Serration characteristic of shaving cutters

Transcription

1 Serration characteristic o shaving cutters The cutting edges o the shaving cutters are perormed along the lank o every tooth through a slotting process which uses tools as per type o igure N 1. Fig.N 1- Example o a serration slotting tool used on serrating machine The results is that each lank o the cutter tooth is grooved by serrations with rectilinear sides, whose intersection corners with tooth surace generate the cutting edge (ig. N 2). Fig.N 2- Indication o the caracteristic sizes o a shaving cutter serrations

2 Serrations are characterised by the ollowing parameters: P c = serrations pitch P p = solid serration P v = serration space P r = serration depth The serration pitch and the pattern splitting between solid and space, mainly depends on module and also on the stock removal. Data normally used are indicated in table 1: Tab. N 1 Serrations pitch in unction o the module Normal module m n Serrations pitch P c Notes Up to 1,50 From 1,80 to 2,00 Provided module is the same,i stock removal increases, From 1,50 to 2,75 From 1,90 to 2,19 From 2,75 onward From 2,00 to 2,20 serrations pitch will be bigger.e.g. i you pass rom a stock removal o 0.04 to one o 0.08 mm per lank,serrations pitch increases o ca. 10% The width o solid serration P p and the space width P v are normally the ollowing: Tab. N 2 Serrations sizing Serrations pitch P c Width o solid Serration P p Serration space P v 1,80 1,05 0,75 1,85 1,10 0,75 1,90 1,15 0,75 1,95 1,15 0,80 2,00 1,10 (or 1,20) 0,90 (or 0,80) 2,05 1,10 0,85 2,10 1,10 0,90 2,15 1,20 0,95 2,20 1,20 1,00 As ar as the choice o P p value (the width o solid serration), it is necessary to make an important consideration. The sum o the serrations width o the teeth that are in contact with the gear determine the surace along which the orce that pushes the cutter against the gear is distributed. The wider the serrations, the wider the contact area and the more pressure (i.e. the ration between orce and contact area will be smaller. I we reduce the pressure (i.e. orce per mm 2 ), then the cutting edges have diiculties in penetrating the steel and in orming chips. Then, in those cases that create diiculties in steel removing, or in the cases where a gear made o high resistance material, it is important to reduce somehow the P p value. When you choose a slightly lower value o P p you also have the advantage to have a wider space, to enable it to contain chippings and to improve the lubricant action. Nevertheless, serrations must not be too small running the risk to make serrations teeth too week and subject to a premature breaking. The serrations depth value P r depends on module and on pressure angle. Values most commonly used are those listed in Table 6 Tab. N 6 Serrations Depth

3 Normal Pressure Angle Up to rom to rom onward Serrations Depth Min = 0,20 + 0,22 m n Max = 0,20 + 0,28 m n Min = 0,20 + 0,28 m n Max = 0,20 + 0,32 m n Min = 0,20 + 0,25 m n Max = 0,20 + 0,28 m n It is important to speciy that the serrations have not a constant depth all along the tooth height. This peculiarity does not come rom a wrong manuacturing o the serration, but rom the act the every serrations root is rectilinear, i.e. it does not ollow the tooth involute proile outer surace. Practically all slotting machines that perorm serrations give the tool a rectilinear movement, because i you wanted to ollow the involute proile, the machine kinematism would be greatly complicated without granting the tool a real advantage in perormance. As you can see in igure no.23, serration depth is normally bigger in the central / lower area o the tooth. This is because once you resharpen, and reduce outside diameter accordingly, then you remove the portion o tooth where serration depth in narrower. Thanks to such trick you increase the number o possible resharpenings and, meantime, you manuacture a more resistant tool close to outside diameter area. Fig.N 3- The bottom o every serration is rectilinear, thereore serrations depth is not constant The position o serrations on the shaving cutter tooth also depends on the shaving method, i.e. on the way the cutter moves with reerence to the workpiece. During parallel and diagonal shaving the cutter moves axially with reerence to the gear, and thereore the cutting edges (the serrations corners ) move along the gear tooth lank in such a way to avoid that the same serration tooth never all in the same position o the gear tooth lank. During underpass and plunge shaving you do not have a longitudinal movement o he cutter with reerence to the gear; in this case i you had the serrations teeth as per igure no.13a, then every serration tooth would all in the same position during every part revolution, with the result that cutter tooth serration would leave markings on the gear tooth lank; in other words serrations teeth would remain "imprinted" along gear teeth lank. It is then necessary to oset the serrations o one tooth with reerence to the those o the next gear tooth, i.e. to perorm an helical slotting as in igure N 4(b)(c).

4 Fig.N 4- Serration oset in conventional shaving cutter (a) and in underpass + plunge cutters (b) The serrations teeth oset o one tooth with reerence to the next one is indicated with S in igure N 4(c). There is an important relationship between serrations teeth oset S, Serrations pitch P c and the gear number o teeth. It is in act necessary that on one tooth trace on a generic gear tooth (e.g. tooth Z 1 ) another tooth trace is overimposed, slightly oset, ater on gear revolution (see igure N 5) Fig.N 5- Serrations traces coming one ater the other on a gear tooth must overlap The trace o one serration on tooth Z 1 will be in a determined position, while the trace o the serration o the same row on the next tooth o the cutter on tooth Z 2 will be shited o the value S, on teeth Z 3 will be shited o 2 S,and when the gear has completed the tour returning on tooth Z 1 the serration tooth trace will have been shited o Z S. In this interval, anyway, you can it in many pitches P c, e.g. n pitches, and so, in order to determine the value o P i you can use the ollowing relationship: P i = Z S - n P c where: n P c Z S (n + 1) P c

5 The most commonly used value or P i is mm. In this way you guarantee an overimposing o traces o: S p = 0,8-1 mm. The value you have ound or P i is only temporary,because you still have to establish how many starts should the whole group o serrations be made o. I you consider the sequence o the serrations, i you ollow the progression o the single shits you will see that they ollow an helix along the cylinder ormed by the shaving cutter and the pitch o this helix will be p = S Z1 as you can see in igure N 6. Fig.N 6- Oset serration are placed along an helix. The no. o starts must be an integer number In this interval, you can ind many serrations and everyone corresponds to an helix belonging to a dierent start. The number o starts must be an integer number. The ollowing condition must be satisied: S Z1 = P c (integer number) You will then choose as the integer number closest to the ratio recalculate the value S Z 1 S Z1 P c and the Pc = veriying (just to be sure) that the P i value is acceptable. We now slightly touch the subject o serrations helix direction. As you understand rom the a.m. expression you need to choose son parameters in such a way that the number o start is an integer number and the overimposition o the traces o the single teeth o the gear has values included in that interval. Sometimes this can be obtained with a serration helix having the desired direction, but other times you have to assign a negative value to S ; o course the serration helix will have a direction opposite to the desired one. In some instances this solution does not give good results and then it is necessary to reconsider the serration design, by varying the pitch P c and the value o P p and P v. The general consideration leading to a choice o serration helix direction are linked to the cutting direction o each cutting edge.

6 I you consider igure no.27a the cutting edges sliding direction is Right to Let i cross o axes angle is as per igure no.27a, conventionally indicated as positive ( + sign). I cross o axes angle has opposite direction, cutting direction will be the opposite. Fig.N 7- Oset direction must take into account the cutting direction (1) Gear (2) Shaving cutter (d t ) Cutting direction (x) Trace o a generic cutting edge (y) Trace o the ollowing cutting edge I you consider igure N 7a you see that, in case the shiting o the trace ater on revolution o the gear is positive (positive P i value), i.e. it alls into an area already worked by the previous cutting edge, during its path, inds a step which makes the entry o the corner onto the steel easier; cutting action will be easier and the surace inish will be good. On the contrary, i with the same cross o axes angle the shiting o the trace ater on revolution o the gear is negative (negative P i value), i.e. it alls into an area not worked by the previous cutting edge, the corner would not correctly bite the steel and quality o the surace will be more irregular. I the cross o axes o angles has a negative direction as in igure N 7b, the best cutter would be the one having the shiting arranged in such a way to have a negative P i value, while the tooth quality surace will be very low in case o positive P i value. We have already outlined that it is advisable to avoid, wherever possible, that serrations cross at the tooth tip, creating a situation like the one on igure N 8a. Just at the top o the tooth the solid part o the serration is ree, i.e. not supported by the internal core and thereore the serration itsel will be more subject to breaking. This inconvenient is particularly requent on shaving cutters with small module. This eature enables deeper serrations and, in practice, a higher number o resharpenings. This method normally used has proven so positive that, even i initially developed to solve the problem o the crossing o serration on tooth tip, it has been extended also go the cutter having a suicient top land.

7 Figure N 8 shows the various instances that might happen and the related standard pattern o serrations rom one lank to the other. Fig.N 8- Indication o teeth oset along the two lanks o the same tooth in dierent cases to reinorce teeth tip. Fig.N 9- Some examples o serration On cutter with module smaller than 1 mm, considering the extreme diiculty in slotting the serrations, sometimes the so called cut serrations are perormed:the principle consists in carrying out circular cut, parallel to cutter aces as indicated in igure N 10.

8 Fig.N 10- Cut serration or modules smaller than 1 mm These cuts leave serrations without the internal solid supporting portion and, thereore, are quote delicate, but unortunately there is n other way to create that type o serrations. These circular cuts can be perormed beore heat treatment with thin turning tools or, better, with thin CBN wheels ater heat treatment. With this latter system you could also obtain better cutting edges. It is obvious that this type o serrations can be used only with diagonal and parallel methods. The helical serration o underpass and plunge cutters have the teeth at the extremes with width varying rom 0 to P p size. The tiny teeth are dangerous because they can easily brake and sometimes the bits can all into both gear and cutter teeth during the rotation, running the risk to endanger proile and cutter itsel. Incomplete teeth are also dangerous rom the saety point o view because they can cause injures to the operators handling cutters. From the a.m. reasons it is necessary to get rid o the incomplete teeth rom both sides, as shown in igure N 11. Fig.N 11- Incomplete teeth are removed