Precise Measuring of Angles for Segmented Bowls

I am a woodworker and recently, I have started to make Segmented Bowls. In order to do this, one makes a ring out of smallish wood segments that are cut to very precise angles. An error of even a tenth of a degree is multiplied by the number of segments - typically 8 to 12 or more and up to 144 for some very complex bowls. This causes the ring to not meet at the end. There are ways to compensate for this, but I prefer to get my angles more precise.

Initially, I purchased a high precision digital protractor. It was not high precision enough.
My first ring ended up with a gap of several degrees.  I viewed this experiment as a failure.

My First Sled

So I moved to geometry. That worked much better. Here is a photo with the clamps in place and a workpiece up against the stop ready to be cut.

Here is the same sled with the clamps removed and the distances marked:

So - what about the math? First of all, the base line (closest to the saw operator) must be perpendicular to the blade. That should be easy on a well-aligned saw.

Next measure the "Line parallel to base". I suggest metric system since the rulers are usually more precisely calibrated and also in decimal units.

To get "D", multiply the this distance by the Tangent of the Angle. In this example we are looking for a 12 segment bowl, so we can go to Google and enter "Tangent of 15 degrees" to get .26795.

Draw the lines on the wood and position the fence board. You should get an exact 15 degree cut.

Using Yardsticks

Last weekend, I attended the Segmented Woodturner's Symposium. There, I was introduced to Wedgies - a better way to make a segment cutting sled. This seems to be an improved technique. It uses two fences. The angle with the base line is not as important as the difference in angle between the two fences. I have not yet built one of these, but a major problem seems to be getting this precise angle. My solution again is using geometry - this time using yardsticks. 

Well, not exactly yard sticks - more like meter sticks. I bought three (that's right - three) 40" rulers from Harbor Freight. These are just over one meter long. I arranged them as follows using three clamps:

On the far end, I clamped the two yardsticks so that the 1 meter marks exactly lined up:

This time, I am looking for a thirty degree angle. So, I 

1. Cut the angle in half (30 / 2 = 15 degrees)
2. Get the Sine of this (Sin(15) = ,2588)
3. Multiply this by the aligned marks on the ruler (.2588 x 100 cm = 25.88 cm)
4. Double this (25.88 x 2 = 51.76 cm)

So, rather than trying to use the ends of both rulers, I opted for setting the left side at 10 mm and the right side for 61.76 mm.

I then measured this with my "High Precision" angle ruler

and it measured 29.8 degrees. Does this mean that my system was off by 0.2 degrees? NO! It means that my precision angle protractor is off by .2 degrees. Multiply this by 12 and we have a total error of 2.4 degrees. Not good enough for a closed segmented bowl ring.  (To be fair, the protractor is specified for .3 degrees, so it is within spec). How do I know for sure which is correct? 

Well, my wife's father was a printer and he left us a large metal 30 - 60 - 90 triangle which I think is probably pretty accurate. For years, my wife has insisted that we hold onto this as a souvenir of her father's business. Well, I tried that and it exactly fit in the angle I created using geometry. 

BootMgr not found

I got this message when I tried to start one of my computers. It was running Windows 7 Home Premium. There are 2 disks in it. One is the original 1 GB disk and the other is a smaller SSD.

I figured that the problem is with the SSD, so I tried swapping the two drives. It seemed to boot one time, but I noted that the two drives were swapped. I expected the larger drive to be C and the smaller one to be D, but they were reversed. I don't understand why.

Later, I tried this again and got the error message.

I found that it would book OK with only the smaller drive installed on SATA0, but it would not boot with the larger drive on SATA0.

After booting on the smaller drive, I made a Windows 7 System Repair Disk. I booted from the larger disk and tried to use the repair disk, This failed.

I next tried booting with SATA0 on the smaller SSD and SATA1 on the larger disk. It failed.

When my system boots, I see a brief options screen from the boot ROM with an option for "ESC=BootMenu". I clicked ESC and then in the boot menu, selected the smaller drive. This worked.

Next time, I selected F10 to get into the Setup Program. Under advanced, I found that the system had been set up to boot from larger hard drive, even though it was on SATA0. I changed the boot order to prefer the smaller SSD. I rebooted. This worked. Hurrah!

The older larger hard drive had originally been the only drive on this machine and was fully bootable. My guess is that my system had been using the boot manager on this drive all along and that somehow, the boot manager on this drive failed. So, the problem is not really fixed, but rather made irrelevant.

Next step was a bit of an experiment. I downloaded AOMEI Backupper Standard (free). and cloned the SSD to the larger hard drive. This took several hours. BUT - when I was done, I configured the computer with only the large (newly cloned large hard drive) on SATA0. It booted. Voila - Success.

After that, I put the whole thing back together and found that it booted from the SATA0 SSD. The other drive was there as Drive D, but was showing with a capacity the same as that of the SSD.

I guess I could run a partition program to get at the remainder of that disk. I think I will save that for another day.

UPDATE - I believe that eventually this stopped working and a repeat of the process did not help. Eventually, we learned to work around the problem.