Re: [CAD_CAM_EDM_DRO] Making your own Ball nuts

Well, I can try to explain the reason for preloading.

We are all used to using lathes, mills, etc that have lots of slop
in the leadscrews. We just machine in one direction to take it
up. Like the little jerk you give on the handle of a lathe when
doing a threading pass to insure the carriage slop is eliminated.

But with cnc it is desired to have a machine that can do arcs,
circles, and other curves and motions. The axes are reversing
motion all the time. If there were any play in the drive each reversal
would cause undetermined motions until it had moved enough to
take it up.

Doing software manipulations can't really correct for this lost motion.

Now with preloading no lost motion occurs until you overcome
the preload force. That force is often hundreds of pounds or more.
Low friction becomes important when bearings are loaded like
this all the time... hence the use of ballscrews.

Now cast polymer nuts can have pretty low friction as well when used
with oil. They are not really preloaded. The play is just reduced to a small
value; perhaps a thousandth or so. The oil film damps lost motions
so you won't have wild uncontrolled rattling around with an axis
reversal.

For critical applications preload is certainly needed but I understand
that the cost of the ballscrews are a major problem for hobbyists.
So I am thinking of easily fabricated methods to cut that cost.

Ok, lets check the catalogs!

I'll grab the MSC big book.
Ok,a 5/8-5 48" Nook acme (4140 medium hardened 0.003 lead accuracy)
is $23.81!

A THK ballscrew without nut 48"x0.601" 5 tpi is $57.

A Thomson 0.625"x 72 ballscrew (no nut) is $85 .

These aren't quite apples to apples as I picked sizes with
good prices. Some sizes are unusually expensive prob
because they aren't made in as large a quantity.

But the Nook is $5.95/ft, the Thomson is $14.17/ft and the
THK $14.25/ft. Seems pretty good! Even Nook preloaded
polymer nuts are pretty cheap, but the preload is only 9
lb. Not quite enough.

The ballscrews are harder and have a higher polish than
the acme. But the Nook is 200Br - that might be machinable
on a small lathe without grinding. Other than that they
are similar materials and accuracy. And these are not the
lowest prices around i'll bet.

So it looks like for hobbyist or light use applications the
cast nut concept might save some significant $$$. For
industrial users (like me) that need high accuracy running
8 hours a day preloaded ballscrews are the best bet- as
expensive as they are.

As to whether ballscrew stock or acme would be better
with cast nuts... I don't know. I think both will work.
I can talk to the Philly people about this.

The goop is expensive, but you don't need much of it.

Les

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html

----- Original Message -----
From: "Marv Frankel" <dcdziner@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, October 18, 2002 10:53 AM
Subject: Re: [CAD_CAM_EDM_DRO] Making your own Ball nuts


> Les,
> I hadn't even thought of using the ballscrew material with the
polymer
> nut. I know that you can buy the screw material by the inch, from
> McMaster-Carr, but other than it being a new screw, would there be any
great
> advantage over the acme screw? You might also ask the product people if
> they've done any significant wear tests, to estimate the effective life of
> such a nut. Could you explain preloading, and what it does?
>
> Marv Frankel
> Los Angeles