Lathe Autostop

An automatic, locking motor switch for threading. Turns the motor off when the carriage has reached a pre-determined point.

 

 

 

 

 

 This is an add-on for the minilathe. It consists of three pieces, powered by a 12 volt supply. Construction requires a milling machine or an adaptor for the lathe. The first piece is a sensor head with a position sensor for the carriage and three indicators. It is wired to an electronics box that has a reset switch. The electronics box is wired to the main motor control box for the lathe.

The lathe, when operated at very slow speeds for threading purposes, will stop within a quarter of a turn or so, even without a load. This modification uses the carriage position to throw a relay to turn off the motor at a pre-determined location. The motor will remain off until the circuit is reset. An indicator will light if the motor is latched off.

WARNING: This circuit is not designed for high motor RPM. Use with caution!

Having given the proper warnings, and such, and adding the disclaimer that any use of this design is at the risk and sole responsibility of the user, and also adding that this design may be freely used for private purposes, but may not, without permission, be sold or used for commercial purposes; now we start with the design.

This is based, in part on the work of Varmit Al and others. Without their webpages I wouldn’t understand this lathe as much as I do.

 

Note: The gridwork in the background is 1/4 inch by 1/4 inch.

 

 The mechanical design of the sensor head is a bit involved, but not too much. There is a clamp, shown on the right side, that attaches to the lathe bed. The left hand side is the indicator section, with three LED indicators. There are also a microswitch and several resistors in this box. The clamp and the indicator box section are held together with two stainless steel screws. The material of both clamp and indicator box is aluminum. The clamping screw is brass, to avoid marring the lathe bed.

 
The clamp started life as a 1.5 by 1.5 by 1.0 block of aluminum. The central slot is milled to be the height of the lathe bed’s T cross arm. The step section on the left fits against the T column. The two countersunk holes fasten the clamp to the indicator box. The sensor post hole is 5/16 in diameter, drilled through. From the other side, drill in about 3/16 of an inch with a 3/8 inch drill. This keeps the post from popping out of the clamp. The view here is with the clamp on its back side.

The arm below the lathe bed is roughly 1/2 inch thick, and the support of the top is also 1/2 inch thick.

 
While not easy to see, the sensor post hole is drilled to allow a 3/8 inch section to be inserted in the block to a depth of 3/16 of an inch. The idea is to allow the post to travel back and forth about an eighth of an inch.

 
Barely visible in the top photo is the 1/4 by 20 tapped hole for the clamp screw. This is drilled in the middle of the clamp, and centered over the lower arm of the clamp. The lower photo is viewed from the headstock towards the tailstock.  The top view of the clamp shows the hole for the clamping screw.

 
 The smaller hole is tapped 6-32 and is positioned so the sensor post may slide back and forth, but cannot turn. You can use a setscrew or adjust the length of a screw to match. This no-turn feature was going to be used for a fine adjust, but that just didn’t work well. This view shows the clamp as seen from the tailstock looking towards the lathe headstock. The clamp fits on the back of the lathe bed.

 
This is the sensor post. It is made from 3/8 inch steel rod. The lower part is turned to 5/16 or a little under. The wider part is very slightly under 3/8. The top post is turned a little less than 1/4 inch. Note the slot milled for the “no-turn” screw. The length from the top of the 3/8 inch section to the bottom is the width of the clamp. The actuator post (the 1/4 inch part) is long enough to go through the indicator box wall and into the box by about 1/8 or so. It will trip a microswitch when the carriage is too close.

 
This view shows the sensor post, the clamp, and the indicator box in approximately correct positions. The microswitch is installed on the bottom of the box, which metal is about 1/8 inch thick or so. Major design goal is to have the sensor post clear the bottom of the box. The 6-32 screws should clear the microswitch, too.

Note the small but important clearance between the sensor post hole and the box bottom. Note the offset on the top 6-32 screw to clear the microswitch and also give the most clearance for the sensor post. The position of the lower screw is less critical.

 
The sensor screw is installed. The sensor itself is a brass 10-32 screw, with the head turned mostly flat. The spring is from an old floppy drive, and used to help clamp the spindle onto the disk. Note that the back end of the sensor post is visible, it must go into the indicator box. The base of the spring has to be wider than 5/16 of an inch to clear the sensor post. Total sensor travel is a little less than 1/4 inch.

The sensor post must move freely and not bind.

 
The indicator box, already attached to the clamp. The box is a piece of 1 1/2 by 1 1/2 by 1 inch aluminum. The wider sides are approximately 1/4 inch, the narrower sides are 1/8 thick. The microswitch is mounted in the bottom so that the maximum extension of the sensor post just falls short of the maximum depression of the plunger. The wall of the indicator box stops the post. The post can be seen almost contacting the arm of the microswitch.

 
The wall of the indicator box. This is the side that mates to the clamp. The larger hole is 1/4 diameter, the microswitch is just visible in the hole. The shoulder of the sensor post is stopped by the box wall. The other two holes are tapped 6-32 to hold the box to the clamp.

 
The indicator box alone with the lid. The lid is shown in top view. The LEDs are placed between the microswitch and left box wall, giving them full depth. Note the small milled ledges on the edge of the box: they are about 1/8 inch deep by 1/8 inch tall. The width is about 1/2 inch.

 
Bottom view of lid. The nuts for the LED holders come uncomfortably close to the walls of the box. The slots milled in the box walls are to clear these nuts. The four holes are countersunk on the top and are for 4-40 flathead screws. The holes are approximately 1/8 inch from the corners in both directions. This barely fits, but does work.

 
A view of the clamp and indicator box as seen from the bottom. The tapped hole for the clamp rod is clearly visible. The screw visible on the indicator box is the 2-56 screw that holds the microswitch in place. It is countersunk so it is flush with the box bottom. The lower surface floats above the lathe bed.

 
This shows the approximate orientation when being used. The brass screw and spring are not installed, but the countersunk screws are visible. The one on the right is elevated to clear the microswitch and the sensor post as much as possible.

 
The clamp screw is a piece of 1/4 by 20 brass. The knob is commercially available. It may be too long, and if so, is easily cut down. The end is ground flat to minimize possible damage to the lathe bed.

 
The business end of the assembly. Since it hasn’t been wired yet, the leads that will come out the right side of the indicator box (behind, in this picture) are not shown. Note the sensor post, the clamp, and the LED indicators in the background.

 
Top view of the mechanical assembly. The box lid was cut slightly oversize, fastened to the indicator box itself, and was milled to size as the edges and sides of the box were given a finishing cut. Other than the microswitch, no electronic components are yet installed in this view.

 
Sensor as wired, this is relatively simple to do. There is a small piece of PC board material glued to the underside of the lid. It is the common point for the 12 volt input. The microswitch will be bolted back into the bottom of the box with a 2-56 screw.

 
Bottom view of the lid. The main idea is to get the resistors close enough to the board so they don’t hit the microswitch. The resistor leads are just connected to the LED leads.

 
The sensor tested. This is the normal indication with the system not triggered. The next light down is yellow, the bottom one is red. Note the wires coming from the back of the sensor.

 
This view shows the sensor box in the triggered state. The yellow light indicates triggered, the red light indicates that the motor is shut off.

 
The sensor head completed and wired. The connector on the end is a 5 pin DIN connector, the same that is used on a computer keyboard (old style). The connector on the electronics box is a match for this and is a 5 pin jack. The wire is twisted pair, two pairs total. There’s a yellow, red, green, and black wire. Over those wires is a spiral wrap that one of the local home hardware stores was closing out. It’s about 1/8 or so collapsed, and binds the two twisted pairs together.

 
This is the electronics box. The sensor connector is on the left. The power switch is the paddle switch between the two lights. The two lights indicate the operating mode. More on that, later. The pushbutton is the reset. On the right side is the power connector and the motor relay connector.

 
Close up view of power and motor relay connectors. I put the motor relay connector (a 1/8 inch stereo phone connector) up a bit too high and had to shave off some of the plastic supporting the lid. It could have been down about 1/8 of an inch, minimum.

 
View of lights and sensor connector. The power switch is a DPDT center off. This simplifies the wiring considerably. The large empty space has been reserved for the small electronics board. The lights are green and red 12 volt incandescent lights. They are visible from a wide angle.

 
This is the circuit board. I considered making a photoetched board, but that’s too much trouble. This was cut with a cutoff wheel in a dremel tool. The cathode (K) and gate (G) of the SCR connect to the board, the anode is just bent up into the air and connections are made to it. The parts you can’t see are the input, +12 volts (above the input) and a blank for the anode (no connection).

 
The long ground connection from the power jack is soldered directly to the board.

 
This shows the location of the board within the box. The box lid will be mounted on the lathe, which means that you are viewing this in the “normal” orientation.

 
This shows the detail of the power and motor relay connections. The outside (ground) of the motor relay connector is unused. The two leads that are used are the tip and the ring. The connector on the motor box is also female, so the cable can be removed without leaving an exposed hot connection.

 
The box and lid. The lid has two tapped aluminum studs, used to provide threads for soft materials. These are tapped 6-32. The two studs go into unused locations between the parts.

 
A somewhat larger view of the electronics box. Heat shrink tubing is used where needed to insulate the connections.

 
The electronics box completed and with lid attached.

 
A note about the electronics: With the exception of the microswitch, there were no parts that could not be bought at Tandy/Radio Shack. The relay is part 275-218C, the SCR is 276-1067. The diodes are 1N4001. RS does sell a microswitch, it should fit with little modification if the sensor arm is removed. All the connections with the exception of the phono jacks are sealed connections. All of the connections have been designed so that there will be a minimum possibility of exposed voltage of any sort. A well stocked electronics junkbox will reduce the price of this project by quite a bit, but the parts are readily available, so that is not a problem.
Location of the miniature phone jack for the motor relay. It’s located down at the bottom left side of the motor controller electronics box. Use heat shrink tubing to make sure that there is no possibility of shorts.

 
The motor relay fits precisely between the reversing switch and the box wall. Connections are made with 1/4 inch terminals. The only modification to the lathe that stays is the small hole in the box. The motor leads that used to go to the switch now go to the arm of the relay. Do observe polarity, otherwise the motor will run the wrong way.

 
There’s a small diode across the relay coil. Do not omit this. The cathode (banded) end goes to the +12 volt line. The purpose of the diode is to damp the spike caused by the magnetic field of the relay coil when the relay is turned off. This protects the SCR.

 
This is an overall view of everything. The only thing to change is that the cable to the sensor head needs a clip on it to keep it out of the way. The electronics box is mounted on the chip tray.

 
The motor relay connector goes into the box in a very out of the way location (so far). This does suggest that the cover be put back on the gears after they are changed for threading, no?

 
The sensor head in position, and shown relative to the chuck. Notice that the whole thing clears the 3 inch chuck, and can be moved back almost to the motor box.

 
The lathe motor stopped and triggered. This is just after the motor has been latched off.

 
When the carriage is moved back and the tool is disengaged (note that there is a gap between the carriage and the sensor screw). The green light is on, indicating that the sensor is clear. However, the motor is latched off (and the red light remains lit). The motor only starts when the reset button is pressed.

 
This shows the positioning of the whole sensor with respect to the lathe bed. The brass clamp screw is trimmed down so that it will clear the chuck and the electronics part of the sensor.

 
Operation:

There are two modes. Each is selected by the position of the power switch. The lights on the electronics box indicate the mode. The mode with the green light on will stop the motor. Use this for threading.

The mode with the red light on will not stop the motor. It does indicate that the limit has been reached by turning the yellow light on.

 
Setup:

Adjust the stop position so that the yellow light turns on and the green light off when the lathe carriage is in the desired position. The sensor can be used as a mechanical stop, but not under power. It is somewhat sturdy, but probably not sturdy enough to withstand the lathe carriage running into it under power. (That’s why the light for this mode is red.) The stop is not intended to monitor the position of the cross slide, so be careful.

 

Running (autostop mode only):

With the sensor adjusted, make sure that the red light on the sensor (motor latched off) is out, and the green light is on. Adjust the lathe tool to the desired position and turn on the lathe. Adjust the motor speed for slow. Engage the half nuts for threading. When the carriage has advanced enough, the sensor is triggered. The yellow light and the red light both go on and the motor stops. Disengage the half nuts and move the tool back to the starting position. Once the carriage is moved, the yellow light goes out, but the red light stays on. The motor stays off.

Once the tool is at the starting position, press the reset button. The motor will restart. Since this is in series with the motor switch, it is just as easy on the motor as the forward/reverse switch.

 

Theory:

The sensor uses a microswitch to determine the carriage position. Switch status is indicated by the green or yellow LED’s. These are high brightness LED’s to be easily visible under bright shop lights.

The latching mechanism could have been a relay with a third pole holding power on to itself. However, all the RS relays that had heavy duty contacts were DPDT, which meant one less set of contacts than needed. The solution could have been a set of digital electronics, but a simple SCR works as well. An SCR is a diode that is off until the current into the gate lead exceeds a trigger value. The SCR then goes from off to completely on, and must have power removed from it to be reset.

  

The red pushbutton shorts out the SCR, and that effectively removes power. The two diodes might be puzzling at first. The one going from the gate of the SCR back into the sensor box is used to allow the green LED to turn off. The voltage on the SCR gate is either (in this circuit) 0 volts or about .6 volts. For the green LED to be turned off, it needs to be disconnected. Without the diode, the current going through the green LED would either go to ground through the switch or through the gate of the SCR. The SCR would turn on, but the LED would not go out. The diode prevents that.

Now the diode in the cathode lead of the SCR might be a bit more puzzling. Here’s the problem it solves.

The diode going to the green LED drops about .6 volts or so itself. So there is no difference to the gate of the SCR if the switch is open or closed. The current through the 1000 ohm resistor always goes through the gate. We need a way to detour that through the switch.

By putting a forward biased diode in series with the cathode of the SCR, we raise the gate voltage about .6 volts again. So the voltage at the gate is either about 1.2 with the switch open, or .6 with the switch closed. That effectively grounds the gate lead, so there’s no current flowing in through it, and the SCR remains off.

Note on the sensor head:

Because the power to the relay is routed through the power switch, but that’s all, the red light on the sensor head will indicate that the motor has been turned off regardless of the operating mode.

To make certain that a wire has not been inadvertently disconnected, press the red reset button with the lathe running. The lathe should stop. If the lathe does not stop, then the sensor system will not turn off the lathe.