Sine Bar Intro

This is a short introduction taking what we learned from the Gage block stack page and applying it to actually measuring something on a sine bar using a test indicator. In this example, there is a part that was machined, that has a 90 degree included point in the center similar to a small shear blade.

Since its a included angle, we know that each individual angle of the piece is actually 45 degrees from the horizontal. Therefor we shale first quickly put together a Gage block stack to measure it using our sine bar.

On our calculator, SIN45 is .7071 now we take that number and multiply it by 5.000 for the hypotenuse side of our sine bar to get a stack height required of 3.5355. Now that we have that measurement we will select the following gage blocks eliminating that number down to zero.


And with these 4 blocks we shale connect them together properly by wringing them together. Allowing the surface friction of the gage blocks to "glue" them together. Since we have done the first step of the process, we will now use a sine bar and a angle plate to assemble our inspection assembly as shown.

As you can see we are using the angle plate to hold the sine bar in relative position compared to the surface plate and holding the part on the surface of the sine bar and the angle plate. This is due to how we are supposed to measure the part.

When inspecting a part we must follow the rule of datums. That is 3 planes that will constrain the part for inspection. Think of your graphing paper, how you make X and Y graphs, but with a 3rd dimension of a Z axis.

So if we look at our typical X-Y-Z plane, we can compare our surface plate to being the X axis, our sine bar is actually our Y axis holding the part relative on a angle (imagine the sine bar being put on a 90 degree angle and then bent down to our 45 degree datum plane we wish to use it as. And finally our angle plate, keeps our Z axis of the part constant by giving us a axis to reference off when putting the part on the surface plate.

Now we can take a test indicator and check both sides of the part, although small we would still be able to see if its off the angle we require.

As you can see, the indicator has been "zeroed" to that side at the 5 mark, but by the time we get to the other side we get.

We have only went one division away from our 5 marked zero on the other side. This is actually a .0005" indicator, so our angle over the roughly .750 is .0005 off on the angle. We can go on and calculate the taper per foot, or the angle difference, however for this small part our .0005 that the angle is different is completely acceptable.

Below is a attachment of GGG-B-121B the Military Specifications for Sine Bars.

Dimitrios Simitas

Dimitrios Simitas,
Feb 27, 2010, 10:42 PM