Plain Indexing and Cutting Spur Gear (1941)

gears provide one of the most common methods of transmitting power to meet different requirements Gears of different designs are produced by skilled Craftsmen in machine shops the simplest type of gear is the spur gear its teeth are parallel to its axis this type of gear is often cut on the milling machine the average blueprint of a gear usually contains terms peculiar to gear designing therefore an understanding of these terms is helpful to The Machinist the basic during all tooth dimensions on a Gear is an imaginary line called the pitch line it is on this line that the thickness of the tooth is always measured the distance from the pitch line to the face of the tooth is called the addendum the distance from the pitch line to the base of the tooth is called the dundum to provide sufficient clearance when the teeth are engaged the Dum must be greater than the addendum these are the terms applied to the parts of any single gear tooth in the gear as a whole the pitch lines of the teeth form a circle called the pitch Circle the number of teeth per inch of pitch diameter is known as the diametral pitch this can be simply Illustrated this gear has a pitch diameter of 3 Ines and it contains 12 teeth for every inch of pitch diameter there are four teeth therefore the diametral pitch of this gear is four or as more commonly expressed this is a four pitch gear this gear has the same pitch diameters but contains 15 teeth since it has five teeth for every inch of pitch diameter this is called a five pitch gear in other words the diametral pitch is always equal to the number of teeth divided by the pitch diameter or the number of teeth per inch of pitch diameter the selection of the proper gear blank stock is determined by the outside diameter 4 and 3 3/4 and the width 1 and 3/4 the full depth of tooth is to be 2695 th000 the depth of cut to be made the number of teeth 36 and the diameteral pitch 8 determine the selection of the proper cutter this can be learned by means of a handbook there are a number number of Cutters for each pitch depending on the number of teeth to be cut in this case 36 teeth are wanted so a number three cutter is indicated the cutter selected should always be carefully checked the specifications of every cutter are stamped into the metal to cut a gear on the milling machine a dividing head is used it consists of two parts the headstock and the tail stock both are located in the same table slot it is between these two centers that the work is held the tail stock can be adjusted at various Heights and angles for a straight cut as in Milling a spur gear the tail stock is set horizontal this can be done accurately by means of a calibrated scale on the rear of the tail stock to ensure a rigid cutter setup the table is moved as close as possible to the head of the machine the cutter is lined up with the approximate Center of the headstock after applying the key and the remaining collars and Arbor support bushing the operator rigidly secures the cutter setup while there is still sufficient clearance around the arbor the center of the headstock is lined up accurately with the center of the cutter the centers of cutter teeth are marked for this purpose the saddle is locked in this position next the table is lowered to provide room for setting up the work to hold the gear blank in place place a mandrel with an expanding sleeve is used the mandrel is tapered the operator centers the expanding sleeve in the gear blank inserts the mandrel and drives it solidly into place with a soft Hammer which should be either of lead or raw hiide before placing the mandrel between the centers of the dividing head the tail stock Center is lined up squarely with the headstock Center sufficient room is left between them to hold the mandrel before the tail stock is secured to the table then the centers are checked again with a surface gauge next the operator secures a dog to the mandrel the mandrel is held against the headstock by the tail stock the dog is held solidly in position by a set screw notice that the large end of the tapered mandrel is next to the headstock so that the thrust of the cutter will tend to tighten the gear blank on the mandrel the operator centers the work directly under the cutter the table is raised by hand until the cutter just scrapes the gear blank it is from this position that the full depth of the cut is calculated the index dial of the vertical feed screw is set at zero then the work is moved away from the cutter the second function of the dividing head is to accurately locate the desired number of cuts on the gear blank each full turn of the crank rotates the gear blank one 14th of a turn in other words there is a 40:1 ratio between the movement of the crank and the movement of the work suppose we had to cut four equally spaced slots in a disc 10 turns of the hand crank will turn the work 1/4 of a turn locating the second slot accurately 10 more turns will turn the work another quarter of the way around locating the third slot and 10 more turns will turn the work the last quarter of the way around locating the fourth slot the operating principles of the dividing head can be further simplified by imagining the hand crank to be centered directly in front of the work if 40 equally spaced Cuts were wanted each full turn of the crank would move the work 1 14th of a complete Revolution thus locating each of the 40 cuts if 20 Cuts were wanted two full turns of the crank would move the work 24ths or 1 12th of a complete Revolution locating each of the 20 Cuts if 10 cuts were wanted four full turns of the crank would move the work 44ths or 1/10th of a complete Revolution locating each of the 10 cuts 8 5 four or two cuts could be located just as easily because they too can be divided equally into 40 in other words dividing 40 by the number of Cuts required will give the number of turns of the crank necessary to locate each cut this establishes the formula 40 / n the number of cuts to be made equals T the required number of turns of the crank up till now we have only considered making Cuts in numbers that go into 40 evenly but this job calls for 36 teeth which means making 36 cuts from the formula 40 / n = t 40 / 36 = 1 and 1 nth turns after making the one full turn the problem is to make the additional one nth of a turn and make it exactly to do this it is necessary to divide the plate in back of the handle into nine equal parts turning the handle the distance of one of these divisions provides the additional 1 nth of a turn standard indexing plates are provided which make it possible to accurately locate whatever part of a full turn of the handle may be required each number on the plate indicates a circle consisting of that number of holes the crank can be adjusted so that the locating pin will enter the holes of any desired Circle because there is no nine hole circle on the plate the operator selects the smallest number that can be divided evenly by nine in this case 54 since 9 goes into 54 six times moving the pin six spaces on this circle turns the handle the required 1 nth of a turn to simplify the operation a sector is used as a gauge adjusting the sector to six spaces eliminates the necessity of counting out the number of spaces each time a cut is to be located since the distance between both Blades of the sector equals 1 nth of a turn moving the sector against the back of the pin automatically locates the starting point for the next 1 and 1 nth of a turn as the pin approach approaches the proper hole the handle should always be moved slowly if it is moved past the hole always move the handle back at least half a turn and then bring it up to position again slowly this will take up any play that may be present in the gears of the dividing head and prevent over indexing before the First Cut is taken the indexing calculations must be carefully checked for this purpose small Nicks about 1/4 in deep are made around the entire gear blank after the last Nick has been made the operator counts the number of Nicks and checks with a number of teeth called for in the blueprint 36 the head is indexed for another tooth then the cutter is brought up to the first Nick by hand to be sure it TRS up in this Nick this entire procedure Mak trial Nicks counting them and checking the location of the cutter in a Nick is one of the most important steps in cutting a gear with the dividing head correctly adjusted for locating each cut the depth of cut is next calculated the full depth of tooth is to be 2695 th000 since the gear must be finally finished two cuts are requ required the second one a finishing cut of 1116 therefore the first cut will be the full depth minus 1116 or 207,000 each division on the vertical index dial represents 1,000 of an inch so the operator raises the table 207 divisions for the roughing cut a conservative cutting speed for the cold rolled steel gear blank will be used 90 ft per minute at the teeth of the cutter since the cutter is 3 in in diameter its circumference is almost 8/10 of a foot therefore the cutter speed should be 115 RPM the nearest speed to that calculated is selected 116 RPM for the first cut a chip thickness of 2,000 can be used the cutter has 12 teeth and in one revolution will remove 24,000 of metal with a cutter turning at 116 RPM it will remove 2 and 784 th000 of metal per minute the operator select the nearest figure on the feed dial 2 and 3/4 in the work is further supported by a table Jack the first roughing cut is then made across the gear blank notice that a flow of coolant has been provided to lubricate the teeth of the cutter and to keep them cool when the first roughing cut is completed the operator stops the cutter with the table in this position an automatic table stop is located to prevent the indexing head from running into the cutter on the subsequent cuts another stop is also located to save time when the work is moved back to the starting position for each new cut now the operator is ready to make the balance of the roughing cut around the gear blank for for the 11/16th in finishing cut the table is raised an additional 62 and 1/2 divisions from the starting point of seven giving a final reading on the vertical index dial of 69 and12 since the depth of cut is small the speed is increased and the rate of feed is decreased to ensure a fine finish next two trial finishing cuts are made just deep enough to check the thickness of one tooth the thickness of the tooth is always measured on the pitch line the reading must correspond to the specifications called for on the blueprint the finishing Cuts can then be made around the entire gear it should be noted that the actual cutting of work on the milling machine is the smallest part of any job for if the proper cutter is selected a cutter stamped with the specifications called for on the blueprint if both the work and the cutter assembly are rigidly set up if the location of each cut has been acurately calculated with the dividing head if the proper speed and feed have been set and if the entire setup has been carefully checked before making a single cut then the last step actually producing the work called for is a relatively simple procedure and the result is a spur gear cut exactly to specifications a gear that will pass every test made to check it before it is put to work to perform its important function of transmitting power from one moving part to another