Subject: Re: Lead Screws & Encoders

From: [email-masked] (Paolo Zini)
Date: 20 May 1996 16:58:52 GMT

In article <[email-masked]>, Doug Bryant <[email-masked]> says:
>
>I have an imported milling machine with sloppy lead screw nuts (over 15 
>thou backlash).  To fix the problem I designed and built a set of 
>anti-backlash nuts that automatically take up any backlash.  They work 
>very well as I can now do climb milling with no problems.  The only 
>backlash is about one thou most likely due to the space between thrust 
>bearings and the leadscrew handle.
>
>I expected I might run into a problem if the leadscrew pitch varied even 
>slightly or if the thread/root thickness varied.  Turns out there are 
>slight variations which causes some binding in certain areas.  I've tried 
>making the threads uniform by filing the flanks of the "thick" threads 
>but had no luck with this.
>
>Does anyone know of a good source for accurate lead screws?  I'm also 
>interested in outfitting the machine with a DRO. Anybody know where I can 
>get these and about how much the encoders (preferably linear) and DRO 
>would cost?
>
>Thanks,
>Doug

Can you describe better your anti-backlash nuts? 
It would be intersting for my lathe.

If you want to find shaft encoders, look at
http://www.usdigital.com/ 
You can find there several models of shaft encoders, proces under 50$
to have a readout, i tink that the simplest solution is to feed the 
pulses of shaft encoder to the PC and write a small program to display
the reading. If your leadscrew is worn-out, I tink that is possible to
tabulate the errors in the program and to automatically correct the 
reading according to the table.

Paolo

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From: Doug Bryant <[email-masked]>
Date: Sun, 19 May 1996 09:02:39 -0700

Thanks,
Doug

---

From: Steve Wellcome <[email-masked]>
Date: 21 May 1996 15:23:49 GMT

Backlash in leadscrews is normal; it's not necessarily
a problem.  Just always approach work from one direction
and, if you go too far, be sure to back up beyond the range
of the backlash and then approach the point from the
original direction.  This is standard operating procedure
with leadscrews.

For example, if you want to move 2.25" and you accidentally
go to 2.30", back up to 2.1" or whatever it takes to get 
beyond the backlash and then approach 2.25" from the original 
direction again.  You can't just back up from 2.30" to 2.25"
and have it come out right, unless you have zero-backlash
ball screws or something.  Or a DRO with separate linear
scales.  Both of those are big bucks though, and unnecessary
if you use proper technique as described above.

---

From: [email-masked] (Peter Drumm)
Date: 20 May 1996 00:07:58 GMT

In message <[email-masked]> - Doug Bryant
<[email-masked]>Sun, 19 May 1996 09:02:39 -0700 writes:
:->
:->I have an imported milling machine with sloppy lead screw nuts (over 15 
:->thou backlash).  To fix the problem I designed and built a set of 
:->anti-backlash nuts that automatically take up any backlash.  They work 
:->very well as I can now do climb milling with no problems.  The only 
:->backlash is about one thou most likely due to the space between thrust 
:->bearings and the leadscrew handle.

015 isn't that much, a friend of mine has a Bridgeport built in the
early '50s, it has about .040 backlash in the screws, and he has no
problems with it.

:->
:->I expected I might run into a problem if the leadscrew pitch varied even 
:->slightly or if the thread/root thickness varied.  Turns out there are 
:->slight variations which causes some binding in certain areas.  I've tried 
:->making the threads uniform by filing the flanks of the "thick" threads 
:->but had no luck with this.
:->
:->Does anyone know of a good source for accurate lead screws?  I'm also 
:->interested in outfitting the machine with a DRO. Anybody know where I can 
:->get these and about how much the encoders (preferably linear) and DRO 
:->would cost?

The same guy put DRO on the Bport last year(I installed it for him)
cost was about $us1200 for X + Y axis, no Z axis, it works great. He
especially like the 'bolt circle' function of it. (for the life of me
I can't think of what brand it is tho...) For better leadscrews, you
could possibly have some made, or go to ballscrews.

[email-masked]
Using Warp/Neologic News/PMMail/iLink2/WebEx
Wausau, WI. 44d 58m 00s N x 89d 36m 45s W
All opinions are my own, even if they are wrong!

---

From: Doug Bryant <[email-masked]>
Date: Sun, 19 May 1996 22:14:40 -0700

Peter Drumm wrote:
> Unfortunately, I experienced catastophic results when climb milling at 
0.015 backlash.  I like the option of climb milling on occasion as it 
often leaves a better surface finish.  Also having only a thou or two 
backlash makes it much easier to re-establish a position as I don't have 
to remember to subtract or add backlash to my destination reading.

> That's a bit rich for me.  Sort of like putting a $50 saddle on a $10 
horse!!  I've done a fair bit of digital electronics work before so I 
might try to make some of my own components.  I have found a place that 
sells the "raw" encoder i.e. it isn't packaged so I'd have to make a shop 
proof enclosure.

I don't think ball screws are an option as I want to maintain manual 
feed.  Ball screws usually have to be made with a fairly coarse lead so 
they tend to be non-locking.

Thanks for your help.  If you have any other thoughts on the above let me 
know.

Doug

---

From: Mike Rehmus <[email-masked]>
Date: 22 May 1996 03:27:24 GMT

Replacement leadscrews are available from companies who
specialize in these items.  Usually you can buy two
different grades of 'normal' leadscrews and two grades
of ballscrew parts.  In both cases, the higher quality
units are ground.

You might look into Aha Engineering's DRO package for
your mill.  Components for two axis are $345 . . . you
supply the computer.  A 286 will suffice.  

I have a friend who has their setup on his mill.  Seems
to work very well. 

Their approach is to use rotary encoders but to drive the
encoders with aircraft cable wound around the encoder shaft
with both ends fastened to the table.  Much like the old
daisy wheel printer drives in the days before gearbelts.

Best regards,

Mike Rehmus

---

From: [email-masked] (WRitt)
Date: 21 May 1996 01:11:21 -0400

>to remember to subtract or add backlash to my destination reading.
Try snugging your table binders when climb milling, this has always worked
well for me. You are not going to do  much better than .003" backlash. If
you are going to a DRO you need not be concerned with your screws. If you 
have  problems  remembering which way the backlash goes mark the surface
above the dial with an arrow. Red magic markers work fine.

Bill  Rittner

---

From: Doug Bryant <[email-masked]>
Date: Tue, 21 May 1996 17:39:31 GMT

> reading according to the table.

Paolo,

Thanks for you advice on encoders.  Here's a brief description of the anti-backlash nuts 
 I made for my milling machine.  Note that ball nuts would be a better choice for 
machines with high speed lead screws.

Backlash can be taken up in a couple of ways.  One way is to split the nut 
longitudinally (i.e. along the length of the lead screw) and then change the position of 
one half of the nut relative to the other half.  This forces the thread flanks of each 
half of the nut to be in contact with opposite thread flanks on the lead screw.

The trick is how to adjust the spacing of the two halves of the nut without introducing 
additional backlash.  This could be done by fixing one half of the nut to a mounting 
body and moving the floating half with a screw mechanism such as a bolt that passes 
through the body of the nut.  To lock the nut in place after a manual adjustment, a 
second bolt on the other side could be tighened.  There are numerous other ways of 
achieving relative movement between the two halves of the nut, but I found it difficult 
to design it as an automatic adjustment without adding backlash through the adjustment 
mechanism.  Besides, in my case I didn't have enough room for this design.

A second method splits the nut laterally (i.e. perpendicular to the length of the lead 
screw).  The best way to describe this is to imagine a bolt with two nuts on it.  If the 
nuts could be pressed apart then you would have the thread flanks of one nut (the 
floating nut) in contact with one side of the bolt threads and the flanks of the other 
nut (the fixed nut) in contact with the other side of the bolt threads. 

Once again the trick is how to apply this pressure without introducing additional 
backlash.  Some designs use a heavy spring to push the nuts apart, but I didn't feel 
that would work on a milling machine which puts substantial forces on the nuts.  
Instead, I threaded the outside of the floating nut and put another nut over it which 
was seated against the mounting block holding the fixed nut.  By rotating this external 
or backlash adjustment nut I can force the leadscrew nuts apart and take up any 
backlash.  

To keep the floating nut from turning, it is mounted on a plate which has two reamed 
holes in it in the corners.  Accurately turned rods extending from the mounting block 
pass through these holes allowing the floating nut to move only along the length of the 
leadscrew.  An O-ring anchored at one end to one of the rods and connected at the other 
end of the backlash adjustment nut provide the torque needed to screw the floating nut 
away from the fixed nut located in the mounting block.  Note that very little torque is 
needed and in fact you don't want a lot of pressure pushing the two nuts away from each 
other as that will cause unneccessay wear.

This arrangement is very sturdy as the floating and fixed nuts basically press against 
each other via an external nut -- all metal to metal contact with no backlash in the 
adjustment mechanism (as long the rods mentioned above have a very close fit to their 
holes).

Space was a major consideration as I didn't want to have to machine the bottom of the 
milling machine table to make room for a bulky design.  Instead I exploited that fact 
that I had a fair bit of room to use along the length of the leadscrew.  The nuts I made 
for my milling machine are about 1 1/2" x 1 1/2" by about 2 1/4" long and fit on the 
existing mounting holes.

The problem with any automatic take up is that it requires very accurately machined lead 
screws otherwise it will tend to bind.  I am currently considering modifying the design 
to make it a manual adjustment or purchasing better lead screws.

---

From: Richard Hager <[email-masked]>
Date: Wed, 22 May 1996 13:13:47 -0400

Paolo Zini wrote:

 i tink that the simplest solution is to feed the

-- 

Hello to Paolo and others interested in DRO's, in hopes of saving you a pain of 
a programming job!...

For a number of years, Ah-ha! had a deluxe DRO program that ran on any 286 or 
better PC.  This was actually the product which launched the company way back 
when.

Although we dropped this product from our catalog a year ago, in seeing the 
amount of interest here in low cost DRO's, we'll make it available to members 
of this group with the understanding that we cannot offer extensive phone 
support for it as we do for our CNC software.

It is quite simple to setup and use, handles 3 axes, and accepts standard TTL  
quadrature encoders (either rotary or linear).  The cost is $149 US.  It runs 
under DOS, NOT windows.  While DRO is running, no other software can be 
running.

DRO contains a number of nice features including: radius/diam switching for 
lathes, a 'beep when close to zero' function, two sets of counters for absolute 
and relative, ability to handle any resolution in counts/inch, and backlash 
compensation.

I believe it's switchable to metric also, but don't hold me to that!  As I 
recall, it also had a 'tach' input, to display spindle RPM on-screen.  Another 
nice feature is that the readouts are large 1" high numbers that you can see 
>from  6 feet away.

Once again, phone support would not be available and documentation is minimal 
(8 pages plus wiring diagram if I remember right).  If you have some knowledge 
of PC's and the ability to read a schematic, it will be intuitive and self 
explanatory.  If you don't have the knowledge, you should probably buy a 
commercial unit.

If you'd like to get this software, please call the main number.  I'm totally 
swamped right now and will be unable to answer any email about it.

Whether you get this software or roll your own, good luck with it!  A DRO is a 
wonderful addition to any machine and well worth the effort.

        Richard Hager

+ Ah-ha! Design Group, Inc.           -
+ Precision CNC Technology, since 1991 -
+ 612-641-1797, Fax: 612-641-8681       -
+ "I just like to build stuff" So...     -
+ for CNC info, don't email me, call      -
+ or email Ah-ha! directly.  Thanks!       -
+ www.gdic.com/ahha  email: [email-masked]   -