TESTING SCREWS The screws cut in engineers' shops are sufficiently true for all practical purposes. But the fact remains that no guide screw yet made is true, and no true screw can be made apart from the use of devices which are unknown in the machine shop. The microscope is brought into requisition for testing standard screws, but commercial screws simply have to pass the test of gauges. The microscope class of measuring machine makes use of cross hairs, adjusted to various portions of the screw, and a micrometer apparatus for fine reading of differences. At the National Physi cal laboratory, Teddington, England, there are fine machines for testing pitch errors by the system of magnification of the move ment of a style brought into contact with the threads of the screw undergoing examination. The magnification is 75o in one case. The results of errors in any screw sent for test are shown on a chart.
An accurate mode of testing effective screw diameters is by means of what is termed the "three-wire" method. Three hard steel wires lapped to a high state of accuracy are laid in the grooves of a screw, two on one side and one on the other, and measure ment is taken over the lot with a micrometer calliper, or in a measuring machine.
Screw thread gauges are considered in the article TooL, but a new kind of testing has arisen, which does not come under the head of a tool, since it depends on optical methods. Briefly, a magnified image of the screw thread, as much as fifty times larger, is thrown on a screen by a lantern. The screw rests in a cradle in front of the lens, and any number of screws can be tried in succession. First a master screw or gauge is put in and its shadow projected on to a tolerance chart on the screen some distance. When the cradle has been adjusted so that this master screw appears correctly on the chart, the screws to be tried will reveal, by their position on the chart, whether they are too large or too small, or of inaccurate pitch. Short framed lines indicate the boundaries of a good thread while a faulty shadow is seen to be undersize. Errors of pitch are revealed by the shadow stand ing too far to one side.
The problem of producing a true screw has occupied investiga tors since the days of Henry Maudslay (1771-1831). The great difficulty is that of attaining accurate pitch, so that the distances between all the threads shall be uniform, and consequently that a nut on the screw shall move equably during the rotation. The im
portance of this point is felt in the dividing engines of various classes employed for ruling, and in measuring machines used for testing standards of length.
There are a number of methods of correcting the errors in screws; the principal one is that of retarding or accelerating the traverse motion of the screw-cutting tool by means of a com pensating lever bearing on a compensating bar, which is formed after observations have been made on the degree of accuracy of the leading screw used to propel the tool carriage. The original errors in the leading screw are therefore eliminated as far as possi ble. The inspection of the screw is done by means of the micro scope working in conjunction with a line measure fastened down parallel with the axis of the screw, so that the coincidence or otherwise of the screw pitches with the subdivisions of the meas ure may be compared. ( J. G. Ho.) SCREWDRIVER, the tool which drives screws, must be made of good steel and suitably tempered so that it will not snap off through being too hard, or twist or bend through being too soft. The chief interest in this connection in manufacturing processes and repairs attaches to the mechanically operated screw drivers which vastly speed up the rate of removing or inserting screws. Some are driven by a flexible shaft actuated from a cord and pulley, the shaft being manoeuvred about anyhow to suit the situation of the screws. Or a pneumatic or electric drill is fitted with the blade, and held in the hands anywhere required.
Some small articles demanding the use of numerous screws are placed under a fixed screw-driving machine, the spindle of which carries the driver blade, pressed down by a lever. In wood work ing, screw-driving machines are largely used in mass production.
For hand use only the ratchet and the spiral types are very convenient. The former enables the grip to be maintained on the handle the whole of the time, while the screw is turned in either direction, leaving one hand free. The latter sends the screw home or runs it out on giving a straight push to the handle. Some drivers combine both actions for selection as desired.