"Roger Shoaf" wrote in message
...

Do you feel that there is an incremental benefit in this method rather
than
just making 8 passes of 0.075" from the initial hole increasing the depth
by
0.050" each time?


Since you don't have a very rigid set-up, you have to avoid doing the
things
that promote flex. To start off with a center drill spots the beginning
of
the hole as close as possible.

When you drill the hole under size, it removes the bulk of the material
yet
still permits a bit of wandering to occur.

OK, so far the procedure is identical to mine with the exception that I did
not drill the hole undersize. Something I really should have thought about.
As it happens it did not matter in this application.

When you then switch to the end mill to size the end of the slot, you are
not removing very much material and even on a non rigid machine you get
very
little walking if the end mill, so your hole will be as true as you can
get.


Now when you offset one axis by only .010", that should allow enough
rigidity to keep the cutter from walking on you.

When you then switch to an undersize mill to clear the tits, the tendency
of
the cutter to walk will be deflected into the waste area of the slot, and
as
material is removed the spring in the cutter relaxes and straightens out
the
slot wall.

Let me see if I uderstand what you are saying: The plunge is less likely to
walk if you remove only 0.010" of one end of the slot even if you go the
whole thickness (3/8" in this case).

The process of tit removal is something I am a little concerned about (it
may be a routine procedure for you but bear with me here): The idea is that
the tits are removed completely and *only* the tits are removed. I can see
in my hands two possible outcomes: One, the tits are removed incompletely
and the inside of the slot is wavy, and two, the tits are removed with
excess material making the slot wider than specified. In my case the latter
probably would not be such a disaster if limited to a few thou but for the
argument's sake how do you ensure that the latter does not happen? I assume
you line yourself up on the widest diameter of the initial hole at right
angles in the direction of the slot - but that cannot be easy! I do not see
an edge finder being much help.

By milling shallow passes you are going left to right and then back so you
are getting two slightly different feed paths and increasing the amount of
walk and flex that will widen the slot on you.

The question is if on your machine the end result is acceptable to you, is
one of practical application.

True - my tolerancies are quite - tolerant. However, the reason I am beating
this topic to death is to know the right way the next time when the job is
not so forgiving.

You can try it both ways and see what the limits of you machine are. I
have
done some pretty close work on some really tired old machines and have got
**** poor results on great machines and it seems that the key to getting
the
good result consistently is to work within the limitations of what you
have
got to work with at the time you are doing the work.

True. At this point it is sometimes difficult to tell which are the
limitations of the machinery and which of the operator. What I got so far is
a mixture of both.


--
Michael Koblic
Campbell River, BC