RE: [CAD_CAM_EDM_DRO] Re: Cam making....

I've plotted many cams, not with a DTI, but from the tappets, lifters, or
valves themselves. We used to use vernier timing pulleys and spend hours
getting the cam timed spot on, but most only plotted one lobe, plot all of
them and you get quite a surprise. I used to build many performance engines,
computer model them for the drag strip guys and for the aircraft industry,
and broke quite a few once believed power limits for a particular engine.

It is not uncommon to find manufacturers cams that are 3 or 4 degrees out on
some lobes compared to others, and 20 thou deviation in lift at the valve.

Some of this of course will be engine block machining errors, is the cam
square to the center line of the block, are all the lifter ports in a
straight line, and at the same angle etc.

Error on lifter bores isn't too bad, but again a couple of thou error on the
cam centre line is not uncommon, and blocks that are several thou taller at
one end than the other.

Strangely all the performance blocks I have built have been very square, but
my colleagues seem to find plenty that were out of square and quite a few
heavily warped blocks.

On BL A series blocks it is not uncommon to find a 5 thuo bend being applied
to the crank, the crank on these is so weak it seems to survive unless you
tune the engine. 5 thoughh on a V8 will take the crank out quite quickly, I
was brought a 400 Pontiac V8 block that was eating cranks, more than fuel,
the journals were over 20 thou out of line, I think it had been cooked at
some stage.



So what does all this mean. I suppose in a production block the tolerances
vary that much, that a little error in the cam is not uncommon. For a race
engine, it is nice to have everything as accurate as possible, nice square
block, same height at both end, align bore all the journals etc., but again
a little cam error won't make that much difference. Normally the lift is so
hi, a few extra or fewer thou won't make much difference, and the angles are
normally fiddled with in 10 degree changes, so one degrees which is a pretty
big increment to a machinist won't make that much difference. The big
differences come from bore and deck heights, and matching all the air flow
paths to get it in and out of the engine as fast as possible whilst
maintaining even velocity throughout a cross section of the track.

For cams in all if you do the best job you can, it will most likely be OK



Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Peter Reilley
Sent: 24 March 2007 23:39
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: Cam making....



I am curious about your method for measuring your existing
cam. Using a dial indicator to measure the lobe shape will
not give accurate results unless the radius of the probe
tip is taken into account. Also, any slop in the indicator's
stem will contribute to errors.

Are these errors of concern? Perhaps such accuracy in an
antique engine is not necessary. Is the object just to get the
engine running and not to get the original horsepower?

Pete.

----- Original Message -----
From: "xj5373" <ian.c.haynes@ <mailto:ian.c.haynes%40btinternet.com>
btinternet.com>
To: <CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups.com>
Sent: Saturday, March 24, 2007 6:03 PM
Subject: [CAD_CAM_EDM_DRO] Re: Cam making....

Well thanks everyone for the excellent advice, what a great group!
Summarising there is a technique for reverse engineering existing
cams;
Rotate in 5 degree intervals, measure with a dti and tabulate then
input to CAD as a series of lines
A technique for working from a hand drawn cam;
Import a scan of the drawing on top of a template made up of radial
lines at degree intervals using OSNAP intersection to find the points
To create the cam outline it is recommended to use arcs wherever
possible then use polylines to fill the gaps.
To smooth the polylines use the fit command.
The ideal method though is to draw the cam directly in CAD.

My next step is to get the first sample cam drawing from my colleague
and see what we are starting with, this is going to take a few days
though as he is away for a while. I will also investigate CV mode, I
assume this is CV mode in Mach 3. I have a bit of an issue with my
DXF conversions of JPEGs coming out as 3D polylines, I need to
explore converting them to 2D polylines before I can use the fit
command.

Just out of interest, the plan is to cast the cams in wax then
assemble them in the wax at the appropriate angles before committing
to casting. These are for JAP V twins where the inlet and exhaust
cams share the same shaft.

Dissappointed I didn't see the CNC cam grinder in Ebay, I would have
put in a bid!

I will give you an update when I get a real drawing to work with.

Thanks again

Ian





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