Re: [CAD_CAM_EDM_DRO] re:Re: New guy needs advice!

ballendo@... wrote:

> >>This is already a tall order. There is machine table accuracy,
> >>spindle accuracy and tool deflection to consider. I have a
> >>machine that is pretty accurate (for an oldie) but I can't maintain
> >>this sort of accuracy without taking a lot into consideration.
>
> So nice to see someone beating this drum! It has always amazed me how
> many people think that if the resolution is .001(or better), then my
> parts WILL be, too !?! REAL machinists know that accuracy is gained
> and lost in small increments, from a NUMBER of places. And much more
> easily LOST than gained, IME!

I was amazed to see out lead machinist working on a part I'd designed that
had narrow deep slots in a 3/8" aluminum plate, and needed pretty square
corners. He was running a 1/16" end mill in halfway from both sides.
What amazed me was seeing the end mill bent at about a 20 degree angle
while taking those cuts! Obviously, the part was not anywhere NEAR size,
and the walls of the slot were REALLY tapered. I thought he knew better!
After taking the part back to him twice, he finally figured out he could use a
1/8" end mill to make the entire part, then use the 1/16" to sharpen the
radius at the corners. That one worked. I'm pretty familiar with tool deflection,
but I've never seen anyone just blithely cutting a part with the tool deflecting
20 degrees!

>
> >You should really look at used, full size machines.<snip>
> >If you are near Michgan or Long Island, there are dealers selling
> >machines every week.
>
> But, PLEASE, take someone with you who KNOWS machines! It IS good
> advice to look for "old iron", but remember there are TWO major
> reasons machine tools are "put out to pasture".
>
> 1)They're worn out. Not good for you, since as a new machinist, you
> won't know how to compensate for this inherant inaccuracy. A good
> machinist CAN make excellent parts on a poor machine(Not that he
> wants to :-)...) A beginner will be in the unfortunate position of
> not knowing where( or who) the problem really is...
>
> 2)They're obsolete. This one is better, but tricky. They can be
> obsolete for the "right" reasons (for our purposes). Things like
> outdated or "blown" controls, Lack of Ballscrews, etc. Or...
> For the "wrong" reasons, like the Gorton mill spindle/collet
> mentioned awhile back. ("Standard" parts aren't, collets not
> available, Bad year/ model of machine, etc.)

Yup, thanks for adding these additional concerns! I should have
added them myself.

Jon