There is nothing in an assembly file other than links to the part files and assembly constraints. You need to include ipt files with the iam.
There is a rack and pinion in the samples folder that one of my students did a couple of years ago. Samples/models/assemblies/Arbor Press. Suppress the angle constraint on the pinion shaft. Isolate the pinion shaft and ram (rack). Suppress the rotational/translation constraint. Line up the teeth by eye. Unsuppress the rotational/translation constraint and move the ram.
The pitch diameter of the pinion was 1", therefore the circumference was PI. The student could have entered PI*1 as the ratio. I think he tried PI but didn't understand the units and ended up rounding off.
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