This section describes screw positioning errors, the common cause for troubles in automating screw fastening.

(1) Screw positioning errors and causes for the troubles

・While causes for screw positioning errors are related to characteristics of both screw shapes and workpiece onto which the screws are fastened, factors on the automation equipment are more influential. Fig.1 shows factors for screw positioning errors.

・Many causes for the troubles are related to the quality of workpiece onto which the screws are fastened.Secondary factors for these troubles are possibly related to inappropriate clamping of the screws.Important issues, thus, to be addressed on the automation equipment side are countermeasures for (1) screw positional deviation, (2) vibrational shift, (3) inappropriate clamping of the screws.Items (1) and (3) of the three failures above are also related to countermeasures for screw feeding errors, as shown in the previous section.

(2) Relationship between screw positioning errors and feeding conditions/clamp mechanisms

・In order to properly position a screw, the screw must be oriented and positioned correctly in the screw chuck section after the screw is transported via an air chute, etc.If the orientation or position of screw is wrong, positioning error will probably occur.

・In order to ensure proper positioning in the screw chuck section, the following three requirements must be met: a) orientation of screw during transport is correct, b) screw is stopped and stable after transport, c) screw chucking is reliable after screw is stopped and stable.

・To ensure proper positioning in the screw chuck section the following three chucking functions are required:

・The following process can be referenced as the procedure for designing the chucking mechanism:

・When the spring force is used for the stopper mechanism for screw in transport, or for clamping mechanism for screw, the conditions (transporting speed of screw, etc.) and mechanism must be adjusted to maintain the following spring force:

・Positional deviation caused by vibrations also needs some countermeasures.Selecting a parts feeder or alignment unit, and optimizing the feeding speed, etc. are issues to be addressed as they are related also to the noise generated by the automation device.

(3)Significance in process design for screw fastening process

・The following chart shows a typical process design.Determination process of "Positioning check" is set as an independent process, since this process is necessary for ensuring the reliability of positioning and the stable control of screw fastening torque.