cutting cams

[brickyParticipant @bricky]

I am at the cam making stage on my opposed piston engine.I have made cams in the past but there were always plans to position the offset for machineing.I have read  several articles on the subject but they refer to using cad which I don’t have and I have exhausted attempts at trial and error on paper,so help is required.The cam is 10mm overall with 1.3 of lift and a 6mm shaft,I assume the base circle will be 8.7 can someone help please.

[JasonBModerator @jasonb]

Does it have flat or radiused flanks? What angles are given?

Probably easier to post a bit of the drawing.

[JasonBModerator @jasonb]

Found the drawing in the magwhich is quite useless. I’ll see if there is anything better in MiM if not I will have a go at tracing the image in Alibre and pulling some sizes off it. Or if you have a better drawing post an image.

 

How do you want to cut it? Jig in the lathe, Boring head or just the facets like he did?

[Nigel Graham 2Participant @nigelgraham2]

CAD would certainly make it quicker and easier, but cams can be plotted on paper, accurately, without trial-and-error.

You need determine the high and low dwell angles, which will bound arcs struck from the shaft axis, divide the rest into closely-set but equal divisions, then put the radius on each from a graph of the “unwrapped” locus alongside the cam plot itself.

The graph angle (x-axis) scale is not important, the radial (y-axis) scale is, matching the cam’s maximum radius even if the plotting is to a large-scale. The number of angle divisions should of course be the same on both parts of the plot. The more the better, within reason.

The technique is somewhat similar to plotting a development in sheet-metal fabricating.

The slopes can be of various types: linear, harmonic, etc., depending on the requirement but what drawings you do have should give some idea. They might simply be as tangents to the two dwell arcs.

Once plotted as a set of polar co-ordinates, the cam itself can be milled using a rotary table (a DRO is great help here for setting the radial travels). The resulting, small waves or facets are blended smooth by hand, by file or vertical linisher.

 

(I’ve not tried it myself but that describes how I was taught to design them, and how they were made in the printing-machine factory for which I once worked. They made both axial cams, by profiling a cylinder’ length, and radial, cut from gauge-plate discs.)

[brickyParticipant @bricky]

As I have done before I would cut them in the mill with a boring head.The angles are at 101 Degrees .Sorry about no image but since the change to the mag I can’t put images on the posts.Scaling the printed image of the cam from the one given measurment I worked it out like most of the engine to 10mm overall with 1.3 of lift and I get 8.7 base circle with a 6mm shaft.

Frank

[JasonBModerator @jasonb]

The 101 degrees is the offset of one cam from the other not the duration that is normally given.

As the side view has neither cam at max lift the 10mm is not useful

Lift is 1.3mm as given given

As the 10mm is not to the peak then deducting 1.3mm from that won’t give the base circle.

Now I know you are using a boring head I will complet the CAD. This is what I have so far

Base Circle Radius 4.625

Flank Radius 8.75

Tip Radius 2.375

Duration 136.3deg

 

Scaled the 31mm length as it gives the least amount of error

[JasonBModerator @jasonb]

So if we round the duration to 136deg that makes for simpler machining.

12mm stock would be the minimum you could use, turn to 11.9mm which will set the tip height

Set the boring head to swing a diameter of 17.5mm

Offset Y by + 1.535mm and bring in the boring head until the X offset is -3.829 which will have it cutting the right hand flank.

360deg less the 136 deg = 224deg which is how far the rotary table needs to be rotated assuming a 90:1 worm then 56 cuts at 4deg (one handle rotation) works out quite nicely. and will produce the cam. Just blend the tip radius with a file.

Check which end you are working from as this may need to be swapped round to get the two cams spaced correctly but assuming first cam cut from zero to 224 deg of rotation then second cam is cut from 101deg to 325 deg

[JasonBModerator @jasonb]

Do you need the various other lengths that make up the 31mm and their diameters?

[brickyParticipant @bricky]

Thank you for all the trouble you have gone to that is going to make the job easier.Thanks again Jason.

Frank

[bernard towersParticipant @bernardtowers37738]

Why not use a Westbury jig???

[JasonBModerator @jasonb]

I think the boring head method is quicker. Rather than having to loosen the clamps and reset the cam blank multiple times on the Westbury jig you simply wind the handle of the rotary table 1 turn or however many degrees you want and cut.

The cutting action is basically the same except the Westbury has the work moving and the other has the boring head moving.

Probably comes down to what you have and are familiar with, I’d just bash them out on the CNC 😊

[bernard towersParticipant @bernardtowers37738]

Not really the option for most swarf makers

[Nigel Graham 2Participant @nigelgraham2]

Interesting method. I didn’t know that approach – using a boring-head and rotary table though I know of the technique for using a boring-head for creating a sphere.

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