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rtr@...
This procedure isn't difficult, but must be understood to successfully
install a rotary encoder where none have been before.

The end of a screw usually has a centerdrilled hole, the screw is usually
hardened , but the core is quite soft to take shock. A good way to do this
is to drill the appropriate pin size ( a hardened drill blank works well)
while the lead or ball screw is assembled in the machine. Don't use a
carbide bit, because if it shatters, you are in deep trouble. Try a slight
interference fit first, It won't come out that way when you use a hand
electric drill. Try for at least a one inch depth. If the screw has
enough metal sticking beyond the bearing, you can go for 1 and a half or
two inches. This means the pin will tend to cock less in the hole.
Obviously, if there isn't going to be much wall thickness left in the
screw, make an adapter pin so you can use a smaller diameter hole and still
have the same sized pin end at the encoder as the encoder shaft.

Then drill and tap for a couple of set screws at 90 degrees to allow you to
indicate it in. Put the indicator near the end of the pin, because that's
where the coupling will fit. The pin doesn't needto stick out more that an
inch when you finish. You can cut it off and file the end when you have it
in place, indicated, and clamped with the set screws. Do thiswork on the
pin gently then indicate it again. Encoders don't like a lot of vibration
so bring it within 3 thousandths or better. I'm always happier if it
comes within one and a half thousandths.

You can connect it with an oldham coupler or similar low inertia couplers.
Surprisingly, if you mount it so the shaft pin and the encoder pin come
within a few thousandths beyond the slop in the system, you can use a piece
of surgical tubing for the coupling. You can add an outer layer of heat
shrink tubing if you think the surgical tubing is too compliant, but I've
never had to. The extra stiffness will transmit more vibration to the
encoder, a bad trade off. There is little friction in good encoder
bearings so the surgical tubing wall thickness is adaquate for a coupling.
Obviously, you can't do this if you need to drive the screw or brake it
through the encoder shaft, a bad idea in any case.

Besides eccentricity in the rotation of the encoder coupling pin, the error
that kills the most encoders is failure to allow enough room between the
ends of these two shafts. All sorts of distortions occur when an axis is
stopped after a rapid movement. Ten thousandths of longitudional slop is
not unusual in a ball screw, more in an acme screw. I take a leather or
wooden mallet or a carefully handled babbit hammer and carefully tap the
screw longetudinaly toward the encoder to get some idea of how much
longitudinal slop must be allowed for. Do this on the other end of the
screw, driving it toward the end with the pin. You aren't worried about
how much it retreats from the encoder. If you can't get to the other end
of the screw, clamp something to an accessible part of the slide and tap it
toward the encoder end from there.

Once you have some feel for this longitudinal travel, use shim stock to set
the dial indicator away from the end of the pin by a your estimated amount
and make sure that the same taps do not show up on the dial indicator. If
they do, increase the pin to encoder shaft clearance.

You need to indicate the encoder mount so its shaft is concentric with the
pin you just installed. Sometimes its easier to leave the pin you
installed full length until you have indicated in the encoder mount. It's
a good idea to drill and ream for taper pins when you have the encoder
mount indicated in. If you have a choice, countersink the encoder mount so
the encoder fits snugly, and use servo clamps if the encoder will accept
them. Clamp the indicator to the pin and indicate the countersunk encoder
locating hole wall.

The last trick works for feeling eccentricity here and other places you may
want to check for it. CAREFULLY rest a screwdriver blade on the pin, then
on the encoder shaft so the screwdriver blade is pulled away from your hand
when the shaft is rotated, and so it won't interfere with your coupling.
If, when you are moving the axis rapidly, you feel a bounce on the shaft,
you did something wrong. Start over.

One fix you don't want to have to do is locktite an oversized pin into the
end of the shaft and then machine it while you traverse the axis. Don't
use a hardened drill blank for this approach. I've installed lots of
encoders and this is a distillation of those experiences.

Ted Robbins

From: "Ted" <rtr@...>

This procedure isn't difficult, but must be understood to successfully
install a rotary encoder where none have been before.


Then drill and tap for a couple of set screws at 90 degrees to allow you to
indicate it in. Put the indicator near the end of the pin, because that's
where the coupling will fit. The pin doesn't needto stick out more that an
inch when you finish. You can cut it off and file the end when you have it
in place, indicated, and clamped with the set screws. Do thiswork on the
pin gently then indicate it again. Encoders don't like a lot of vibration
so bring it within 3 thousandths or better. I'm always happier if it
comes within one and a half thousandths.

Most encoder warranties are voided by NOT using an appropriate coupling.
I use couplings made from a single piece of metal, slotted helically to provide
radial and axial compliance, but no torsional compliance.

You can connect it with an oldham coupler or similar low inertia couplers.
Surprisingly, if you mount it so the shaft pin and the encoder pin come
within a few thousandths beyond the slop in the system, you can use a piece
of surgical tubing for the coupling. You can add an outer layer of heat
shrink tubing if you think the surgical tubing is too compliant, but I've
never had to. The extra stiffness will transmit more vibration to the
encoder, a bad trade off. There is little friction in good encoder
bearings so the surgical tubing wall thickness is adaquate for a coupling.
Obviously, you can't do this if you need to drive the screw or brake it
through the encoder shaft, a bad idea in any case.

Actually, many good encoders have a fair amount of drag in their
bearings. They use a pair of angular contact bearings with preload, to
stiffly constrain the shaft from any radial movement, which would
show up incorrectly as rotation. They also have at least one, usually
two shaft seals, to keep crud out of the bearings, and grease out
of the optics. That all adds up to inch-ounces of static drag.
I'm using 1000 line encoders, counting all transitions, so that is
4000 counts/rev, or more than a count for every tenth of a degree.
It would be foolish to waste that accuracy with a homemade coupling
that allowed twist to develop. Also, the metal coupling is good for
many years, what if the surgical rubber turned to gum? Do you know
what OIL does to LATEX? Yucck!

Besides eccentricity in the rotation of the encoder coupling pin, the error
that kills the most encoders is failure to allow enough room between the
ends of these two shafts. All sorts of distortions occur when an axis is
stopped after a rapid movement. Ten thousandths of longitudional slop is
not unusual in a ball screw, more in an acme screw.

I don't know how a precision machine could possibly keep any accuracy
if there is .010" longitudinal movement of the screw! I have a total of
.001" of slack in my mill, and I'd like to find the sources, and reduce it.
But, that is the sum of ALL the sources of slack, like torsion of the
leadscrew, shaft couplings, bowing of the screw, slack in the angular
contact bearings, slop in the anti-backlash ballnut, etc. etc.

Jon

rtr@...
Jon,
I have no disagreement with anything you have said except for the method of
measuring axial displacement. When an axis is quickly stopped after
traversing under load, even heavily built cast iron milling machines
exhibit more than the static displacement measured by pushing and pulling
the axis. That's why I suggest a shock (Gentle, of course) method of
measuring axial displacement.

I have often used helical cut couplers and find them satisfactory. I
suggested the surgical tubing as a low cost alternative for encoders in the
50 to 200 cycle range. When you get to very high resolution encoders, you
run out of error budget with surgical tubing. The shrink tubing and a
small metal tubing sleeve might help keep oil off the rubber.

Ted
----------

From: Jon Elson <jmelson@...>
To: CAD_CAM_EDM_DRO@...
Subject: Re: [CAD_CAM_EDM_DRO] Installing rotary encoders
Date: Tuesday, June 15, 1999 12:01 AM

From: Jon Elson <jmelson@...>

From: "Ted" <rtr@...>

This procedure isn't difficult, but must be understood to successfully
install a rotary encoder where none have been before.


Then drill and tap for a couple of set screws at 90 degrees to allow

you to

indicate it in. Put the indicator near the end of the pin, because

that's

where the coupling will fit. The pin doesn't needto stick out more

that an

inch when you finish. You can cut it off and file the end when you

have it

in place, indicated, and clamped with the set screws. Do thiswork on

the

pin gently then indicate it again. Encoders don't like a lot of

vibration

so bring it within 3 thousandths or better. I'm always happier if it
comes within one and a half thousandths.

Most encoder warranties are voided by NOT using an appropriate coupling.
I use couplings made from a single piece of metal, slotted helically to

provide

radial and axial compliance, but no torsional compliance.

You can connect it with an oldham coupler or similar low inertia

couplers.

Surprisingly, if you mount it so the shaft pin and the encoder pin

come

within a few thousandths beyond the slop in the system, you can use a

piece

of surgical tubing for the coupling. You can add an outer layer of

heat

shrink tubing if you think the surgical tubing is too compliant, but

I've

never had to. The extra stiffness will transmit more vibration to the
encoder, a bad trade off. There is little friction in good encoder
bearings so the surgical tubing wall thickness is adaquate for a

coupling.

Obviously, you can't do this if you need to drive the screw or brake it
through the encoder shaft, a bad idea in any case.

Actually, many good encoders have a fair amount of drag in their
bearings. They use a pair of angular contact bearings with preload, to
stiffly constrain the shaft from any radial movement, which would
show up incorrectly as rotation. They also have at least one, usually
two shaft seals, to keep crud out of the bearings, and grease out
of the optics. That all adds up to inch-ounces of static drag.
I'm using 1000 line encoders, counting all transitions, so that is
4000 counts/rev, or more than a count for every tenth of a degree.
It would be foolish to waste that accuracy with a homemade coupling
that allowed twist to develop. Also, the metal coupling is good for
many years, what if the surgical rubber turned to gum? Do you know
what OIL does to LATEX? Yucck!

Besides eccentricity in the rotation of the encoder coupling pin, the

error

that kills the most encoders is failure to allow enough room between

the

ends of these two shafts. All sorts of distortions occur when an axis

is

stopped after a rapid movement. Ten thousandths of longitudional slop

is

not unusual in a ball screw, more in an acme screw.

I don't know how a precision machine could possibly keep any accuracy
if there is .010" longitudinal movement of the screw! I have a total of
.001" of slack in my mill, and I'd like to find the sources, and reduce

it.

But, that is the sum of ALL the sources of slack, like torsion of the
leadscrew, shaft couplings, bowing of the screw, slack in the angular
contact bearings, slop in the anti-backlash ballnut, etc. etc.

Jon


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bill, List Manager

OK, at the risk of beating a dead horse, If you have access to .0001" DRO
scales, wouldn't they provide a more accurate method of locating the table,
than indirectly with encoders?
If not I'll be installing encoders on my mill, if so can the programs you
guys are running be set up to read from scales?
Finally, does anybody have info about how well the BOBCAD program works? And
can it be used with the Linux programs for a source of the G codes?

Don't know about Bobcad, but I am using Vector to generate g-code for use in
EMC. I have been very pleased with what Vector offers and as I get use to
what EMC wants in the way of g-code I am having no problem getting Vector to
produce a code that will work. go to (the US
distributor) and check it out. You can also get some more info at
(the developer).

Tim
[Denver, CO]

From: TADGUNINC@...

OK, at the risk of beating a dead horse, If you have access to .0001" DRO
scales, wouldn't they provide a more accurate method of locating the table,
than indirectly with encoders?

Yes! But, who can afford these? There are a number of innacuracies introduced
by using ballscrews and encoders. but, you should be aware that linear
scales are not a panacea! If the ways are worn such that the axes are not
orthogonal, or that an axis doesn't travel straight, then the linear scales won't
provide accuracy, either.

If not I'll be installing encoders on my mill, if so can the programs you
guys are running be set up to read from scales?

The computer should not be able to tell the difference.

Finally, does anybody have info about how well the BOBCAD program works? And
can it be used with the Linux programs for a source of the G codes?

It does not run on the Linux machine. they do have a DOS emulator, but I hear
news that the Windows emulator may be up and running. But, anyway, I
use Bobcad/CAM Ver 16.1 on a Windows 95 machine, and then send the
files over by network to the Linux CNC machine. I really haven't set Bobcad
up to have all the beginning and end functions I should have there, but it definitely
does work. I have cut several parts using Bobcad designs run through the
G-code generator of the CAM function. I find Bobcad cumbersome, and
the added things I have to do to specify roughing passes and finish offsets
to be pretty difficult. But, if the geometry is complicated, it definitely does
the job!

Jon