Lathe Tachometer

 

This is still in progress, so there’s more software debugging to be done. The front panel is still under construction
I’m still learning to use the lathe, and one thing that has made me wonder how to set things up is the speed of the lathe. Now there is a very recent (came to my notice after I started this project) lathe tachometer that is available from one of the vendors.

However, I felt that I could roll my own for less cost, and get some features that were unique.

My design goals were as follows:

1) Tachometer readout from 8 rpm to the lathe’s maximum rpm, 3500.

Moderate size, and based on the 8741 microprocessor, which I have a few of….

3) SFM readout if the diameter is known, diameter choices are 1/8 inch to 5 inches.

4) Simple photo pickoff from the lathe spindle.

5) expansion capability, perhaps to talk to another controller that can make the lathe a CNC.

6) Capability to implement the Autostop for the lathe.

 
This is a wirewrap board. They can still be had for very little if you hit the right hamfest or computerfest at the right time, and the person is selling them. This is a cut down Augat board that is too thick (1/8 inch) for standard connectors. It has had the connector fingers cut off, and the board itself has been cut in half because it’s a small project. 
This is wirewrapping. The wire is 30 gauge silver plated wire with kynar insulation. Wire is placed in a special tool, which is then placed over the .025 inch square post. The tool spins the wire around the post, and the wire pressure welds to the post. It is a gas tight joint and is quite easy to do, if you have the parts. This wirewrap board has 0.3 inch rows of pins spaced 0.1 inches apart. One plane is power, the other, ground. 
Here’s the case, exploded view, but without wiring. The power regulator is on the lower panel. The switch panel is to the left, and the connector panel is to the right. The back and top are left off, because there’s no wiring here. The board is connected to the front panel, which is a mistake. Wiring is changed less than the programming. The board now is attached to the back panel. 
Here’s the bottom, or top, of the case being drilled. You can see that the braces and angle plates are used to support the pieces of the case at a proper right angle. 
Match drilling the top for the countersunk screws. The visegrips are good for holding the side in place. The front side (to the right) is there only to support the top, and isn’t attached yet. Why make a spacer when you already have one? 
The switch plate is another piece of 1/4 inch aluminum. I wanted this to be rugged. The toggle switches are for SFM/RPM mode, and latch mode. The two push buttons increment or decrement the diameter for SFM calculations.

 
Note that the panel is 1/4 inch thick, and the switches are not made for this. They’re much happier when connected to a 1/8 inch plate. I had to leave off some little bits and pieces.

 
The connector panel has a 5 pin DIN connector to the RPM sensor and the position sensor. The subminiature phone jack goes to the lathe motor lockout, and the jack on the right is a metal coaxial power plug jack that takes 12 volts in to run the whole thing.

 

 
Here’s the switch panel during the inspection process. The subminiature phone jack had to be recessed in the panel. The inspector apparently approved, or perhaps because nothing was edible, lost interest and left. Nothing negative was said, however.

The power supply regulator is to the right.

 
3/4 inch spacers mount the board on the front panel. However, making changes to the chip were a problem, so the spacers were removed and used to hold the board off the back panel. This leaves 3 holes on the front panel that I’d just as soon not have.

However, the sensor disk and assembly needed to be done next.

 
The sensor disk is a piece of .060 aluminum. It is chucked in a 1/2 inch 5C collet in a spin assembly which gives accurate rotations to 1 degree. This is the second slot getting machined. The holding fixture is a 1/2 inch bolt. The outside has already been turned to 2 inches.

 
The sensor plate. The inside was bored out on the lathe, with the outside held in the 3 jaw chuck, reversed. The quick change toolpost was used to hold a standard boring bar, one of the 1/2 inch cylindrical bars that cost about $20.00 a set. Works just fine. The bright spot is due to the buffing wheel that was used to remove the edge burrs from the slitting saw.

 
The sensor assembly itself. It came from an old floppy drive, and consists of a phototransistor and an infrared LED. The small aluminum block was made to mount the assembly on the lathe. The wire is two conductor shielded, and the heatshrink is the kind that has two layers. The outer layer contracts, and the inner layer melts and fills the voids. It’s a waterproof seal and quite strong.

 
Here’s a shot of the IR led in action. This is taken with an infrared camera, so you can see that the blue and yellow colors of the wire are off.

 
This is the mounting position of the sensor assembly. The lathe gear assembly was drilled and tapped for an M6 cap head screw, so all the stuff is designed to be metric on the lathe. The wires will be routed to the left and down.

 
Here's the side view of the sensor, showing the clearance and the slot in relationship to the sensor.

 
This picture shows the sensor and the sensor plate. There are two slots, which give one pulse each 1/2 of a revolution. This works because the tachometer is a period counter.

 
The tach working on the bench. This is part of the self test sequence. The processor is the large chip on the bottom left. The two lookup proms are immediately below the display. There are three LEDs to the left of the display to show modes. The plugins to the board are the I/O connector to the upper left, and the power connector on a pigtail at the bottom of the box. The board can be completely removed by unplugging.

 
The low range RPM reads to tenth of an RPM. The lowest range is 8 RPM, and the low range ends at 99.9 RPM. The high range runs from 100 to 9999 RPM, although resolution is limited at the higher resolutions, it’s not going up in 1 RPM steps. The switch between the two ranges is automatic and is just a difference in the display.

 
A high range rpm reading. The front panel is not finished, but the holes are filled with flat head screws.

 
The tachometer in the underrange (stopped) position. This is what happens if the period counter overflows or the reading would be below 8.0 RPM. The red light is for the latch mode, and the green light indicates SFM mode.

 
The tachometer being tested. The ham station is in the background for now.