HOW SPECIAL CAMSHAFTS ARE MADE.
HOW SPECIAL CAMSHAFTS ARE MADE.
By F. T. BERSEY, M.I.A.E. (Managing Director of the Laysta11 Motor Engineering Works, Ltd.)
IF the modern sporting motorist were to be asked -11what supreme quality he would desire for his engine he would probably reply, “Why, all the revs. in the world, of course,” and more than likely he would supplement this modest wish by a request for a few more revs. still.
*In the continual search for increased engine efficiency the sporting car owner is ever on the alert to discover fresh improvements, and will maintain the desirability for increased compression ratios, the more accurate balancing of reciprocating and rotary components, and in some cases such parts as pistons and connecting rods are so greatly lightened that the machinist expert trembles for the owner when the car is first submitted to its road tests.
The methods whereby improvements are effected in any of the above-mentioned directions are more or less known to the technically minded motorist, who, indeed, may be equally familiar with special features relating to camshaft designs, but immediately this class of work is contemplated the owner is confronted with serious practical difficulties.
While it is common knowledge that most racing cracks and successful competitors in hill-climbing events use non-standard camshafts which give widely differing valve timing, the exact process by which the alterations are made is more or less shrouded in obscurity.
At the request of the Editor of THE BROOICLANDS GAZETTE the writer, having had long and somewhat unique experience in this class of automobile engineering, now has pleasure in lifting the veil to explain the entire process.
The First Steps to Improved Timing.
At the very outset it must be stated that the exact setting adopted by various racing men is the result of long and expensive experimental work, and is, therefore, guarded most jealously ; but, without betraying any confidence, the general lines upon which the improvements are obtained may be disclosed. For example, if a man comes to the Laystall Works, he may either give very definite instructions as to the camshaft setting he requires, or, on the other hand, he
may merely ask for a new setting, giving so much extra lift, with rough indications as to other alterations considered to be beneficial.
By the aid of modern machinery and a close theoretical study of this particular problem, one can usually meet the requirements ; but, obviously, when no definite data are supplied, the customer must be prepared to take some degree of risk as to the actual improvements effected.
Making the Cam Model.
In cases where the re-grinding of the cams is merely intended to eliminate wear or ridging on the working surface, the process of correction is very simple, for the re-grinding does not, as some people imagine, have any effect whatever, either on the normal setting or the existing lift of the valves, provided no alterations have ,been made to the design of tappets. This is done on a special machine, to be described later in connection with camshaft alterations. Fig. illustrates a standard cam setting, as well as the modified setting to give extra power, and the way in which the
work is carried out is explained as follows. By the aid of the fixture shown in Fig. 2 the original setting is noted down, the camshaft being mounted between the centres as shown, and the opening and clos:ng position for both inlet and exhaust valves is checked with the aid of a special recording instrument, which is very sensitive. Supposing the inlet opens at 50 after top centre, this instrument shows the exact position, which is noted on the large graduated disc shown on the lefthand side, other characteristics of opening and closing being noted in a similar way.
Take the case of the tabulated settings shown on p. 351. It may be possible to reduce the cam contour by a grinding process, without any need for a new camshaft, though with very wide variations the latter course is desirable, and in such a case the first part of the process consists in making a camshaft model, largely by hand. By successively removing metal from the cam contours the desired opening and closing is eventually obtained, and, needless to say, this part of the operation calls for the greatest accuracy.
SPECIAL CAMSHAFTS ARE
The cam model is first ground on the circular part of the contour by means of an extremely ingenious workshop method, then the pre-determined amount of lift is gained, also by machinery ; thus only the sides of the cam require treatment by hand.
In making the cam model it is only necessary to evolve the contours of one inlet cam and one exhaust cam, which means that the cost of the hand work is kept down to a minimum, this work being entrusted to an engineer whose skill almost amounts to legerdemain.
Forming the Camshaft from the Model.
The next part of the process of camshaft construction is equally mysterious to those unacquainted with the technical details. In a few words, what happens is this :—The model camshaft is formally introduced to the cam copying machine, which immediately sets about to cut two large formers, of which the contours are an exact reproduction of the cam model. As with all other parts of the process, save that of shaping the sides of the two cams on the model, the possible element of human error is absolutely eliminated. For the benefit of those unversed in the arts and mysteries of
modern machine shop practice, the details of this peculiar process may be described as follows :— First of all the cam model is mounted between the centres of the copying machine (shown in Fig. 3), and a large circular disc is brought into contact with the contour of the first cam. As the machine is set in motion the holder for the disc moves transversely to and fro. By the aid of cutting tools mounted on the moving holder two large metallic discs are cut to the exact contours of the cam model so as to form master cams, and when the second cam is copied in the same way the first part of the process is terminated. In the meantime a blank camshaft with circular collars, representing the embryo camshaft, has been prepared on another machine, in this case an ordinary centre lathe or automatic lathe, according to whether the order is for a single camshaft or a quantity. Following the turning operation, the new camshaft is milled roughly to shape in readiness for the second process on the cam-copying machine. For this purpose a change is made on the moving holder, a grinding wheel taking
the place of the rolling disc, the latter being so mounted as to follow the contour of the newly made master cams. On the machine being set in motion the camshaft is ground by treating each cam separately until the shaft is nearing completion, when the hardening of the cams is undertaken.
Hardening the Camshaft.
All new camshafts are made from a carefully selected ease-hardening chrome-nickel steel, the hardening process being effected by placing the shaft in a steel box which is packed with a chemical compound. The whole is then placed in a muffle, the heat of which is controlled by a pyrometer, the effect being that the surface of the shaft absorbs a high carbon content, and, when cooled out, presents a glass hard surface with excellent wearing qualities.
Ensuring Final Accuracy.
If the hardening is carried out carefully the shaft will not suffer distortion ; but if any slight distortion should take place it can be corrected by the use of a special appliance designed for the purpose, and then
the final accuracy of the cam contours is ensured by further grinding on the cam-copying machine. Thus. in addition to the high degree of accuracy inseparable from such a system of manufacture, well made special— or even ” freak ” camshafts, if you like—can be relied upon as possessing good wearing qualities.
Recent experience with the modern high speed high efficiency engine has had the effect of throwing much interesting light on camshaft design, and while one could continue discussing so fascinating a subject almost indefinitely, the writer must bring the article to an end with a few general remarks.
At one time the cry was for quick lift cams, then some wanted slow opening and quick closing ; but the general tendency is in the direction of slow lift cams both for inlet and exhaust valves, and mushroom tappets in place of those of the roller variety, which cannot possibly follow any fast moving cam without risk of valve dither.
HOW SPECIAL CAMSHAFTS ARE
A Camshaft Danger.
When members of the public see a car suddenly burst into flames at Brooklands, the cause of the outbreak is usually attributed to a choked jet, a stuck valve, or similar occurrence, but, as a matter .of fact, racing drivers have cut down valve-timing to such a degree that the safety line is often overstepped.
A 5o to r chance of a fire is long odds compared with other risks forming part and parcel of the racer’s calling, but the public should know of this, so that their ideas on the safety of ordinary touring models need not be disturbed.
.’ABLE No. I.
COMPARATIVE VALVE SETTINGS ON A HIGH EFFICIENCY
ENGINE. Before Alteration of Camshaft.
Inlet valve opens to° after top centre.
Inlet valve closes 15° after bottom centre.
Exhaust valve opens 300 before bottom centre.
Exhaust valve closes 50 after top centre.
After Alteration of Camshaft.
Inlet opens ••• … 5° before top centre.
Inlet closes … 450 after bottom centre.
Exhaust opens … … 45° before bottom centre.
Exhaust closes … … 8° after top_centre. LI FT Alik I0Pc4 LI FT
Dotted lines show cam before alteration. Full lines show cam after alteration with
period increased by 200 and lift by 1 us m. 2 m mit 11,1 Shape of cam after-alteration to retain original opening and closing, but giving
increase of 2 mm.
SPECIAL VALVE SETTINGS. TABLE No. 2.
Inlet valve closes
Inlet valve opens 300 before top centre.
60° after bottom centre. 65° before bottom centre. Exhaust valve opens 350 after top centre. Exhaust valve closes
9 mm. lift on all valves.
TABLE No. 3..
Inlet valve opens
Inlet valve closes 50° after bottom centre.
to° before top centre.
450 before bottom centre. Exhaust valve opens 200 after top centre. Exhaust valve closes
The two extreme timings both give equally good results on engines of similar capacity.