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.

 

1 comment:

  1. Quality bit of workmanship as always hope to catch up with you when the weather warms up regards Paul

    ReplyDelete