|Project by splintergroup||posted 03-23-2015 06:39 PM||2244 views||11 times favorited||8 comments|
I’ve had a number of comments and questions on the clock face used in the making of my Mantle Clock. There are enough steps involved that I thought I’d dedicate a writeup to it.
Clock faces can be made in many ways, what I tried to do is sub-divide the face into wedges, one for each hour. The challenge is keeping a tight joint between all the parts. This can be done rather easily using just veneers and a sharp knife, but I choose to use wood from my stash.
One wedge per ‘hour’ on the clock face means 12 joints between parts. That’s a lot of chances for gaps to show up if the angles are not cut perfectly.
Table saws are good for cutting only one angle with exactness, 180 degrees (a straight line). You can get ‘perfect’ 45 degree miters for a picture frame using a special 45 degree jig, but this only really works because you are cutting both sides of the miter at the same time on opposite sides of the blade. Even if your miter angle comes out at 42 degrees, the other part has a 48 degree cut. The results are the same, put the parts together and they form a 90 degree angle. The joint may not run exactly corner to corner, but the frame will be square.
This doesn’t work as easily for the clock face with 30 degree angles, but the same principle applies.
My clock face needs to be an 8 inch square with an additional 1/4 inch on each side for fitting into a groove. I’ll make it 9”x9” and trim it to size later.
Step 1. Prep the wood. Here I run a 7/8” thick blank of Walnut through the drum sander (120 grit) to get things evened out.
Next I resaw the Walnut to double my yield. This also lets me book match the adjacent hour wedges for a more consistent look. This is more important for flat sawn wood like the Walnut I’m using.
A cardboard template allows me to avoid bad areas like the sap wood on these pieces. The template is a 30 degree wedge, about 6” long which will cover the distance from the face center to a corner. The wedges for 12, 3, 6, and 9 o’clock are cut shorter to save some wood.
I orient the wedge so the grain is aligned, otherwise it would look kind of weird.
With a table saw set up as best as possible for an absolutely straight up (90 degree) blade, I use my miter gauge to cut one straight edge along the template tracings (two on the piece shown).
I then crosscut the ends just to get rid of any rough edges.
At this point I could use my miter gauge to complete the cuts, giving me the required 30 degree wedges. The problem is miter gauges are difficult to set ‘exactly’ to a particular angle and they can wiggle around a bit in the miter slot as the cut is made. Each small error in angle will be multiplied by 12 when cutting the wedges. This would also require the miter gauge be set to 60 degrees. I want to make the error as small as practical to get the best fit. If I used my miter gauge for the cuts, I would have set the gauge up with an accurate reference. Plastic drafting triangles are very accurate and work well for setups. Why use one to set my miter gauge when I can use it directly?
This is the jig I use to cut the wedges cleanly. Double sided tape holds the triangle to a piece of tempered hardboard. Note that the triangle overhangs the hardboard against the saw fence. This allows the triangle to reference directly off the fence and give me the best accuracy. The small stop is also taped to the hardboard.
The jig is used just like a typical table saw taper jig. The workpiece rests against the stop and is held in alignment with the side of the triangle. I like to work from the side of my saw when making these cuts, that is why I have the fence to the left of the blade.
The work is positioned with the straight cut side against the jig and aligned so the cut produces a sharp point. The jig base acts as a backer to eliminate chipping. Flip the workpiece to cut out the other wedge.
When all is done, I have twelve wedges, as close to 30 degrees as possible and with smooth, tooth-mark free cuts on the sides.
Now for some layout. Note the book matched pairings. I also placed the parts in their final order on the clock face. The wedges will be glued together in pairs and then these pairs will be joined. The last glue up will join the two semicircles as seen in this picture.
These pair glue ups will have perfect fitting joints. I use masking tape stretched across the joint (both sides) for clamping. There is enough elasticity in the tape to close the joint and hold it until it is dry. I like to work on a scrap of melamine coated particle board. It is dead flat and glue doesn’t stick. Note that I have pieces of tape at the ready.
Glue is applied to both surfaces and spread out.
The wedge points are aligned and tape is stretched as it is applied across the joint (on both sides). I let this dry for about 30 minutes before moving on.
Next I reposition the parts to confirm what gets glued next. Note that I trimmed any tape edges that will interfere with the next glue up.
For each semicircle, three of the pairs are joined up and glued. This was it for the day so these parts dried as shown overnight.
A straight edge along one of the semicircles shows the error accumulation in my 30 degree cuts. I ended up a bit shy of 180 degrees. The gap you see is about 0.020” which works out to an error in my angles of about 0.1 degrees. Try setting a miter gauge to within 0.1 degrees!. The triangle jig was simple to make and simple to use. As a bonus I could cut my angles to within 0.1 degrees of ‘perfect’.
It’s interesting that the other half circle ended up slightly over 180 degrees. The added thickness of the glue could cause this.
Even if was only off by 0.05 degrees, the two halves would not join up without a slight gap.
Here is the trick to getting perfectly tight joints all around the circle.
Table saws can only cut one angle perfectly, 180 degrees. I take advantage of that and use another piece of hardboard that has had both sides cut parallel on the saw. Leaving the fence in the same position used to cut the hardboard, I again use double sided tape to hold the semicircle to the hardboard. This is positioned such that the saw blade will make a cut that just skims the entire length of the workpiece. If the workpiece came out slightly over 180 degrees, I position it so the center just touched the edge of the hardboard and the tips overhang equally. If the workpiece was under 180, I make the tips touch the edge (the center will overhang).
After these cleanup cuts, the parts are laid together, nice fit!
I then tape-clamp these two halves together like before. Note the extra strips of tape near the outer edge. These keep everything level across the clock face while things dry.
The joints show no gaps, just a fine glue line. I ran the clock face through the drum sander (120 grit) to even everything out before this picture. you can see the sanding scratches running top to bottom.
I now square up the piece to 9”x9” using another piece of hardboard to ride along the fence. The board is double sided taped to the clock face and carefully aligned along the center axis of the 12 and 6 o’clock wedges.
Shown is the now squared face, still rough from the drum sander. I wiped with mineral spirits to simulate the look of a finish. At this point the clock face is about 3/8” thick
For structure I glue this face to a piece of 1/4” plywood. In this case I glue a piece of plywood onto each side.
A resaw pass through the bandsaw and viola! Double the pleasure. After all the cleanup sanding I should end up with a 1/8” layer of the Walnut on the plywood.
I use a square and the same 30 degree triangle to mark for the hour pips, tape allows me to see my pencil marks. The radius of the circle is set to the length of the hour hand.
I drill the holes for the pips on the drill press with Forstner bits, 3/8” for 12, 3, 6, and 9 o’clock, 1/4” for the rest. The holes are close, but not quite all the way through
Slugs of 1/4” and 3/8” copper rod cut at the bandsaw are epoxied into each hole. The tape is left in place to catch any ooze-out. Places like Enco sell the copper, Catch them when there is a 20% off coupon and free shipping to save a bundle.
Another few passes through the drum sander to flatten everything out. It takes a few minutes between passes to allow for the copper slugs to cool off.
Some sanding with a random orbital sander to 320 grit followed by a buffing with 0000 steel wool and the work is done. Again, some mineral sprits wiped on to get a feel for what the finish will look like. I still need to trim it to final dimensions and apply a good sealing finish to keep the copper from tarnishing.