Control Device for Controlling the Rotational Speed of a Portable Power Tool

The present invention relates to portable power tools and in particular to a control device for automatically controlling the speed of rotation of the electric motor of a portable power tool as a function of the degree to which the chuck of the tool is opened to accommodate the tool bit.

It is well known in the art that the cutting speed of the tool bit of a portable power tool, as for example the drill bit of an electric drill, is a function of both the rotational speed of the motor and the diameter of the tool bit. In particular, it will readily be appreciated that for a fixed motor speed, the cutting speed of a drill bit will increase proportionately with increases in the diameter of the drill bit used. Accordingly, prior art electric drills are known to include motor speed control knobs which can be manually adjusted by an operator to set the speed of the motor to the recommended speed for a given bit diameter as provided on a reference table. With prior art electric drills, therefore, it is incumbent upon the operator to remember to adjust the speed of the motor each time the size of the drill bit is changed. Failure to correctly set the speed of the motor can cause less than optimum operation and a consequent premature wearing of the tool bit.

Accordingly, it is the primary object of the present invention to provide a control device for a portable power tool, such as an electric drill, that will automatically adjust the speed of the motor in accordance with changes in the diameter of the tool bit used.

In addition, it is a further object of the present invention to provide a control device for a portable power tool, such as an electric drill, which also includes means for permitting an operator to initially preset the speed of the motor to an optimum reference speed for a particular type of material so that subsequent motor speed changes caused by changes in bit size are made relative to the preset reference speed.

FIG. 1 is a diagrammatical elevational view partially in section of an electric drill incorporating the speed control device of the present invention; and

FIG. 2 is a circuit diagram of the speed control device of the present invention.

FIG. 3 is a circuit diagram of the speed control device of the present invention incorporating alternative potentiometer circuit.

FIG. 4 is a plan view of a portable electric drill equipped with the speed control device according to the invention.

FIG. 5 is an elevation view of the portable electric drill of FIG. 4.

Referring to FIG. 1, an electric drill 10 incorporating the speed control device of the present invention is shown. The electric drill 10 is otherwise conventional in construction and includes a keyless chuck 12 which is rotatably driven through a gear assembly 17 by an electric motor 15. The drill chuck 12 can be of the kind known as a keyless Validus chuck and includes a plurality of jaws 14 which are opened and closed under the control of a ring 16 for securing a drill bit in the chuck. Ring 16 acts upon jaws 14 via a thrust plate 18 which moves fore and aft with adjustments in ring 16 to open and close jaws 14 as is conventional. The position of the thrust plate 18 is therefore indicative of the size of the opening defined by the jaws 14.

In accordance with the present invention, the shaft 13 of chuck 12 has an axial bore formed therein to accommodate a sensor rod 20 whose forward end is maintained in constant contact with thrust plate 18 under the bias of a spring 22. Sensor rod 20 is mechanically coupled via element 24 to a connecting rod 26 which is in turn connected to the wiper arm 28 of a potentiometer circuit 30. The output of the potentiometer circuit 30 is provided to a speed control circuit 32 which controls the amount of current supplied to the motor 15 in accordance with the setting of the potentiometer circuit 30. Thus, it can be seen that as the chuck jaws 14 are opened and closed to accommodate various sized drill bits, the position of the wiper arm 28 of potentiometer circuit 30 is adjusted via the mechanical connection through sensor rod 20, element 24 and connecting rod 26, thereby varying accordingly the speed of the motor 15.

In addition, the preferred embodiment of the present invention includes a multiple-position speed select switch SW1 for manually selecting an optimum speed range for a given type of material.

Turning now to FIG. 2, a circuit diagram of the speed control circuit of the present invention is shown. The speed of the motor 15 is controlled by controlling the amount of current supplied to the motor. This is accomplished in a conventional manner by controlling the firing angle of a semiconductor switching device, herein a thyristor 34, which is connected in series with the motor 15. The firing angle of thyristor 34 is controlled by varying the magnitude of the voltage provided on line 38 to the gate of thyristor 34, which is in turn determined by the setting of the potentiometer circuit 30. In the preferred embodiment, the potentiometer circuit 30 comprises four parallel connected variable resistors 36a-36d which have their wiper arms mechanically tied together and their wiper terminals tied in common to line 38. One side of each of the variable resistors 36a-36d is tied in common to one side of the voltage source and the other side of each of the variable resistors 36a-36d is connected through another resistor, 35a-35c respectively, to one of the select terminals of the speed select switch SW1, except for variable resistor 36d which is connected directly to switch SW1 as shown. The movable contact terminal of select switch SW1 is connected to the other side of the voltage source. Thus, it can be seen that the setting of select switch SW1 determines which series connected resistor pair--i.e., resistor 35a and potentiometer 36a, resistor 35b and potentiometer 36b resistor 35c and potentiometer 36c, or potentiometer 36d--will be switched in circuit to control the voltage signal on line 38. Since the values of resistors 35a-35c are unequal, it will be appreciated that the voltage range of the signal on line 38 can be altered by changing the setting of select switch SW1. In other words, the range over which the speed of the motor will vary pursuant to changes in the position of the chuck jaws is determined by the setting of speed select switch SW1.

Accordingly, in practice an operator will initially set the position of select switch SW1 in accordance with the type of work material. This serves to establish the appropriate optimum speed range for that particular material, given the range of bit sizes which can be acommodated by the drill. Thereafter, as the position of the chuck jaws is changed to accommodate various sized drill bits, the resistance values of the potentiometers 36a--36d will change accordingly as previously described, to thereby automatically adjust the speed of the motor 15 relative to the speed range selected. In the embodiment illustrated, four different speed ranges are provided. However, additional speed selections for accommodating a wider variety of materials can readily be added to the control circuit if desired.

According to another embodiment of the invention, the potentiometer circuit can be configured as shown in FIG. 3 wherein only one potentiometer 36 is required. Thus, when the position of the chuck jaws are changed, the resistance value of potentiometer 36 will change accordingly.

Referring now to FIG. 4, a portable electric drill 10 is equipped with a housing 42 and a keyless chuck 12. The top portion of the housing is cut away to show the speed-control circuit 32 mounted on a printed-circuit board 46. The printed-circuit board 46 is mounted in the upper part of the housing 42 as shown in close proximity of the connecting rod 26 slideably mounted in the housing. The connecting rod 26 is connected to the sensor rod 20 as shown in FIG. 1.

The potentiometer 36 is mounted on the board 46 and includes resistance element 50 and a wiper arm 28 attached to the connecting rod 26. By adjusting the chuck 12 to accommodate a drill bit of a size selected by the operator, the rod 26 is moved in a direction along its longitudinal axis as previously described so that the voltage taken off of the wiper arm is indicative of the drill size.

The switch SW1 of FIGS. 2 and 3 is also mounted on board 46 as are other components of the speed-control circuit.

While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the accompanying claims.