Re: [CAD_CAM_EDM_DRO] Re: wrong guy, but have servo question.

Marcus & Eva wrote:

> I run a Bridgeport clone .
> With a Sandvik 2 flute positive rake cutter, a 1/8" pass and a 95% stepover
> in P20 is the kind of cut I take all day long, when I'm roughing out a
> cavity.
> Admittedly, my machine is near new, and has 3 horses to drive it.
> Any less, and I can't justify owning the machine, because it is not
> competitive.

Well, it depends entirely on what you are doing. Yes, the 3 Hp makes a
big difference. I don't have much need to make large pieces in hard
steels. Most of my commercial work is cutting holes in 1/8" aluminum
instrument panels, drilling and milling aluminum instrument housings, etc.
A lot of it is with relatively small tools, 1/8 to 1/2" typically. I occasionally
make some tooling or something, but again, rarely need massive metal
removal rates on these parts.

> The position of the quill matters a lot when I take these kinds of cuts.

No doubt about it!

> My point was not that you will not get a useable CNC by driving the quill as
> your Z axis; what I was pointing out is that this choice leads to some
> significant compromises in the performance of the machine.

My machine is already compromised, by 40+ years of use without a re-scraping.
The fixed knee way has a good deal of wear on it, and at some locations the
table moves several thousandths of an inch in the Y axis when you change
directions of knee travel. This would need to be scraped before you did
any work where the Z needed to be moved during a cut. (I always move it
in the raise direction last when changing Z height, to keep this position as
consistant as possible.)

> I was making my comparisons from the point-of-view of a commercial user
> whose choice has to compete with other brands of CNC on the market.
> Just to put everything into perspective here:

Yes, as a job shop, the performance of the machine makes a big difference.
My machine only needs to do a few hours of work a month do satisfy
most of the commercial needs.

> I can buy a brand new Haas VMC with toolchanger for $30,000.00 CDN.

Hmmm, sure those are Canadian $? I think their cheapest machine goes
for US $30K.

>
> A Haas against a kneemill conversion is like Secretariat against Elsie the
> Cow, in terms of performance.
> My question to you all:
> Why are you spending this inordinate effort on the CNC guts and then bolting
> it on to such a limited machine?

Most of us are NOT job shops, but closer to hobby-type work, or hobbies
that have grown into serious businesses. And, if you are doing light work
on aluminum, a knee mill is adequate.

> Has anyone of you out there considered building a decent machine base to
> take all this incredible effort on the CNC guts?
> Even if you farm out all of the milling and surface grinding, I bet you
> could come up with a pretty decent machine base for less than the cost of a
> kneemill.

Well, I don't know. Some people who were in the right place at the right time
have gotten decent knee mills at great prices. I've had mine about 18 years,
now, so it was already there. I suspect that's the same for many of us.

Jon