Just IN- Hi everyone

[rosewhite]

I'm retired from a lifetime in English manufacturing but still interested in engineering and last night I was chatting online with a guy in Los Angeles who is thinking of getting a home workshop to make model locos and one thing we got round to is my describing a job I saw in the first workshop back in 1962 when making textile machinery.
The job was to make a large semicirular open ended trough in a big iron casting so that a big roller could roll in the trough but separated by just a few thou. trough and roller were heated and woollen cloth was fed through to get that classy gloss for mens suits.
Job was done on a big planer with a rotating toolpost on the crossslide.
The guy couldn't believe the job could have been done quickly as of course he's never seen a planer used.
So tell me please, how could such a semicircular openended trough be cut today with a big cnc mill?
problem was, in order to set the tool radius the old guy used to make two vertical cuts either side of the roughcast groove and set the dimension to what was already known from the gauge that was cut from a disk the same diameter as the roller.
centreline of roller was above the top of the trough so the actual trough was only a segment of a circle.
how would that be programmed today?
roller was 30inch diameter and trough was only about 10inches deep. what is the formula for calculating the width of the finished trough?