On Thursday 29 August 2019 01:24:19 John Dammeyer wrote: > > -----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)
I have made adjustable, double nutted nooks, on 1/2" screws, but the nuts are around $50 a copy. Got backlash in the thou range but it took a big motor to spin it too. I have a 1450mm long 25mm screw on my Sheldon 11x54. I put covering bellows over the screw on each side of the nut to keep the swarf at bay. Then I mounted the nut to a newly made apron plate such that the grease fitting was reachable. The only fly in that setup is that the left bellows is compressed enough to start lifting the carriage off the v-way when the carriage is under the chuck, and that effected the accuracy of the re-grind of the MT5 in the spindle bore a wee bit. Seems the spindle was bent about 7 thou by the weight of the mounted chuck when it fell over and broke the compound beyond repair. Long before it followed me home. My z motion is limited to around 60 ipm by the motor, a 1600 oz/in nema 34 whose inductance severely limits its top speed when only 55 volts is available. Taking that motor off the go704 and replacing it with a 954 oz/in and an AC powered driver took the go704's z from 27 ipm to over 100 ipm, and I then used that motor on the sheldon, I should do the same with the sheldon. Its a poor motor for almost anything but the knee of a big mill. I have no problems with screw whip at the speeds I can move it now. I have considered remaking the apron as there is about 3/4" of room the nut could move to the right, which would ease the squeeze on the left hand bellows but while a new plate is on site, I've misslaid my round tuit. :) I'd also have to arrange an extension grease fitting. It worked well while replacing the barrel on "old meat in the pot" which a smaller bore in deference to the old mans shoulder telling him the 30-06 Ackley Improved he'd shot out 4 of since back in the 60's, was way more gun than needed in WV's hills. So its now a 6.5 Creedmore, much easier on the shoulder and I'm still looking for that magic 1/2" load. 1" is easy, does that with almost everything I feed it. Since the nut on the Z is stopped at a foot or more from either ends hanger bearings, I don't see much of a chance at whip being a huge problem on mine. OTOH, I'd need to see how the apron on that one could be modified (or replaced as I did my old Sheldon) to mount the nut. I've even considered reversing the nuts install on the screw and putting it on the right hand edge of the aprons back, but that just moves the bellows crush problem to the tail stock end of the screw. Sigh... > 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#_parameter > > > > >s_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 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
