Omni Mast Not Aligned

Initially I had made the assumption that the major components of the Omni – such as the Base Plate, Swing Arm and Mast – would not require any adjustment or modifications.

However as I progressed in trying to align my machine I came to the conclusion that I could trust nothing and set about measuring and testing each component.

The most perplexing problem was that as I swing the quill and dop across the lap, the tip of the dop would travel in an arc over the lap. And that arc would not be the same height depending on what position the swing arm was in.

I don’t have any photos to demonstrate the problem so you’ll have to visualize this with my little sketches.

Imagine you are looking across the lap at the end of the dop. Adjust the dop so it is just barely touching the surface of your lap at the 6 o’clock position. Now swing the dop across the lap to the far side. When I did this, my dop travelled in an arc with the highest point somewhere near the hold-down screw in the middle. But the dop did not touch the lap at the far side!

Dop travelling in an arc over the lap.

After puzzling over this for a while, I tried moving the swing arm out and tested on different parts of the lap – still getting the arc but not the same amount of arc.

I realized that this indicated that the mast was not true – that depending on what direction the mast was facing, the dop and quill would move in a different arc. The sketch below will give you an idea what I’m talking about. The dop will follow the edge of the disk, so you can see how the arc the dop travels will change as the orientation of the mast changes.

Visualizing the dop travel arc

So the first thing I needed to do was Align the Mast. This was complicated significantly upon my discovery that the Base Plate is warped, leaving me nothing reliable to measure the Mast alignment against.

By the way, if your Mast is leaning the opposite way (towards the lap), the arc the dop tip traces will look like this:

Dop travelling in an arc over the lap.

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