"Michael Koblic" wrote in message
...

"Roger Shoaf" wrote in message
...

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).

Sure you will get less walk. The surface incontact with the work is only
the small portion of the end face of the tool. Your X any Y gibs are locked
and your quill gib is snugged, so you have very little room for wiggle.
The feed is linear to your hand and you can feel if you start to over do it
easier than when you are turning a crank.

Also bu unly taking .010" at a time you will be able to plunge fairly
quickly and you only have to do it 7 or 8 times.

If the mill does flex sideways, you will end up with only a small deviation
twords the bottom of the slot and it will only be a couple of thou wide and
deep. This gives you a straight wall of at least 80%.




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?

Ok let me see if I can explain. You have done all of your plunges, and you
now swap your 1/4" end mill for a 3/16" end mill. Since the diameter of the
new end mill is smaller, You would start from the right side of the slot and
plunge it to depth or raise the table to bring the mill to depth. Your
cutter should be then centered in the slot and you move the table about .025
away from you and the cutter should then be .007 from the kiss point.

You now lock the gib, and loosten the side travel gib, and re-snug it so it
allows you to turn the crank, but with some resistance. Now feed the work
into the cutter. You are going to get a little flex in the end mill but you
are taking a small bite and all of the flex is going to be away from your
final desired dimention so you still have room to make a clean up cut. When
you get near the end of your pass, you are going to run out of metal to cut
as you enter the quarter inch hole. Now you can take a clean up pass back
doing climb milling or extract the cutter and restart from the right side of
the slot but this timetake about a .003 bite. The bottom of the slot will
be narrower than the top but the end mill will be trying to remove less
material so the flex-away will be a lot less and the side wall should be
really close to square.

Now reset to make your final clean up cut, perhaps upping the speed a notch
this time you will be climb milling and you should clean up really close to
what you want to be at.

Now you do the same thing on the other side of the slot.



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.

Your dial is set and locked from when you did the plunge cuts, so you should
be able to just adjust the crank to accomidate the smaller cutter.

When you get ready to cut the back side of the slot, you need to be
cognisant of the backlash in the feed screw so when you make your first pass
onthe back side you can sneak up on the firast pass and you will be able to
keep the slot width from going too wide.


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.

Practice on some scrap to hone the technique, and pretty soon it will become
second nature.

I learned this technique from a very fussy german mold maker when my task
was to cut the pockets in mold bases 6 inches thick. He started by having
me cut to within .005" of the desired size and then he did the final sizing
until I had the tecnique down pat, then he let me finish them up to final
size.

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.


Practice, and observe the results, and be willing to learn from your
mistakes. Soon the techniques will become second nature, you will get over
confident has some part you have many hours into and your mistakes will
become fewer and farther in-between.

Also when you by the next machine that has less slop you will really be
appreciative of all of your futzing time. I remember the firs project I
made on a Hardenge lathe Wow!!!


--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
they come up with this striped stuff.