About to build my frame soon and have read most of these posts, and want to get the best geometry for a 4’ by 8’ sheet total accuracy. I realize their are many issues, but getting the best geometry in my mind gets the better shot at it.

For my shop I do not have any constraints on L, W, or H. The final design will be on wheels to move it around. I don’t anticipate cutting anything larger than 4x8’. I do plan a center horizontal slot to rout mortises on up to 6x6’s, but usually 2x2’s to 4x4’s. That slot will be recessed into the “spoil board”.

But this discussion is about geometry. I’ve made a dynamic sketch in F360 that models your spreadsheet geometry. For instance, this is the “Standard” as referenced by the spread sheet. It dimensions all 9 key cornerpoints and centers with distances and angles.

It correlates with the top center angle of 17.2º, the “lift angle” for top dead center, and 8.6 º as the “pull angle” on the lower left corner. After reading many of the posts I believe these are the two that are causing the most accuracy issues, as far as geometry is concerned.

I have focused on improving both of these angles to get more “lift” and “pull”.

By changing the height from 18 to 30, and the width from 116 to 156, both of the lift and pull angles increase to 21º. This should increase the lift in the top center position, and the pull in the lower corners.

Note: The 12’ (140" top centers) standard pull angle in the spreadsheet is 18.4º. So the increase over that is just 2.6º.

So my question to you is based on your experiences so far, do you think 21º is enough? Lift / or Pull?

I am avoiding chain weight / sag for this discussion. The longest distance from sprocket is 148 3/16" and the shortest is 42 27/64", so the chain motion length minimum is still about 105" plus some extra, so the first 40" or so from the Maslow ring can be an aluminum rod or something light.

Thanks for your thoughts. Do you agree increasing the lift and pull angles will help?

Ed