Checking a 45 degree square?

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loply:
Hi guys,

Thanks for the replies.

In truth I don't have that much use for an incredibly accurate 45 degree square, but precision scraping has become something of a joy to me. I made by 12" master square which appears, as far as I can measure, to be within a few microns of 90 degrees 12 inches high, and I'd love a similarly accurate 45 just to complete the set. Within 10 microns would be nice and if I can get it within 5 I'd be chuffed for the rest of my days :D That kind of rules out any methods involving scribing though.

I'm aware that two 45s should make 90 which can be verified on a surface plate with an indicator easily but you'd need to make 3 to ensure you don't have a 44 and a 46... My next best idea is that I'd have to make a sine bar and set it at 45, then lay the 45 degree square in top and it should be flat... but then the accuracy is limited by the sine bar and the accuracy of all the gauges you used to prop it up. Maybe that's the best we can do though?

I like the idea about using the mill to drill holes in a triangle though. I feel like the general concept may have some mileage.

Will let you know how I get on,

Rich

vtsteam:
Rich, there's a little confusion between distance and angle here.

Do you mean you want a 10 micron tolerance at the end of the new tool's leg, measured from a straightedge set at an absolute 45 degree angle to a baseline?

If so, and you give the length of the leg, the angular tolerance you want can be calculated.

You will obviously need some standard (no matter what method constructed) that is true to that angular tolerance.

Scribing can be extremely accurate if it is checked afterwards by reversal as mentioned earlier. If it isn't true after checking, whatever distance makes up the error can be halved and re-scribed, and then checked again. You can keep doing this until you are at the tolerance of a scribed line's thickness -- which at whatever distance out the leg extends is a tight angular tolerance indeed.

To get the last bit of tolerance possible, placing a straightedge on the scribed line, checking the workpiece against horizontal baseline straightedge. Layi another straightedge against the first angled straightedge and removie the first. This eliminates the visual aspect, and the thickness of the scribed line, and substitutes contact (which is the means of all scraping tests). Then the piece is placed against the second straightedge and checked for contact with the vertical 90 degree block edge.

When they both match side to side, by contact, it is as close as contact can measure. Which is as close as can be scraped by conventional methods.

S. Heslop:
If it's triangular shaped couldn't you measure the lengths of the sides, and just make sure they match and the angle between them is 90 degrees?

mexican jon:
I'm guessing that you can accurately check that you have 1 90 degree angle, therefore using pythagoras you can work out if the other angle you require is indeed 45 degrees. :D :D

--- Quote from: S. Heslop on May 27, 2015, 12:26:21 AM ---If it's triangular shaped couldn't you measure the lengths of the sides, and just make sure they match and the angle between them is 90 degrees?

--- End quote ---

Or you could just do what he said :D

Fergus OMore:

It's a bit early and the streets have to aired but surely Simon you have moved into describing and isosceles triangle which cannot have equal sides and 90 degrees somewhere. If you split an isosceles triangle exactly down the middle you get back to the Euclidian solution of right angled triangles.

But enough 14 to 16 year old Matriculation antics, two practical thoughts emerge. The first is to cannibalise a scrap machine tool to find the unworn sections and the second is to use things like Moglice, Devcon and Turcite in the construction of references.

These were not around when Connelly penned his epistle to the great unwashed.
My old mangle is running on Turcite! :bang:

Back to the coffee

Norman

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