LeeValley digital angle finder guage - Redux

[eccentrictinkerer]

OK, I got my new angle finder and had fun measuring "squareness" of stuff around the shop and I was impressed with measurements to 0.1 degree.

I tried a real-world test on a piece of MDF 12 3/4" by 31 1/8" with two factory edges.

I checked the part with the L-V angle finder and both ends measured 90.0 degrees. Great!

Then, I measured the dimension of each long edge. Edge 1 was 31 1/8" and edge 2 was 31 1/16"!

Checked with draftsman square and proved the error.

I've got two problems.

One, SMT is off 1/16" over length of travel (fixed that courtesy of search on this forum, thanks, guys! ).

Two, L-V angle guage appears to not be able to indicate the error.

Am I doing something wrong or is the devices' resolution not capable of catching this 1/16" error.

I'm 46 years removed from 9th grade trigonometry, so I need help.

Thanks in advance.

J.D.

[LCHIEN]

I can see several points of comment.

1. An SMT "Error of 1/16th" is not very specific. Not being parallel to the blade actually is not a source of out of square errors in of itself. The miter fence being not 90° to the blade will cause squareness errors in a cross cut. If the miter fence is 90 to the SMT but the SMT is not parallel with the blade then that is a possible source of error. Basic Geometry - things that are at an angle to a reference (e.g. SMT) will be at the same angle to something parallel to the reference (e.g. the blade, if it is parallel to the SMT).

2. A 1/16th error over 31 inches is a hair over .1 degree. I believe from former discussions w/o looking this time that the claimed accuracy of the protractor is 1 degree with 0.1 degree resolution. Theres a difference between accuracy and resolution as I have often discussed before here. If it told you 90.0 then you only have the theoretical right to believe that its somewhere between 89 and 91 degrees (accuracy of +/- 1 degree). However it will indicate small changes in angle down to .1 degree (The resolution). In practice I imagine you will find the accuracy to be much better than 1 degree but not perhaps as good as .1 degree. Resolution is really in this case determined by the fact that it has one digit after the decimal point display, so that's a given.

[eccentrictinkerer]

Thanks, Loring, I think I've got it now. This mathematically-challeged sawdust-maker forgot the relationship between resolution and accuracy.

I was 100% sure I'd get the answer on this forum and 99.9% sure it would be you to supply the answer.

Thanks again,

J.D.