Sawdust on the Floor

I was recently asked about the process of scribing stair treads and risers to fit an uneven wall.  While the answer to this particular question deals with installing stair parts, the process  is known as scribing and can be used on a multitude of different woodworking projects.

We all wish that the walls or staircases in our homes were perfectly flat, straight and even.  But unless your home builder went to unusual steps when he constructed the home, your walls will be anything but flat, straight and even.  Wall studs, all have bows, or crowns in them, nothing we can do about that.  If your walls are plastered, as most are here in North America, then the thickness of the plaster used to seal joints, will vary, giving us unevenness in the wall surfaces.  Or if your sheet rocking skills are less then professional, then you will have uneven surfaces to deal with.

Yet we as woodworkers and home handymen want to install stair treads, risers or cabinets against these uneven, not flat and sometimes not straight walls without any gaps showing between the ends of the piece and the wall.  Problem is that we made the pieces (risers, treads and cabinets) flat and straight.  One option that a lot of people employ is to simply cover the gap with trim work, and this may work out ok on some cabinet installs, but it is not the correct method for all applications.  A better method is to simply scribe the piece to meet the contours of the wall.

Let’s talk about a stair riser as our example piece.   And for the sake of our discussion, I am assuming that the tread already has been cut to its final width.  Let’s also assume that the stair stringers or frame work are already in place. So our only task is to fit the riser to the wall.  And let’s make one more assumption that the wall we are working against is on our right hand side.  Yeah, I know that I am always preaching about not making assumptions, but in this case it is only for clarification. 

The first step to getting a good fit is to take your riser back to the shop and cut a 45º bevel or miter along the back side of your riser.  We want this miter on the right end of the board, the end where it meets the wall. Now take the riser back to the stair stringers.  Securely hold the riser in place, and check the fit along the right hand edge.  If your wall were perfect, there would be no gap, and you could continue making up the riser.  But there probably is a gap.

What we want to do is transfer the shape of the gap to the end of the stock.  The best tool to transfer the shape is a drafting compass. This simple tool is the same compass we all remember from our school days.   Take the compass and open it to the widest opening in the gap.  Lock the compass at this width. Before we scribe the shape of the gap, we need to move the stock back to your left, the amount of the compass opening.  If for example, the widest spot on the gap equaled ½”, then you would move the stock, ½” to the left, away from the wall.  Now starting at the top edge of the board, with the fixed leg of the compass along the wall and the pencil leg on your stock, slowly move the compass down the entire face of the riser.  The pencil line scribed by the pencil will be a duplicate of the shape of the gap.

Depending on the amount or size of the gap the next step is to remove the wood the right side of the pencil line. Some tools of choice would be a file, block plain or power sander.  Work slowly, and check the fit as you proceed.  When you can fit the riser completely against the wall with no gap showing, you can then proceed to work the left end of the riser.

The only problem with this method would be if you were fitting a riser between two walls. Then you would have to determine the angle, slope, shape of the walls independently.  The method I use is use two dummy or scrap pieces of stock as trail risers.  I would determine the shape of each ends, and work the trial pieces to fit that end.  When I had a good fit, I would copy that shape onto the actual riser.

Going back to the first step of this process, back cutting the ends of the stock at 45º was actually a step to save me work.  The thickness of the bevel gets smaller as it approaches the edge of the stock.  If I have to file, plane or sand stock from this area, I would rather just have to remove the smaller thickness of the bevel than to have to work through the entire thickness.

Now this process will work the same if you were installing the treads of the stair case instead of the risers.  The only difference would be on the bottom riser.  Then not only would you want to scribe the sides of the riser, but also the bottom edge of the riser where it meets the floor. 

Scribing a cabinet to a wall uses basically the same process, only with a small variation. This is a trick I learned from my late Father in law. Again, for the sake of discussion only, let’s assume that the width of the opening is 24” and once again we are going to scribe this cabinet to a wall on the right hand edge of the cabinet. The trick is to make the cabinet itself only 23 ¾” wide. However, the outside width of the stiles for the face fame would be 24” wide. The stile for the left side would fit flush against the left side of the cabinet; the right hand stile would overhang the cabinet by ¼” along the right hand side of the cabinet.

Making the cabinet a ¼” narrow gives us a little wiggle room when we install the cabinet, just in case the corner is a little out of square. The ¼” over hang along the front is where we will scribe the cabinet to fit the contours of the wall. One additional trick is to back cut the right hand edge of the right hand stile to the 45º bevel, just as we did for the stair riser.

While this method will ensure a great fit of the cabinet along the wall, it does require a little extra work when you are installing the cabinet (Beyond scribing the edge).  Logically, this end cabinet may not be the first cabinet you install in a room, but you want to temporarily install it to determine the scribe line of this cabinet.  Remember that we have to move the cabinet or riser back to the left the width of the widest gap before we transfer the wall contour to the edge of the cabinet.  If the cabinet to the left were already installed and bolted in place, you couldn’t do that, so a little extra work may be involved.  But the fit of the scribed edge is really worth the extra work.

It must be very frustrating being a tool manufacturer.  As woodworkers, we demand that our tools perform flawlessly each and every time we use them. We also demand that in all but some very rare cases that the tool be capable of doing more than one task.  You wouldn’t be very happy with your tablesaw or miter saw if it were only capable of making cuts at a pure 90° angle. Nope, we expect that these tools not only make these cuts, but in the case of the miter saw, we demand that it also makes beveled and compound miter cuts, our tablesaws are expected to not only perform rip cuts, but cross cuts and beveled cuts. At the same time, we also demand that the cuts made by these tools be smooth and have no tear out. And in a weird way, it’s this demand that the tools in our shop do more than one task that has helped to create problems like tear out. But not totally. Tear out is really a two part problem.

Here is the first part of the problem.  Wood is basically a series of fibers stuck together.  These fibers run in the direction of the grain of the wood.   When you rip a piece of wood, you are slicing these fibers along their length and tear out is not a problem. However when you cross cut a piece of stock, you are cutting the ends of these fibers. These fibers have very little strength on their own, they rely on the next set fibers for support. For example imagine cross cutting a 2″ wide piece of 4/4 oak. As the blade cuts the first fiber, it is held in place by the next fiber.  This mutual support continues until you have cut almost through the stock.  As the last fibers are cut, there is nothing to support them.  These fibers will normally tear rather than be cut.  This produces tear out.  The sad part is that there is very little we can do about this, wood is simply wood.  And we can’t change the makeup of it no matter how hard we try.

The second part of the issue involves some good old trigonometry. At all points of the blade other than when it’s at true 90°, the height and angle of the blade will form a triangle, the base of this triangle is the top of the tablesaw or miter saw.  The length of this base changes, depending on the height and angle of the blade.  To accommodate this fact, the manufacturers of our tablesaw saw and miter saws have shipped the saws with inserts wide enough to allow the blade to be tilted to its limits and the blade at full height.  This wide open insert is the second part of the tear out issue. This wide opening in stock inserts does not provide adequate support for the end fibers, creating a breeding ground for tear out.

Rather than complain to the manufacturers of our tablesaws and miters saws, a better solution would be to simply replace the stock insert with a zero clearance insert.  These Phenolic Resin inserts are offered for many different brands of tablesaws.  The idea is to remove the stock insert, don’t discard it, you will need it when you do any work not at 90°, and replace it with an insert that is sized for the blade you are using.  If you typically use blades with different kerfs, then it is recommended that you have inserts for each blade. 

To install, first raise the blade to its highest position.  Verify that the blade is really at 90°.  I would recommend that you use a good digital protractor for this task.  If the blade is not at 90°, refer to your tablesaw manual to adjust the zero stops of your particular saw.  Once you are satisfied that you are really at 90°. then completely lower the blade. Now remove and store away the original insert, and replace it with the Zero Clearance insert.  When you get the Zero Clearance insert, it will not have any opening for your blade. We take care of that next. 

The next step is for you to SLOWLY raise the spinning blade, so that it cuts a clearance channel in the Zero Clearance insert.   I have found that using the rip fence to hold one edge of the insert, and push sticks for the other edge works quite well.  Just be patient, don’t rush to get the blade to its final height.   Some woodworkers, who feel that the Zero Clearance insert is important enough, and that they cannot tolerate any tear out, will have inserts for not only different kerfs but for difference ranges of blade height.

It’s important that you only use this insert for work at 90°, as soon as you try to use it for a beveled cut, you will either bind the blade or deform the Zero Clearance insert.  Use the stock insert for all of these cases.

If your tablesaw is one of the major brands, or you like to do it yourself, there is a Zero Clearance insert  kit available that contains parts and tools needed to make your own inserts.  Simply use your original insert as the template for the outer shape of the insert. Use a 2 flute flush trim bit in your router or shaper to create a perfectly matched insert.  

The same problem occurs on our compound miter saws. The wide mouthed insert on most compound miter saws, just doesn’t provide the needed support for those wood fibers as we cut through the stock.  I haven’t located any aftermarket inserts for the compound miter saw yet.  But trust that I am looking.  In the mean time, when I absolutely have to prevent tear out, I will normally place a sacrificial piece beneath the stock piece I want to cut. 

So to the manufacturers, thanks for all of the options that your tools provide.  I just wished you could read the minds of all of the woodworkers, craftsperson’s and trades people who use your tools and provide us with that perfect tool that meets ALL of our needs at the same time.  Oh yeah, keep the price down at the same time.  Is that too much to ask for?