Cams in good condition for early cammy Velos are hard to
come by. I had a number of decent K-17/2 cams (which superseded the early K-17
cam) that were reground by Newman Cams and used on both the AJcette and
the V-Twin (see here)
and I have one more of these but, ideally, I would like to use a K-17/5 cam.
There are 2 reasons for this: the later K-17/5 cam is considered superior in the
respect that it has a smaller base circle diameter and consequently a lower average
contact speed with the rocker skid – which is better for the overall mechanical
integrity of the cam/rocker combination and, secondly, and this is my own
opinion, it is much easier to work with in the respect that it has a threaded
portion that allows it to be pulled off the camshaft much more easily that the
K-17/2, which has no such threaded section.
A K-17/5, on the left, and K-17/2 cam are shown in the picture below for
comparison.
Just for the record, the K17/2 cam (which is often stamped
24C) needs a Sykes-Pickavant type bearing puller to get behind the cam to remove
it from the camshaft. This is what mine looks like.
They are pretty cheap and worth every penny to avoid damage to
the large K-12 bronze bearing,
I only have one K-17/5 cam, shown in a different camera angle below,
with the internally-threaded puller that I made when I took it off a camshaft
long ago.
This cam was reground some years ago by Newman and is in
excellent condition. However, I only have this one and decided that, as I would
like to fit K-17/5 cams on more than one of my engines, I would make another by
replicating this one. I have never made a cam before and this has proved to be
an enlightening experience.
The top-down approach that I adopted to copy this is:
1) Measure
the existing cam
2) Machine
a blank (ie all the “round bits” and the thread) on the lathe using O1 tool
steel
3) Machine
the inlet and exhaust cam profiles on the milling machine
4) Use
an external machine shop to put in the keyway for the woodruff key (I can’t do
this with my equipment)
5) Clean
up as necessary and heat treat.
Simple really.
However, the question then arises as to what exactly
to measure and how will it be machined?
The picture below shows a depiction of the cam positioned
under an end mill with the Z axis aligned with the vertical axis of the milling
cutter and the Y axis being the direction of a traverse of the table beneath
the cutter.
To machine the cam from a blank, ie a round section, the cam
is now rotated in the rotary indexer or dividing head by some angle, θ, the cutter is lowered by
an amount Z and the table of the milling machine traversed in the Y direction
so that the end of the cutter will remove material at the highest point and
which is tangential to the cam surface, shown below,
….and then continue to rotate the cam incrementally through
one revolution and remove material until the cam profile has been completely
machined – with the equipment that I have this is the only realistic way I can machine the
cam.
The key question is: what is the depth-of-cut for any given
value of θ?
The depth-of-cut does not have a simple relationship to the
cam profile, as will be discussed in a later blog. I do not have the cam lift profile so
it wouldn’t help anyway.
To machine a new cam by copying an existing one the
depth-of-cut needs to be measured for small increments in θ - I have chosen to measure
and subsequently machine the cam using 20 of rotation. Yes, this is
a very small increment but this is the level of definition that is needed to
capture accurately the periods of maximum acceleration (and deceleration) and to avoid significant "flats" on the nominally smooth contact surface of the cam.
To measure and locate the position of the tangent point I
made an electrical probe, shown below (together with a mandrel to hold the
cam), in which there is a 1/8” diameter hardened and tempered pointed silver
steel probe that is inserted into a ½” diameter shank and held in place with
epoxy resin so that it is electrically isolated from the shank.
The cam/mandrel/rotary indexer was then set up on-centre in
the milling machine and the probe connected to a multimeter such that an
electrical circuit is made when the probe point touches the cam. The multimeter
has a setting that provides both a measurement of resistance and sounds a
buzzer when contact is made.
I do not have a DRO to give a digital reading for the
vertical position of the probe and so I needed to rely on the micrometer reading
graduated in 0.001” and which I found measurements to be
very repeatable.
The process for deriving the depth-of-cut data is as
follows:
1) Rotate
the cam through an angle Δθ
2) Lower
the probe carefully and traverse the table in the Y direction until electrical
contact is made.
3) Note
the readings of the vertical position, Z, and the Y coordinate (referred to
below as the Yoffset )
4) Repeat steps 1- 3 for 3600 of cam rotation
As I mentioned above, I have chosen Δθ = 20 of
cam rotation to take measurements. The cam nearer the threaded end is the
exhaust cam and I positioned this so that 00 is at (approximately) peak lift and
then rotated clockwise to 2, 4, 6 degrees ….etc until the base circle is
reached. This was then repeated from the same start position but rotated in the
opposite direction (360, 358, 356, …etc) to complete the data collection for
the exhaust cam. Without repositioning the cam, the inlet cam was then measured
in the same way; it is very important that the same angular coordinate system
is used to maintain the overall relationship between the inlet and exhaust
cams.
One practical point on this measurement method: it is not
possible to detect one single, unique point of electrical contact for any given
position of the cam and, in practice, the buzzer will sound for 2 positions –
one approaching for increasing “Y” and the other when the table is traversed in
the opposite direction, ie decreasing “Y”.
The situation is shown below.
The 2 measurements were averaged to give a value of Yoffset
midway between the readings. I found that the vertical coordinate could be
repeatably measured to 0.0005” and the horizontal position to a couple of thou,
depending on the angular position of the cam.
It should be stressed that the Y measurement of the tangent
point is not required to machine the cam; it is only required to derive the cam
lift profile as described below.
Here, the following variables are defined:
Θ the prescribed angle of rotation of the rotary indexer
Z the measured depth-of-cut
Yoffset the measured offset of the location of
the tangent point
Φ the angle between the Z axis and the location of the
tangent point
L the cam lift at rotation angle Θ
Rbase the radius of the base circle
Llift,max the maximum cam lift
The following relationships allow the cam profile, ie the
relationship between the cam lift and the angle of rotation, to be determined:
tan Φ = Yoffset / (Llift,max + Rbase
- Z)
from which
Φ = tan-1 (Yoffset / (Llift,max
+ Rbase - Z))
The angle Φ can also be defined in terms of the cam lift, L,
as:
cos Φ = (Llift,max + Rbase - Z) / (L
+ Rbase)
giving the cam lift:
L = (Llift,max + Rbase - Z) / cos Φ -
Rbase
And so, after many hours of taking measurements, I have
ended up with the following information, which has been input and plotted from
an Excel spreadsheet.
The raw data from the exhaust cam, the first one measured,
looks like this.
I have indicated the locations of the exhaust opening and
closing and the direction of rotation of the cam in the engine – the same
angular coordinate system applies to all of the following pictures.
The data from the inlet cam is shown below.
The derived cam lift profiles are shown below.
Now, I realize that this is not the traditional way in which
cam profiles are shown but this is what you get when the angular coordinate system
is referenced to the position of maximum exhaust cam lift at zero degrees –
and, to machine both cam profiles, they need to be in the same coordinate
system. They are also shown with the direction of cam rotation in the engine
going from 3600 to 00.
It is interesting to compare what I have measured with
published data about Velocette K-series cams, although as you will see, it is
of limited use. The following table is reproduced from Fishtail #80 (April
1969).
The data in the above table is in degrees crank angle and
the clearance is measured at the valve. To compare the periods with my measured
cam data the clearance should be modified by the rocker ratio, which is pretty
close to 1 and makes very little difference (see below)
and the period divided by 2 to convert from crank degrees to
cam degrees.
In the table below I have compared the periods, in cam
degrees, for both the K17/5 and K17/4 cams using the valve timing data from the
table with the measured data from my cam.
|
K
17/5 Cam (Fishtail)
Period Period
(Crank) (Cam)
|
K
17/4 Cam (Fishtail)
Period Period
(Crank) (Cam)
|
Measured
Period
|
IVO
|
390 BTDC
|
2880
|
1440
|
430 BTDC
|
2930
|
1470
|
1600
|
IVC
|
690 ABDC
|
700 ABDC
|
EVO
|
600 BBDC
|
2800
|
1400
|
680 BBDC
|
2960
|
1480
|
1600
|
EVC
|
400 ATDC
|
480 ATDC
|
The differences are almost certainly due to the fact that,
because both the rocker skid and cam are curved surfaces and the skid does not
bear directly onto the top of the cam, the contact line between them does not
stay in the same place as the cam rotates and the rocker lifts.
It is also possible to compare the phasing of the cams. ie
the angular difference between the peaks of the inlet and exhaust (strictly,
these are not necessarily the locations of the peak lift but rather the mid
points between the opening and closing). Using the valve timing given in FT #80
I end up with 102.50 cam angle difference between the peaks for the
data quoted for the K17/5 cam; if I do the same thing with my data then it
gives only 620.
Why such a big difference? The explanation is because taking
the difference in the angular locations of the inlet and exhaust mid-points on
the cam does not translate in a simple way to the valve timing. The contact lines
of the rocker skids are not on the centreline of the camshaft when everything
is installed in the cambox – see the picture below (this cambox is from another
engine that I’m working on).
I have put 3 lines onto this picture to show the centreline
of the camshaft and, on either side and as best as I can judge, the position of
the contact lines of the inlet and exhaust rocker skids with their respective
cams which clearly shows they are not coplanar. It is for this reason that the phasing
of the inlet and exhaust valve timings do not have a simple relationship to the
cam profiles with the inlet valve opening earlier and the exhaust closing later
than that suggested by the measured cam phase angle.
As the machining data for both inlet and exhaust cams
profiles is now known the last exercise is to make a drawing for a blank. The
blank is essentially the complete cam but without the profiles, shown below.
The one detail not shown is the keyway: this is 1/8” wide
and 1/16” deep.
All the required information is now collated to be able to
start cutting metal.