Once a while back I was really into making rocking chairs. Although I would really like to make more of them but my focus has temporarily gone elsewhere. Well, some of you know me from my time in the TWW Chat Room and the forum there, well I can be a little.....precise.....anally so at times. In drilling the holes for the spindles and legs I made a slight discovery, compound angles are a pain to drill on a drill press with a table that only tilts left and right.
Sin A = opposite/hypotenuse
Sin A = a/h
Cos A = b/h
Tan A = a/b
Then I thought to myself that when all is said and done a compound angle is just two angles in a rigid 90 degree plane. Well instead of two angles in planes that are 90 degrees off (7 degrees to the left, and 10 degrees to the rear), why not use one angle that is based on another angle. It actually works, but takes a bit of computations. Instead of measuring an angle back and an angle off to the side, all from 90 degree axises, you can measure one angle off 90 degrees from dead center and then measure the actual angle from that point. Well that can be a royal pain to figure out.....but I did it the first time though.
Then I went to my SketchUp book of tricks to come up with a better solution. Always a better way, right? Well a 1/4 sheet of Ply or MDF and 18" of piano hinge later I had my better way.
Now unfortunately for you the reader I have already made mine. At some point I am going to blog a project as I make it and you'll get more play by play photos. But it is all pretty simplistic and the SketchUp drawing that you can download should give you enough insight to make your own with little difficulty. But you do have to be precise.
Unfortunately there is no easy way to calculate the angles. Grab a calculator. Use the 8" from the hinge pivot to the inner inset in the equation: Tan(desired angle) * 8" = measurement from the bottom of the inner inset to the lower table. From there you are going to need to either convert to approximate fractions, generally to the nearest 1/16" or at most 1/31", or just use the decimal values.
To reduce the number of times that you have to do this you might want to mark the common angles as you calculate them on the side. To download the SketchUp model click the below graphic.
The first step in this is to cut the top and bottom tables. Simple enough, the top can be whatever size is needed for your situation. I used a fairly large table of 24" x18" . But if you alter the distance between the pivot point (where the hinge is) and the inner cutout section you will need to adjust the above equation: tan(A) * 8" = lifted distance, where "A" is the desired angle. Get it? It's really not that hard if you have a calculator handy. You are also going to need to adjust the lower table size so try not to make it too hard.
Next on the list of things to do is the smaller table that will fit onto the drill press table. This will need to be about 14 13/16" long and 18" wide and it helps a lot if it is a perfect rectangle. If you make a wider table you may want to widen this, if you make a narrower table you are going to need to reduce the width. The idea is to not have the lower table too narrow that you need a flashlight to see your settings and not too wide to prevent you from lets say clamping your work piece down on the sides. This section you are going to need to align perfectly parallel to the drill press table's back edge. But when you align it leave room to see the angle dial of the press's table. You are going to need to bolt this section down. Mark, drill, and inset holes for bolts and washers to hide flush to the top surface.
Then check your piano hinge for its thickness and depth, and create a rabbet on the front edge of the top of the small table. Alternately you can reverse the piano hinge and fix it to the front edge of the board, this would probably be a preferred method but one that I didn't put in the SketchUp plans. Using this method would depend on the depth of your piano hinge. If, for example, you have a piano hinge that has the holes set at 1" from the pivot point, this isn't going to work on a 3/4" table unless you drill more holes. But if your piano hinge is only 3/4" in depth and the holes are about 1/2" out you're good to go.
If you have followed the plans religiously (small chuckle inserted here) you should be able to find the center of the inside edge of the top and bottom tables (the edge that is going to be closest to the drill press shaft). Mark and line them up, then carefully arrange the tables to mark the holes needed to affix the top table to the hinge. I found that carpet tape works great for this. Drill pilot holes for the screws and permanently mount the two tables together.
Now we are getting good. The last piece you are going to need to cut are the sides. I did this out of two 10 1/8" x 12 3/16" pieces. The best way to explain this puppy is to download the model, you'll get the idea. Download from here. Essentially, on the 10 1/8" side, draw a line 1 11/16" in. This line will reference both your pivot point on the hinge and the start of your arc.
From the point where the line insects the bottom of the piece, mark 2 1/4" up. This will mark the starting point of your arc cut. From the point where the line intersects the top of the piece, mark 8 1/4" out and 1/4" down. This will mark the ending point of your arc cut. Mark lines to outline a 1/2" wide arcing cut from one point to the other keeping the 8 1/4" radius throughout. Then cut the arc using a 1/2" straight bit with a router (for you galoots out there, time to dust off the coping saw). Don't forget to keep 1/4" of the piece at the top for stability and 1 11/16" at the bottom. And the last step is to cut off about a 10" from the bottom to the top. This is just to get added material out of the way. Do this for both side pieces.
From here it is just assembly work. A carriage bolt and wing nut on both sides to hold the angle. Cut or route a recess for sacrificial inserts plus some mounting holes to the drill press. Maybe add a T-track.
I think this will give you the general idea and a starting point to add your own spins on this. Mainly it is all in the plan and time on this is about to run out so I am hitting the Post button.