?) looked pretty comprehensive when I browsed it in the store, but it's more expensive."Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
AlbionWood,
The post is a 4X4 post (3 1/2" X 3 1/2") so the width of the tenon is 1". If I made the tenon 2" then the mortise would also be 2" wide in a 3 1/2" wide post. I will be gluing, no pins. I have read often that a rule of thumb is to make the tenons 1/3 the thickness of the wood for the strongest joint. In my case the wood is 3 1/2" so I think I'm pretty safe with a 1" wide tenon and 1" wide mortise.
Thanks for the pictures. You obviously have done some more advanced M&T joinery that I have. Actually none of the joinery or the base of this island, for that matter, will be seen as it will be covered with a beadboard-type material. I'm just looking for strength. Thanks for your post.Regards,
Buzzsaw
I misunderstood - what you call "width" I would have called "thickness." (Tenon-centered thinking, I guess.)
So the mortise is 1" wide, 2" deep, how long? It's the surface area of the cheeks (the walls of the mortise) that count for glue joint strength.
The upper limit for mortise length depends on grain orientation; most M&T joints are cross-grain, so a glued tenon should not be more than 3" wide (across the grain). Pinned tenons can be wider, because the pins flex a little. "Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
This tenon would be 2 1/2 inches long, 2 1/2 inches deep, and 1" wide. This is assuming I am going to try a blind mortise. Thanks.Regards,
Buzzsaw
Plenty of beef, should be way-strong as long as you get good fit between M&T. Have fun!"Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
Thanks,
I'll let ya know how it goes. I appreciate all of your experienced advice...Regards,
Buzzsaw
Yes, there is a rule of thumb for mortises, but like most rules of thumb, it's only a guide. Depth of 2/3 of the part and a width of 1/3 is considered "standard". At one extreme is the through mortise, and at the other is a 1/4" groove for a stub tenon.
Thanks Mangler. That makes sense...Regards,
Buzzsaw
You can buy an entire set of forstener bits or multispur bits for under $40.00. Grizzly has them with a hex shank for $24.95, from 1/4" to 2 1/8" 16 pieces. The 1" and larger bits are saw toothed. I have not used the Grizzly bits but have used similar chinese made forstner bits. They work fine for most users. I also have several bits made by Connecticut Valley ( I think I have the correct name) these bits are definitely several notches higher than any chinese bit I have used. Most of these bits have seen heavy use over the years and stay sharper because thet do not overheat as easily. I have sent the conecticut valley bits out for sharpening, the chinese bits I touch up best I can, then trash them if the results are not good.
mike
Be careful when you buy the forstner bits when you buy them. There are two (2) types with one having a long spur ( American style) on it and the other having a short spur ( european style, I think.) on it. You want the short spur style with blind mortises so the spur doesn't break thru the other side.
Dave in Pa.
Thanks Dave. I'll make sure I buy the proper bits.Regards,
Buzzsaw
Just wanted to fill everyone in on using forstner bits for the first time for mortising. I LOVE them! First of all, I have been using spade bits but I really like the forstner bit especially because I was doing blind mortises and the flat bottom was great. The forstner just seemed so much easier to use with a true cut. These are the best mortises I've ever made. Thanks everyone for your input.
Regards,
Buzzsaw
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