Planer Sled - #1 The Beginning & Starting the Base

For a long time, well actually always, whenever I have had a twisted board or one with a curve, I used a hand plane or more recently a power hand planer to remove the twist and flatten the face of the board.  The most common, easiest and fastest way to do those operations is to use a long bed jointer however, I don’t have one.  To get what I want which is one with an 8” wide helical head and a 6’ long bed like the one below runs about $2,600 and up.  That’s more than I want to spend which is why I am still using hand tools.  I do have a planer but a planer does not flatten twisted boards it just makes opposing faces parallel and thinner.  Using my thickness sander does the same thing so it won’t work to remove a twist from a board either.

A couple years ago, a friend sent me a link to a YouTube video that described how to make a sled that can be used with a jointer to truly flatten a twisted board up to four feet long.  I kept the link and put it on my to-do-someday list.  Well, after the work and aggravation required to flatten the boards for the just finished Glider/Swing fresh in my mind now seemed like the time to go ahead and build it.  As I almost always do the project started out by building a 3D model in SketchUp modifying the YouTube design to fit my planer.  The finished model is shown below.  It’s about four feet long and twelve inches wide.

To use the twisted board is set on the sled with a series of six leveling bars.  The bars are then adjusted so they are at an angle that matches the underside of the twisted board so it is fully supported and does not rock.  The drawings below illustrate the board set on the sled before any of the leveling bars (red arrow) are adjusted.  Once adjusted the sled and board get run through the planer flattening the top face of the board a little at a time.  Once it’s flat the board is removed from the sled, the flat face laid down on the planer bed then run through the planer multiple times to make that face flat and parallel to the first face.  It may not make a lot of sense for me to try explain it here but when it’s done, I will take a twisted board and go through the process to flatten it.

The sled starts with the base which is a torsion box using ¼” thick MDF for the top and bottom skins.  They are sandwiched over a ladder skeleton made from ¾” thick pieces of MDF.  An exploded drawing is shown below.

The base starts by cutting the bottom MDF skin to size on the tablesaw.  The skeleton is next which begins by ripping the ¾” MDF to the correct width strips then cutting them to length.  To make sure the lengths are the same a stop is set on the chop saw.  Shown below is the setup for cutting the short cross pieces or “rungs” within the ladder.

After laying out the spacing for the rungs on one of the long strips both of them are clamped together and the centerlines transferred to the second strip using a small square.

With the rungs location marked a test clamping is done to make sure all the pieces fit tight and the completed ladder is square.  Everything checked out with no problems.

Starting the glue-up is next.  Because the ladder assembly has to be accurately located, I scored a reference line on the bottom MDF skin using a measuring gauge with a blade rather than a pencil.  The result is shown in the bottom.  A nice fine line cut into the face of the MDF. 

Unfortunately, two problems occurred to me as I thought through the glue-up process.  First the line is so light that it’s going to be difficult to get the ladder right on it.  Second and more important there are 13 pieces that make up the torsion box and all of them except for the ladder rungs need to be really precisely located when glued in place or it’s going to cause problems later on in the build.  Thinking about trying to locate the pieces where they go when they are all slippery with glue at once sounded like a recipe for disaster. 

To reduce the number of pieces being glued at once I thought about preassembling the ladder’s parts so there would only be the ladder plus the two skins to worry about in the final glue-up.  Three pieces is a lot easier to work with than 13.  Just gluing up the ladder as in the clamping test is a possibility but I was concerned about a glued MDF joint being strong enough to hold together during the skins glue-up.  Reinforcing the joint with pocket hole screws would solve that problem so with some scrap MDF I made several test joints with different length screws and different depth pocket holes.  The bad news is they all failed.  When the screws are run in, they split the MDF.  You can see that in the photo below.  If the ladder was made with solid wood that would not have been a problem but I want to use MDF because of its dimensional stability.

What I finally decided to do is glue up the torsion box in stages.  The first step is to glue one of the long strips to the bottom skin.  One edge will go on the scribed line talked about earlier but the line is not the only thing used in setting the long strip in place.  Three spacers (red arrow) one at each end and one in the middle will also be used to locate the strip.  The photo below shows how the spacer gets clamped into the bottom MDF skin along with the clamp that holds the strip tight to the spacer.  This way I don’t have to worry about the strip slipping around while I tighten the clamps.

The gluing process once the spacers are in place is to spread glue on the strip, clamp it to the spacers to locate it then clamp a thick caul on top of the strips to spread the clamping pressure.  Here is what the glue-up looks like with all the clamps in place.

After letting the glue cure overnight all the clamps get removed and here is the result.

Next Up – Completing the Sled