Re: [CAD_CAM_EDM_DRO] NEW conversion project....

"D.F.S." wrote:

> I'm wondering about simply making a new one from scratch rather
> then trying to straighten the old one.

Making a new leadscrew, or the bracket, dial and handle?
Making an accurate leadscrew that long might be a challenge, without
the proper equipment.

> I want to keep the manual functionality and would like to be able to
> easily and fully disengage the new drives.
>
> I have some fairly heavy steppers and am a computer and electronics
> geek so I'm not afraid of that as such, but by the same token I'd
> rather not reinvent the wheel or fall in the same pits.
>
> This mill is a 1978 Kent Knee mill.
> The bed is 9 X 42 with a 2 hp motor and an R8 collet.
>
> Is there any rule of thumb on how big of motors it
> takes to drive the axis on one of these.
> This is for personal use, and I don't need a super fast system
> and something that is designed to handle 3 shifts a day
> on a production floor and run for 30 years.

If you use ballscrews, there is about a 2:1 reduction in torque required

to turn the screw under load. So, if you are going to keep the Acme
screws,
you will need bigger motors. Also, it is pretty hard to get rid of the
backlash with Acme screws.

> On the other hand, I'm impatient and want stuff to work after
> as reasonable amount of messin-with-it.
>
> What does everyone use on the bridgeports and how are the
> steppers geared? Obviously a guy could run a small stepper thru a
> 100-1 reduction gear setup or direct drive with a 2 hp DC motor,
> but either is a bit extreme, what middle ground is there?
> What works?

Servos can have an extremely wide range of speeds, but steppers have a
pretty severe limit at the high end, as the torque falls off severely at
high
speeds. Much over 1000 RPM is not possible. So, just about anything
over a 2:1 belt reduction will end up reducing your rapid feed to a
crawl.

> Not to start a debate about Axis naming conventions but what mechanism
>
> do people use to move in the "Z" or (up and down standing facing the
> machine)
>
> Both my table AND the Quill will move in this axis.
>
> What are the advantages and problems with automating one or the other.
>
> Off the top of my head, the Quill could move a lot faster, but
> the table would have a lot more travel.

Yup, that is the tradeoff. It is guaranteed (unless the head has been
wrecked)
that the quill travel is parallel to spindle rotation, but this is not
necessarily so
for spindle to knee movement. If you keep the machine precisely
trammed,
then this is so until the next crash. If the head is not precisely
trammed, then
drilled holes will not be perpendicular to the XY plane. But, you know
all thqat
already.

I prefer moving the quill, and have CNC control over that, with nothing
but the
hand crank and dial on the knee. The quill is moved by a leadscrew
directly
attached to the quill where the stop ring normally connects. When doing
a
lot of spotting, drilling and tapping, the quill goes up and down a
great deal.
Also, when drilling smaller holes in aluminum, the feed rate needs to be

fairly high. When peck drilling harder materials, it is sometimes
necessary
to withdraw the drill all the way for chip removal, and then plunge
almost to
where the drilling last progressed, andvance a bit, and repeat. This
would
really be a horrible thing to do with the knee! (I also drill prototype
PC
boards with my mill, and that is lots of up and down at high feed rates,

too.)

You have to consider wear on the knee slides, too. These are the most
highly stressed slides on a Bridgeport-style machine, and frequently
show
a lot of wear in a small region. This causes 'rocking' of the knee in
the
loose area, causing the table to shift in the Y direction, and also rock

(rotate) around the X axis. The quill, by its design (Bridgeport, at
least)
wears symmetrically. Due to the head design of the Bridgeport, quill
wear
is VERY slow.

> Another issue, is someone overtightened a bolt or stud in the center
> T slot right at the center of the table.
>
> It broke a hole the width of the slot and about 3" wide along
> the length.
> It is not a structural issue I don't think, but it is right over the
> lead screws and also down into the knee.

GASP! Yes, it IS structural! Most of the table rigidity is in the
front and back
edge of the table, and I wouldn't worry about the edges of the T-slots
being
broken out. But, this hole is all the way through! You might want to
dig up
a precision straight edge or surface plate and see if the table is
warped.
You might also want to see about Magnafluxing or dye testing the area
around
the hole to see if cracks have developed in the table.

Man, I sure hope you got a good deal on this machine! I wouldn't have
paid much for it with this much plainly obvious damage!

> I don't want to continue to drop chips and other crap down there.
> I'm afraid welding or brazing a patch on there will warp the table.

Yes, I would be very hesitant to involve anything that could cause
cracks
to worsen.

> How does JB or other types of epoxy hold up to on these machines?

IF JB Weld will hold water jackets together on water-cooled gas engines,

I can't see how it won't hold in this application.

Good luck, sounds like you will need it!

Jon