> -----Original Message-----
> From: Gene Heskett [mailto:ghesk...@shentel.net]
> Sent: August-28-19 9:32 PM
> To: emc-users@lists.sourceforge.net
> Subject: Re: [Emc-users] MESA 7i92H Saga
>
> On Wednesday 28 August 2019 23:18:28 John Dammeyer wrote:
>
> > Yeah the backlash is pretty bad but even back in May 2007 when I
> > bought it the cost for all that mass was quite low. Now Grizzly
> > doesn't even sell that model anymore and the closest unit is almost
> > double. Add an exchange rate of over 30% and installing ball screws
> > will be cheap and has always been in the plans.
> >
> > I bought mine in Canada from House of Tools which is no longer around.
> > But this link will get you to it at Grizzly
> > https://www.grizzly.com/search?q=(g3616)
>
> And the spec sheet doesn't list backlash. And backlash above 10 thou can
> get quickly messy. And those screws aren't Nooks. I'd be looking for a
> ball screw kit on fleabay. That will reduce the backlash to around a
> thou and things will be much happier. They will also need 5% of the
> motor torque to turn them compared to the acme or square thread screws
> in it now.
>
I put ball screws on my JGRO CNC router. I found the 1/2" ACME screws I
started with and the nylon nuts caused too much problems along with them not
really being straight.
What I found with the ball screws was they are much nicer but the longer one
still suffers from whip if it turns too fast. The plan is to add the second
ball nut to remove what little backlash there is but also about 4" away from
the first essentially making the ball nut 6" long. That should reduce the whip
considerably.
I think for the Y axis on the mill it won't be a problem but the X is fairly
long and the ACME screw is currently 25mm in diameter. I'd want at least 5/8"
(16mm) and preferably 20mm. Most of the cheap ball screws out there are far
smaller. And I have to fit the ball nut into the same place as the ACME nut.
(which doesn't have any sort of backlash adjustment)
John
> > > -----Original Message-----
> > > From: Sam Sokolik [mailto:samco...@gmail.com]
> > > Sent: August-28-19 7:52 PM
> > > To: Enhanced Machine Controller (EMC)
> > > Subject: Re: [Emc-users] MESA 7i92H Saga
> > >
> > > You must have a substantial amount of backlash? The backlash is
> > > taken out at max axis acceleration. that is why you need to have 2X
> > > acc for the stepgen_maxaccel. There is a chance that the axis
> > > could be accelerating and the backlash taken out at the same time
> > > causing 2x the acceleration being sent to the stepgens.
> > >
> > > On Wed, Aug 28, 2019 at 9:24 PM John Dammeyer
> > > <jo...@autoartisans.com>
> > >
> > > wrote:
> > > > Gene,
> > > >
> > > > You've got it backwards. The parallel port version is correct
> > > > which is why that line is there.
> > > >
> > > > > > setp parport.0.pin-07-out-invert 1
> > > > >
> > > > > remove the above line, or set it false
> > > > >
> > > > > > net zdir => parport.0.pin-07-out
> > > >
> > > > Peter Wallace suggested I do this:
> > > > "If you are using LinuxCNC 2.8 you can set the stepgens dir
> > > > polarity parameter."
> > > >
> > > > Can't find a "dir polarity" parameter here:
> > > > http://linuxcnc.org/docs/html/drivers/hostmot2.html#_parameters_3
> > > >
> > > > However there is an "invert_output" parameter . Still nothing I
> > > > can find online really guides me in the direction of doing it with
> > > > port inversion or how to use that invert_output since the dmesg
> > > > instructions in the above
> > >
> > > web
> > >
> > > > link don't help.
> > > >
> > > > However the other suggestion from Peter which was to set the
> > > > stepgen
> > >
> > > scale
> > >
> > > > to minus. Did that to both Z and Y axis and now homing works.
> > > >
> > > > I suspect because the parallel port implementation always reported
> > > > "unexpected real time delay" on start-up that it wasn't able to
> > > > actually keep up with the acceleration parameters in the .ini
> > > > file. Now with the 7i92H, the acceleration on the Z axis is way
> > > > too high for the Gecko run stepper motor. It's not losing steps
> > > > but doesn't sound happy. A bit of tweaking with values and it's
> > > > much smoother.
> > > >
> > > > I've also played around with the Y axis acceleration since it too
> > > > has a lot of mass to move; not only the X axis but also the X axis
> > > > table rotation mass. That's why the DC Servo has 4:1 instead of
> > > > 3:1 reduction like the X. Max ipm is therefore 120ipm for Y and
> > > > 150ipm for X. For the knee with the Gecko and 3:1 to the bevel
> > > > gear I get 24ipm. Any higher
> > >
> > > than
> > >
> > > > that and the stepper will skip.
> > > >
> > > > The backlash compensation is also far harsher. A series of jogs
> > > > in the same direction create almost no vibration on the head of
> > > > the mill (hand placed against it) but change direction and my hand
> > > > feels the bump as the backlash is first taken up. This was
> > > > especially noticeable on the knee with the slight bevel gear
> > > > connection to the knee screw until I dropped acceleration to a
> > > > much lower value than I had on the parallel port version.
> > > >
> > > > The 7i92H requires the STEPGEN_MAXACCEL to be double the
> > > > MAX_ACCELERATION. Setting it lower than that caused strange
> > > > motion problems so I suspect the only way to get rid of the hard
> > > > bump as backlash is taken out is to also reduce axis acceleration.
> > > >
> > > > Or buy ball screws which is on the list...
> > > >
> > > > John
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > _______________________________________________
> > > > Emc-users mailing list
> > > > Emc-users@lists.sourceforge.net
> > > > https://lists.sourceforge.net/lists/listinfo/emc-users
> > >
> > > _______________________________________________
> > > Emc-users mailing list
> > > Emc-users@lists.sourceforge.net
> > > https://lists.sourceforge.net/lists/listinfo/emc-users
> >
> > _______________________________________________
> > Emc-users mailing list
> > Emc-users@lists.sourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/emc-users
>
>
> Cheers, Gene Heskett
> --
> "There are four boxes to be used in defense of liberty:
> soap, ballot, jury, and ammo. Please use in that order."
> -Ed Howdershelt (Author)
> If we desire respect for the law, we must first make the law respectable.
> - Louis D. Brandeis
> Genes Web page <http://geneslinuxbox.net:6309/gene>
>
>
> _______________________________________________
> Emc-users mailing list
> Emc-users@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/emc-users
_______________________________________________
Emc-users mailing list
Emc-users@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/emc-users
