13-04-2011, 08:13 PM
Thanks for reading my post Alan, and for your interesting comments, and showing off your posh RPM meter!
Frankly, I don't need an RPM meter - my woodturing lathe has seven speeds by changing the belt on the pulleys, which only takes a few seconds, and I know instinctively what speed it should be at for a given diameter of wood. Too fast, and it gets scary! You'll know of course that the smaller the diameter, the faster the rotational speed can be, because what matters is the speed at which the material passes the cutting tool. For each rev, a 1" spindle being turned on a lathe will pass 3" of material per rev past the tool, but a 10" diameter workpiece will pass about 30 inches, so though the revs are the same, the amount of material passing the tool is ten times greater. For sanding and friction polishing of wood on the lathe, the speed needs to be much slower, or thin walled vessels can soon overheat and crack.
I'm making the rev counter more as an intellectual excersise than anything - it stops them putting me in a home!
I have the basic circuit from a model engineering mag, but there were no PCBs, so I've designed my own. There are three modules to it - a 12V stabilised power supply, the interface to the cheap and cheerful frequency counter module, which needs 12 Volts and 5 Volts, and the photo-interrupter which fits on the lathe headstock. Because the frequency counter module counts Hz rather than revs per minute, to get it to count correctly it's necessary to fool it by sending 60 pulses per rev, (hence one per second), so to get a correct readout at say 1000 rpm, the frequency counter module needs 60,000 pulses per minute. To accomplish this, a disc about the size of a CD is slotted with 60 1mm slots, which passed through a photo-interrupter (which costs less than a pound) mounted on the headstock. At 1,000 RPM, the beam is thus interrupted 60,000 times by the revolving slotted disc, and this signal is fed to the frequency counter via the interface, whereupon the frequency counter sees what it thinks is 1kHz, so it displays 1,000.
That's the theory, but of course, the difference between theory and practice in practice, is often greater than the difference between theory and practice in theory! I've designed and made up the boards, and the slotted disc, but I'm waiting for the frequency counter from the PRC, if ever it gets here. If it all works, I'll post it under homebrew. The 12V stablised PSU could be used for other projects, or could be altered to any other voltage with a different regulator; 9 Volts for exmple, as a battery eliminator for transistor radios.
Hope that's of interest.
Night night,
David
Frankly, I don't need an RPM meter - my woodturing lathe has seven speeds by changing the belt on the pulleys, which only takes a few seconds, and I know instinctively what speed it should be at for a given diameter of wood. Too fast, and it gets scary! You'll know of course that the smaller the diameter, the faster the rotational speed can be, because what matters is the speed at which the material passes the cutting tool. For each rev, a 1" spindle being turned on a lathe will pass 3" of material per rev past the tool, but a 10" diameter workpiece will pass about 30 inches, so though the revs are the same, the amount of material passing the tool is ten times greater. For sanding and friction polishing of wood on the lathe, the speed needs to be much slower, or thin walled vessels can soon overheat and crack.
I'm making the rev counter more as an intellectual excersise than anything - it stops them putting me in a home!
I have the basic circuit from a model engineering mag, but there were no PCBs, so I've designed my own. There are three modules to it - a 12V stabilised power supply, the interface to the cheap and cheerful frequency counter module, which needs 12 Volts and 5 Volts, and the photo-interrupter which fits on the lathe headstock. Because the frequency counter module counts Hz rather than revs per minute, to get it to count correctly it's necessary to fool it by sending 60 pulses per rev, (hence one per second), so to get a correct readout at say 1000 rpm, the frequency counter module needs 60,000 pulses per minute. To accomplish this, a disc about the size of a CD is slotted with 60 1mm slots, which passed through a photo-interrupter (which costs less than a pound) mounted on the headstock. At 1,000 RPM, the beam is thus interrupted 60,000 times by the revolving slotted disc, and this signal is fed to the frequency counter via the interface, whereupon the frequency counter sees what it thinks is 1kHz, so it displays 1,000.
That's the theory, but of course, the difference between theory and practice in practice, is often greater than the difference between theory and practice in theory! I've designed and made up the boards, and the slotted disc, but I'm waiting for the frequency counter from the PRC, if ever it gets here. If it all works, I'll post it under homebrew. The 12V stablised PSU could be used for other projects, or could be altered to any other voltage with a different regulator; 9 Volts for exmple, as a battery eliminator for transistor radios.
Hope that's of interest.
Night night,
David
