Re: [CAD_CAM_EDM_DRO] Doesnt ANYONE have this drawn up??? was: John's CNC plans???

Comments on the whole conversion thing below.

On Thu, 2003-08-14 at 11:08, Jon Elson wrote:
> Vince Negrete wrote:
>
> >Jon, you've very much enlightened me on using the existing screw dimensions.
> >Stupid me... I never thought of that! very cool!
> >
> >
> All this stuff is hard to make. Some things, like the shape of the end
> bracket, and
> the J head on a Bridgeport, are free-form, hand-made surfaces ground into
> mold patterns 50 years ago. I tried to measure many of these parts with
> granite surface plates, angle plates, vernier height gauges, etc. and
> still came
> out way off, so holes had to be drilled out, shims inserted, all that messy
> stuff. So, any way you can continue to use the parts that ALREADY fit
> fine has got to be OK.

I highly disagree with using existing parts.

First of all, the bearings used on the ends of ACME screws generally
aren't the sort that would be used on a zero backlash ballscrew setup.
The runout, etc. on them is not suitable. In my opinion they need to be
replaced with a more suitable bearing, like a 7302, a precision angular
contact bearing.

Once you realize the need to replace those bearings, the existing
bearing holders are useless. Making new bearing mounts also allows the
motor to mounted for direct drive.


> I made my own yoke (the block in the center of the
> saddle that holds the leadscrews) because I thought the Bridgport part was
> flimsy. Well, it was a big mistake, and I had a HELL of a time aligning
> things so the screws ran straight, etc. A number of ballscrew retrofits
> make cylindrical adaptors that either fit the nut right inside where the
> original bronze nuts went (that would require an undersized screw) or
> the nut sits just outside the yoke on the end of the cylinder. I now
> realize why they do that - to preserve the factory alignment of the screws
> and nuts.

Let me state something else: doing a retrofit takes a TON of measuring
and designing. On our retrofits we replaced the bearing mounts and ball
screw mounts on every axis. Our servo setups will easily push 1000
pounds if we let them and the stock screw nuts on most machines were
never designed to take those sort of forces. They need to be replaced.
All of our ballnut mounts are rectangular in shape. Sometimes we had to
machine clearance on the machine to make them fit and sometimes we had
to machine away parts of the ball nut for clearance. You will be
shocked to see how flimsy the stock ball nut mounts are on some of these
machines.

A really handy shop accessory is a DRO. One of our mills has one and it
was used extensively during the retrofit machining. Retrofitting
involves a huge amount of drilling and tapping and the more precise
things are the easier it is to get proper alignment.

I don't mean to brag, but all the axes fit perfectly except one where a
hole was manually drilled in the machine in the wrong location.

I doubt there is any machine that can't be retrofitted, but they will
all take time and patience.

Before you start thinking this is more than you want to undertake, let
me state that the benefits and time saved once the machine is in
operation FAR, FAR outweigh the time (and cost) of doing the conversion.

Machining jobs that used to be a pain are now 15 minutes of editing G
code and a few minutes running through it in single step mode and the
part is done. The best thing is that the next part only takes a few
minutes of your time to get it set up in the machine.

Some other thoughts:

a) convert as large a machine as you can. The speed with which things
can be done on a CNC machine are only limited by the cutting power, so
bigger = faster.

b) don't overlook coolant and chip handling. Coolant is an absolute
necessity on these machines because the cutting rates are easily
adjusted via feed override to be high. Chips will accumulate faster
than you can imagine when running multiple parts and high cutting
speeds.

Enjoy !

> Just a rambling comment on some of the pitfalls I ran into when doing my
> own retrofit.
>
> Jon
>
>
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