RE: servo mill for possible conversion

The Excello is a good machine - heavier than a Bridgeport. The one listed
is a 2-1/2 axis machine and would be quite expensive to convert to full Z
axis control. However, most of the NC machines used in tech schools get
practically no use because a) the instructor is afraid of letting students
wreck it and b) the instructor is afraid of having to pay $115 an hour to
have a repairman come fix the control each time it breaks down. Thus,
someone will probably be getting a nearly new machine.

My guess is step motors (X&Y), probably Slo-Syn and coupled directly to the
ballscrews. Most of these older NC knee mills were factory retrofits of
manual machines and many of them had substantially less travel on the X axis
than their manual counterparts. Looking at the pictures I would say the
machine is early to mid-70's, as step motors and tape drives were common at
that time. The machine likely cost the school about $40K new.

This machine looks like a good buy. Count on scrapping the controller and
investing into a Z axis ballscrew.

Doug Harrison

> -----Original Message-----
> From: Matt Shaver [SMTP:mshaver@...]
> Sent: Tuesday, November 23, 1999 5:57 PM
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: Re: [CAD_CAM_EDM_DRO] servo mill for possible conversion
>
> From: "Matt Shaver" <mshaver@...>
>
> > > From: "David M. Munro" <munro@...>
> > > Here is a mill on eBay, for a possible PC control conversion. It is
> > > located in Ark. and looks to be in really clean condition. Any
> thoughts?
> > > Is this a project you'd like to take on? Is this a decent quality
> machine
> > > worth pouring some money into?
> > >
> > > http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=202770474
>
> > From: Jon Elson <jmelson@...>
> > I couldn't tell how big this machine is. It most likely has DC servo
> > motors, but may have resolvers instead of shaft encoders. These will
> > require a converter/interpolator, or need to be changed to encoders.
> > Servo amps may need to be changed. The CNC control has got to
> > be one of the oldest I've seen, must be mid 60's.
> >
> > Note, this is a 2-axis CNC. The quill feed is air operated.
>
> My comments are:
>
> 1. What Jon said.
> 2. In its current condition, especially if it still works, it's a real
> museum
> piece. It would almost be morally wrong to alter it.
> 3. Notwithstanding #2 above, there are a few practical considerations:
> a. X & Y may be stepper actuated, not servo. Jon is right, this is
> really
> old, so that's possible.
> b. I wonder how much backlash there is in the quill drive mechanism.
>
> Conclusion:
>
> The machine is apparently in great mechanical shape, and the price ($850
> last
> time I looked) is low, but if you buy it the only thing you can count on
> getting is an X & Y ballscrew equipped knee mill in good condition. If you
> can reuse any of the electronics or motors then you can take that as a
> bonus.
> Excello is a good brand so that counts for something as well.
>
> The machine I recently retrofitted (that you saw at my shop) is
> tentatively
> sold and should be leaving within a week or two, so I have the room for
> another project. If you do buy that machine I could certainly do the
> retrofit
> job. My main reservation is what to do with the quill drive if the
> existing
> arrangement isn't satisfactory. From what I can see in the pictures, they
> have replaced the manual quill feed handle on the right side of the quill
> housing with a purpose built rotary actuator. Downward motion is limited
> by
> the stop rod assembly on the front of the quill housing. Unless they have
> found a way to eliminate the backlash in the quill drive rack it's likely
> we'd have to scrap that whole contraption in favor of something like what
> Jon
> built for his Bridgeport Z axis:
>
> http://artsci.wustl.edu/~jmelson/CNCconv.html
>
> (look near the bottom of the page)
>
> Another possibility is to have a 2-1/2 axis machine by controlling the X &
> Y
> and living with a Z axis that is either "up" or "down" like a pen plotter
> or
> signboard cutter (you probably don't want this...). Let me know what you
> think and I'll keep looking around as well.
>
> Matt
>
> > Welcome to CAD_CAM_EDM_DRO@...,an unmoderated list for the
> discussion of shop built systems in the above catagories.
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