# Powering the knee on a knee mill for Z-axis travel



## 7milesup (Oct 14, 2022)

I found this video the other day and was rather intrigued by it.  The guy in the video cut a hole in the side of his knee and installed a servo motor and cogged belt driving a ball screw.  He had replaced the ACME screw with a ground ball screw and nut.   Cutting into the knee would concern me only from an aesthetic standpoint and not a structural standpoint because I feel that there is enough cast iron that the hole would not matter.
He (Owen) utilizes gas struts to offset the weight of the knee.   Utilizing the knee for the Z-axis movement eliminates all of the clutter associated with a quill Z-axis conversion and also maintains all of the manual controls.
The one downside I see to this is the possible wear on the knee ways, although as a pure hobby machinist, the mill will probably outlast me in that role.


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## Aukai (Oct 14, 2022)

Impressive


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## pontiac428 (Oct 14, 2022)

For sure!  The knee is for milling, the quill is for drilling, tapping, boring.  I see one issue, the backlash is taken out of the knee elevator by the gas struts, which push up.  Unless he has CNC way locks, every time the z axis is under load, there will be a backlash issue as the slack is taken out between the  gas struts and the elevator assembly.  That negates the value in those ABEC 9 bearings and premium Japanese ball screws, in those cases where the shoe fits.

Edit:  The partition wall around the mill is... interesting.  Probably does great to keep chips off the electronics and what lies beyond.


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## Shotgun (Oct 14, 2022)

He's shooting alcohol through a paint sprayer for a 2hr cycle time!?  I wonder if he knows that this is how thermobaric bombs work?


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## Aukai (Oct 14, 2022)

I'm wondering if there is a momentary pause during a Z operation the gas struts will exert enough pressure to push the table back up to starting point before backlash?


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## pontiac428 (Oct 14, 2022)

Aukai said:


> I'm wondering if there is a momentary pause during a Z operation the gas struts will exert enough pressure to push the table back up to starting point before backlash?


Or if it can even push it back home consistently.  This is more a hair-splitting over design to me, backlash on a lead screw on a heavy knee can be very large but not an issue in manual operation, since gravity does its part in the same direction that the z pushes against the work.  I know I'm going full Karen in another thread about useless leading zeros in precision, but this is a completely different issue to me, it's an issue in design, not quality/cost of components.


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## 7milesup (Oct 14, 2022)

pontiac428 said:


> I'm going full Karen


Bahahaha.  That is funny. 


Aukai said:


> I'm wondering if there is a momentary pause during a Z operation the gas struts will exert enough pressure to push the table back up to starting point before backlash?


As @pontiac mentioned, some fine-tuning of the gas struts would/could eliminate any perceptible backlash.



Shotgun said:


> He's shooting alcohol through a paint sprayer for a 2hr cycle time!?  I wonder if he knows that this is how thermobaric bombs work?


Yeah, as soon as I saw that, I wondered if he knew if he was making a very explosive mixture.  At least he used "clean burning" fuel so his machine and shop wouldn't get all sooty when it started on fire.


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## pontiac428 (Oct 14, 2022)

7milesup said:


> Yeah, as soon as I saw that, I wondered if he knew if he was making a very explosive mixture.  At least he used "clean burning" fuel so his machine and shop wouldn't get all sooty when it started on fire.



I took some liberties on estimating his room volume and emission rate (minimal air mixing, typical shop size, 2 meter near field zone, 10 grams/minute emission rate), but the critical point of 10% of the lower explosive limit for ethanol on this simulation lies around 16,000 ppm.  After 2 hours continuous, he'd reach only half that under these conditions.  Still might have flash fires, though, ethanol = gasoline as far as flammability goes, but probably no grand explosion


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## Karl_T (Oct 14, 2022)

I have powered the knee two different ways. Ten years ago i added ball screws  and balancing air cylinders to my excello 





						Knee ballscrew toolchanger
					

One big weakness on a CNC knee mill is lack of travel in Z making it very hard to do toolchanges without moving the knee and losing your part 0 setting



					www.cnczone.com
				




A couple years ago, i bought a vectrax with a power knee. just added a VFD and scale to it.


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## 7milesup (Oct 14, 2022)

Karl_T said:


> I have powered the knee two different ways. Ten years ago i added ball screws  and balancing air cylinders to my excello
> 
> 
> 
> ...


That is awesome.  I like the idea of driving the ball screw from the bottom.  One more solution to ponder.


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## davidpbest (Oct 14, 2022)

7milesup said:


> I found this video the other day and was rather intrigued by it.  The guy in the video cut a hole in the side of his knee and installed a servo motor and cogged belt driving a ball screw.  He had replaced the ACME screw with a ground ball screw and nut.   Cutting into the knee would concern me only from an aesthetic standpoint and not a structural standpoint because I feel that there is enough cast iron that the hole would not matter.
> He (Owen) utilizes gas struts to offset the weight of the knee.   Utilizing the knee for the Z-axis movement eliminates all of the clutter associated with a quill Z-axis conversion and also maintains all of the manual controls.
> The one downside I see to this is the possible wear on the knee ways, although as a pure hobby machinist, the mill will probably outlast me in that role.


You really have to admire this guys resourcefulness.  I winced when I saw the image of the hole in the side of the knee, but I guess with the addition of the gas springs that Z-axis works pretty well.  I'd love to see some repeatability tests on tolerances achieved.  The air brush mist coolant system is inventive, but it's not MQL, and this the amount of alcohol he's spitting out has no chance to evaporate quickly enough to prevent a fire hazard.  As an aside, finding denatured alcohol (my coolant choice as well for aluminum, but delivered in a Fogbuster) has become a real supply challenge - at least here in the PNW.  IMO he made the right choice with the Acorn kit, but I think I would have gone with a new Nuc PC instead of that antique, if for no other reason than to eliminate the additional enclosure.  But all in all, very impressive.  If/when I do my CNC conversion, I'll try to hire this guy as a consultant.  Thanks for posting.


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