A sine bar is a precision fixture for holding work at specific angles.
It can be used in the milling vice to produce an angled face or add a chamber to an edge. It can also be used to drill holes at specific angles.
The sine bar used in this case was a 5" one, which is exactly 127mm.
This distance is the measurement between the centre of the two cylinders on the bar.
The sine bar can hold a workpiece at any angle, provided a packing piece of the correct height is used. In a professional machine shop the packing piece(s) would be slip gauges which are blocks precisely ground to size and polished, so they can be combined to create any thickness. The packing can then be placed under one end of the sine bar to tilt it at the exact angle.
Gauge blocks are expensive but drill shanks are quite precise and may sometimes provide the right lift, or a piece of scrap can be machined to the required size.
In this example, the sine bar needed to be tipped to provide an angle of 18°.
To work out the packing we can use the Sine trig function.
In a right angle triangle.......
Sine of the angle = opposite/hypotenuse
So in this case:
Sin 18° = X/127mm
So. X = Sin 18° x 127mm = 39.245mm.
The lathe can be used to create a part of this length.
A phone app can be used to sanity check the packing piece. Here the phone is zero'd with the sine bar flat.
This image shows the packing piece under one end of the sine bar, and the resulting angle.
This set up could then be used to hold a workpiece in the milling vice or clamped to the mill table, to create a face on a part at 18°.
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