Most sanders on the market appear to be based on two principles. First, the power device needed for its operation - in general an electric motor - is integrated into the sander. The integration of such a device with sensible power leads to a heavy sander which makes sanding for a long time exhausting, in particular when sanding overhead, e.g. when sanding ceilings.
Second, the sanding forces are not internally compensated. Rather, the low-pass behaviour of the sander mass and the hands of the user are required to attenuate the sanding vibration caused by the periodic sanding forces which may be proportional to the product of exciter frequency and amplitude. In the case of a band sander, the high sanding forces have to be compensated by the operator directly.
As far as exciter sanders are concerned, the sanding performance is achieved by rather high frequencies and velocities by small amplitudes. The effect is that on many surfaces, the sanding paper appears to hover on the surface to be sanded. The abrasive paper does not easily penetrate the surface due to the high velocities.
A concept is proposed which includes two alternative principles:
The sander is separated into two parts, the sanding device proper, and an actuator/vacuum device. Both are connected by a multi-purpose hose serving as power, control and dust transmission, as shown in the picture below.
The objective is to keep the sanding device as light as possible, yielding a tool which allows to sand in any position without great physical effort. The energy needed is provided by the actuator and transmitted through the connecting hose. In addition, a vacuum device built into the power/actuator device removes the generated dust. Controls on the sanding device transmit a control signal to the actuator, such as on/off and the sanding speed.
In order to keep the sanding effort to a minimum for the user, the forces generated by the sander have to be self-compensating. This is achieved by a counter-movement as shown in the animated picture below.
In theory, two sanding pads moving in opposite direction would result in two cancelling forces, but with a remaining torque. Four sanding pads will also cancel out the torque, yielding zero force and zero torque. The only force a user has to put in is a slight push against the object to be sanded. The vacuum device the sanding device is connected to will help in this respect as the vacuum will try to glue the sanding device to the object.
Based on the principle above, the sanding amplitude can be higher as for conventional sanders, because sanding forces are compensated to be zero external forces. This leads to a sanding behaviour as when sanding by hand: sand uniformously in one direction with moderate velocity. Da abrasive paper will not hover over the surface to be sanded, but acts as it is supposed to do with a given sanding paper grade.
The sanding pad is composed of housing and its internal mechanics as described below.
The housing is a light plastic structure in two half-shells, in the shape of a trowel, with a good, rubber-coated grip. It allows to be held with one or two hands. The housing provides a connector for the vacuum hose. Air is sucked in between the sanding pads and around.
As mentioned above, four sanding pads are required to compensate all sanding forces. Levers mounted on a frame, which like all rigid parts can be made in Aluminium, guide these pads. Flexibility in the joints is provided either by some brass bearings, or rubber. This needs to be investigated further, as the bearings must resist against the abrasive sanding dust.
An important subject in the design is the width of the sanding pads. It is proposed to use 1/4 of the standard 115mm sanding sheets. The sanding paper will stick to the pads by using Velcro style fastening. During production, conventional 115mm sanding sheets can be easily cut into four strips.
The life span of a sander is among other factors determined by the life span of any bearings used. Abrasive residues enter end destroy bearings eventually. So the best design would avoid bearings at all. For this reason, The moving parts are suspended by springs.
Similar to brakes in a bicycle, two steel ropes are used to move the sanding pads, one for a forward movement, and the second for the counter-movement. The following drawing should be self-explaining.
Permanent magnets are mounted on the sanding pads. Stator windings mounted on the sanding pad frame will generate Electro-magnetic forces. The current of the coils is controlled from the actuator/vacuum device, all together similar to the principle used in permanent magnet servomotors and drives. The additional weight has to be investigated.
The actuator/power side will comprise an actuator depending on the sanding actuator used, a vacuum device, similar to the ones used in conventional vacuum cleaners, and a compartment for vacuum cleaner bags. The design must be simple and robust, and standard vaccum cleaner bags should be used.
If the sanding pad is based on two steel ropes, the actuator could be either a conventional motor (combined with the vaccum motor) with a piston rod, or, alternatively, a separate Electro-magnetic actuator. In the case of an elector-magnetic sanding pad, only a power stack and a simple control board have to be developed.