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Old Jun 10, 2007, 01:50 PM
John Montrose
Guest
n/a Posts
CNC router (stepper motors in parallel)

Hello folks,

I'm slowly amassing the parts to buiild an 8' x 4' CNC router and most
of the designs I've seen have dual leadscrews on X.

What are the mechanics of driving/synchronising these?

If we construct a crude 'flow diagram', it would go:

software -> parallel port -> breakout board -> stepper driver board ->
stepper motor

Where should the signal be split? Can TurboCNC or Mach 3 send
simultaneous and identical signals to two sets of parallel port pins?

If I split at the breakout board and send to two driver boards, is the
signal in any way weakened by doing this?

If I split before the driver board, are there any synchronisation
issues?

If I split after the driver board, do I need to consider, if you like,
'power issues' i.e. if my motors are 2.5A, does the board need to
deliver 5A?

How important would things like keeping the wires the same length be?

Thanks very much.
Old Jun 10, 2007, 03:01 PM
dave sanderson
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On 10 Jun, 19:50, John Montrose <johnmontrosetoena...@yahoo.co.uk>
wrote:
> Hello folks,
>
> I'm slowly amassing the parts to buiild an 8' x 4' CNC router and most
> of the designs I've seen have dual leadscrews on X.
>
> What are the mechanics of driving/synchronising these?
>
> If we construct a crude 'flow diagram', it would go:
>
> software -> parallel port -> breakout board -> stepper driver board ->
> stepper motor
>
> Where should the signal be split? Can TurboCNC or Mach 3 send
> simultaneous and identical signals to two sets of parallel port pins?
>
> If I split at the breakout board and send to two driver boards, is the
> signal in any way weakened by doing this?
>
> If I split before the driver board, are there any synchronisation
> issues?
>
> If I split after the driver board, do I need to consider, if you like,
> 'power issues' i.e. if my motors are 2.5A, does the board need to
> deliver 5A?
>
> How important would things like keeping the wires the same length be?
>
> Thanks very much.


If it was me Id split the feed into the driver, and use 2 (identical)
driver boards.
The input impedance to the driver boards *should* be high enough that
the drive from the parallel port will be sufficient. A quick look at
the data sheets should confirm this, and if not then a buffer amp to
split the signal is an easy fix. I wouldn't trust 2 pins to drive 2
drivers and rely on them being synchronized, Thats hard enough to
achieve in properly dedicated hardware with properly dedicated
software (my day job) that anything lesser is not likely to work 100%,
and its going to fail just when you dont need it to (assuming it works
in the first place).

Keeping the wires the same length is theoretically very important,
however unless your very unlucky (or have some expensive test
equipment) differences in the order of a couple of feet will be hard
(impossible) to detect.

Have you considered a mechanical split? ie use 1 stepper (approx 2X
the size?) and a timing belt to drive a pair of leadscrews. that
simplifies the wiring, and I suspect that if you do the calculations
you probably dont need twice the size of what you are currently
planning on.

hth

Dave

Old Jun 10, 2007, 04:04 PM
John Stevenson
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Sun, 10 Jun 2007 19:50:26 +0100, John Montrose <johnmontrosetoenails@yahoo.co.uk>
wrote:

>Hello folks,
>
>I'm slowly amassing the parts to buiild an 8' x 4' CNC router and most
>of the designs I've seen have dual leadscrews on X.
>
>What are the mechanics of driving/synchronising these?



Two ways of doing this, mechanical or software.
Mechanical requires two screws to be driven by timing belts or similar and geared
together.
Software approach below.

>
>If we construct a crude 'flow diagram', it would go:
>
>software -> parallel port -> breakout board -> stepper driver board ->
>stepper motor


Correct but after the breakout board we have N x stepper driver boards to go to N x motors
where N = the number of driven motors, not always the number of axis, again more later.

>
>Where should the signal be split? Can TurboCNC or Mach 3 send
>simultaneous and identical signals to two sets of parallel port pins?


Can't comment on Turbo CNC only Mach3.
In Mach 3 you have 6 axis X, Y, Z, A, B and C
X, Y and Z can be slaved to A, B or C quite simply by selecting from a menu which ones
need to slave to which. So in your case on a router assuming no rotary axis you select X
to slave with A and wire the two identical motors up to Z and A step and direction pins
from the breakout board to 2 of your 4 drivers, then onto the motors.
You only need to setup the X axis as the slaved axis [A] mimics the X axis.
All driver settings like microstepping has to be the same for both axis.

>
>If I split at the breakout board and send to two driver boards, is the
>signal in any way weakened by doing this?


No it treats the supply as being 4 motors just that two are slaved.

>
>If I split before the driver board, are there any synchronisation
>issues?



Doesn't apply.

>
>If I split after the driver board, do I need to consider, if you like,
>'power issues' i.e. if my motors are 2.5A, does the board need to
>deliver 5A?


You need to add all your motors together to get a total amperage and aim for 2/3 of that.

>
>How important would things like keeping the wires the same length be?


Under normal build constraints this shouldn't cause a problem but if you feed the two
slaved motors centrally and then go different directions the cable lengths will be more or
less equal.

>
>Thanks very much.

--
Regards,

John Stevenson
Nottingham, England.

Visit the new Model Engineering adverts page at:-
http://www.homeworkshop.org.uk/
Old Jun 10, 2007, 05:09 PM
Tony Jeffree
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

John S has given you the SP on doing "slave" configurations - 2 motors
driven from the same axis signals. This can work OK under normal
conditions; however, it is worth considering what happens when (as
will inevitably happen) you drive the slaved axes too hard. One of
them will lose steps first, at which point, the axis will be driven
slightly "crab-wise" - one motor leading the other - till it locks up
good & solid. At that point you will need to unwind one of the motors
to get that axis back in operation. In contrast, if you take the
mechanical approach and drive both leadscrews from a single motor via
toothed belts or whatever, this particular problem goes away.

Regards,
Tony

On Sun, 10 Jun 2007 19:50:26 +0100, John Montrose
<johnmontrosetoenails@yahoo.co.uk> wrote:

>Hello folks,
>
>I'm slowly amassing the parts to buiild an 8' x 4' CNC router and most
>of the designs I've seen have dual leadscrews on X.
>
>What are the mechanics of driving/synchronising these?
>
>If we construct a crude 'flow diagram', it would go:
>
>software -> parallel port -> breakout board -> stepper driver board ->
>stepper motor
>
>Where should the signal be split? Can TurboCNC or Mach 3 send
>simultaneous and identical signals to two sets of parallel port pins?
>
>If I split at the breakout board and send to two driver boards, is the
>signal in any way weakened by doing this?
>
>If I split before the driver board, are there any synchronisation
>issues?
>
>If I split after the driver board, do I need to consider, if you like,
>'power issues' i.e. if my motors are 2.5A, does the board need to
>deliver 5A?
>
>How important would things like keeping the wires the same length be?
>
>Thanks very much.

Old Jun 11, 2007, 03:40 PM
John Montrose
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Sun, 10 Jun 2007 13:01:42 -0700, dave sanderson
<david.sanderson@bem.fki-et.com> wrote:

>If it was me I'd split the feed into the driver, and use 2 (identical)
>driver boards.

....

>Have you considered a mechanical split? ie use 1 stepper (approx 2X
>the size?) and a timing belt to drive a pair of leadscrews.


Dave,

Thanks very much for the suggestions. As a comment only, and not
wishing to start an argument, your advice given on the basis of your
'day job' (i.e. not possible electronically) does seem diametrically
opposed to that of later replies.

If I do use a (long, flappy) timing belt, how do I cope with backlash?

Thanks.
Old Jun 11, 2007, 03:49 PM
John Montrose
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Sun, 10 Jun 2007 21:04:16 GMT, John Stevenson
<john@stevenson-engineers.co.uk> wrote:

>In Mach 3 you have 6 axis X, Y, Z, A, B and C
>X, Y and Z can be slaved to A, B or C quite simply by selecting from a menu which ones
>need to slave to which. So in your case on a router assuming no rotary axis you select X
>to slave with A and wire the two identical motors up to Z and A step and direction pins


John,

Thanks for the reply. Please read this with the implied huge smilie:
wiring identical motors to Z & A will surely have, shall we say,
interesting results!

I'd prefer to do it this way as it makes the build simpler, if more
expensive. Directly connecting each motor to its leadscrew means I
don't need to worry about coping with backlash in a (minimum 8') long
timing belt.

Thanks again.
Old Jun 11, 2007, 04:03 PM
John Montrose
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Sun, 10 Jun 2007 23:09:28 +0100, Tony Jeffree <tony@jeffree.co.uk>
wrote:

>John S has given you the SP on doing "slave" configurations - 2 motors
>driven from the same axis signals. This can work OK under normal
>conditions; however, it is worth considering what happens when (as
>will inevitably happen) you drive the slaved axes too hard.

....

>In contrast, if you take the
>mechanical approach and drive both leadscrews from a single motor via
>toothed belts or whatever, this particular problem goes away.


Tony,

Thanks for the reply. As you say, once steps are lost, there is a
problem. I'm hoping to minimise the possibility of this by
overbuilding using Arc's bigger motors and drivers, with a belt or
possibly a planetary reduction box.

The thing I don't quite understand is how one deals with backlash in a
long, toothed belt set-up. If the router can cut 4' boards, the belt
needs to be minimum 8'. It won't be like my car where the belt always
runs in the same direction; there will be continual reversals.

Thanks for your time..
Old Jun 11, 2007, 04:45 PM
Mark Rand
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Mon, 11 Jun 2007 22:03:31 +0100, John Montrose
<johnmontrosetoenails@yahoo.co.uk> wrote:


>The thing I don't quite understand is how one deals with backlash in a
>long, toothed belt set-up. If the router can cut 4' boards, the belt
>needs to be minimum 8'. It won't be like my car where the belt always
>runs in the same direction; there will be continual reversals.
>
>Thanks for your time..



The answer is simple.. If you have, say, 3mm of backlash and 50mm pulleys
that's about 7degrees. With a 5mm pitch leadscrew that's about 0.1mm of
error. That's bugger all on an 8x4 sheet of wood. It's equivalent to a 1 deg C
change in temperature of the steel frame of the router.

From experience with tape libraries using 16 foot toothed belts direct driving
the picker arm, I'd expect 3mm to be on the outside of the possible backlash.
1mm would be closer to normal.


Mark Rand
RTFM

Old Jun 11, 2007, 05:11 PM
Tony Jeffree
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Mon, 11 Jun 2007 22:45:37 +0100, Mark Rand
<randm@internettie.co.uk> wrote:

>That's bugger all on an 8x4 sheet of wood. It's equivalent to a 1 deg C
>change in temperature of the steel frame of the router.


Is bugger all an Imperial unit or Metric? <G>

Regards,
Tony
Old Jun 11, 2007, 05:17 PM
John Stevenson
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Mon, 11 Jun 2007 21:49:59 +0100, John Montrose <johnmontrosetoenails@yahoo.co.uk>
wrote:

>On Sun, 10 Jun 2007 21:04:16 GMT, John Stevenson
><john@stevenson-engineers.co.uk> wrote:
>
>>In Mach 3 you have 6 axis X, Y, Z, A, B and C
>>X, Y and Z can be slaved to A, B or C quite simply by selecting from a menu which ones
>>need to slave to which. So in your case on a router assuming no rotary axis you select X
>>to slave with A and wire the two identical motors up to Z and A step and direction pins

>
>John,
>
>Thanks for the reply. Please read this with the implied huge smilie:
>wiring identical motors to Z & A will surely have, shall we say,
>interesting results!


No no, TWO identical motors, not two THE SAME motors <bg>

>
>I'd prefer to do it this way as it makes the build simpler, if more
>expensive. Directly connecting each motor to its leadscrew means I
>don't need to worry about coping with backlash in a (minimum 8') long
>timing belt.
>
>Thanks again.


From your other post to Tony on about lost steps if you are thinking about using the Big
Bertha's from Arc and the large drives then just direct couple them via a coupling and
don't worry about reductions, belt drives etc.
The loads on a router are quite small unless you are building it from RSJ's and these
motors will be more than enough.

As an idea I have motors slightly less powerful than these of Ketan's direct driving
0.200" pitch ball screws with similar drives to Ketan's on 2 axis and one of Ketan's on
the other.
This is on a 3 tonne machine that does some serious work.

If you was going the belt drive route the best arrangement would be to mount the stepper
central and using two separate pulley and belt sets, drive each screw.
This way you will have approx 2' centre's and the belts would be less.
Backlash is dealt with by using an idler and getting the belt tight.
Timing belts are designed to be run tight, and I mean tight.

Other methods that may bear looking at is to hang the gantry off high level rails as
opposed to the normal method of bed level rails that everyone follows.
Upside of this is two fold, one the rails are out of the crap and secondly you can have
the X motor mounted centrally on the gantry and drive both side via two jack shafts and a
spring loaded rack and pinion.

Some of the high end laser cutters run this method as it's faster than balls screws due to
whip over an 8' length.

Other alternatives is to replace the rack with a long length of timing belt stretched and
glued to a rail and a spring loaded timing pulley as a pinion.
The same has even been done with lengths of chain and sprockets but that smacks of being
designed by Harley Davidson and I'm sure anyone can do better than that.

--
Regards,

John Stevenson
Nottingham, England.

Visit the new Model Engineering adverts page at:-
http://www.homeworkshop.org.uk/
Old Jun 12, 2007, 01:48 AM
Mark Rand
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Mon, 11 Jun 2007 23:11:57 +0100, Tony Jeffree <tony@jeffree.co.uk> wrote:

>On Mon, 11 Jun 2007 22:45:37 +0100, Mark Rand
><randm@internettie.co.uk> wrote:
>
>>That's bugger all on an 8x4 sheet of wood. It's equivalent to a 1 deg C
>>change in temperature of the steel frame of the router.

>
>Is bugger all an Imperial unit or Metric? <G>
>
>Regards,
>Tony


Bugger all is metric. The correct Imperial quantity would be three fifths of
five eights of F*** all.


Mark Rand
RTFM
Old Jun 12, 2007, 02:26 AM
John Stevenson
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)

On Tue, 12 Jun 2007 07:48:49 +0100, Mark Rand <randm@internettie.co.uk> wrote:

>On Mon, 11 Jun 2007 23:11:57 +0100, Tony Jeffree <tony@jeffree.co.uk> wrote:
>
>>On Mon, 11 Jun 2007 22:45:37 +0100, Mark Rand
>><randm@internettie.co.uk> wrote:
>>
>>>That's bugger all on an 8x4 sheet of wood. It's equivalent to a 1 deg C
>>>change in temperature of the steel frame of the router.

>>
>>Is bugger all an Imperial unit or Metric? <G>
>>
>>Regards,
>>Tony

>
>Bugger all is metric. The correct Imperial quantity would be three fifths of
>five eights of F*** all.


AND in Fahrenheit !!

>
>
>Mark Rand
>RTFM

--
Regards,

John Stevenson
Nottingham, England.

Visit the new Model Engineering adverts page at:-
http://www.homeworkshop.org.uk/
Old Jun 12, 2007, 06:56 AM
dave sanderson
Guest
n/a Posts
Re: CNC router (stepper motors in parallel)


John Montrose wrote:

> On Sun, 10 Jun 2007 13:01:42 -0700, dave sanderson
> <david.sanderson@bem.fki-et.com> wrote:
>
> >If it was me I'd split the feed into the driver, and use 2 (identical)
> >driver boards.

> ...
>
> >Have you considered a mechanical split? ie use 1 stepper (approx 2X
> >the size?) and a timing belt to drive a pair of leadscrews.

>
> Dave,
>
> Thanks very much for the suggestions. As a comment only, and not
> wishing to start an argument, your advice given on the basis of your
> 'day job' (i.e. not possible electronically) does seem diametrically
> opposed to that of later replies.
>

Nope, you misunderstand, not impossible, just likely. In widoze, iirc
you cannot guarentee anything to less than 10mS accuracy, adn given
the scheduler and other programs running it is likely that you will
get pins waggling at different times. whether this *actually* affects
you in the real world I dont know, but given that you would get in a
mechanical lockup if it happened and the other simpler alternatives
which cant give this problem. Dayjob wise errors in the >1mS range can
be a problem, I once spend a week chasing the differenct between 2
inputs on a chip...<Tears hair out>

> If I do use a (long, flappy) timing belt, how do I cope with backlash?


Simple, dont use a flappy timing belt
Timing belts are designed to give acurate relative rotation, its there
purpose. In a car they only rotate 1 way, so you can get away with a
simple tensioner, there is no backlash, so a simple spring will take
out the flap and the tension side is always the same.... on a mill you
have more space,and are less constrained by packaging / robustness
etc, so design a tensioner that keeps the belt tight in both
directions, soemthing like a fixed tensioner pulley on the backside of
the belt, or even the correct length belt. The Yanks seem to use
timing belts a lot on their CNC conversions so they can mount the
motor out of the way.

Dave

 


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