Turning and Boring / A specialized treatise for machinists, students in the industrial and engineering schools, and apprentices, on turning and boring methods, including modern practice with engine lathes, turret lathes, vertical and horizontal boring machines

by Franklin Day Jones

Fig. 12. Views illustrating how a Double Square Thread is Cut

Cutting Multiple Threads.—When a multiple thread is to be cut, such as a double or triple thread, the lathe is geared with reference to the number of single threads to the inch. For example, the lead of the double thread, shown at B, [Fig. 12], is one-half inch, or twice the pitch, and the number of single threads to the inch equals 1 ÷ ¹/₂ = 2. Therefore, the lathe is geared for cutting two threads per inch. The first cut is taken just as though a single thread were being cut, leaving the work as shown at A. When this cut is finished the work is turned one-half a revolution (for a double thread) without disturbing the position of the lead-screw or carriage, which brings the tool midway between the grooves of the single thread as indicated by dotted lines. The second groove is then cut, producing a double thread as shown at B. In the case of a triple thread, the work would be indexed one-third of a revolution after turning the first groove, and then another third revolution to locate the tool for cutting the last groove. Similarly, for a quadruple thread, it would be turned one-quarter revolution after cutting each successive groove or thread.

There are different methods of indexing the work when cutting multiple threads, in order to locate the tool in the proper position for cutting another thread groove. Some machinists, when cutting a double thread, simply remove the work from the lathe and turn it one-half a revolution by placing the tail of the driving dog in the opposite slot of the faceplate. This is a very simple method, but if the slots are not directly opposite or 180 degrees apart, the last thread will not be central with the first. Another and better method is to disengage the idler gear from the gear on the stud, turn the spindle and work one-half, or one-third, of a revolution, as the case might be, and then connect the gears. For example, if the stud gear had 96 teeth, the tooth meshing with the idler gear would be marked with chalk, the gears disengaged, and the spindle turned until the chalked tooth had made the required part of a revolution, which could be determined by counting the teeth. When this method is used, the number of teeth in the stud gear must be evenly divisible by two if a double thread is being cut, or by three for a triple thread, etc. If the stud is not geared to the spindle so that each makes the same number of revolutions, the ratio of the gearing must be considered.

Setting Tool When Cutting Multiple Threads.—Another method, which can sometimes be used for setting the tool after cutting the first groove of a multiple thread, is to disengage the lock-nuts from the lead-screw (while the spindle is stationary) and move the carriage back whatever distance is required to locate the tool in the proper position for taking the second cut. Evidently this distance must not only locate the tool in the right place, but be such that the lock-nuts can be re-engaged with the lead-screw. Beginning with a simple illustration, suppose a double thread is being cut having a lead of 1 inch. After the first thread groove is cut, the tool can be set in a central position for taking the second cut, by simply moving the carriage back ¹/₂ inch (one-half the lead), or ¹/₂ inch plus the lead or any multiple of the lead. If the length of the threaded part were 5 inches, the tool would be moved back far enough to clear the end of the work, or say ¹/₂ + 5 = 5¹/₂ inches. In order to disengage the lock-nuts and re-engage them after moving the carriage 5¹/₂ inches (or any distance equal, in this case, to one-half plus a whole number), the lead-screw must have an even number of threads per inch.

Assume that a double thread is being cut having 1¹/₄ single threads per inch. The lead then would equal 1 ÷ 1¹/₄ = 0.8 inch, and if the carriage is moved back 0.8 ÷ 2 = 0.4 inch, the tool will be properly located for the second cut; but the lock-nuts could not be re-engaged unless the lead-screw had ten threads per inch, which is finer than the pitch found on the lead-screws of ordinary engine lathes. However, if the movement were 0.4 + 0.8 × 2 = 2 inches, the lock-nuts could be re-engaged regardless of the number of threads per inch on the lead-screw. The rule then, is as follows:

Divide the lead of the thread by 2 for a double thread, 3 for a triple thread, 4 for a quadruple thread, etc., thus obtaining the pitch; then add the pitch to any multiple of the lead, which will give a movement, in inches, that will enable the lock-nuts to be re-engaged with the lead-screw.

Whenever the number obtained by this rule is a whole number, obviously, the movement can be obtained with a lead-screw of any pitch. If the number is fractional, the number of threads per inch on the lead-screw must be divisible by the denominator of the fraction.

To illustrate the application of the foregoing rule, suppose a quadruple thread is to be cut having 1¹/₂ single threads per inch (which would be the number the lathe would be geared to cut). Then the lead of the thread = 1 ÷ 1¹/₂ = 0.6666 inch and the pitch = 0.6666 ÷ 4 = 0.1666 inch; adding the pitch to twice the lead we have 0.1666 + 2 × 0.6666 = 1.499 inch. Hence, if the carriage is moved 1¹/₂ inch (which will require a lead-screw having an even number of threads per inch), the tool will be located accurately enough for practical purposes. When the tool is set in this way, if it does not clear the end of the part being threaded, the lathe can be turned backward to place the tool in the proper position.