# Is It Realistic To Cnc A Knee Mill Z Axis?



## coolidge (Nov 1, 2015)

I'm talking a PM 935, its smaller but the knee is still pretty heavy.


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## JimDawson (Nov 1, 2015)

My initial reaction is no.  But it has been done successfully, a counter balance would be in order I think.  I've never seen it done with a ball screw but that would be in interesting experiment.  The one thing that you would lose is being able to make minor Z adjustments on the fly when running as a CNC, but I guess you could adjust the quill to accomplish the same thing.

I have also see the Z axis done by moving the entire head vertically on a set of ways that were installed between the ram and the head with a large ball screw, this allowed the head to still rotate and nod, as well as keeping the quill free for manual operation.  This was on a machine the same size as mine, with about 400 lbs of head.


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## arvidj (Nov 1, 2015)

Here are several threads that might be of interest. You may need to dig a ways thru each of them but trust me, there is useful 'CNC the knee' information in both of them.

http://www.cnczone.com/forums/vertical-mill-lathe-project-log/25895-cnc-forum-posts.html

I have thought about his air cylinder approach and really liked it based on the minimal amount of mass and complexity added to the system over a counter weight\pully\cable system, with the added benefit of the counter force being easily adjusted. I've also wondered "would there be an advantage to making the air cylinder and the ball screw and thread assembly co-axial?" ... as in the shaft of the air cylinder was replaced with the ball screw and thread assembly.

I also believe that in the thread are references to commercial machines that CNC the knee but use weights and cables as the counter balancing method.

http://www.cnczone.com/forums/vertical-mill-lathe-project-log/176547-cnc.html

A different implementation of a solution.

Arvid


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## Eddyde (Nov 1, 2015)

arvidj said:


> I've also wondered "would there be an advantage to making the air cylinder and the ball screw and thread assembly co-axial?" ... as in the shaft of the air cylinder was replaced with the ball screw and thread assembly.
> Arvid



I don't see any advantage to such an arrangement, First, I think it would be difficult to integrate a ball screw into an air cylinder. Second, when fully extended the where would the bottom of the cilynder be? Assuming a knee travel of 18" the Z screw cylinder arrangement would be 36" long when fully extended?
From what I have read, two cylinders, one on each side of the Z screw is the best way to go.


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## Primitive_Pete (Nov 24, 2015)

What about a servo drive? Could you put a gear on the shaft the moves the bevel gear? I wouldn't be very fast but I wouldn't want all that mass flying around anyway.


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## chevydyl (Nov 27, 2015)

The problem with that is backlash, there's a lot of backlash in a used bevel gear set, even when new there's still a substantial amount. If you could get that part of it worked out you'd be good, just replace the acme screw with a ballscrew with a rotating nut, throw on some gas springs to counter some of the weight


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## RonGinger (Dec 4, 2015)

I have an older Jet knee mill, a bit smaller than the small bridgeport, but still has a substantial knee. I decided to control the knee since it is simpler. My plan was that if it didn't work well for Z I cold always just treat is as power fed and do the quill.

It has worked great, and I really like having the quill free for delicate drilling jobs. I did not add any counter balance, weights, springs or whatever. I have considered that a gas spring might add some speed, but I get fast enough Z moves. I have done some pretty delicate engraving job and the knee is going up and down rapidly without trouble.

When I first started I was having trouble loosing steps, but I switched to a smothstepper and it solved it.

I now run Mach4 with the ethernet smoothstepper, but I am working on a PoKy 57E-CNC board. Nothing wrong with the ESS, I just want to try the PoKey, I know Brian Barker has high regard for it.


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## arvidj (Dec 4, 2015)

RonGinger said:


> I have an older Jet knee mill, a bit smaller than the small bridgeport, but still has a substantial knee. I decided to control the knee since it is simpler. My plan was that if it didn't work well for Z I cold always just treat is as power fed and do the quill.



Ron,

Any pictures on what you needed to do to make it work? How you set things up?

Thanks,
Arvid


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## Muzzer (Dec 11, 2015)

There are 2 Z-axes on Bridgeport clones like this - the knee and the quill. There are quite a few commercial conversions out there and you can check out the manuals for some of them. Mostly they motorise the quill which is fine if you can live with only 5" of vertical movement.

Here's one example for the Prototrack. Scroll down to around page 152 for the Z-axis. Most of these conversions replace the feed stop with a yoke that is driven by a ballscrew.  This is what I'm doing on mine.

The problem with motorising the quill lever is that there is a lot of backlash in the gear so you need to load it with a weight or spring that is bigger than any load it's likely to see. 

Motorising the knee probably needs a counterweight but there will be a lot of inertia - and friction unless you replace the leadscrew.


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## JimDawson (Dec 11, 2015)

Muzzer said:


> The problem with motorising the quill lever is that there is a lot of backlash in the gear so you need to load it with a weight or spring that is bigger than any load it's likely to see.



See what I did here:  http://www.hobby-machinist.com/threads/z-axis-cnc-conversion.21060/#post-199564

The quill is driven by the rack & pinion, but is biased upwards with an air cylinder.  it has 0 backlash, and consistently holds 0.0001


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