SC QUALITY GUILDSine bar and height gauge record
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Work out the gauge block stack for a sine bar angle, check the angle on the part, and record height gauge and comparator readings against tolerance.
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How a sine bar sets an angle
A sine bar is a hardened, ground bar with two rolls of equal diameter a precise distance apart, commonly 5 in, 10 in, 100 mm or 200 mm. One roll rests on the surface plate and the other on a stack of gauge blocks. The center distance L is the hypotenuse of a right triangle and the stack height H is the side opposite the angle, so H = L × sin θ and, from a stack, θ = asin(H ÷ L). For 15° on a 5 in bar, H = 5 × 0.258819 = 1.2941 in.
Build the stack with the fewest blocks: clear the last decimal place first, then the next, and so on. Each wrung joint adds a small error. Above about 45°, a small stack error makes a larger angle error, so the sine bar is best used for smaller angles.
Checking the angle on the part
With the bar set to the nominal angle, the surface under test should be parallel to the plate. An indicator run between two points a known distance apart shows the error: the angle error is about the change in reading divided by the distance, in radians. A change of 0.0006 in over 2.000 in is 0.0003 rad, about 1.03 minutes.
Height gauge and comparator readings
- Direct: zero the height gauge on the surface plate and read the height. Any reading on the plate is a zero error to subtract.
- Comparator: set the indicator on a gauge block master close to the nominal, then read the part. Actual = master + (part reading − reading on the master). Short indicator travel keeps the indicator's own error small.
- Drift: re-check the master at the end. A changed reading means every reading in between is in doubt by that much.
Good surface plate practice
Use a clean, calibrated plate, let parts and blocks reach the same temperature, wipe blocks before wringing and handle them as little as possible.
On the exam
CQI questions give a bar length and an angle and ask for the stack, or give a stack and ask for the angle. Use sine, not tangent, and the center distance between the rolls. Watch the calculator mode: degrees, not radians.