Part 9 (1/2)
[Illustration: Fig 5 Thread is for a Number of Successive Cuts]
When the tool is withdrawn at the end of the first cut, if the carriage is disengaged from the lead-screw and returned by hand, the tool ain engaged with the lead-screw If the nu cut is a e can be returned by hand and engaged with the lead-screw at random and the tool will follow the first cut For example, if the lead-screw has six threads per inch, and 6, 12, 18 or any nu cut that is a ed at any tiinal cut This is not the case, however, when the nu cut is not a multiple of the nu the carriage back to the starting point, when cutting threads which are notthe overhead driving belts) in order to bring the tool back to the starting point without disengaging the carriage; in this way the tool is kept in the saed froood th of say two or three inches; but when they are longer, and especially when the diae (which means a slower speed), it is rather slow as considerable ti point This is due to the fact that the carriage is ed, it can be traversed quickly by turning handle _d_, Fig 2
A e by hand when the nu cut is not a multiple of the number on the lead-screw is as follows: The tool is ht end of the work and the carriage or split nut is engaged with the lead-screw The lathe is then turned forward by hand to take up any lostthe position of the carriage The positions of the spindle and lead-screw are alsoa tooth on both the spindle and lead-screw gears, which happens to be opposite a corner or other point on the bed After a cut is taken, the carriage is returned by hand to the original starting point as shown by the line on the bed, and is again engaged when the chalk inal position; the tool will then follow the first cut If the body of the tailstock isthe first cut, the carriage can be located for each following cut by ainst the tailstock, and it will not be necessary to have a line on the bed
[Illustration: Fig 6 Indicator used when Cutting Threads]
=Indicator or Chasing Dial for Catching Threads=--On so threads,” as this is called in shop language This is a siraduated dial _D_ and a heel _W_ (see Figs 2 and 6) which meshes with the lead-screw, so that the dial is revolved by the lead-screhen the carriage is stationary, and when the carriage is moved by the screw, the dial re the carriage when one of the graduation lines is opposite the arrow e is returned by hand and when one of the graduation lines again moves opposite the arrow, the half-nuts are thrown into mesh, as before, and this is repeated for each successive cut, thus causing the tool to always coht with the thread If the nuement can be made when any line is opposite the arrow, but for odd nu or nu cut is a ement can be made at any time, as previously mentioned
=Principle of the Thread Indicator=--The principle upon which the thread indicator operates is as follows: The number of teeth in heel _W_ is some multiple of the number of threads per inch of the lead-screw, and the number of teeth in the heel, divided by the pitch of the screw, equals the nuraduations on the dial For example, if the lead-screw has six threads per inch, the heel could have twenty-four teeth, in which case the dial would have four divisions, each representing an inch of carriage travel, and by sub-dividing the dial into eighths (as shown) each line would correspond to 1/2 inch of travel The dial, therefore, would enable the carriage to be engaged with the lead-screw at points equal to a travel of one-half inch To illustrate the advantage of this suppose ten threads per inch are being cut and (with the lathe stationary) the carriage is disengaged and moved 1/6 inch or one thread on the lead-screw; the tool point will also have roove in the work as the pitch is 1/10 inch If the carriage is moved another thread on the lead-screw, or 2/6 inch, the tool will still be out of line with the thread on the work, but when it has moved three threads, or 1/2 inch, the tool will then coincide with the original cut because it has passed over exactly five threads This would be true for any number of threads per inch that is divisible by 2 If the thread being cut had nine threads per inch or any other odd number, the tool would only coincide with the thread at points 1 inch apart Therefore, the carriage can only be engaged when one of the four graduations representing an inch of travel is opposite the arrohen cutting odd threads; whereas even nuht lines
This indicator can also be used for ”catching” fractional threads As an illustration, suppose 11-1/2 threads per inch are to be cut, and the carriage is engaged for the first cut when graduation line 1 is opposite the arrow; engagement would then be made for each successive cut, when either line 1 or 3 were opposite the arrow, or in other words at spaces equal to a carriagefractional threads is liable to result in error, it is better to keep the half-nuts in engage the lathe
=Replacing Sharpened Thread Tool=--If it is necessary to sharpen the thread tool before the thread is finished, it should be reset square with the work by testing with the thread gage as at _B_, Fig 1 The carriage is then engaged with the lead-screw and the lathe is turned forward to bring the tool opposite the partly finished thread and also to take up any backlash or lost ears or half-nut If the tool-point is not in line with the thread groove previously cut, it can be shi+fted sidewise by feeding the compound rest _E_ in or out, provided the latter is set in an angular position as shown in the plan view, Fig
2
If the thread tool is ground flat on the top as at _A_, Fig 4, it is not a good tool for rees has any slope In order to give each cutting edge a backward slope, it would be necessary to grind the top surface hollow or concave, which would be impracticable When a course thread is to be cut, a tool shaped as at _B_ can be used to advantage for rough turning the thread groove, which is afterward finished to the correct depth and angle by tool _A_ This roughing tool is ground with a backward slope froer
=Use of Co=--Another forood for cutting V-threads especially of coarse pitch When this tool is used, the corees, as shown, and it is fed in for the successive cuts by handle _w_ in the direction indicated by the arrow
It will be seen that the point a of the tool rees with the axis of the work, thus fore _a--b_, which can be set as shown at _B_, fore is given a backward slope, as shown, it cuts easily and enables threading operations to be performed quickly Threads cut in this way are often finished by taking a light cut with a regular thread tool The cutting edge _a--b_ is ground to an angle of 60 degrees (or slightly less, if anything) with the side, as shown by sketch _A_
When cutting threads in steel or wrought iron, some sort of lubricant is usually applied to the tool to preserve the cutting end and give a smooth finish to the thread Lard oil or a mixture of equal parts of lard oil and paraffin oil are often used for this purpose If the thread is small, the lubricantcoe threads, it is better to have a strea upon the tool-point This constant flowto the tool, on a bracket at the rear of the carriage
[Illustration: Fig 7 Cutting Thread by using Co 8 (A) V-thread (B) U S Standard Thread (C) Square Thread (D) Left-hand Thread (E) Double Square Thread (F) Triple Square Thread]
=Threads Commonly Used=--Three forms of threads or screhich are in co 8; these are the V-thread (_A_), the U S
standard (_B_), and the square thread (_C_) The shapes of these threads are shown by the sectioned parts The V-thread has straight sides which incline at an angle of 60 degrees with each other and at the sale with the axis of the screw The U S standard thread is similar to the V-thread except that the top of the thread and bottoroove is left flat, as shown, and the width of these flats is made equal to 1/8 of the pitch The square thread is square in section, the width _a_, depth _b_ and space _c_ being all equal All of these threads are right-hand, which ht so that a nut will have to be turned toward the right to enter it on the thread A left-hand thread winds in the other direction, as shown at _D_, and a nut is screwed on by turning it to the left
=Multiple Threads=--Threads, in addition to being right-and left-handed, are single, as at _A_, _B_, _C_ and _D_, double, as at _E_, and triple, as at _F_, and for certain purposes quadruple threads or those of a higher multiple are ele thread in that it has two grooves, starting diarooves cut as shown at _F_
The object of these multiple threads is to obtain an increase in lead without weakening the screw For example, the threads shown at _C_ and _E_ have the same pitch _p_ but the lead _l_ of the double-threaded screw is twice that of the one with a single thread so that a nut would advance twice as far in one revolution, which is often a very desirable feature To obtain the sale thread, the pitch would have to be double, thus giving a much coarser thread, which would weaken the screw, unless its diameter were increased (The lead is the distance _l_ that one thread advances in a single turn, or the distance that a nut would advance in one turn, and it should not be confused with the pitch _p_, which is the distance between the centers of adjacent threads Obviously the lead and pitch of a single thread are the sa a U S Standard Thread=--Thea U S
standard thread is the sa the lathe is concerned The thread tool must correspond, of course, to the shape of a U S standard thread This tool is first ground to an angle of 60 degrees, as it would be for cutting a V-thread, and then the point is9 As will be recalled, the width of this flat should be equal to 1/8 of the pitch By using a gage like the one shown at _G_, the tool can easily be ground for any pitch, as the notches around the periphery of the gage are marked for different pitches and the tool-point is fitted into the notch corresponding to the pitch wanted If such a gage is not available, the width of the flat at the point can be tested by using, as a gage, a U
S standard tap of the sa the thread, the tool is set square with the blank, and a nu fed in until the width w of the flat at the top of the thread is equal to the width at the bottoht size provided the outside diameter _D_ is correct and the tool is of the correct form As it would be difficult to measure the width of this flat accurately, the thread can be tested by screwing a standard nut over it if a standard thread is being cut If it is being fitted to a tapped hole, the tap itself is a very convenient gage to use, theto caliper the tap and then co 9 U S Standard Thread, Thread Tool, and Gage]
A good iven size is as follows: First turn the outside of the blank accurately to diameter _D_, and then turn a small part of the end to dia cut for the thread is then taken with the tool point set to just graze diameter _r_ If ordinary calipers were set to diaroove, the size would be incorrect owing to the angularity of the groove, which le whenTo determine the root diameter divide 1299 by the number of threads per inch and subtract the quotient fro this rule as a formula,
/1299 _r_ = _D_ - ( ----- ) _N_ /
in which _D_ equals outside diameter; _N_, the number of threads per inch; and _r_, the root diameter The number 1299 is a constant that is always used