Last update December 25th. 2014

Watchmaker's Lathe Motor & Speed Control
 

May 5th. 2006

 
  
Electronic Motor Speed Control
  
Watchmaker's lathe requires motive power just like any other lathe, although driving power is in proportion to its size. 

Watchmaker's lathes are usually powered by small mains operated universal motors, rated from 40W to 200W which are often exceeding 5000 RPM. Speed of rotation is in most cases controlled with a foot operated rheostat, and in some cases the reduction of revolutions per minute is achieved with an arrangement of pulleys driving belts using device known as watchmaker's countershaft. 

The latter arrangement is used to reduce number of revolutions and preserve the motor torque, needed for normal lathe work.

Instead of the elaborate arrangement of pulleys and belts I have decided to have motor speed controlled electronically. To achieve it without buying an expensive device I chose a German made module which needs only a few components to get it to work. 

The finished motor speed controller is capable of working at 110 through to 240 VAC driving a motor rated:

 600VA -  max current 6A @  110VAC 
1400VA - max current 6A @  230VAC

Maximal load can be achieved when the module is mounted on a heat sink. 

2000VA - max current 18A @  110VAC 
4000VA - max current 18A @  230VAC

Kemo Electronic GmbH - http://www.kemo-electronic.de/
 
Module MO28N is made in Germany, by Kemo. 
 
It is a phase control circuit. 
 
Electrical loads, such as electric motors with carbon brushes, heaters, solenoids, soldering irons, lamps etc. can be controlled with it.
Motor Speed Controller Module M028N
  
The M028N module comes with a potentiometer which when connected to the module controls the motor speed of rotation. Motor Speed Controller Module Pot
 
Here is a diagram showing how to connect the module. Motor Speed Controller diagram
 
This image shows module placed in the aluminium box with a potentiometer mounted on a panel with the on/off switch.
 
On the back side panel are mounted two mains sockets.
 
Motor Speed Controller inside
 
The mains inlet socket is provided with a fuse. Mains socket with fuse
 
Here it is shown how the fuse is accessed. Mains socket with fuse
 
The completed unit shown from the front side with Potentiometer and mains on/off switch.
 
The box I used to house motor speed controller is a standard Euro card housing, 
 
measuring: 
2-1/2"x4-1/2"x6-1/2"
Motor Speed Controller Front Psnel
 
Here is the unit with the back panel shown, mains inlet and outlet sockets.  Motor Speed Controller Back Psnel
  
One of my watchmaker's lathes mounted on a Borel aluminium stand with a sewing machine motor mounted and ready to be used with motor speed controller utilising Kemo phase control module.

Normally the sewing machine motor would get quite hot when running any prolonged period. However, when used with electronic motor speed controller the motor never gets hot, it doesn't even get warm.
Boley & Leinen in Borel mount
 
Typical Sewing Machine motor, rated 150W 0.8A @ 220 VAC. 
 
With no load it will spin at more than 5000 RPM therefore to use it to power watchmaker's lathe the speed must be reduced, often it needs to be manyfold.
Boley & Leinen in Borel mount
  
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    Universal Motors
Universal motor is a series-wound or a compensated series-wound motor designed to operate at approximately the same speed and output on either direct current or single-phase alternating current of a frequency of (not greater than) 60 Hz and of approxi­mately the same RMS voltage. 
  
Very popular applications for universal motors are portable drills, saws, routers, vacuum cleaners, sewing machines, food mixers, blenders, and many others.
   
It is to be noted that the speed drops off rapidly with an increase in load and increases with a decrease in load. 
    
This characteristic is most desirable in vacuum-cleaner service, for if the cleaner is used under conditions which decrease the volume of air han­dled the load on the motor decreases. This decrease in motor load is accompanied by increased motor speed and increased vacuum, so that the cleaner will actually handle more air than it would if a constant- speed motor were used. Likewise, this characteristic of speeding up on light loads is very desirable in the case of portable drills, for the motor will drive small drills at high speed and larger drills at a lower speed.  
  
At the very low speeds, as achieved with rheostat, torque falls drastically, often to a level that makes the motor unusable.   
  
Motor, with characteristic such as this is undesirable for driving the watchmakers lathe, hence we resort to the use of electronic control circuits to allow reduced speed with more or less unchanged torque, although unless very elaborate and expensive devices are used the success is only partial. 
  
  
  
  
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  Repulsion Motors
 
Repulsion motors are often used where speed adjustment is required, for their speed can easily be adjusted by varying the brush shift, giving them adjustable varying-speed characteristics. 
  
A repulsion motor is a single-phase motor, which has a stator winding arranged for connection to the source of power and a rotor winding connected to a commutator. Brushes on the commutator are short-circuited and are so placed that the magnetic axis of the rotor winding is inclined to the magnetic axis of the stator winding. This type of motor has a varying-speed characteristic.   
  
In case of the Multifix repulsion motor the speed is controlled from 0 to 5000 RPM with practically constant torque, as well as having the ability to change direction of rotation by the use of the same lever, which controls the motor speed.
 
These characteristics make repulsion motor the ideal choice of a motor driving watchmaker's lathe. Without use of additional electronic speed control circuits or the use of the  countershaft, still capable of delivering the full rated torque of the motor to the lathe at any desired speed from zero to maximum motor's RPM.
 
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 Multifix M80 - Repulsion induction motor
 
Picture to the right shows German made Multifix M80 mono-phase repulsion induction motor. 

M80 motor weighs 5.5 kg (~12 lb) and is rated 80 W @ 2800 RPM i.e. 1/10 HP, with full power of 110 W i.e. 3/20 HP at full speed of rotation, at either 110 or 220 VAC.

Both, speed and direction of rotation are controlled by a lever setting, CW or CCW at 0-5000 RPM.

 

 
I have acquired Multifix M80 repulsion induction motor as a second hand, previously used motor. 
   
Have I known earlier of its quality and performance I am sure that I would have bought it new, years ago.   
   
These motors are not cheap, they come with a price tag of over 500 euros, nevertheless they are worth every cent one pays for them.  
 
Another view of the same motor capable of running in either direction at the speed of 0 to 5000 RPM. 

Direction and speed are selectable by setting lever position, delivering almost full torque at any chosen speed. 

 

 
By moving the lever one actually changes the angular position of brushes related to the magnetic axes of rotation, thus changing the speed and direction of rotation. 

Setting the lever in middle position stops rotation of the motor.

Moving the lever to the opposite side changes direction of rotation.

   
View of the nameplate label giving details of the motor.      
 
Nice thing about the motor is that it is made in such a way that in order to work with 110 VAC mains, motor needs only a simple rewiring of the mains connection that is located at the bottom of the unit.    
   
Wiring diagram is included in a form of a sticker found on the inside of the bottom cover. 
 
  
One of the features of the motor's virtually constant torque over the whole speed range 0-5000 RPM allows motor use for polishing by mounting the buffing wheel on the motor shaft. 
   
Tapered shaft and buffing wheel can also be mounted on the opposite side as well. 
 
The image on the right gives us view of the receptacle used for mounting a flexible shaft.    
   
When polishing, flexible shaft allows us to reach into the tight places which normally are inaccessible when using just the buffing wheel.    
   
Note that there is a cross pin inserted through the motor shaft.
 
 
The two tapered shafts are not identical, one is left hand and the other is right hand threaded. 
  
The right hand tapered shaft has a slot which  allows mounting inside of the receptacle. 

On both sides of the motor shaft is 8.00 mm in diameter. 

 
    
   
Image shows tapered shaft mounted on the right hand side of the motor inside of the receptacle, also used for mounting of the flexible shaft. 
   

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 Multifix Record MR25 - Repulsion induction motor

 
The Multifix Record MR-25 is mono-phase repulsion induction motor. It looks identical to the M80, except that it is much larger, twice as heavy and is more than twice as powerful. 

MR25 weighs 11 kg (25 lb) and is rated 185 W @ 2800 RPM i.e. 1/4 HP, with full power of 300 W i.e. 2/5 HP at full speed of rotation.

Speed and direction of rotation are controlled by a lever, 5500-0-5500 RPM.

 
Motor nameplate label showing basic specs of the motor

On both sides of the motor shaft is 14.00 mm in diameter. 

 
Speed vs. Torque as function of the angular position of brushes.

Angular position of 0º - no rotation and no torque, 

 
The MR 25 motor Torque vs. Efficiency as function of the angular position of brushes.
  
My lathe bench with the Favorite II lathe and M80 with MR25 placed next to them.

The MR25 size becomes apparent only when placed next to the M80 which I use to drive my Favorite II lathe. 

I use the MR25 motor for polishing of watch cases, bracelets, and for whatever else is in need of polishing.

 
One of the smaller repulsion motors, the Multifix RE60 induction  repulsion motor, capable of running in either direction, having both, speed as well as direction of rotation lever controlled. 

Practically constant torque across whole speed range from 4800-0-4800 RPM. 

Continuos duty, rated power 60 W (1/12 hp) powered from mains, either 110 or 240 VAC.

Quite suitable for driving smaller watchmaker's lathes of the D-bed variety.  

   
   
   

  References: 

'Fractional-and Subfractional-Horsepower Electric Motors' - Cyril G. Veinott - McGraw Hill 1970

  

Multifix Record MR 25 Pamphlet page 1 
  Multifix Record MR 25 Pamphlet page 2 

Excerpt from Bergeon catalogue pages 5072 A & B

 
 
 

 

 

  

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