Replies

1. 
   MBL | May 14, 2004 06:27pm | #1

   Dear Mat,

   Solid wood would be better for stepping on it.

   If you had to use MDB, it'd be good to trim the front edge with a hard-wood strip, at least 1.5" wide, with buiscuits, before it be put into the dadoes. This to add some rigidity and to protect the MDB edge, else it could crubmle with use. A trim in the back edge would also be adviseable. But then you may want to reconsider using just solid wood.

   MDB would make the stool heavier to move around, too and if it is to be used often, that'd be a factor.

   Most sheet materials are not meant for loading unless they have a support frame underneath or around it. Gluing into dadoes should not be a problem.

   -mbl-

1. 
   MatthewSchenker | May 14, 2004 06:39pm | #2

   MBL,

   I should have said that all the major structural parts of this project are going to be solid wood. The only parts I am considering MDF for are the strips that get laid into the dados. And those are just in the walls.

   I guess another way of putting this is -- does MDF form a bond with the face grain of solid wood that is as strong as other face-grain-to-face-grain bonds?

1. 
   jackplane | May 14, 2004 08:01pm | #3

   In answer to your last question, yes. MDF is quite porous and will allow you to make a strong bond with wood, assuming you want to use regular wood glue.

1. 
   MatthewSchenker | May 14, 2004 08:28pm | #4

   Jackplane,

   Thanks for the advice. The MDF is in a hidden spot in the project, so my only concern is that it is strong. My reason for using MDF in this spot is that it is stable. If it forms a strong bond, then I have my answer.

   Thanks again for your help!

1. 
   MBL | May 14, 2004 09:42pm | #5

   Friend,

   Hope you are not saying that some strips of MDB are edge glued to the edge of a solid piece of wood, and then the mdb strip fits and is glued into the dado.

   If that's the case, it wouldn't be too good. If a complete piece of MDB is fit into dadoes with no structural discontinuity, and it is not loaded on its surface, there's no problem. If it is loaded in anyway, it needs help from solid wood frame or support.

   Otherwise, glueing MDB into a dado is not an issue.

   Good luck.

   -mbl-

2. 
   jackplane | May 14, 2004 09:48pm | #6

   From your description, I can't really tell what you're trying to build. Yes, face grain to face grain, will work fine with MDF or solid wood or a mix. But as a structural member, MDF offers very little strength. In this case you'd be far better to use straight grained wood.  MDF cannot bend much without breaking, and should not be relied upon here for joinery.

   Edited 5/14/2004 2:54 pm ET by JACKPLANE

3. 
   MatthewSchenker | May 14, 2004 10:23pm | #7

   I'm not so good with CAD software, or I would create a picture. I think I can describe it better:

   Imagine a typical table top, with three pieces of wood edge-glued together. Imagine running a dado across the table top, so you have a dado 1/4" deep and 1" wide. Then you place a piece of wood (or MDF) into that dado for support. Since the piece of wood is the same dimensions as the dado, it is flush with the surface. Also, because it is face-gran-to-face-gran, it should add a lot of strength to the structure.

   Now, with my project, the structure I'm supporting is not a table top, but the walls of a stepstool. Not the steps, but the walls. The glued-in MDF would be there just to secure the three sections of the wall to each other. There will also be a rear support added to this structure.

   I hope this helps. It is very difficult to describe something like this with just words, as all of you probably know!

4. 
   shall | May 14, 2004 10:37pm | #8

   If I understand what you are saying, you want to run this dado perpendicular to the grain of the solid wood then---effectively---inlay a strip of MDF flush to the face of the board.

   Two things strike me immediately:

   1) the strength of the edge glue joints (long grain to long grain parallel) should be strong enough and not need "support".

   If you are wanting to prevent cupping of the wood, would breadboarding work in this instance?

   2) if you apply glue along the entire length of the inlaid strip of MDF you will have wood movement problems.

5. 
   MatthewSchenker | May 15, 2004 05:44am | #10

   Shall,

   Yes, you do understand what I am saying! Thank you.

   I'm glad you are saying that the joint should be strong enough without extra supports. It makes sense, since tabletops are glued the same way.

   Thank you for your help.

6. 
   ian | May 18, 2004 01:03pm | #14

   Matthew

   If I've got the right view of your project, the treads of the steps will provide all the support the sides of the stool will need to control cupping.

   Ian

7. 
   MatthewSchenker | May 18, 2004 11:21pm | #15

   Ian,

   Thanks for your help.

   This post shows that one can learn something even from a simple project like a footstool!

   Thanks again!

8. 
   ian | May 19, 2004 12:19pm | #16

   Of course, you could make your stifiner a decorative detail by putting it on the outside and attaching it with dovetail keys.

9. 
   HowardAcheson | May 17, 2004 04:24am | #12

   >>Imagine a typical table top, with three pieces of wood edge-glued together. Imagine running a dado across the table top, so you have a dado 1/4" deep and 1" wide. Then you place a piece of wood (or MDF) into that dado for support.

   The way I read the above, you are inlaying an MDF strip across the grain of a glued up panel. If this is the case, you are headed down a road of disaster. You can not glue in a cross grain restrainer like MDF. The solid wood will want to expand and contract with seasonal changes in humidity. MDF is not affected by humidity changes. The panel will want to self destruct or warp severly.

   Why do you want to do this? There isn't anything to be gained.

   Maybe I'm not seeing what you want to do.

10. 
    MatthewSchenker | May 17, 2004 05:27pm | #13

    Howie,

    Yes, you're right!

    I've abandoned the idea. At first, I thought it would provide some extra strength, but it seems that is is clearly not necessary.

    Thanks for your input.

2. 
   TomMGTC | May 15, 2004 02:59am | #9

    The edge glued joint will be plenty strong for the application. The mdf or hardwood spline really does nothing but add unnecessary complexity to the project. Shakers edge glued their two steppers long before mdf was around with great success. 

   Tom

   Douglasville, GA

1. 
   MatthewSchenker | May 15, 2004 05:46am | #11

   Tom,

   Thanks for your input.

   Good point about the Shaker-style stepstools. I'm actually copying one I saw recently in an antique store. Mine will be three steps (at 8", 16", and 24") since I have very high kitchen cabinets! But your point remains the same.

   Thanks again for your help.

3. 
   CharlieP | May 20, 2004 05:33am | #17

   Matt, if I am understanding what you are saying, the MDF could weaken or destroy your stepstool. 

   First, I assume that you meant to say you were gluing the edge grain to edge grain of the wood (like a table top), not the end grain to end grain.  Glue on end grain has almost no strength.  A well prepared edge grain glue joint is stronger than the original wood - if you try to break it, it is unlikely that the break will occur at the glue line. 

   Once you have your three pieces of solid wood joined, with the grain going the same direction, note that there will be practically no expansion/contraction of the length of the wood, but there will be non-trivial expansion/contraction in the width.  For rough numbers assume at least 1/8 inch change in a board a foot wide.  If your steps are attached so that they expand/contract the same way, everything is fine. 

   However, if you attach something across the grain, that doesn't expand and contract, the wood will eventually split and crack.  What doesn't expand?  MDF like you suggested.  Plywood.  Or regular wood where the grain is at a right angle to the first piece. 

   Bottom line:  If you have no support, you will be fine.  If you add a support, your project will self destruct.  Or maybe I didn't understand what you were planning!

   ________________________
   Charlie Plesums  Austin, Texas
   http://www.plesums.com/wood

1. 
   MatthewSchenker | May 20, 2004 05:38pm | #18

   Charlie,

   Yes, you're right. I meant edge grain, like a table top.

   I get the picture that MDF is a BAD BAD BAD idea and I'll never think of using it this way again, believe me!

   But now I have another question. I'm thinking of building the footstool out of cherry and ash -- ash for the steps and cherry for the walls. Then I was thinking of cutting a 3/4"-wide x 3/8"-deep dado in the walls and gluing in a strip of ash for contrast. According to your last post, this would be a bad idea, since the inlayed ash would be at a 90-degree angle to the walls pieces. The other option is to lay the ash in a groove that runs at an angle so it is not at 90 degrees to the walls but more like 45 degrees.

   What do you think of this?

   Edited 5/21/2004 9:59 am ET by Matthew Schenker

1. 
   ian | May 21, 2004 03:43pm | #19

   I can see two ways you could set the ash in as a contrast.

   1)  cut the ash as short grain pieces (i.e strips from across the board rather than along the board) and set into a shallow dado.  The grain direction would match that of the cherry so the problem of differential movement would be minimised.

   2)  use long grain strips of ash, but attach them across the cherry using dovetail keys.  These keys allow a moulding to stay put whilst the case side expands and contracts

2. 
   CharlieP | May 21, 2004 04:34pm | #20

   Ian's answer is exactly right. 

   The problem is not that MDF is bad, but the problem of wood movement.  You can't fight it - no amount of clamping, sealing, or steel bands will keep the wood from shrinking and expanding across the grain.  Before there were explosives (reportedly back to the early Romans, but I am not old enough to confirm it) the way rock was quarried was to drill a hole (like they do for explosives), thoroughly dry a piece of wood, then carve it to fit the hole.  Once it was in the hole, pour water in the hole, and the expansion of the wood (across the grain) would break the rock. 

   MDF and plywood don't expand and contract like normal wood, so they can be glued to each other. 

   Wood pieces can be glued as long as they expand the same direction, so your accent would work fine if it were cut so the grain matches.  Your step-stool steps can be glued into the sides of the stool (assuming the grain on the sides is vertical) because both will expand front-to-back

   To attach wood where the grain goes in different directions, a joint must be used that can slide.  Hence Ian's recommendation of a sliding dovetail.  A breadboard end is only glued at one point, and the rest slides.  Tops are attached to tables or cabinets with sliding joints or elongated screw holes.  Mortise and tenon joints have lots of side grain to glue, but limit the cross grain area - the tenon may be an inch or two wide but not 12 inches wide

   In addition to Ian's suggestions, there is one more option that I can thin of.  If the layer of wood is very thin (i.e. a veneer), it doesn't have enough strength to push sideways (on a microscopic level it does push sideways, but since it cannot move sideways gets thicker).  So if your dado were about 1/16 inch deep or less, the accent could run across the grain of the primary wood. 

   Putting your accent at a 45 degree angle would reduce the stress it causes to about 70%, but would not eliminate it.  (Cosine of 45 degrees is .707)________________________Charlie Plesums  Austin, Texashttp://www.plesums.com/wood

1. 
   MatthewSchenker | May 21, 2004 05:10pm | #21

   Charlie & Ian,

   Thanks guys, you are teaching me some great new things here. I've recently started studying workshop math, and this is a good hands-on lesson for me!

   I like the idea of gluing the contrasting ash at a 45-degree angle to the cherry, and making it only 1/16" thick. This way, it still contributes to the design, and adds a little bit of strength, without worries of wood movement.

   But I also like the idea of just cutting the contrasting wood so the grain runs in the same direction. Of course, this option means I would have to cut at least three separate pieces of contrasting wood to lay in the dado, since the stool's walls are composed of three 6"-wide pieces, and I don't have access to an 18"-wide piece of ash! I'd try to cut pieces so they overlap sections of the cherry, to add strength.

   Edited 5/21/2004 10:12 am ET by Matthew Schenker

1. 
   CharlieP | May 22, 2004 12:59am | #22

   Since the whole idea of a strip only 1/16 inch thick (or less) is that it doesn't move or constrict the movement of the other wood, then it doesn't matter if you have 18 pieces 1 inch wide.  It is only decoration.  You certainly don't need an 18 inch wide board to start.  (If you do the 45 degree trick, your strip will be over 25 inches long)

   Another rule of thumb that you might find useful... A good design for steps is twice the height of the step, plus the depth, should be about 25 inches.  If you have a ladder with 12 inch steps, the depth is about 1 inch.  An 8 inch high step should be about 8-9 inches deep.   ________________________Charlie Plesums  Austin, Texashttp://www.plesums.com/wood