Wednesday, 22 January 2025

Making a K-17/5 Cam for KTT 55 – Part 2: Machining and Heat Treatment

All the information for machining the cam has been determined (see Part1) and it’s now time to start cutting metal.

A 100mm length x 51mm diameter piece of O1 tool steel was ordered – but it turned out the supplier was out of 51mm and 56mm diameter turned up instead. Oh well, a bit more swarf.

Then next step was to transform the piece of metal on the left into a cam as shown on the right. Unfortunately, Father Christmas was not paying attention to my wish-list for a 4-axis CNC machine so all machining was done on my 77 year old Harrison lathe and modified (so I could machine theV-Twin crankcases) Tom Senior milling machine.

I had already made a drawing for the cam

and the first step was to machine the 2 main diameters for the cam itself and the threaded section and to put a 9/16” drill down the centre.


The 20 TPI thread was then screw-cut before finishing with a die and a thread chaser, the machined part separated from the bar and machined to length, the internal diameter bored with great care to give a good interference fit on the camshaft and, finally, the groove separating the inlet and exhaust cams machined using a 3mm wide profiling tool, shown in the picture below.


Although a parting tool was used initially for this ~3/8” deep groove, it is finished with the profiling tool (tip radius is 1.5mm) to avoid sharp stress-raising corners.

It was now time to machine the cam profile on the blank.


The 2 pictures below show the depth-of-cut for the exhaust and inlet that I had measured from the original.


To make life easier, I printed the data and added another column for the depth-of-cut - 0.010”, shown below.


The purpose of this was to be able to make a number of selected cuts at various angles to remove the bulk of the material and with a machining allowance before applying the final depth-of-cut at 20 intervals.

The picture below shows the exhaust cam with large chunks removed in this way

The next 2 days was spent laboriously rotating the indexer 20 and machining to the final depth-of-cut. Needless to say, this is a pretty boring repetitive process but is not an occasion to let the mind wander and needs concentration to avoid the cam ending up in the scrap bin!

Anyway, it went according to plan and this was the result …without the surrounding pile of swarf, removed from the milling machine but still on the mandrel.



Even with an increment of 20, small flats can be seen either side of the nose of the cam where there is a rapid change of curvature.


This is unavoidable with any finite sized increment and can be easily removed by careful use of #150 grit emery cloth to give a continuous profile.

(If you look carefully at the picture, the same “flat feature” can be seen on the original reground cam on the right close to the nose of the inlet cam).

The final machining operation was to put in the keyway. In the past, these have been spark-eroded but my spark-eroder has recently retired; I guess at the age of nearly 86 he’s earned a right to stop inhaling paraffin fumes.

Luckily, a company DSA Products Ltd., which is a 20 minute drive from my house, offers an EDM wire erosion service and Mark Skilton, who owns the business, was extremely obliging in setting up the cam on one of his machines to put in the keyway.


Machining of the cam is now complete

and the last operation is heat treatment. Before heat treating the cam a few checks were made on the final dimensions - base-circle diameter, max lift etc.. and these were within a few thou of the original so I was quite happy that the machining had gone according to plan.

The cam was coated in anti-scale compound and heated to 8200 C in my high temperature furnace.

before quenching in vegetable oil.

After tempering in my other furnace at 2200C for a couple of hours the cam was ready for a final clean-up and testing for the level of hardness.

I can judge the hardness fairly well with a fine file (I call it the FFT – Fine File Test – not to be confused with the more usual meaning of the abbreviation – Fast Fourier Transform) but you can’t beat a proper quantitative measurement …so off to my friendly manufacturer up the road to get a proper hardness measurement.


which came out at 58 HRC.


58 HRC is quite acceptable for a cam and, at this point, it is ready to be pressed onto the camshaft.

 

Wednesday, 15 January 2025

Making a K-17/5 Cam for KTT 55 – Part 1: Reverse Engineering an Existing Cam

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.

Wednesday, 1 January 2025

A Start on Refurbishing the Cammy Engines

I have spent the last 4 months rebuilding the clutches, gearboxes and positive stops for the 3 immediate cammy Velo projects plus the 2 flat-tankers for the follow-on project. I’m sure there will be a few details yet to sort out but, in the meantime, it’s time to start building/rebuilding the engines.

So, what is the starting point? ….and the plan?

The starting point for KTT 305 is simple – it’s the engine that I removed from the frame a year and a half ago and have been tripping over in the workshop ever since.

The only bit that is obviously missing is the cambox oil pump, which I removed some months ago to reverse-engineer to make some new ones.


So, all 3 engines will get a nice new cambox oil pump.

The bottom-ends of KTT 55 and the Model K engine that will form the basis of the cammy special were rebuilt many years ago by Alpha Bearings



Both of these engines have complete new crankshafts/con-rods/bearings that the late Max Nightingale masterminded.

 

and which have been balanced to new pistons and rings manufactured in the US by JE Pistons and Total Seal respectively.


A batch of 4 of these was made based on an original KTT piston but redesigned using modern technology. I have used one of these previously in the AJcette project (see here); I took this bike to the Manx GP back in 2019 and found it went like stink for a little 350 and so I’m optimistic that this performance will be repeated here. I have 3 of these pistons left and, because the ring pack is designed to run in a Nikasil plated bore, I have 4 cylinders (I only need 2 for the immediate projects) that have been Nikasil plated. KTT 55 and the cammy special will use these.

I have also been working on rebuilding cylinder heads in the background. All of these have new valves, guides, springs etc. and some of the heads have valve-seat inserts where it was necessary. 3 of these heads were used on the V-Twin and AJcette projects and I have just completed 4 more for the current projects (I managed to sneak into the workshop for a few hours over the Christmas period).



The mating surfaces of the head and barrel have been ground together with fine/medium grinding paste in the time-honoured fashion ….and before the valves were fitted. Incidentally, I always measure the depth of the recess in the head

and the protrusion of the spigot on the barrel

before grinding – they need to be pretty well the same value, because there is no point in trying to make good contact surfaces if there is a large disparity. The worst case is if the protrusion on the head spigot is significantly greater than the depth of the recess because this will result in bending loads around the periphery of the head joint when it is bolted down.

These are the other heads, barrels and one cambox which have been completed.


Most of these have 5/16” diameter inlet valves although I used a 3/8” valve (part number k2/12) for the most recent one.  I also found that the K4/5 top collars do not fit the Terrys VS 147 valve springs – they are designed to fit an inner spring with a larger ID. So, if you plan to use NOS VS 147 springs, as I did, make sure you look after your original K4s!

I have 3 period carburettors – 2x AMAC 15 TTs and a slightly earlier AMAC T15 HXDM, all of nominal 1” choke size. All of these were rebuilt by Martin Bratby some years ago.


The diameter of the stub fittings on the carbs is slightly larger than the stubs currently fitted to the cylinder heads but it won’t be too much work to make and silver solder a sleeve onto the stubs where necessary.

One of the heads has a cambox that I refurbished some time ago but I need to complete the rebuild of at least one more cambox.

I have plenty of crankshaft shock absorbers that are in good condition – some new ones that Dave Smith in the US made for me some years ago (one of these is fitted to the AJcette).

These have a different spline to the original and the parts are not interchangeable with original parts. I also have a number or original shock absorbers that are in good condition and which I have cleaned up and chemically blacked, which is why they are hanging up to drain off the excess oil.  

I’m not expecting that much work will be needed on these.

There are a lot of ancillary parts – bevels, oil pumps, sprockets, casings etc that I won’t go through in detail here



and a new pair of bottom bevels if needed.

thanks to the Velo Owners Club spares scheme.

Most of these parts are in good condition, considering their age.

Square ML magnetos with anticlockwise rotation were generally fitted and I have a number of these

all of which have been rebuilt (armature rewinds, remagnetised etc.) by Paul at APL.

So, this is the starting point – a pile of bits! Lots of them. As with previous projects, the plan (loosely) is to rebuild large “chunks” – cylinder heads (already done), cam boxes, complete bottom-ends, complete bevel drives etc and then to ensure that everything fits together properly. I also need to strip KTT 305 to see what lies inside.

This should keep me busy for a while.