I received the protractor today and for the price, it's an amazing little gadget and I have only a couple of criticisms which I'll address later. As everyone has noted, the protractor has a vernier scale that allows reading down to angular minutes, although this is going to be difficult for those of us with ancient eyes. However I was able to make readings with the aid of a magnifying glass.
Using this device on a surface plate will be a piece of cake but measuring small parts while holding both protractor and part in one's hands is not easy ! it's almost as clumsy as making thread measurements with the three-wire method. The part tends to want to slip out of the protractor jaws and I dropped a couple while evaluating the tool.
Practice improves this, as you'd expect. And speaking of practice, remember how it took a bit of that to be able to obtain consistent and accurate readings with telescoping gauges ? The protractor is something like that, although not as picky. When I first started using it, the consistency of my measurements was somewhere between two and seven minutes of angle.
The more that I used it, however, the more improvement I saw and eventually (after about twenty minutes) I was able to repeat measurements within a minute or two of angle. I expect that the device will, like a telescoping gauge, require a minute or two practice when it's time to make a measurement to confirm that a consistent result is being obtained.
I limited my brief evaluation to the measurement of a set of precision angles, from one to thirty degrees as shown below:
The maximum angle that can be measured is slightly over fifty degrees and I felt that the larger the angle, the more awkward the measurement technique. This may not be a problem in normal usage, like on a surface plate or with part clamped in a milling vise - maybe it's just because I was holding the protractor and the part and didn't have that third hand that would so often be useful !!!
If I had only one or two samples to measure, then I'd not know if the measurements obtained were indicating errors in the tool or errors in the parts. But with eight samples of different angles, I'm confident that my measurements were a good indication of protractor accuracy.
The measurements were consistently undersize but just barely, the mean error was -1.5 minutes of angle and the standard deviation was 1.9 minutes of angle so one can be reasonably assured - at least with this particular instrument - that a measured angle would be in the range of 0.6 minutes to 2.4 minutes HIGHER than the indicated measurement. That's pretty darned good accuracy for a tool that is available for less than $30 USD !
Oh yeah, I made a note of the measured error and placed it inside the plastic case for the protractor.
This leads me to a problem that could have been addressed with a simple design change. There are no means of zeroing the protractor; whatever the existing error, you have to live with it. Would have been nice if, by loosening a couple of screws and slightly nudging the vernier dial, I could move the low reading of -1.5 minutes to 0 minutes on the scale. As is, all of the movable parts that would permit zeroing are screwed and then pinned, having been "aligned" by the manufacturer.
But here's my major criticism and I just can't find any excuse for this shortcoming. The "reference bar", the sliding bar with the sharp chamfer on one end, has a sixty degree vee on side that is clearly meant to ride in a sixtee degree groove on the mating part. These are the parts (and note that I have yet to thoroughly clean the protractor, LOL):
The lack of the vee groove allows the reference bar to wobble from side to side slightly with modest pressure, even when the clamp is securely tightened. This doesn't affect measurement accuracy but is irritating when trying to hold an odd-shaped part in one hand, the protractor in the other hand while the part is moving. (BTW, flipping the reference bar doesn't improve the wobble.) The device is fairly well-made, incredibly well-made for the cost, and the lack of this helpful feature detracts a bit from total satisfaction.
Addendum: Been playing with the protractor for another twenty minutes and apparently some of the previous errors were operator-related. I'm getting zero errors (within the limits of the instrument) on some of the angle blocks now. It would be interesting to hear other owners' experience regarding this device.
Using this device on a surface plate will be a piece of cake but measuring small parts while holding both protractor and part in one's hands is not easy ! it's almost as clumsy as making thread measurements with the three-wire method. The part tends to want to slip out of the protractor jaws and I dropped a couple while evaluating the tool.
Practice improves this, as you'd expect. And speaking of practice, remember how it took a bit of that to be able to obtain consistent and accurate readings with telescoping gauges ? The protractor is something like that, although not as picky. When I first started using it, the consistency of my measurements was somewhere between two and seven minutes of angle.
The more that I used it, however, the more improvement I saw and eventually (after about twenty minutes) I was able to repeat measurements within a minute or two of angle. I expect that the device will, like a telescoping gauge, require a minute or two practice when it's time to make a measurement to confirm that a consistent result is being obtained.
I limited my brief evaluation to the measurement of a set of precision angles, from one to thirty degrees as shown below:
The maximum angle that can be measured is slightly over fifty degrees and I felt that the larger the angle, the more awkward the measurement technique. This may not be a problem in normal usage, like on a surface plate or with part clamped in a milling vise - maybe it's just because I was holding the protractor and the part and didn't have that third hand that would so often be useful !!!
If I had only one or two samples to measure, then I'd not know if the measurements obtained were indicating errors in the tool or errors in the parts. But with eight samples of different angles, I'm confident that my measurements were a good indication of protractor accuracy.
The measurements were consistently undersize but just barely, the mean error was -1.5 minutes of angle and the standard deviation was 1.9 minutes of angle so one can be reasonably assured - at least with this particular instrument - that a measured angle would be in the range of 0.6 minutes to 2.4 minutes HIGHER than the indicated measurement. That's pretty darned good accuracy for a tool that is available for less than $30 USD !
Oh yeah, I made a note of the measured error and placed it inside the plastic case for the protractor.
This leads me to a problem that could have been addressed with a simple design change. There are no means of zeroing the protractor; whatever the existing error, you have to live with it. Would have been nice if, by loosening a couple of screws and slightly nudging the vernier dial, I could move the low reading of -1.5 minutes to 0 minutes on the scale. As is, all of the movable parts that would permit zeroing are screwed and then pinned, having been "aligned" by the manufacturer.
But here's my major criticism and I just can't find any excuse for this shortcoming. The "reference bar", the sliding bar with the sharp chamfer on one end, has a sixty degree vee on side that is clearly meant to ride in a sixtee degree groove on the mating part. These are the parts (and note that I have yet to thoroughly clean the protractor, LOL):
The lack of the vee groove allows the reference bar to wobble from side to side slightly with modest pressure, even when the clamp is securely tightened. This doesn't affect measurement accuracy but is irritating when trying to hold an odd-shaped part in one hand, the protractor in the other hand while the part is moving. (BTW, flipping the reference bar doesn't improve the wobble.) The device is fairly well-made, incredibly well-made for the cost, and the lack of this helpful feature detracts a bit from total satisfaction.
Addendum: Been playing with the protractor for another twenty minutes and apparently some of the previous errors were operator-related. I'm getting zero errors (within the limits of the instrument) on some of the angle blocks now. It would be interesting to hear other owners' experience regarding this device.