Part 11 (2/2)
=Change Gears for Metric Pitches=--When screws are cut in accordance with the ive the lead of the thread in millimeters, instead of the nuears for cutting lish lead-screw, first deteriven lead in millimeters
Suppose a thread of 3 lish lead-screw and a screw constant of 6 As there are 254 millimeters per inch, the number of threads per inch will equal 254 3 Place the screw constant as the numerator, and the number of threads per inch to be cut as the denominator:
6 254 6 3 ------- = 6 ---- = ----- 254 3 254 ---- 3
The numerator and denoear ratio are next ears The first whole nuet a whole nuear having 127 teeth is always used when cutting ear required in this case has 90 teeth Thus:
6 3 5 90 --------- = --- 254 5 127
Therefore, the following rule can be used to find the change gears for cutting lish lead-screw:
_Rule--Place the lathe screw constant multiplied by the lead of the required thread in millimeters multiplied by 5, as the numerator of the fraction, and 127 as the denominator The product of the numbers in the nuear, and 127 is the nuear_
If the lathe has a iven number of threads per inch is to be cut, first find the ”metric screw constant” of the lathe or the lead of thread in ears of equal size on the lead-screw and spindle stud; then the ears is si a lish lead-screw
_Rule--To find the change gears for cutting English threads with a metric lead-screw, place 127 in the numerator and the threads per inch to be cut, multiplied by the metric screw constant multiplied by 5, in the denoear and the product of the numbers in the denoear_
=Quick Change-gear Type of Lathe=--A type of lathe that is30 This is known as the quick change-gear type, because it has a systeears and replace the threads of various pitches Changes of feed are also obtained by the sa e by the rod _R_, whereas the screw _S_{1}_ is used for screw cutting As previously explained, the idea of using the screw exclusively for threading is to prevent it fro worn excessively, as it would be if continually used in place of rod _R_, for feeding the carriage when turning
[Illustration: Fig 30 Lathe having Quick Change-gear Mechanis 31 End and Side Views of Quick Change-gear Mechanisearthreads of different pitch, will be explained in connection with Figs 30, 31 and 32, which areparts Referring to Fig
30, the ear _s_ on the spindle stud through idler gear _I_, which can be ears _a_, _b_ or _c_, Fig 31 This cone of three gears engages gears _d_, _e_ and _f_, any one of which can be locked with shaft _T_ (Fig 32) by changing the position of knob _K_ On shaft _T_ there is a gear _S_ which can beto the spline or key _t_, both the sliding gear and shaft rotate together Shaft _T_, carrying gears _d_, _e_ and _f_ and the sliding gear _S_, is mounted in a yoke _Y_, which can be turned about shaft _N_, thus ear _S_ into31 The shaft on which the eight gears are ear _n_ on the feed-rod, and the latter, in turn, drives the lead-screw, unless gear _o_ is shi+fted to the right out of engage threads
[Illustration: Fig 32 Sectional Views of Quick Change-gear Mechanises for different threads or feeds are obtained by siears in cone _C_ As the speed of shaft _T_ depends on which of the three gears _d_, _e_ and _f_ are locked to it, the eight changes are tripled by changing the position of knob _K_, ear _I_, three speed changes ether, so that the twenty-four changes are also tripled, giving a total of seventy-two variations without reear _s_ were placed on the spindle stud, an entirely different range could be obtained, but such a change would rarely be necessary As shown in Fig 30, there are eight hardened steel buttons _B_, or one for each gear of the cone _C_, placed at different heights in the casing When lever _L_ is shi+fted sidewise to change the position of sliding gear _S_, it is lowered onto one of these buttons (which enters a pocket on the under side) and in this way gear _S_ is brought into proper ear of the cone _C_ To shi+ft lever _L_, the handle is pulled outward against the tension of spring _r_ (Fig 32), which disengages latch _l_ and enables the lever to be lifted clear of the button; yoke _Y_ is then raised or lowered, as the case ear is shi+fted laterally to the required position
[Illustration: Fig 33 Index Plate showing Position of Control Levers for Cutting Threads of Different Pitch]
The position of lever _L_ and knob _K_ for cutting threads of different pitches is shown by an index plate or table attached to the lathe and arranged as shown in Fig 33 The upper section _a_ of this table shows the different nuear _I_ is in the position shown by the diagraear is ives the changes for position _C_ of the idler
The horizontal row of figures froht positions for lever _L_, which has a plate _p_ (Fig 30) just beneath it with corresponding numbers, and the column to the left shohether knob _K_ should be out, in a central position, or in
In order to find what the position of lever _L_ and knob _K_ should be for cutting any given number of threads to the inch, find what ”stop”
number is directly above the number of threads to be cut, which will indicate the location of lever _L_, and also what position should be occupied by knob _K_, as shown in the colueared for cutting eight threads to the inch
By referring to section a we see that lever _L_ should be in position 4 and knob _K_ in the center, provided the idler gear _I_ were in position _A_, as it would be ordinarily, because all standard numbers of threads per inch (U S standard) fro 4 inches in diaear in that position As another illustration, suppose ant to cut twenty-eight threads per inch This is listed in section _c_, which shows that lever _L_ must be placed in position 3 with knob _K_ pushed in and the idler gear shi+fted to the left as at _C_
The si operation of rera nineteen threads per inch A 20-tooth gear is placed on the spindle stud (in place of the regular one having 16 teeth) and one with 95 teeth on the lead-screw, thus driving the latter direct as with ordinary change gears Of course it will be understood that the arrangeear mechanism varies somewhat on lathes of different make
CHAPTER V
TURRET LATHE PRACTICE
Turret lathes are adapted for turning duplicate parts in quantity The characteristic feature of a turret lathe is the turret which is e and contains the tools which are successively brought into the working position by indexing or rotating the turret In many instances, all the tools required can be held in the turret, although it is often necessary to use other tools, held on a cross-slide, for cutting off the finished part, facing a radial surface, knurling, or for some other operation After a turret lathe is equipped with the tools needed fora certain part, it produces the finished workan ordinary engine lathe, principally because each tool is carefully set for turning or boring to whatever size is required and the turretposition Turret lathes also have syste the travel of the turret carriage and cross-slide, in order to regulate the depth of a bored hole, the length of a cylindrical part or its dia machines of this type areduplicate parts, unless the quantity is sht be much more than offset by the cost of the special tool equip up” theMachine”)
[Illustration: Fig 1 Bardons & Oliver Turret Lathe of Motor-driven Geared-head Type]