page 198

35.7.2 Squares

•Squares use known angles as a measurement reference. Generally a square is used to measure 90 degree angles (i.e., square corners)

•The basic types are,

-Combination Set - This has a sliding blade and is used for layout.

-Standard Square - There are three grades: 1. Reference, 2. Inspection, 3. Workshop

blade

beam

-Toolmakers Square

-Cylindrical Square

		90°	Both the object to be measured, and
			the square are placed on a reference
			plane. The square should provide and
			90° angle to the reference plane.

- Direct Reading Type

•The advantages of the Toolmakers, and cylindrical squares are,

1.There is a line of contact between the part and the square.

2.More resistant to damage.

3.Can be checked by rotation.

•Standard Squares can be checked for errors using a reversal test. In this test an angle plate is placed on a reference plane, and a standard square is placed against the angle plate. A dial indicator is run along the square from one end to the other, and the drop/rise is measured. The square is now rotated so that the other side is now measured. The drop/rise in height can be used to calculate the angles of both the square, and the angle plate.

page 199

test A

drops 0.0007”

4.00”

test B: With the square reversed

rises 0.0003”

2.00”

Some values of drops, and distances are given above for illustration. The first step in calculating the angles is to find the angles in the first, and second tests.

				–0.0007			·
θ A =			asin	-------------------		=	–0.010°
					4.00
θ		=	asin		0.0003	=	0.009°
	B				---------------
					2.00

Based on these values, the angle of the square is,

θ		= 90° +		θ	A	– θ	B	= 89.99°
	SQUARE			-----------------
						2

Likewise, the angle of the angle plate is,

θ		= 90° +		θ	A	+ θ	B	= 90.00°
	ANGLE			-----------------
						2

page 200

35.7.2.1 - Coordinate Measureing Machines

•generally measure x-y-z coordinates using touch probes

•these measurements can be made by positioning the probe by hand, or automatically in more expensive machines.

•reasonable accuracies are 5 micro in. or 1 micro metre.

•The method these machines work on is measurement of the position of the probe using linear position sensors. These are based on moire fringe patterns (also used in other systems).

1.The Pattern - two sheets with thin fringes are put at right angles, the optical effect is a darkened strip that runs along the strips. As one of the strips is moved the band will move up or down. When optically magnified the moving strip can be used to determine direction, and distance of motion.