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Author Topic: Z AXIS KNEE  (Read 19109 times)

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Re: Z AXIS KNEE
« Reply #30 on: November 09, 2008, 02:54:41 PM »
My size 42 stepper drive BP sized CNC mill has Z on the quill, but my 55 year old back is tired of cranking the knee up and down so I'm adding an 1800 in-oz stepper motor to the knee also. I'll configure two start up files so I can use the knee for Z for heaving milling (quill locked) and the quill for Z for drilling, tapping, light milling.
So I'm going to have my cake and eat it too.

Ray L., I'd like to put some gas springs on my knee like you did, to you have any model numbers and the supplier info for the ones you bought?

Thanks,

Paul T.

Paul,

   The gas springs are from McMaster-Carr (www.mcmaster.com).  Just search on "gas spring".  I used two of the longest, heaviest ones they had - only $20 each!

Regards,
Ray L.
Regards,
Ray L.
Re: Z AXIS KNEE
« Reply #31 on: November 11, 2008, 07:45:14 PM »
I just finished powering my knee (Birmingham BP clone) with an 1840 oz-in stepper motor using the stock acme screw. Its working pretty well, 15 ipm even though I'm underpowering the stepper with only 48 volts, it barely gets warm. I suspect by adding gas springs and bumping my power supply up to 72 volts it would probably make 25ipm. The stepper motor has 2:1 reduction pulleys on it.

One thing I"m pleasantly surprised about is less backlash than I thought I would have, only 0.0035" total, although I'm expecting this to increase as the bevel gears wear in some, but that's way less than I expected.

I already have a powered Z quill on this machine, but I got real tired of cranking the knee up and down manually so I added the powered knee as a 4th axis, it works real well with Mach so far.

If I was doing it over I'd likely use one of the servo motors Ray L. used, but that aside I'm pretty happy with how the knee is working so far, its a real step forward from hand cranking, and although most of the time I've been able to live with 5" of quill travel this will give me more Z clearance under program control when needed.

I don't know if I could live with a knee powered Z only, I do a lot of spotting/drilling/tapping and the limited speed and acceleration with the knee would slow that down,  but so far even with the stock acme screw it looks like it would work pretty well.

Paul T.

Offline fast89

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Re: Z AXIS KNEE
« Reply #32 on: November 14, 2008, 07:59:22 PM »
The whole knee z axis is a pretty dumb argument. I've been using mine at 120ipm for months. In that time i've made 200+ parts with MILES of peck drilling. up,down,up,down,up,down thousands of times. i replaced the screw with a ballscrew and a custom pedestal. I replaced the original straight cut bevel gears with hardened spiral cut ones. If i was to do it over again, i would just go belt drive and loose alot of noise. I also installed a grease zerk on the side of the knee and a hard line to the bevel gears. I also covered the gears with a custom pvc cover to keep the chips from getting to them. The only reason i'm not moving it faster is the number of pulses per inch. I've had tons of problems with this machine and just recently found out that parallel ports become less stable and relaible over 40-45khz (where i am). New encoders are on the way and we'll be moving at 180ipm with more reliability. BTW, i've got 500lbs worth of gas springs under the knee. All the real cnc bed mills i repair  have 1000lb+++ heads. They all use counterweights except for the Haas, they use a single nitrogen charged gas spring. The little 30 taper  bed mills like a Robodrill or  Brother don't have anything.

Offline Sam

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Re: Z AXIS KNEE
« Reply #33 on: November 15, 2008, 12:25:09 AM »
Quote
The whole knee z axis is a pretty dumb argument.
>:( No, it isn't. I know I've learned a few things from reading about peoples experiences on this topic. I will bet I'm not the only one, Ed included.

Quote
i replaced the screw with a ballscrew and a custom pedestal. I replaced the original straight cut bevel gears with hardened spiral cut ones. BTW, i've got 500lbs worth of gas springs under the knee.
So basically you've made some significant modifications that will allow the knee to be used as a Z-axis dependably.

When I first read Ed's post, I agreed 100%. I have never seen a knee used in place of a quill feed. Of all the mills in the small shop I work, I couldn't imagine using the knee as a Z axis. Not in the worn out abused shape that they are in, anyhow. It just seems that using the quill would be the logical choice. I'm glad to learn that using the knee is indeed an option. With the right combination of mods, it CAN be done, and achieve results just as good or better than a quill. With the right combination of mods, pieces of metal can fly to the moon, and return to earth, too.
I have full confidence Ed was talking about a standard mill running on a cheap screw on a brass nut. (Ever seen a brass nut fail? It's not pretty!) If given the task of converting our J-Heads to CNC, would I choose the knee as the Z instead of the quill? No, I wouldn't. I suppose it's just my personal preference. I would feel more at ease driving the quill, instead of diving into unknown territory. To be done reliably, it seems there is quiet a bit of work involved, and most likely more costly. If given the task to convert our J-Heads to CNC and use the knee as a Z-axis....Could I do it? Oh heck yeah, I know that now for a fact! So case in point...It's not a dumb argument. 
In no way should anybody think that Ed has a screw loose, or is inexperienced in any way from reading this topic. Those of us that have been here for a while know for a fact Ed is a very capable machinist, and could teach us all a thing or two. He does some super good work, and is a valuable part of the forums. He's a forum "regular" and helps when he can, so "Thank you, Ed"
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."
Re: Z AXIS KNEE
« Reply #34 on: November 15, 2008, 01:45:00 AM »
Sam,

Thanks for the comeback,  and  , Yes , I too ,have seen a brass Z axis nut fail , and the knee dropped as far as it could go. OOPS. ;D
Box ways, maybe. Dovetails, no way.
I've got 50 years in the trade. I've seen it all, and did it all, ( well, almost.) but still learning. :D

Ed


« Last Edit: November 15, 2008, 01:55:37 AM by edvaness »
Ed VanEss
Re: Z AXIS KNEE
« Reply #35 on: November 15, 2008, 10:03:20 PM »
Ray,L can you post a photo of the gas springs you mounted on the knee? Id like to see exactly where you put them.

I have continued to work with my new AC servo on the knee. I have run it up and down very smoothly many times, but every once in a while it will hang and fault the motor going DOWN. I have also heard a bit of a growl, or chatter going down. I suspect I have a gib problem. I think it may be shifting slightly and acting as a wedge.

Eliminating the gears and driving the screw directly works very well. The crank shaft came out through the side in a bolt on casting, so I simply removed that and have a nice access hole into the knee. I was able to get the motor inside the knee casting. with a short belt to the screw. Mechanically it looks very neat, almost like it was designed that way.
Re: Z AXIS KNEE
« Reply #36 on: November 15, 2008, 10:22:35 PM »
Ray,L can you post a photo of the gas springs you mounted on the knee? Id like to see exactly where you put them.

I have continued to work with my new AC servo on the knee. I have run it up and down very smoothly many times, but every once in a while it will hang and fault the motor going DOWN. I have also heard a bit of a growl, or chatter going down. I suspect I have a gib problem. I think it may be shifting slightly and acting as a wedge.

Eliminating the gears and driving the screw directly works very well. The crank shaft came out through the side in a bolt on casting, so I simply removed that and have a nice access hole into the knee. I was able to get the motor inside the knee casting. with a short belt to the screw. Mechanically it looks very neat, almost like it was designed that way.

Ron,

    Some bad cell-phone pictures below.  I placed the gas springs next to the dovetails.  I thought a lot about the merits of putting them there, or alongside the pedestal, and decided this was best, to avoid the problem you seem to be having.  I figured this was more likely to keep the dovetails moving smoothly. 
    Mine does make more noise coming down than going up.  Up is actually pretty quiet.  On the way down, I get a small amount of "clunking" that I believe is the knee lagging slightly, then dropping.  The noise, I believe, comes from the bevel gears as the small amount of backlash opens and closes.  I haven't had a chance yet to really get to the bottom of it, but I don't think it's anything to be too concerned about.  And it also happens only at high speed (above ~50 IPM).

Regards,
Ray L.
Regards,
Ray L.

Offline fast89

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Re: Z AXIS KNEE
« Reply #37 on: November 16, 2008, 06:47:46 AM »
I had some chunking and jerky movement of the knee on the down move with the acme screw. I just figured it was protesting this kind of movement. After the ballscrew upgrade, everything smoothed out.  I couldn't be happier with it, well after this week i will be happier becasue it's getting a new encoder and then it will be moving 180ipm. It's also getting an automatic lube pump.
Re: Z AXIS KNEE
« Reply #38 on: November 16, 2008, 10:47:25 AM »
I had some chunking and jerky movement of the knee on the down move with the acme screw. I just figured it was protesting this kind of movement. After the ballscrew upgrade, everything smoothed out. I couldn't be happier with it, well after this week i will be happier becasue it's getting a new encoder and then it will be moving 180ipm. It's also getting an automatic lube pump.

Do you still go through the bevel gears, or belt drive?  Any problems with back-driving when the motor is not powered?

Regards,
Ray L.
Regards,
Ray L.

Offline fast89

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Re: Z AXIS KNEE
« Reply #39 on: November 16, 2008, 10:51:33 AM »
I put spiral cut hardened bevel gears in it. I never use it manually so i don't know if it would drop down or not if the servo is unpowered. I wish i had gone belt drive but hey, it works pretty good and makes money.