Messages

161

General Mach Discussion / Re: 3D acceleration problems and choppy cuts

« on: July 14, 2010, 07:32:41 PM »

Here's an update for anyone that's following this. I finally got some time to go through the  mechanics and electronics. I had a bunch of stuff to take care of on this thing that I've been putting off so I jumped right in. The bottom line is the ball screw nut on the Y axis came loose from the flange. I had rebuilt the entire Y axis a few months ago and forgot to put lock tight on the threads. Big head slap moment. That would explain why it would loose more with higher acceleration.
I just figured that out at quitting time so I didn't have time to do a proper test but the quick one I did do came out dead nuts on.
I'm still going to replace the servos and drives with something more robust. I ordered three Dugong drives and their HDBB breakout board. I'm going to change out the servos to the 1125 oz models.

When I get that all set up then I'll attack the chopping issue again. Maybe it will be gone!

Thanks again everyone for all the help.

Derek

162

General Mach Discussion / Re: 3D acceleration problems and choppy cuts

« on: June 23, 2010, 02:48:37 PM »

First of all Thanks for trying to help me on this.

I didn't reply specifically to the other posts as I think I cover them in this post and will cover them by the end of the tests.


I'm trying to keep forward momentum on this pattern while I'm trying to fix the chopping.

Here is a screen shot of what I'm carving

It's 24” in diameter and 10” deep. I'm carving the interior.
 I was waiting for some material to dry so I did a few tests. I set the acceleration to 30 on the X and Y and ran my roughing file with no cutter. It's a horizontal strategy with a .125” step. I set up a dial indicator on the Y axis and relied on the optical sensor on the X. After running the file the Y axis was off by .015”.  I reset the axis and ran the file again but stopped it before it got to the boss at the bottom. It was right on the money. I continued to finish the run and it was off by .010.  When the bit approaches the boss and makes the sudden change in direction it is quite violent. I believe this is what is causing to loose position.  I don't think it's mechanical as the toothed pulleys are keyed to the shafts. I then set the acceleration back to 12 and ran the file again. It came right back to "0" on the dial indicator.

I plan on doing a few tests on the finish cut on this pattern. 

Test 1. cut 1.5" with the same strategy as I used on the other pieces. this will be choppy.

Test 2. Increase the acceleration to 30 and cut another 1.5"

Test 3 convert lines to arcs and cut anther 1.5"

Test 4 reduce acceleration and cut another 1.5" with arcs. 

At this point I'll probably finish off the pattern as I'm starting to fall behind. Hopefully it will be a good visual as to what's going on.

I'll be doing this tomorrow.

Thanks again
Derek

163

General Mach Discussion / Re: 3D acceleration problems and choppy cuts

« on: June 22, 2010, 11:31:47 AM »

Your comment about 'loosing steps' is a bit confusing. This should not be possible with servo motors.

What is your encoder resolution and what are your steps per inch in Mach. Something I have seen a lot is people converting from steppers and using stepper appropriate reductions or even direct coupling. Gearing for a 1,000 RPM stepper with max torque at zero RPM and a 4,000 RPM servo with relatively continuous torque should be very different.

Just for an example, if you have a 600 oz-in motor at 2:1 ratio,then you get 1,200 oz-in at the lead screw. If you increase the gearing to 3:1, then you get 1,800 at the screw. Huge difference. If your max desired travel is say 200 IPM, the you might look at gearing for the servo motor to be at about 80% of it's rated RPM at 200 IPM of travel. If you over size the power supply , then you can use 100% of the motor's RPM rating. That should give you the max available acceleration from the motor . . provided you give the motor it's max amps.

Stepper motors sort of pour power where servos sip, sip, sip . . . but servos want a BIG straw.  I did a lot of testing with DC brush servo motors from little36V NEMA23 to 90V NEMA34. With the bigger motors, the difference in acceleration between 17.5A max drive output and 35A max drive output is incredible.

The servos top out at 900-1000 rpm. I'm running them at 2:1 with .250 per turn lead screws. The best I can get out of this arrangement is 120 ipm so I make the max 100 ipm. As far as loosing steps well I guess in theory that should be correct. I know that if I set up a dial indicator and run a bunch of moves with the acceleration at 20 I loose position. If I turn it down to 12 I can run it for hours and It always returns to 0. I've always read on the boards about loosing steps if your acceleration is too high. If this only applies to steppers then maybe I have drive tuning issues.

I'm not cheap I'm poor   Since I don't know what the hell I'm doing I figure put the biggest motors and drives I can afford. I just feel like I'm asking too much from these motors. I built this machine when all Art was still working the bugs out of Master 5 (yes I've upgraded to Mach)  so I have probably gotten my monies worth out of these servos! Even If it doesn't solve my chopping problem it will help out my rapids. 

Derek
 

164

General Mach Discussion / Re: 3D acceleration problems and choppy cuts

« on: June 22, 2010, 10:06:58 AM »

Thanks Simpson
My motors max speed is 100 ipm with an acceleration setting of 12. This is the most I can run without loosing steps. I've done experiments previously with increasing the acceleration and the chopping really improves. I really feel the router is really under powered for what I'm asking of it.

I'm really not worried about the surface finish in that it looks a lot worse in the picture than it is. The problem with contour profiling with small step overs (as I see it) you can never get a proper chip load to get a great finish. Your always just skimming the surface of the piece. If I can cure the vertical lines I'll be a happy camper! The problem really only shows up when I'm carving symmetrical objects like  rings and such.
I looked at the power supply you suggested but unfortunately the spec sheet link is broken. I'll email them and see what I can get.

Here's a picture of the piece I'm doing the finish work on and had over a million lines of code. It's a tri-power manifold for an early Lincoln.



Thanks again for the help
Derek

165

General Mach Discussion / Re: 3D acceleration problems and choppy cuts

« on: June 21, 2010, 10:01:02 PM »

Thanks for the info guys.

The drive system is tight as far as backlash but I have a tiny bit of shake in the gantry. The gantry is pretty ridged and it only shakes in the curves so this may be related to the chopping.
Here is a shot of a piece I just finished. I sprayed a black lacquer guide coat and gave it a quick sanding to show the chops.

This is a 24” diam ring. It has a 2 degree draft. I'm cutting it with a horizontal strategy with a .010” step down. I plotted out a couple of the line segments that the G code generated. The line segments are about .090 in length with a .0004” deviation from the arc.
In the picture you'll see the vertical lines that form the chop. The distance between those lines is about .380". The smaller stuff between them is just chatter from the 8” bit and the lack of smoothness in the gantry. That I can live with as a quick sanding gets rid of it.  The chop is at it's worst where the X and Y are working together and then smooths out when it gets to parallel.
The first piece I cut was with a feed rate of 80ipm. The second one (pictured) was cut at 50 ipm.
There really was no difference in the chop. 
I'm roughing out the inner pattern tonight and I'll have it ready for the finish pass tomorrow night. I read a post by Hood that suggested reducing the max speed in the motor tuning section should help the problem so I'm going to give that a try.

Simpson I've read that test a few times before and we're in complete agreement on the Geckos and Rutex drives. I have had the exact same experiences. The geckos would fault if I farted too loudly.
The Rutex 2020 drives are really over rated  in the amp department. I've popped a couple and Tom has been really good about replacements but I just have lost my trust in the Rutex product.
Thanks for the explanation on the amps and torque. I'm going to be purchasing a new power supply to run the larger servos and I'm going to be buying bigger drives but I was hoping to do it in stages. I plan on only changing out the X and Y axis.

The servos I'm looking at are 1125 oz  90V 7.8A continuous and 40A peak. Can you make a recommendation for a power supply?


Thanks
Derek

166

General Mach Discussion / 3D acceleration problems and choppy cuts

« on: June 20, 2010, 01:38:48 PM »

I've been battling this problem for ages. And it's only getting worse because the complexity of my work is increasing.  Plus I'm really getting tired of sanding out the chops.
I have a 4' x 5' Gantry router using 600 OZ Ametech servos driven 2:1 and Rutex 990H drives.
I do fairly large foundry patterns. My last pattern had over a million lines of code and took 33 hours to run. And that was just the finish pass!

The problem is related to acceleration for sure and I've read all the posts about Tempest. I even gave Quantum a try and although it smoothed things out it was just too slow to be practical.  If I raise the acceleration the cuts get better but I start loosing position.

My plan is to upgrade the servos to something that will give me a buttload (that's a technical term look it up) of torque. In a perfect world I would change the servos, drives and power supply. But in this economy it far from a perfect world.
My thought is to switch to the Keling KL34-180-90 1125 oz servo and a new power supply. I'm hoping to retain the Rutex drives and limp it along until I can replace the drives with a set of Dugongs down the road.

So My questions are:
Am I kidding myself about using the rutex drives with the new servos.

Will this intermediate step actually help increase my acceleration.

Thanks
Derek

167

General Mach Discussion / Re: THIS IS DEDICATED TO "HOOD"

« on: May 04, 2010, 01:14:00 PM »

Hey Hood you suck.

Just a little balance to keep your head the right size


Thanks for all the help over the years.
Derek

168

General Mach Discussion / Re: Feedhold stops spindle

« on: January 21, 2010, 05:05:16 PM »

Thanks Phil.

This brings up something that's bothered me for a while. Why are the hot keys  different for mill and turn. That's dumb.

Derek

169

General Mach Discussion / Re: Feedhold stops spindle

« on: January 21, 2010, 04:34:03 PM »

Hi guy's
This is a fresh install. I've been putting up with this for  while. I just don't feed hold during a cut. It would work a heck of a lot better if it operated like the mill. Having to make a choice between feedholding during a cut and loosing a carbide or waiting til the end of the cut saving the carbide but possibly loosing the peice sux.

Derek

170

General Mach Discussion / Feedhold stops spindle

« on: January 21, 2010, 03:06:33 PM »

Hi
I installed Mach on a lathe and when I hit the spacebar the spindle shuts off. Is there a setting to keep the spindle on. It stays on when I feedhold on my router but I can't find a way to make it work on the lathe.

thanks
Derek