New Machine Build- BP CNC Knee Drive Report

[79TigerPilot]

Yesterday I finished up the knee drive for 66’ J head BP and got it mounted on the mill. The initial tests look very good, I am getting better than 20 IPM on the knee with Keling 1810 oz-in stepper and Gecko 203V. I was a little worried the motor would not be up to the task of moving 700 + pounds of cast iron but so far I am very pleased with the results.

My design is sort of unique from other knee drive systems I have seen. Rather than drive the crank handle shaft and suffer the losses and backlash in the bevel bears I directly drive the knee lead screw with a 3:1 belt reduction to the stepper motor. There is a 72-tooth timing belt pulley mounted to the lead screw just below the bearing and a 24-tooth pulley on the stepper motor. The motor is mounted on the right side of the knee and the belt passes through a 1” x3” window cut in the knee casting. The motor mount bolts to the side of the knee casting covering the window. All the parts for the motor mount were cut on the mill using 2D under Mach3 control.

To assist the motor two 200# gas springs are mounted inside the knee just aft of the lead screw pylon. They fit perfectly in the little keyhole cutout at the rear of the main lead screw opening.

Initial testing at 50 IPM resulted stepper lockup while raising the knee even at low acceleration settings. I believe this is just a case of the stepper loosing torque at 750 RPM. Backing off to 30 IPM worked fine and backing off further to 20 IPM improved the smoothness. I still have some additional motor tuning and gib adjustments to do.

The only issue so far is some resonance from the knee crank handle shaft due to the slop in the bevel gears. I need to find a way to dampen the shaft or I will have to remove it.

I would upgrade from a stepper to a servo but I been working with Ray to add on of his Quill drives to the mill so the knee will become less of an issue.

More photos at: http://picasaweb.google.com/cschaffter/CNCMillProject#

Craig

[Sparky_NY]

I powered my knee recently. Last year, in preparation, I mounted a couple of those gas springs from McMaster. Earlier this year I found one of them completely lost its tension, last week the other one lost its tension. It might be worth keeping a eye on them to make sure they continue to do their job.

Just for your info, if you later decide to servo the knee..... I used a 40lb cont / 100lb peak allen bradley brush type servo I got new off ebay. I used a dugong servo drive. Belt driven at 2.46:1 it works great. It runs the knee at 80ipm without any gas struts which is exactly the speed I designed for. (servo motor is a lower rpm unit).

[mactec54]

79TigerPilot
You can ajust the bevel gear backlash, by just moving the gear on the shaft to remove any of your backlash

[79TigerPilot]

I already have the backlash set at less than .001". I am going to try and get it a little tighter but I don't think that is going to help. I believe its the torque pulses from the stepper that are causing the torsional vibration in the crank handle shaft. The shaft gets accelerated and then the bevel gear hits the next tooth and bounces back to the previous tooth. This will eventually destroy the gear. Would like to figure out a way to engage and disengage the gear by building a new shaft assembly. I would like to keep the manual handle just in case my drive goes TU.

Craig

Originally Posted by mactec54 79TigerPilot You can ajust the bevel gear backlash, by just moving the gear on the shaft to remove any of your backlash

[79TigerPilot]

Thanks Sparky, those are good data points for any future updates. I am somewhat worried about the gas springs, had lots of hydraulic chairs go bad over the years. Good news is the replacement costs is fairly low at $22. With them mounted side by side the cocking force will be low if one should go bad.

Was your servo a geared uint?

Craig

Originally Posted by Sparky_NY I powered my knee recently. Last year, in preparation, I mounted a couple of those gas springs from McMaster. Earlier this year I found one of them completely lost its tension, last week the other one lost its tension. It might be worth keeping a eye on them to make sure they continue to do their job. Just for your info, if you later decide to servo the knee..... I used a 40lb cont / 100lb peak allen bradley brush type servo I got new off ebay. I used a dugong servo drive. Belt driven at 2.46:1 it works great. It runs the knee at 80ipm without any gas struts which is exactly the speed I designed for. (servo motor is a lower rpm unit).

[Sparky_NY]

Originally Posted by 79TigerPilot Thanks Sparky, those are good data points for any future updates. I am somewhat worried about the gas springs, had lots of hydraulic chairs go bad over the years. Good news is the replacement costs is fairly low at $22. With them mounted side by side the cocking force will be low if one should go bad. Was your servo a geared uint? Craig

No, it is a Allen Bradley 1326 series, standard brushed DC servo. It is driving the knee's original crankshaft through a 2.46 :1 timing belt drive. I also see some backlash but under .001 at the knee crankshaft.

ALso, the gas springs failed before I ever put the servo drive on. I was still manually cranking during that time.