HRP Gears - Model Engineers Digital Workshop

There is no intention here to give a detailed account of milling single gear teeth in conjunction with a dividing head. The writer firmly believes that no engine builder should mind having specialized operations carried out for him, where the type of work demands it. It is appreciated that some modellers will get great pleasure from machining their own gears, and certainly the machining of the spur gears is practical enough, provided one has the necessary range of cutters, however, a very positive dividing head is also required.

Bevel gears are rather a problem to produce without the correct equipment, and it cannot be too strongly emphasised that it is best to have these machined for you by specialists; since due to the tapered teeth, and the angles involved, it is not a practical proposition to produce them by milling. Probably the nearest approach to correct form is obtained by milling each side of the tooth separately, and the cut, overlapping in the middle. However, even this cannot produce the diminishing section of tooth which is produced by a bevel gear shaper.

The gears encountered in the engines are of two forms, one being Circular Pitch, and the other Diametrical Pitch gearing. The Circular Pitch is used on all of the differential bevel gears of my engines, and the Diametrical Pitch on all of the spur gears. When talking about these pitches, if a gear is stated as having £" circular pitch, then this is the exact dimension of the pitch measured at the pitch line. If a gear is 10 D.P., it means that a gear having a pitch diameter of 1" will have 10 teeth on it. To calculate the outside diameter of this gear, it is usual, in gearcutting language, to add on two teeth

i.e.: 2 = 1" = .2" TO~D.P. T~

therefore this 10 toothed 10 D.P. gear will have an O.D. of 1.2". Thus the 25 toothed second shaft pinion for the 3" Burrell would be 2.5" pitch diameter plus .2", giving an outside diameter of 2.7". with gear details, as they affect the modeller preparing his turned blanks, prior to gear cutting. The gear blanks should be first and foremost, concentric; so as to be sure of this, the bore and O.D. should be machined at one setting, wherever possible. The centre boss should then be faced and also the edge of the gear itself. It will now be possible for the gearcutter to locate your gear true on his machine, whether it be gear hobber or shaper. The concentricity of the bore and O.D. is important for the following reasons:-normally on the larger gears it will be impractical for the gearcutter to locate your work accurately on the bore. It is usual practice to lock the gear blank down on the necessary support, this often being a large piece of steel tube of a size to well support the blank, while leaving sufficient clearance for the cutter. Then, with the gear blank partially clamped, the machinist will use a dial gauge mounted on the machine, and clock the periphery of the gear blank true, before machining the teeth. From this description of procedure, you will realise that the teeth are cut true to the periphery. Often smaller gears (pinions) will be machined on a gearcutter which has a permanently true mandrel, and locates the blanks accurately on the bore, and in this instance, the resultant gear teeth will automatically be true with this bore, regardless of the periphery.

A word on bores:-If a machinist is to turn out accurate gears locating on a bore, it follows that the mandrel carrying the blank must be on size or not more than .0005" undersize, to readily accommodate reamed bores. Your gears, especially those which are to be fitted and keyed onto a shaft must be a good fit, however, do not leave these bores too much undersize or the gearcutter may charge you for reaming them out. BROACHING. Where crankshaft gears are submitted for broaching (as in the case of 3" Burrell and 4£" Tractor), the bores should never be undersize, since if they are, they will need to be machined to correct size before broaching. The broaches used for producing internal splines must not bottom on the bores of your blanks, otherwise breakage could occur. Therefore, to avoid extra machining charges, bores for broaching should not be undersize, aim at making them on size to size +,001".

GEAR TOOTH CLEARANCE On one or two occasions blanks have been submitted for gearcutting, and have been undersize on the outside diameters, the modeller having it in mind that this would allow for clearance. Therefore a description of how working clearance is achieved is called for. All gear O.D.'s should be machined to the size shown on your drawings, since this, in conjunction with the design of the gear teeth, will allow clearance for the top of the teeth against the bottom of the mating recesses in the other gear. In fact this clearance is considerable. By studying the drawing (No. 24), you will see two gears which are to run at an exact, fixed centre distance. These two gears are taken from the 3" Burrell, as being a very practical example. They have 17 teeth and 79 teeth respectively, and are 10 diametrical pitch. Remembering the explanation of D.P. at the beginning of the chapter, we will see that the pitch diameter of the 17 toothed gear will be 1.7" and the pitch diameter of the 79 toothed gear will be J9_ = 7.9". Their collective pitch diameters will

therefore be 9.6" and their working centres, hah* of this (4.8").

All gear teeth have a nominal calculated thickness on the pitch line, this with the teeth fully meshing and no clearance, when the gears are at their exact working centres. This nominal thickness (technically known as chorda! thickness), can be calculated; however, the gearcutter is saved this trouble as he uses a gear vernier to measure the teeth, and works from tables supplied by the manufacturer of this instrument. (See drawing No. 25.)

It will be appreciated by the modeller, on referring to the drawing of a gear tooth, that the way to achieve working clearance, is to cut each gear tooth slightly under nominal chordal thickness. With 10 D.P. gears as an example, having a chordal thickness of .15686" for the 17 tooth gear and .15707" for the 79 tooth gear (we will consider the thickness for practical purposes as .157"). With this dimension in mind it will be usual to make the final chordal thickness of the teeth .157" minus .004" to .005" for clearance, thus collectively over the two gears, one would achieve a clearance of .008" to .010". This practice is the one used for fixed centres, whereas if centres were adjustable (such as in the gear train of a lathe) the teeth could be cut at .157" with no consideration for clearance.

NOTE:-A correction is necessary in gear calculations since the calculated chordal thickness is achieved by reference to the pitch circle, whereas the gear vernier calculations are taken across the gear tooth in a straight line. Gear tables include a correction for this, but we will not concern ourselves with it here. (See drawing No. 25.)

The general turning of the gears has been fairly well dealt with. Undoubtedly the more difficult one will be the final drive (BURRELL), and due to its large diameter a good sized lathe wiU be needed. Turning is straightforward enough, but carded out in either low speed back gear, or at a higher speed, using a tipped tool wherever possible. Sometimes one sees differential housings machined badly out of centre. Always commence, with castings of this nature, by machining the face of the centre boss to the correct length (i.e.: half the length of bore) measuring from the centre of the cast wall; checking at the pinion locations. The machining of the locations for the pinions of the differential are more difficult on limited equipment, so this operation is described in detail. Firstly, the bores to locate the pinions must be diametrically opposite, otherwise it will be quite impossible for the mating gears to engage correctly. Study the drawing No. 26 and proceed as follows:-With a sharp pointed grooving tool, pick up the edge of the centre boss of the casting, and arrange it so that the top slide feed dial is at zero, in this position. Then retract the tool to clear the rim of the casting, return the tool by advancing the slide back to the zero position, and then advance the slide the correct amount to bring the tool to the centre line of the pinion locating pin. (This will be half the length of the centre boss.) It will then be an easy matter to wind in and machine a shallow groove to give you the pinion centres along the boss. Again employing a sharp 'V tool, but now turned on its side (see drawing No. 26). Mark the two intersecting lines to the groove already machined, these will be the pinion centres. To do this, it is best first of all to chalk the rim and mark what would appear to be the centre of the pinion bosses, this by eye only, thus we can engrave the first intersection from one of these marks, and then the second by utilising one of the gears on the headstock of the lathe. A suitable piece of strip metal will be used to locate in the chosen gear and against a convenient shoulder on the head-stock casting. All that is required is a gear with an even number of teeth to give two points at diametrically opposite positions. Following this marking out, carefully centre-pop the intersection of the lines, and then drill a small pilot hole, making sure to keep the drill square and not to let it wander. (A small high speed drill is best for this purpose.) If, for any reason, the hole has wandered, then correct it by taking a slightly larger drill, and drilling at an angle to pull the pilot hole over. Finally clean it out with a slightly larger drill and satisfy yourself that this is now exactly central over the guide lines. When the two pilot holes are satisfactorily drilled in for a matter of about " to 3/16", put the casting up in the lathe. Employing a centre drill hi the chuck, and working between tile taUstock centre, open these holes up with your centre drill. This may appear impossible, due to the overhang of the flange of the casting involved, but a simple way out of this is to use a smaller Jacobs chuck, held in the main lathe three jaw chuck. The pin locations are now correctly positioned, and are ready for stage two.

It will now be necessary to drill and ream the bores for the two pinions. Proceed as follows:-With the final drive casting held between the tailstock centre and a centre held in the lathe chuck (see note below) arrange the necessary packing to fit between the centre boss of the casting, and your top slide (in the case of a big lathe), or the saddle with the top slide removed (in the case of a smaller lathe). Then with this packing carefully in place, and the casting held truly between the centres, tighten the clamping nut, after which withdraw the tailstock, and exchange the centre in the chuck for a two flute cutter or slot drill of suitable size. Then advance the work towards the cutter, and machine a true and central location for the reaming drill to follow, afterwards drilling right through the boss, or as shown on your drawing, and finally reaming. The same procedure will be adopted to machine the second bore, again machining a centre in the lathe (see note below), carefully locating the casting between the centres, clamping down, drilling and reaming. (See drawing No. 26.)

In the final drive of the early design of Burrell agricultural engines, the centre hub of the differential carried holes into which the differential locking pin could be pushed, when it was required to lock the differential. Although unlikely to be needed on a model, to be truly authentic, the model should have this extra differential locking pin which is used in place of the standard length driving pin. In use it was pushed right through the hub and the outer differential gear and passed right into the differential centre boss, thus locking it solid.

NOTE:-Locating centre:-Grip a piece of steel rod tightly in the chuck, and machine a standard 60 degree centre to hold the casting. (This method represents the only absolutely true centre you will probably ever have in a lathe, unless it is new and unused.)

The above method will produce bores exactly opposite. However, the only likely point of error, is the packing height of the casting, and an easy method of checking this, is to turn the casting over, checking that the fit to the packing is the same one side as to the other. This checking will then ensure that your bores will be exactly half way along the centre boss.

Should it not be possible to groove and divide, or mark out, your final drive casting on the lathe (due to its small size), it is probably still possible to carry out the second operation. Therefore, a further method is described.

This time it will be necessary to scribe a line equivalent to the machined groove, previously mentioned. Then chalk the boss centres and mark approximate opposites using a rule, after which, centre-pop and drill a pilot hole carefully on the first intersection. Referring to drawing No. 26, take a half circular piece of wire with a hook on one end (this has been bent to follow the curve of the gear itself). It is easiest now to clamp the gear in the vice, and carefully locate the hook in the pilot hole, pressing the wire close into the drum, then carefully scribing a mark at the end of this wire, then reverse it and mark from the other side. It is likely that you now have two scribed lines, or with luck only one if the wire was cut to exact length. If the wire is much too long, i.e.: beyond the centre, trim it to length and repeat the procedure described above, when it will be easy enough to centre-pop between the marks, followed up by using a pilot drill. The accuracy of the position of this second hole can be checked by trimming the wire so that it will exactly coincide with the edge of the hole, and then when it is reversed to the other side, it should take the same position. With care you now have two exactly opposite holes, and you can proceed to machine the pinion locations (already described).

The writer is well aware that, to a modeller with access to sophisticated equipment, these methods may seem perhaps slow, but it is surprising how quickly results can be obtained, and with limits of accuracy well within those required for the job.

To finish this casting, it will now be necessary to face or mill the inside locations for the pinions. The easiest method is by using a vertical milling machine, but for the less well equipped, it is quite practical to make a pilot facing tool, which will cut on both sides of the boss faces. To do this, take a piece of rod of the same size as your pinion bores. Drill it to take a short length of square tool bit, which can be pressed through a slightly undersized round hole, so that the corners will just cut grooves to locate in. Then, with the tool ground up as in the drawing No. 27, and the casting located and clamped as previously described (during boring) it is an easy matter to machine both faces, by movement of the saddle. The cutter should run slowly, and needs to be double sided, so as to machine the faces flat since the cutter will have a certain amount of movement in it. A more permanent version of this tool can be made, by carefully filing out a square hole in the pilot shaft; but it is hardly called for if only needed to machine four faces.

The boss of the BEVEL GEAR WITH BOSS both for the FOWLER and the BURRELL engines, should be machined to just short of the length of the bore of the differential centre in which it fits (or runs). The inner faces of the differential wheels should bear against the face of the differential housing centre in the finished assembly and the bevel gears should be machined to allow clearance in this position.

Some of your gears will need keyways; these can often be shaped in the lathe quite satisfactorily. Have a good stout tool holder, with a tool bit (like a parting tool) ground to dead size. Advance the cutter by the hand wheel, taking about .002" cuts at a time. Fowler crankshaft gears, and those of the Wallis & Steevens Simplicity roller, have four keyways; these can be machined as above, but utilising a suitable head-stock gear and a locating strip to carry out the accurate division.