Re: [CAD_CAM_EDM_DRO] New guy needs advice!

smcleod007@... wrote:

> I
> saw an ad in a machinist magazine for a MAXNC10 mini mill and was
> surprised at how affordable it was.

The disadvantage with many of these smaller machines is the mechanism
for compensating for tool-holder length variations is raising and lowering
the head on a round column. When you do this, you lose the X-Y
alignment of the spindle position, as the head can swivel on the column.
A knee mill does not suffer from this problem. A small end mill in a collet
or flush tool holder projects only about 1-2" from the spindle nose. A
1/2" drill in a 5/8" Jacobs chuck projects 6" or more. None of the
smaller machines (even a full-size Bridgeport) can accomodate these
changes with the quill, as you only get 5" of quill travel on a Bridgeport.
This is a major complication, and for CNC it is a massive headache.
So, that's one of the hidden disadvantages to nearly all the mini-mills,
mill-drills, etc.

> So I started calling a lot of CNC
> companies to find out what's available in my price range. I
> explained that I needed a machine that could mill to .001"
> consistently for prototyping small engine designs from RC models to
> motorcycle size.

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.

Certainly, you are looking at zero-backlash ballscrews, and would do better
with either standard servos with high resolution encoders, or the Geckodrive with a
high resolution encoder.

> The largest piece I could see cutting right now
> would be a 6 in. diameter turbine wheel made out of stainless-steel.
> Some mini mill companies have told me their mills will do everything
> I need. Others have said I need a bigger machine to cut the harder
> steels.

Yes, the real difference is vibration! If the machine is built out of
little aluminum extrusions, you can forget any serious milling. A standard
(Series I) Bridgeport weighs about 2000 Lbs, and many old-time machinists
(from the days when the horizontal mill was king) will tell you that a Bridgeport
is really a toy, for light work only, and can't handle the 'heavy' cuts due to
insufficient rigidity! Their idea of a heavy cut is different from ours,
of course.

> Unfortunately for me there is a huge price difference
> between mini CNC mills and the next step up. I'm turning a storage
> room in the basement of my apartment complex into a tool room so I
> have enough room for a larger mill. It even has three phase power
> available!

You should really look at used, full size machines. You will spend as much,
perhaps, and it will take some time to do the conversion yourself, but
in the end, 6 months down the road, you will be SO glad you did!
Check what's being sold on eBay for a reference. If you are near Michgan
or Long Island, there are dealers selling machines every week.

Jon