Assessing the warp on the bottom of a rocker box.

[DIYman (Doug)]

Hello all

I am after suggestions how to measure the flatness of the base of a rocker box on a dob. To put my query in context here is some background info.

The bottom of the rocker box on my self-made dob is covered by a 3mm stainless steel sheet. The azimuth base has three legs with each having a roller bearing on which the rocker box rides.

When using the scope I find in certain positions the rocker box will slew out of position in azimuth indicating that the azimuth base is not level. I ran a series of tests after carefully setting up the base ensuring that all the tops of the three roller bearings are on a horizontal plane using a precision spirit level. The slew remained suggesting that the bottom of the rocker box (stainless steel plate) is warped.

The solution to the problem is placing packing beneath the stainless steel plate in the low spots. Instead of using trial and error approach I wonder if anyone has any suggestions as to ‘mapping’ the high and low spots on the stainless steel plate. I thought about using a reference plane or plate but it would need to be 700mm wide. That’s a pretty big area for a reference surface that needs to be accurate to least 0.25mm.

A friend suggested using a water bath but measuring down from the stainless steel plate to the water surface but that would not be easy.

Are there any other solutions that you can suggest?

Thanks in advance.

[GeoffW1 (Geoff)]

Hi,

Given the resolution you are wanting to see, say, departures from perfect flatness of ± 1.0 mm or so at the best, I'd use an engineer's straight edge

http://www.toolstore.com.au/estore/style/agp12.aspx

and a set of feeler gauges

http://www.toolmart.com.au/measuring-tools/rt3100.html

You lay the straight edge across the base in various radial positions and use the feeler gauge to measure gaps beneath the straight edge when it is in a stable location.

Cheers

[DIYman (Doug)]

Thanks Geoff
Your idea using a straight edge and feeler guages is a tried and true method which I should have thought of myself. It is the simplest solution to the problem.
Cheers
Doug