Overhead pitch RPM calculation

[aaron]

I've been trying to figure a formula for this, and I keep confusing myself. Maybe someone here can help.

I'm trying to figure out what RPM range I'll need for the motor to control my overhead. Given a turntable speed (33 or 45), a desired LPI for the grooves on the record (say 200, or any value) and a given pitch of my leadscrew (10 threads per inch), how do I calculate the RPM that the leadscrew needs to rotate at?

[Angus McCarthy]

For the sake of calculation, let's start with 45rpm. We have several established numbers in this case: 45 rpm, 10 threads per inch, 200 LPI. We need to find the RPM of the screw to make it travel a set amount of distance across the disc in one minute.

Given that we can only cut 45 lines in one minute, we divide 200 by 45 to find the time it takes to travel one inch across the disc = 4.44~ minutes.
We then divide that one inch by 4.44~ to find the amount of distance traveled in one minute = 0.2250 inches.
Last, we multiply this number by the 10 threads-per-inch of the leadscrew = 2.25rpm.

The basic calculation would look something like this:

[1 minute / (Disc LPI / Disc RPM)] * Screw pitch = Screw RPM

(Unless I'm being a complete dunce today, which is entirely possible )

[Cubed]

Sweet! A new formula! I needed that to verify this.

(pitch of lead screw / desired LPI) * RPM of the record you want to cut = RPM of lead screw.

so

(10/200) * 45 = 2.25

Thanks Angus for the breakdown, now I can get an estimate of time.

[EmAtChapterV]

What sort of overhead do you have? On my Rek-O-Kut M5, the spiralling crank turns at 1/10 the speed of the record - turn it at 10 rpm (3x faster than normal) when cutting 33 1/3 with the 120 lpi leadscrew installed, and you'll get 40 lpi. Turn it at 156 1/2 rpm while cutting a 78 and you'll get 6 lpi.

I've been told that with a Presto overhead, this ratio is 1/8.

[aaron]

It's a homebrew... Literally just a 1/2-10 acme leadscrew

Thanks for your help guys!

[Angus McCarthy]

Well, we both got the same number, so it's probably correct!