July 11, 2009

8 Sided Roof Tutorial

This is a long posting today, I am showing how to use a table saw with a miter gauge to cut a multi sided roof for a miniature structure. The roof you will see me cutting has 8 sides and a 55 degree pitch. It will have a cupola coming up through the center, therefore you will not see a point on the roof panels. This same cutting method will produce roofs with 3, 4, 5, 6, 8, 10, 12 any number as long as it is more than 2.

Please note that I have the safety blade cover removed from the saw, always follow the safety recommendations from you saw's manufacturer.

I figured out what the angles on the roof panels would be from my design in a CAD program. There are charts for determining these angles on the internet with the most common multi sided numbers and angles of slope given as well as online calculators and formulas to use with scientific calculators.  Click here to see  my favorite online calculator for finding the settings for saw blade bevel angle and the associated miter gauge angles that go with a particular roof pitch.

The first cutting task will be to cut a long plywood strip(s). the width of that strip is equal to the distance from the bottom edge of the roof to the top edge perpendicular to the bottom edge. You can bevel the top and bottom edges at this time but those bevels must be parallel to each other as you will be doing a flip cut technique. The length of the strip is figured out by sketching a layout of the panels. Along your layout sketch strip one roof triangle will face point up, the next point down. There is a small waste triangle at the start of the cuts and also saw blade thickness between the cuts. I usually allow for a few extra panels just in case I goof. OK your basic material strip is cut now lets get to the trickier stuff.

With the blade upright at 90 degrees as high up as it will go I set the miter fence to my needed angle. I made an angle template using acrylic on the laser cutter specifically for this job but you can also use an adjustable bevel gauge instead of an angle template. If you want to use the bevel then use a graphics program or CAD program to draw out the specific angle and print it onto paper. Use a light coat of spray adhesive to  stick that pattern onto a flat piece of wood. Next tack wood strips to each side of the angle being very accurate about getting it right against the lines of the angle. Do leave as shown in the photo below a small gap at the apex of the angle to facilitate the use of the bevel gauge.  Now the bevel gauge  between, open it against the wood strips and then lock it to that angle.  You must have a template or bevel gauge short enough or else thin enough to fit between gaps on the saw blade teeth so that  it does not touch any of the teeth on the blade. It should rest only on the solid body of the saw blade, never on top of a tooth. (This angle setting method will not work on a hollow ground saw blade as it does not have a flat surface.)

The next step is to tilt the saw blade over to the angle you need. You must have a bevel gauge or angle template thin enough that to slip fit into the space between the saw teeth so it can rest against the body of the saw blade. I shine a small flashlight from behind my gauge and if any light comes through I adjust the blade tilt until the angle is a snug fit.

Place your long plywood strip against the miter fence and make a cut that takes off that square end and leaves you a lovely angled, beveled edge. In the photo above you can see that waste piece, the skinny triangular cutoff piece to the far right side. Note before you cut anything that if you have beveled  the top and bottom edges you will first need to figure out how the first panel needs to be positioned so those edges are oriented on the roof panel as you want them to be. Now rotate the board 180 towards yourself so the side that was against the table now faces up towards you. From this time it will be rotate cut, rotate cut until you have enough sections for the roof.

At this time you need to set a couple of stop blocks to control the width of your panel. You will see in the photo above that I have used as stops two magnetic blocks made just for table saw setups. Regular magnets would not be strong enough however you can firmly double back tape wood strips to you table saw top to act as stops. You won't be putting a lot of pressure against them, just gently moving your plywood strip over to them. A stop block of some type is absolutely essential so your panels are always the same width. You will need the blocks to be positioned far forward enough on the table so the plywood panel is clear of the blade when you begin the cut. The block along the side should be fairly short in length, an inch or two is enough.

Postition the panel against the blocks, turn on the saw, hold the panel in position against the miter fence while pushing the miter handle forward, cutting through the panel and moving it on past the blade. Then I turn off the saw, let it come to a stop and remove the newly cut roof panel section out of the way of the next cut. Move the miter back to the position forward of the saw blade, flip the plywood, set the material against the stop blocks and the miter fence, turn on the saw and cut again. Only 8 times and you are done! It does not take long to cut the panels, most of the time is in the setup.

The following photos show how to assemble the panels. I use a quality duct tape from 3M that leaves no residue behind when you remove it. I normally buy it at Lowes or my local hardware store but here is an online source for it.  http://www.amazon.com/Scotch-Tough-Residue-1-88-Inch-20-Yard/dp/B0014LQK58 That duct tape is my only clamp.  I use the body of the bird or doll house to conform the perimeter angles of the roof while the glue dries.

First I lay the panels side by side with the angled bevel facing down then I tape the joints.

Next I pull it up into the shape and tape the last joint. I always do a check fit before I glue them together. Would you look at those joints in the photo above, NO GAPS, PERFECT miter joints! That is the beauty of my cutting method, it really does work.

Now take the tape off one joint and put the taped side against the table. Fill the joints with glue but don't get too heavy handed or you will have a big mess to clean up. The adhesive I am using is from Locktite and it is called Polyseam Seal, it is labeled clear, extra adhesive and it has a lifetime warranty. This is an acrylic caulking product I purchase at a hardware stores or at Home Depot or Lowes. This is my primary glue for birdhouse and dollhouse shell building, it has never let me down. When building a birdhouse I make sure every cut edge of the plywood on the birdhouse is completely covered with glue, this prevents any water getting up into the plies and possibly causing dry rot. I am using water resistant plywood, the flat surfaces are very good but those cut edges are the most vulnerable area for water infiltration soaking into the material.

Now pull the last two edges back together, tape that joint and place the roof on the house structure to dry. Put some waxed paper under those corner so you don't glue it to the house just yet as you will want to do more work on the roof such as painting the underside, installing edge trim or even gutters. Because this is a birdhouse I will put an exterior rated gray primer on the plywood before I shingle it. The shingles will weather to gray and any cracks that may develop won't show with the matching primer under there. I have never lost a shingle on a birdhouse, even ones that have been outside for over 15 years. For birdhouses I make thick shingles, thin dollhouse shingles won't hold up for long.

OK now you know how I make my roofs so go forth and build octagon houses, gazebos and towers.

Leveling throat plates on a saw

When making scale models, engineering models, birdhouses or dollhouses I want precision cuts to a particular size that have accurate angles on the sides of the cuts. It can take a lot of fussing and a lot of patience to make that happen. Many of the cuts I make are done on a 10 inch table saw, I want that saw and my small table saws fine tuned so that what I get is accurate to the dimensions I am trying to achieve. It all starts with a good table saw tune-up but that is not the subject of this blog.

To get an accurate cut it is essential that the throat plate be dead level to the top of the table saw. If your throat plate is too high then the edge of the wood sitting on it will slope down towards the fence and your intended 90 degree cut will end up being made at an angle. If the throat plate is too low you might get sloped cuts on short pieces or the wood might hang up on the back edge of the plate opening and mess up your nice cut. If you are trying to accurately cut a groove of a particular depth it too might turn into a mess.

Every saw is a little different, the one I am working on today has small tabs extending into the opening for the throat plate. There are hex head screws in the throat plate that rest on those tabs. I fiddle and fuss and fuss some more back and forth between the screws until I get it just right. To check if it is level I use a steel rule or some other object that has a long straight edge and sharp corners. If the corner hits onto the edge of the plate or the table top I know the plate is not perfectly level so I tweak the height a little more until I feel nothing hitting. Every time I change the throat plate I have to be sure there is no sawdust on the adjusting screws or on the support tabs. Just that little bit of debris will mess up the level. I always grab a straight edge and check when I change plates or if I have been cutting for a long time as vibration from the table saw can move those screws.

Some table saws including miniature saws don't have adjusting screws. If there are no ajusting screws you will need to resort to using shims of metal, paper, plastic, cellophane tape, whatever you can find that works is OK to use. If your throat plate is too high to begin with you will have to make new ones or reduce the thickness where it rest on the edges of the opening. Making your own throat plates is a good thing to learn. I will cover that in another post sometime later.

A lot of the throat plates for miniatures saws are warped to begin with. You might have to back them up with another layer of material to stiffen them. Of course you don't want that layer to touch onto the recessed ledge the plate rest on and it can't be so thick that the saw arbor hits on it and bends the plate upwards.

Want a dead flat plate on a 4 inch table saw that needs no fussing? Then buy a 4 inch table saw from Byrnes Model Machines, he has taken the time to match the recess in the table top to the thickness of the throat plate. There is a link to Jim Byrnes in my file of links on this blog.