Re: Ball Screw Options

Thanks for the info Jon, It isn't a round ram. I asked what the y
travel was b4 I purchased. You can see it at:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1785720326&rd=1

So it looks like I will be installing real ball screws. I took a look
at Reidtools, as per a previous message by Jim. Why couldn't I use
the AR-315 ball nut? It is on page 366 here:
http://www.reidtool.com/Pdf/category/power.pdf
You can jump right to the page instead of "pgdn". It doesn't say
what tpi they are. I could use 2 of them to do the preload. They
are only $45 bucks. The screws are only $88 for a 4' length. They
arn't double circuit, is that a must? Is there any significance to
right-hand or left-hand thread? I'm assuming it doesn't because the
motors can be programed to turn either direction. Also, since the
screws are hardened, how hard is it going to be to turn the ends for
the bearings and motor/encoder shafting?

Is "angular bearing" a type of bearing that means it can take axial
loading? If not, could splain that a little more? Thanks a bunch.
Mark

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
>
>
> natchamp_87 wrote:
>
> >Well, I've taken the plunge and purchased my mill for the CNC
> >retrofit. I bought an older Bridgeport with an M head. It has a
> >9x32 work table and its perfect for me because I get the full 12"
Y
> >travel but the overall machine will fit in my garage much better
than
> >a full 9x42.
> >
> >
> Is this a round-ram machine? I have a 1938 round-ram that
originally
> came with an M
> head. It only had 9" or so of Y travel. My CNC retrofit involved
> making a new yoke for
> the ballnuts, and I made some errors in it, and now have only 7" of
Y
> travel. Sometime I
> need to pull it apart and remove a little metal on my yoke. But,
I'll
> still be stuck with about
> 9" of Y. Maybe with a little more carving I can get all the way up
to
> 10", but that will have
> the saddle coming at least 1" past the front of the knee. I have a
9 x
> 31" table, approximately.
> My machine is serial # 1388, certainly one of the oldest
Bridgeports
> still running!
>
> >Amoung other things, I'm trying to resolve is what I'm going to do
> >for the ballscrews. I would really like input on what my options
> >are. I've looked into the archives and am starting to get the
angle
> >of the dangle.
> >
> >1) Is one option to install anti-backlash nuts on the existing
stock
> >screws? I've read about making/installing anti-backlass nuts but
> >can't figure out if that applys only to ballscrews.
> >
> >
> This generally won't work, as the screws on all old machines are
> differentially worn. Remove the
> 4 socket head bolts on the Y handle and unscrew the Y screw from
the
> yoke. This is good to
> check for inadequate lube, and see how many tons of chips are
inside the
> knee! I filled a
> trash can with what was inside mine! Looking at the lands of the
Acme
> threads, you will
> almost certainly see they are thinner in the middle of the screw.
Any
> attempt to take up
> all the slack at the middle of the screw will cause binding at the
> less-worn ends.
>
> >2) If I were to install new ballscrews and nuts what specs should
I
> >look for? Is 5 tpi the standard? How about the diameter?
> >
> >
> 5 TPI is very common, but 4 is also seen sometimes. Very fine-
thread
> ballscrews need very
> small balls, making the balls wear out faster. Very coarse threads
> allow rapid motion, but
> need more torque to deliver linear force. So, 4-5 TPI would be the
best
> choice in X and Y.
> I have a 10 TPI ballscrew on the Z axis that works fine.
>
> >3) If I do #2 above am I right in that I have 2 options, either
buy a
> >bolt in kit or buy the ballscrews and fabricate how the nuts
attach
> >to the table?
> >
> >
> Yes. Measuring all the heights to make everything fit right is
> daunting, and I worked VERY hard
> to get it right, and I still ended up putting as much as .060" of
shims
> here and there!
>
> >4) What do I have to do/consider for attaching the ends to my
> >servo/stepper motors? Do I have to do any machining to them for
this?
> >
> >
> You need to use pairs of angular contact bearings at one end of the
> screw to firmly locate it
> against axial thrust. You can use pairs of some standard radial
> bearings for this, but without
> a lot of testing, angular contact guarantees they will restrain the
> screw. Most likely, unless
> you have the screws machined for you, or you get used screws,
you'll
> have to turn steps, etc.
> on the ends.
>
> If you use servo motors that don't have integral encoders, a good
setup
> is to mount the encoders
> coaxial to the leadscrews, and couple them with helical-slit
couplers.
> Then, you can use
> toothed belts to give a speed reduction between the motors and
screws.
>
> Jon