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Discussion Starter · #1 · (Edited)
I'm sure it's been asked before. Here's a little background info first. I'm currently making a display cabinet for toy trains where I'm putting about ten shelves roughly 4" apart in it. I dado cut the sides every 4" to accept a 3/4" thick shelf. We won't discuss how that's going since I'm a novice at dadoes but I ended up brad nailing them all from the outside which I really didn't want to do. Some were looser than others but in my defense I measured the shelves for thickness and some of those are inconsistent, as in not a true 3/4" thick. I probably shouldn't have attempted a project with 20 dadoes for a first experience in this, but I laid each one out at exactly 3/4 and cut the lines with a sliding miter saw set to stop at the bottom of the dado, and then cleaned out the waste with a router afterwards. I did it like this to make for easier setup. Just line up the blade and slide across. No clamping guides for a router or building jigs. It worked but the boards aren't all the same thickness. I'm using poplar 1x4 and 1x6 for all parts. No plywood.

Anyway the thought occurred to me that with a board stuck in a dado and glued in its always end grain to face grain. Does this hold well? Shelf end grain glued to side board face grain, and since it's a dado it's not a smooth bottom. Also there is shelf face grain contacting end grain in the dado from the dado sides. So there's always an end grain being glued on all three sides. I guess this holds well since it's obviously done often, but I just wondered how dependable this joint is?
 

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Smart intuition on your part. You are right about end grain not holding well as a glue surface....and to a rough dado surface. The strength of a dado is mainly in the face grain of the shelf against the sides of the dado, which means the dado fit has to be tight. Without that tight fit the joint is very weak....and unsightly if the gap is on top. Sorry to bring the bad news. That is the crappers of dado cutting. If you put a back on the cabinet and fasten the shelves and sides to the back, that will hold the back side tight. A face frame will act the same for the front. You can nail the sides and the ends of the shelves into the face frame stiles (vertical pieces). The face frame will also hide the dadoes and gaps in the side pieces.
 

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Discussion Starter · #3 · (Edited)
We'll I nailed them all anyway, but from the outside in, meaning all the nail marks will show. I HATE that part about it. I guess wood filler is an option but will it stain to match the poplar wood surrounding it? On the dadoes, they do not look perfect, but they are not hideous either. I can see what amounts to a hair gap on some of them. Others were TIGHT! I had to use a rubber mallet to beat them in. I'm unsure of what the proper assembly procedure should have been, but I put all 10 shelves in place on one side and then put the remaining side on afterwards, so this meant I had to work all 10 shelves into it all at once instead of one at a time like with the first side. I will shoot nails through the back into each shelf, and also into the side boards so this will hold it all well. The shelves are made using 1x4 so they're not very deep. I believe the nails in back will be enough. I hate the nails through the sides showing so I definitely won't be shooting any in the front. I clamped and glued the face frame on. That's side grain to face grain so it should be good.

I'm curious what should I have done to insure better dadoes? I cut them all exactly 3/4 wide, painstakingly taking time to be accurate. When I discovered some loose, some fit well, and some tight, I checked my board thicknesses and found them to be not all perfect. I started out with four 1x4x8's in poplar and cut 12 shelves, needing 10 total, with 2 spares. They were all 31" long each. I got different thicknesses from the same boards. I checked moisture content with a meter and was anywhere from 5 to just less than 7 percent so I thought that was good. In other words, no swelling from moisture. These were planed in a wood shop prior to my purchase also. Did they not do well in it somehow? I noticed a few of my boards had snipe on one end. That could explain some of it but I cut this out anytime I saw it and could do so.
 

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You do have some interesting questions and as stated - you're quite intuitive!

I just finished building 2 window seats with dado cuts, one built with pine and the other with popular. I've built quite a few items using dado cuts and I've learned over the years the best dado grooves are cut with a 3/4 " straight bit. The bit I use is from a "Hickory" set and it seems to be a bit shy of 3/4... but that's OK. I like sneaking male members of all joints down to size (this means forcing my joinery together without clamps or mallets). Router bits leave a flatter bottom then dado blades.

To cut dado joints, I measured the distance from the cutting edge of my bit to the side of my router base (2+ inches). I then ripped a piece of wood to match the distance. Repeat this for every size bit and mark the spacer boards.

Now I mark and draw out my intended cuts... place my spacer board on the edge of the cut and clamp a router guide square to the outside of my spacer. This method gets easier every time...
 

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Discussion Starter · #5 · (Edited)
What do you do if your spacer boards ever swell or shrink? I should probably do something like this, and I'm willing to concede that my laziness probably played into this some since it was my first project where I would be dadoing to hold shelves and I was overwhelmed with needing 20 of them. I marked them all, then thought about how I'd need to measure left of each one and mark for and clamp a guide, taking time to parallel it to the dado marks. I thought this would take too long so I went to the sliding miter and quickly made 20 cuts on each board, 2 per dado, inside the lines of my dadoes so I had an established edge, totally straight and right where it needed to be. Afterwards I used a straight 1/4 inch bit and carefully freehand hogged out the waste that was left in each one.

One failure I saw was that no matter how careful I was at making the router bit exactly 1/4 inch deep, and the saw blade to do a stopped depth cut at 1/4" I was off from the saw blade to the router bit. The saw blade tracks left a slightly deeper channel along each side of the dado than did the router bit when removing waste. I just figured this to be no big deal since it's inside the dado concealed. I even thought it might make a channel to hold any excess glue I may have over applied. But it made me question which one was the actual depth needed, blade or bit? If it's the blade and the bit was slightly shallow then that will throw off my overall width for my piece. But it turns out the blade was cutting too deep slightly. The piece was exactly 32" wide when assembled, with 1/4 depth dadoes and 31" shelves.

I believe some of what contributed to this was varying board thicknesses, but also me taking so many steps to cut the dadoes too. I now think it would have been much smoother if I had done just one step, as in set up a guide and make one pass with a 3/4 bit. I've read that some people don't take all the material out on one pass, but rather make an initial shallow pass, then a good deep one to remove most waste, and one more final very shallow pass to leave a clean bottom. Doesn't making three passes like this leave a lot of room for widening out the dado accidentally from router drift? I think it would be best to make one pass and then get away from it. Much less room for error, if the bit and router are up for cutting through the wood all at once without tearing it up or burning it. That was my fear also with a router so I opted for the saw to mark the edges and the router to just remove waste.

You can also add to this that I was fighting twisted wood too. At least one of my 1x4's was twisted, a fact I did not notice when purchasing since I had another shop prepare the wood for me and they taped my boards together in a stack for hauling. At home I learned of the twist when I cut them into the required lengths and laid them on my bench and noticed a wobble from opposite corners. So much for relying on another "for hire" wood shop to premill any wood for me.
 

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If you didn't plane or sand your boards so all were the same thickness, then you were doomed from the start.

The only real answer is a drum sander which will sand your stock flat and all to the same thickness. I know that isn't what you want to hear. Sorry.

Check out the Wood Whiperer on YouTube. Make a router jig like Marc's and you will get good dados...even if you have to match each board to the corresponding dado.
 

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I'm sure it's been asked before. Here's a little background info first. I'm currently making a display cabinet for toy trains where I'm putting about ten shelves roughly 4" apart in it. I dado cut the sides every 4" to accept a 3/4" thick shelf. We won't discuss how that's going since I'm a novice at dadoes but I ended up brad nailing them all from the outside which I really didn't want to do. Some were looser than others but in my defense I measured the shelves for thickness and some of those are inconsistent, as in not a true 3/4" thick. I probably shouldn't have attempted a project with 20 dadoes for a first experience in this, but I laid each one out at exactly 3/4 and cut the lines with a sliding miter saw set to stop at the bottom of the dado, and then cleaned out the waste with a router afterwards. I did it like this to make for easier setup. Just line up the blade and slide across. No clamping guides for a router or building jigs. It worked but the boards aren't all the same thickness. I'm using poplar 1x4 and 1x6 for all parts. No plywood.

Anyway the thought occurred to me that with a board stuck in a dado and glued in its always end grain to face grain. Does this hold well? Shelf end grain glued to side board face grain, and since it's a dado it's not a smooth bottom. Also there is shelf face grain contacting end grain in the dado from the dado sides. So there's always an end grain being glued on all three sides. I guess this holds well since it's obviously done often, but I just wondered how dependable this joint is?
You forget that the top and bottom of the board are not end grain.

If you had a router, why did you use the "hand method" of a miter saw to cut the dados? That is probably where you got your slop, along with the hand router to lean out.

I guess with a sloppy dado slot only the flue on the bottom of the board and the end grain is holding. Though not ideal, end grain gluing does help.

George
 

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A stack dodo on the tale saw might have been a better choice for the operation you described. They can be shimmed to cut the dado to the exact width needed. Routers are limited to the minimum width by the diameter of the bit and your shelves would have to have been planed to the right width to fit the dado. I don't know why the miter saw got into the operation, which seems inappropriate.

Dados work well to hold up shelves utilizing gravity and a couple of toe nails discretely placed will prevent the shelves from pulling out. But a snug fitting dado properly glued should be fine. Dados are not the ideal joint. Sliding dovetails are more resistant to pulling apart.

Bret
 

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Discussion Starter · #9 · (Edited)
You forget that the top and bottom of the board are not end grain.

If you had a router, why did you use the "hand method" of a miter saw to cut the dados? That is probably where you got your slop, along with the hand router to lean out.

I guess with a sloppy dado slot only the flue on the bottom of the board and the end grain is holding. Though not ideal, end grain gluing does help.

George
The shelf's top and bottom are face grain but it contacts end grain inside the dado. The shelf's end grain contacts face grain at the bottom of the dado. So it's always end grain to face grain for gluing to.

I did the miter saw method first to make sure I hit the lines exactly. I didn't trust myself to run a router and not let it drift away from my guide. I suppose I could have used two guides so it can't drift left or right but on that it would require precise placement of both guides, and also I had 10 dadoes to make per board, 20 in all. That would have taken a ton of time to set each one up. I think what I needed to have done was made a jig that would allow for faster cutting. But inexperience told me a miter saw would have been faster to cut precisely to the lines and then a router bit smaller than the dado width would be faster to hog out the waste. On that much, I was right, but I think inexperience also told me that all those shelves would be right at 3/4 thick too, which they weren't. Also I'm willing to say that cutting accurately to within 1/64 inch 40 times in a row on a miter saw within 10 minutes was a bit ambitious on my part too. :laughing:

I didn't hang around and watch the shop prepare my wood and based on some staining on a few boards I know I got some that had not been freshly planed. Squarish discolorations on some of them tells me they were standing up in front of a window with another shorter board partially covering them. Looks like the sun darkened the ends and part of the face on several boards. They also might have been a perfect 3/4 thick at one time but I suspect some slight swelling or shrinking has occurred. I cut each 1x4 in 3 pieces and sometimes ended up with boards off in thickness to each other more than 1/32.

What my main problems is, in my head I can dream up perfectly designed and executed plans, then I can put that design on paper and if I think on it long enough I can even get all the measurements right for a perfect plan. Sometimes I will miscalculate though and then the fun begins. Then, either with perfect plans or not, in the laying out of cut lines on wood I might be slightly off. Then in cutting I might be slightly off again. Then in assembly I might run into real world problems from not perfectly sized boards, or twist, or maybe a brad nail followed the grain line and exited through the face of the wood, or a host of other things. All of this comes together to produce a less than perfect finished piece and cause me a lot of frustration. :laughing:

I think the lessons I'm going to learn here are to make better dadoes with one pass from a router using a bit the width of the dado (two passes if needed for odd width dadoes hitting one edge at a time using a smaller bit), either take more time to set each cut up or make a jig for faster execution of the work, buy a planer and plane my own boards all to a uniform thickness (I didn't like my first experience at a professional wood shop turning out sniped boards anyway), and I'm going to stop trying to think of shortcuts to perfection. What seemed like a good idea obviously wasn't. It didn't allow for any variables and I don't yet own a planer to have insured there weren't any. Not to mention, I have great faith in hitting the lines with this saw but if my lines were off by even 1/32 then problems abound. I tried to be so perfect I left no room for error and real world conditions at all. And again, 40 perfect cuts is probably impossible to begin with.
 

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This gentleman did dado depth torture test using various depths to try and determine which was the strongest.

http://youtu.be/inYVMnve64Q
Thanks for that. I was taught many years ago to make my dados 1/4" deep and have always done it. The man that taught me evidently knew what he was talking about.

I've done torture tests on other situations for my own curiosity. I have compared the relative strengths of different drawer front-side connections. To my surprise, I found that dovetails are not as strong as a drawer side fitted into a dado in a solid wood drawer front and then toe nailed using a 1/4" crowned stapler. I used a framing hammer and swung vigorously at the drawer front from the inside. The staples dado took as many as six blows before separating and the dovetails took as few as one blow. But the dovetails is much more aesthetic and the world wide standard of quality.

Bret
 

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When you wrote that the boards were different thicknesses in your earlier post I suspected that you had not surfaced them at the same time or bought them from a lumber store like Menards. I run into variation in thickness all the time with their lumber. Now that I read that you had a mill shop plane your boards I suspect that from one side of their planer to the other there is a slight difference either in their planer bed or in their knife set. That will easily cause thickness variation if one board is sent through on the left side and another is sent through on the right. It is true that wood from various parts of the log will shrink or swell at different rates. That can be a factor, also.

As a woodworker we all learn that human error and working with a moving substance both make for lots of head scratching and frustration. If it was easy everybody would be doing it. So, join the ranks of all that have been in your shoes at one time and put another notch in your belt of learning.

As for wood putty or patch, if you put it on raw wood, use wood patch that gets hard and can be sanded smooth. That way the overspread around the hole will get sanded off and not leave a lighter blotch when you stain. If you use color putty, it works best for me to put at least one coat of finish on the wood and then fill the nail holes. This eliminates the blotch around the hole. Be sure to buff off the smear around the hole with a rag.

It sounds like you are very in tune with your work and aware so you are going to learn fast.
 

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Glue works fine in dadoes, any way the wood is oriented. Whether it's plywood or solid lumber. I prefer doing dadoes with a router. The grooves machine much cleaner. Undersized bits can be used, or the next diameter down, and a second pass with a shim. After years of doing the process, it's more like second nature to rout dadoes. I leave a router set up for that, and use a shop made jig, which is a fast setup.




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Discussion Starter · #14 ·
Glue works fine in dadoes, any way the wood is oriented. Whether it's plywood or solid lumber. I prefer doing dadoes with a router. The grooves machine much cleaner. Undersized bits can be used, or the next diameter down, and a second pass with a shim. After years of doing the process, it's more like second nature to rout dadoes. I leave a router set up for that, and use a shop made jig, which is a fast setup.




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A shim, now there's a heck of an idea! Thanks, I'll remember that!
 

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The simplest way is usually the best, as in less chance of screw ups, than any multi-step effort. Make a self sizing jig for a router. This one is self-sizing and has it's own built-in clamp. I use a 1/2" spiral down cut (cleaner edges) for dados 1/2" to 1 3/4". I have another sized for 1/4" bit (covers 1/4" to 1/2") and a 3rd etc, etc...

 

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Discussion Starter · #16 ·
I thought down cut bits were best for laminates and ply woods? I've read in magazines that an up cut is best for solid woods to clear debris out. I guess debris isn't as much of an issue in a shallow dado with a vacuum hooked to the router. Thanks for the info. My whole of woodworking knowledge has come from magazines and online forums. I've got no one else to teach me or work with to learn.
 

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Pre-surfaced lumber always varies slightly in thickness, it isn't flattened before planing and edges aren't always straight. This is because foursider milling machines are used or double sided planers, this is high production equipment. 1/64" isn't much until you try to do joinery where that difference can be doubled on the work. This is why most cabinet and furniture makers buy lumber in the rough and mill it on single cut machines, jointer and single sided planer. Rough lumber may be a little less in initial cost but the labor adds a lot compared to buying pre-surfaced. Many folks don't have the equipment to mill their own. If you do and can control the milling, you won't have varying thicknesses or have to deal with it later in making the joints. The extra labor in milling from the rough more than makes up for fussing later due to inconsistencies. You can get a much better surface than commercial machines and the lumber will be flat and straight, too.

A router guided by a straight edge is one of the best ways to cut dadoes. Way faster and more accurate than your method of nibbling. If you make a series of cuts like you said, the waste can clog the dado as it's being cut and can throw the router out of control. When you use a router against a straight edge with the typical round plastic base plate, different areas of that plate may be slightly different in measurement to the bit. Either mark the router so the same part of the base goes against the straight edge or replace it with a shop made square/rectangular wood one. This can be made to other advantages. By carefully reducing each edge of the wood plate you can have one edge that measures 1/128" to another, one 1/64" and one 1/32" or any increments that work for the situation. This will allow you to use a smaller bit and add 1/64" or whatever to the width of the dado by making a second pass on a different edge of the shop made plate.

Spiral bits have their uses. An up spiral will pull all the waste up and this can interfere with the router fitting unobstructed against jigs or straight edges. Down spirals force the waste down into the cut. In cases, this can lift the router and change the depth of cut. It may also overload the chip ejection function of the bit flutes, causing overheating, dulling in turn, and pushing the bit and router. I prefer ordinary straight bits, they shoot the waste right out the dado cut without any interference. Dadoes are cross grain grooves. If using dado blades on a table saw, making non through cross grain cuts can cause tear out on the face of your work, and you won't know until after the cut. Better quality, sharp dado sets may not be so bad but you never know with crossgrain cuts, especially plywood.

Another tactic with varying thicknesses is to cut shouldered dadoes rather than full thickness. They can be made with one or two shoulders, two creates a tongue. A smaller bit can be used to cut the dado and the tongue or end dado/rabbet can be made to fit. You can either move the router bit that cuts the shoulders up a touch, make a second pass or use a shoulder plane to make final adjustments to the fit. It's best to cut them a little tight if thicknesses vary, then fit. In a horizontal shelf situation, just the shelf being supported in the dado is enough for the shelf function. When you add a shoulder, that increases glue area. It's still end grain to face grain which isn't the best for holding over the long term. In some constructions, the cabinet back as well as face frames hold the width of the cabinet together. In cases like a tall book case, a center shelf or two can also be fastened or a dovetail dado used which won't allow the cabinet to pull apart. You may have seen Norm sneak a few toe nails in the end of a shelf into the dado edge. They can be placed either top or bottom so they don't show. You have to be careful with the angle, or the nails can come through the side on the outer face. Some will use screws through from the outside, countersink and cover them with a plug. Finish nails can be used, as you have, and the hole filled. Make sure to use a filler that will accept stain or use colored putty after a couple coats of finish are applied. Apply the putty, buff off the excess, add additional top coats. Don't use colored putty on raw wood.
 

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I thought down cut bits were best for laminates and ply woods? I've read in magazines that an up cut is best for solid woods to clear debris out. I guess debris isn't as much of an issue in a shallow dado with a vacuum hooked to the router. Thanks for the info. My whole of woodworking knowledge has come from magazines and online forums. I've got no one else to teach me or work with to learn.
First of all, I do woodworking for fun and try to keep my Ooops factor to a minimum. One of the ways I accomplish this is to use techniques and processes that I have developed over the past 60 years.

And down cut spiral bits for dados are one of the things that work for me whether I'm cutting plywood, Zebrawood, pine, walnut, MDF, purpleheart or etc. The down cutting flutes leave a cleaner edged on the surface.

As for the mags; if you read them long enough, they begin to both repeat and contradict. But I still subscribe to 4 mags mainly because of the tips and the project ideas.

It's too bad there is no one nearby, even at the same experience level, that could be a shop buddy.

However, there are a lot of people on this site giving good advice, so if in doubt, don't hesitate to ask. At least one of us will have been where you're going, and still remember the lesson learned.
 

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I try to use a router for all my dadoes. I find using a stacked set on the table saw can be trying, especially for long runs or large pieces. I don't like the idea of running stock face down. To get an effective groove, it has to be held down firmly and against the fence throughout the entire cut. Running face down puts the face against the table, and there's the possibility of marring.

You can't see what's happening when the stock is face down. Most of the stacked sets don't produce a nice clean bottom. For however long it takes to shim the blades correctly, I can just mark and set my jig for a handheld, and run the dado.






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