Thread: Digital protractor recommendation

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Mitutoyo Pro 3600 Digital Protractor

Expect to pay about a grand for a decent one.

Actually I think even these are only good to 0.01º from 0 t 9.99º

Originally Posted by Dengue

Can anyone please advise on an accurate digital protractor that will measure 90.12 degrees i.e., to two decimal places ?

A reading of 90.1deg ( one decimal place) will mean the measured angle could be anywhere between 90.05 and 90.15 ie a possible 0.1 deg error.

When cross cutting a 300mm wide board with a saw set at 90.0 degrees measure to this accuracy means the cut could be out of true by 0.5 mm - not good, esp if you make 2 separate board cuts giving a possible error of 1mm when the two boards are placed end to end.

I'll take your maths are being correct, what I don't get is why you need to measure to the level of accuracy you indicate.

for starters, you are assuming that
1) the saw arbour has ZERO wobble
2) the saw blade doesn't flex a little as it makes the cut
3) the material being cut is held rigidly in relation to a set of rectalinear reference faces AND that those reference faces are of similar scale to the reference faces the cut board will be aligned with.
4) the material being cut doesn't experience a little stress relief after cutting

I'm not sure that you can control away those four factors so the method of work you adopt needs to take account of them.
Adding a compensatory control to allow for a possible inherent +/- 0.1° from 90° in a mitre setting is, in that context, trivial


Note that the $1,000 digital protractor BobL linked to is only accurate to +/- 0.1° for 80° to 90° (it's +/-0.05° between 0° and 10°)


Besides, if all you're after is two decimal places of accuracy at 90° you don't need a protractor

Actually ian, you are correct on all counts. But all I want to do on an SCMS is set the angle of cut so that it cuts a 300mm board at exactly right angles i.e., when I put my try square or a set square on it, I want it to line up, not be 0.5 mm off square. I don't think this is an unreasonable expectation.

A saw set to 89.9 degrees will give an error of 300 x tan ( 0.1) =0.52 mm on a 300mm wide board

If you have a good square you can set the scms to exactly 90º. Cut and check for square, adjust, cut and check for square, repeat. You don't need a wizbang digital protractor to tell you where you are, you need a square to tell you where to go! But as Ian said, there are other errors which mean 0.5mm across 300mm is not too bad for an scms. In addition to what Ian said there is also the possibility of run out on the fence that might give different errors for different length boards and whether the edge registered against the fence is perfectly straight.

Every measurement has an unavoidable error ... so there is also the consideration that a square is unlikely to be *exactly* 90 degrees.

Looking at the old method of drawing a perpendicular with the square, reversing the square and repeating ... always assuming the edge is dead straight ... then a square that is off by 0.1 degree will deviate by 0.5mm over 300mm. The other line will be similarly off, so you will see a 1mm discrepancy. I had to look at a ruler to check that that is a visible difference - which of course it is ... the ruler is marked in 1mm increments.

So I guess you can reasonably expect your square to be accurate to say 0.05 degree.

But thinking about that, even assuming an inaccurate square ... if you marked one perpendicular across your work, reversed and repeated ... then the true 90 degree line could be marked by marking from the starting point to the halfway between the two end-points. Then you would make test cuts and compare to the marking out.

Please send me the $1000.
Thanks *very* much,
Paul

You are right too, mic-d, and I actually have set up the SCMS with a try square and the laser recently - extremely accurate result, far better than factory set accuracy. Probably more luck than good process.

I did check the accuracy of the try square against my Engineer's square that I keep just for that purpose:

I got a piece of Melamine, and using the manufacturers cut edge, drew a cut line with a very sharp marking knife and filled the cut in with chalk rubbed in with a finger to give an extremely fine line against a pure white background. Both the Engineers square and the try square were spot on when the handle was flipped.

But I actually wanted the digital protractor to check on the 45 degree cut on the SCMS for doing mitre joints. That is assuming that being a very accurate 90 degrees doesn't necessarily make the 45 degree cut just as accurate.

I know how to do the "4 cut method" of checking the 90 degree cut accuracy, but don't know how to check a 45 degree cut.

Does anyone know of a method?

To check the accuracy of your mitre cuts, have a look here Basically involves cutting two identical boards at 45 degrees, then turning one to make a right angle, and then test that angle with the inside corner of an accurate try square.

The wider the two boards the better

Originally Posted by JillB

To check the accuracy of your mitre cuts, have a look here Basically involves cutting two identical boards at 45 degrees, then turning one to make a right angle, and then test that angle with the inside corner of an accurate try square.

The wider the two boards the better

This is a good way to check because any error is amplified and is easily detected against your good square. and you still don't need no wizbang doohicky digital thingamejig.

Have a look at this link.
How to Make a Cross-Cut Sled | The Wood Whisperer

It shows a method of squaring which is very accurate indeed and requires hardly any technology at all. The video shows how to square a fence on a table saw cross cut sled but you will be able to adapt the idea to square up your other saws, or at least to check the accuracy of the cuts.

the thousand dollar machinist's measuring device would just be a distraction and a frustration with your SCMS. the saw is not built to anywhere near the accuracy that the protractor is. you will be constantly seeing a slew of errors that you cannot isolate or correct for. wood moves too much with humidity changes for that level of precision to be relevant anyway. with wood the way to make joints tight and long lasting is to design for movement and fit in place.

A very accurate protractor would just be a distraction and a frustration with your SCMS. The saw is not built to anywhere near the accuracy that the protractor is.

Yes, Bridger, I have come to the same conclusion, especially after ian pointed out above all the inherent errors in an SCMS - it is a wonder that it will ever get a true 90deg and 45 deg cuts

I think that the old shooting board might need to get resurrected

Originally Posted by Dengue

....I think that the old shooting board might need to get resurrected

Coming in late here, but that's what I would have advised right from the start. It's nice to have your equipment as accurate as its construction shortcomings allow, but precision fitting, be it with wood or metal, usually involves fine-tuning of some kind. Getting your saw cuts as close as possible reduces the hand work, but doesn't necesarily eliminate it, depending on what you are making. It's good to shoot for perfection, but it's an impossible goal, which is why we invented 'tolerance'. How close is close enough depends on the application.

But take heart, the limit of resolution of the human eye is around 0.2mm, so as long as your pieces fit to within 0.1mm tolerence, it will be 'perfect' .....

Cheers,