Setting precision angles #2: The Sine Bar

The sine bar

The next leap for me in setting angles precisely was the rediscovery of the sine bar. My father was a tool and die maker and although I never worked in his shop I did become familiar with the tools of the trade. The only sine bar I had seen however was 3” long.
Then Fine Woodworking ran an article in the July/August 1992 issue called Simple Instrument Sets Precise Angles by Tom Rose. In it he described how to make a 10” sine bar and use it to set a miter gauge. Ahhhh I was off on my obsession again. I knew it’s application in toolmaking and now I could see it’s value in woodworking. It was an epiphany and I realized a lot of many trades could easily cross over to other trades.
However before I was able to make one of my own the next issue of FWW came out and a couple of letters lambasted the author about his ability and the instructions to construct a perfect 10” sine bar and the needed gauge blocks of exact length. My enthusiasm wained. I still thought it was a wonderful tool and wanted one and continued to think about it. I was after all still happy with the bevel board.
Eventually I bought into the Festool line for their precision and portability and loved the work table the MFT. But to me it was just a work table till I joined the FOG forum and read about people doing things I never imagined. One post commented that the hole pattern in the top was in fact a very precise grid system and could be used to align the fence and the saw rail to 90degrees precisely and repeatably. As good as Festool is the miter head for the table, like most included miter gauges was not very good. The sine bar would be my after market miter gauge. I built a couple and the best one I made was .001” off from ten inches. But here is what should have been stressed in the article and I didn’t understand till later was the fact that it doesn’t matter what the length is as long as it is known. If I was real sloppy and had made the bar 9.834” it will still work perfectly because the sine of the angle is multiplied by the length of the bar. Ten inches just makes the math easy, just move the decimal one place. The angular error ignoring the difference from 10” was insignificant for angles under about 35 degrees, so it was put to use as needed. If I needed 45 degrees on the MFT I used the hole grid pattern, it I knew was perfect. I used a different method for cutting my gauge blocks than the article which I will describe. It alone is useful if you need to cut a piece exactly like another that already exists. I do agree that the method described in the article for the gauge blocks is flawed and not accurate enough.

These are the tools needed. The wooden bar is the sine bar, scientific calculator and calipers. I bought the 12” version from Grizzly and they can handle all the measurements needed. 6” calipers would work if you already have them, it just requires an extra gauge block.

To drill the two holes for the round discs I set up a fence on the drill press with a stop block. One hole was drilled, then the calipers were set for 10” and placed against the stop block to index the sine bar over for the next hole. The profile was then cut on the one side to give it the proper clearance. The discs were then epoxied in what remained of the holes. When that was cured the discs were run against the table saw fence and the top was cut parallel to the discs. On this one the discs are 3/4” aluminum rod. Very easy to sand flush to the wood surface. Before the discs are sanded flush I measured them with the calipers to find out how close I came. That number will be used if it is not real close to 10”.

This photo shows it in use.

Now to calculate the block of wood labeled 26degrees and cut it exactly.
For this example 26 was entered in the calculator and the sin button was pressed the answer is .438… Since the sine bar is ten inches that answer is multiplied by ten (or the actual length of the sine bar). This is easy just slide the decimal over and set that length on the calipers. If this is just a one time odd angle you need to cut you can use the calipers the set the sine bar as in this photo.

But chances are you might want to cut that angle again so let’s make a permanent reference block. Using the miter gauge with a stop block set the block to be several inches longer than the 4+ inches needed and lock it down.

Use the calipers as shown against the stop block to space off a scrap of wood then cut that piece of wood.

Then remove the calipers and slide the small piece of wood you just cut over to the stop block and insert another piece of wood to cut.

You now have a piece of wood the exact same length as the caliper setting because they both occupied the same space at one time.

You now have an accurate and infinitely repeatable reference. This can also be used to set a bevel gauge to set the tilt of the blade also.
If any one is interested in reading how I use this on the MFT they can read my post and discussion on the FOG

http://festoolownersgroup.com/festool-jigs-tool-enhancements/precision-angle-setting/msg195201/#msg195201

Feel free to ask questions.

-- Jim, Long Island, NY Ancorayachtservice.com home of the chain leg vise