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Z AXIS KNEE
« on: November 07, 2008, 07:00:26 PM »
I'm seeing lots of post on other forums about using your knee for your Z.
The knee mill is not designed for it, you crank it to your desired height and lock it. That's the way it's intended.

I've never come across a job I couldn't do with 5" or 6" travel in the quill. Some jobs I have 10 tools set up. It's all
in your tool setup. You will need some extended tool holders for your shorter tools.
Unless, of course ,  you want to bore out your big block Chevy.

The knee mill uses an acme screw and miter gears to move that knee. When you do a peck drill cycle , you are raising havoc on those parts.
You will never get the speed that you get from your quill.  Check any knee mill with some use what happens when you unlock the Knee. It will sag.

A bed mill ,where the whole head moves up and down ,has a counter weight,which is usually inside the casting, and runs on ball screws, no miter gears needed.

Wheather you add air cylinders, gas shocks, or whatever, you'll never get the speed, you get from your quill.

Compare the weight of the head on  a bed mill to the weight of your knee, with saddle and table, and / or vise and part , all sitting on top an acme screw and miter gears.

I WOULDN'T do it, but , then again, it's your machine. Build it the way you want it.
 
besides all, Gecko doesn't make a drive big enough for a servo big enough to do it.

Ed  V
Ed VanEss

Offline Hood

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Re: Z AXIS KNEE
« Reply #1 on: November 07, 2008, 07:14:57 PM »
Ed, not all mills use the quill, think the Series 2 Bridgeports had the Kee as the Z. I am planning on doing the knee on the Beaver mill but it will be as an aditional axis to the quill and will be used for tool offsets, I think Brett is planning similar on his Hurco.

Hood
Re: Z AXIS KNEE
« Reply #2 on: November 07, 2008, 07:18:48 PM »
Sorry, but the knee just works fine, *with* a Gecko, and with the same servo motors that's driving X and Y.  Mine has been working flawlessly for the last month or more, and is very accurate and absolutely repeatable.  Rapids are not blindingly fast at the moment (25 IPM) but that is limited by a bad BOB, not the servo.  Adding $50 worth of gas springs has reduced the torque requirement by half, which will also help rapids.  When I get around to replacing the leadscrew and bevel gears with a ballscrew, I think it will work very nicely indeed.  And, I can still use the machine for manual drilling and tapping when I need to.  There is basically no slop in my knee.  Perhaps you're looking at poorly maintained production shop machines, or ones with many, many miles on them?  Besides, this is a hobby machine, not a production shop machine.  Even the bevel gears will probably last a good long while with the kind of use it sees.  Speed isn't everything, and you *can't* effectively totally automate a machine with only 5" of Z travel, unless all your jobs are pretty small.

I'd be real interested in hearing how you go from working with a small end mill, to drilling 1/2" holes, or working on a piece 3-4" high, with only 5" of travel.  Many of the jobs I do require going almost end to end on the knee travel.

Regards,
Ray L.
Regards,
Ray L.

Offline derek

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Re: Z AXIS KNEE
« Reply #3 on: November 07, 2008, 08:46:24 PM »
Well just because you've never needed it doesn't mean it's a bad idea. At least that's the impression I got from the tone of your post.

I've been running mine for over a year and a half now almost daily. My rapid is 60ipm. I regularly run programs with over 100,000 lines of code.A lot of times it's a Z axis profile.
 I'm using a rutex drive and a surplus treadmill motor. I have gas assist on the knee but it's still heavy enough to reduce the backlash. So for me the knee conversion has been the best mod I've done to the mill.

Derek
Re: Z AXIS KNEE
« Reply #4 on: November 07, 2008, 10:37:51 PM »
;D LOL  ;D 

  I knew there'd be some feed back on this.  :D

Hood, the series 2 Bridgeport is 5" quill travel. With a crank on the knee. If there is one with the knee travel, I'd like to see it.


Ray, like I said in my first post, it's how you set up your tooling. I can use a .187 b end mill and a .5 drill and machine a 5" high block.
Put the small end mill in an extended holder, to keep all your tools close to the same oa  length , which is best in any setup. Keep your longest tool as short as possible,
and keep all tools as close to that length. Saves wasted, and excess z travel. This does call for some very good holders .

Ed V
Ed VanEss
Re: Z AXIS KNEE
« Reply #5 on: November 07, 2008, 11:02:07 PM »
Ed,

    So that would require me to have my 1/2" end mill in maybe a 6" long holder, so I can also fit the drill chuck on the machine with a large bit? That's not going to give a very good result on heavy cuts.  The whole point of endmill holders is short = stiff.  I always try to keep the quill extended as little as possible, for the same reason.  Using long holders will significantly increase flex at the tool tip, and degrade finish quality and tool life.  Doesn't sound like a very good compromise to me.
    Ultimately, I plan to have the best of both worlds, by having both the knee and the quill CNC'd and using the knee then only for tool length correction.  But for now, the knee works beautifully.  So, I don't need to invest in a lot of long holders that I won't need down the road, and don't have to compromise finish qualty.

Regards,
Ray L.
Regards,
Ray L.
Re: Z AXIS KNEE
« Reply #6 on: November 07, 2008, 11:21:12 PM »
Ray,

Your saying; " But for now, the knee works beautifully." :D
That's not what your saying on the other forums. >:( 25 IPM.? I use higher feeds.

I have 40 taper tooling, and the extended holders are no problem, not even for roughing.
 I Only use a drill chuck for center drill. I use collet holders for drills.

Good Luck

Ed V
Ed VanEss
Re: Z AXIS KNEE
« Reply #7 on: November 07, 2008, 11:36:44 PM »
Ray,

Your saying; " But for now, the knee works beautifully." :D
That's not what your saying on the other forums. >:( 25 IPM.? I use higher feeds.

I have 40 taper tooling, and the extended holders are no problem, not even for roughing.
 I Only use a drill chuck for center drill. I use collet holders for drills.

Good Luck

Ed V

Ed,

   I have no idea what you're referring to.  My knee works fine, the only limitation being the relatively low speed, which is caused by a bad breakout board, not the drive.  That will be fixed.  99% of my milling time is spent moving in X and Y.  If I got the knee up to 100IPM like the other axes it would save me a few minutes a day, so who cares? Yiour original point seemed to be that moving the knee is bad, because it'll be slower than moving the quill.  For me, the difference would amount to mouse nuts.  If it doesn't save me time, or give me a better result, there's no reason for me to spend the money on it.
   If you're happy with what you've got, great.  But don't tell me your way is the only "right" way to do it.  What you do works for you, what I do works for me.  What's "right" or "best" depends on your needs and your expectations.  For me, a quill-only Z axis would be seriously limiting.  I don't have a million collets for all my drill bits, and I don't have 40 taper tooling.  Like probably 99% of the people here, I have an R8 spindle, and long holders *don't* work well in R8.

Regards,
Ray L.
Regards,
Ray L.

Offline Chip

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Re: Z AXIS KNEE
« Reply #8 on: November 07, 2008, 11:43:17 PM »
Hi, Ed V

Search for Bridge Port Series 2 CNC, I have 2 of them, There Air Assist Ball Screw Knee.

Chip
Re: Z AXIS KNEE
« Reply #9 on: November 08, 2008, 12:06:19 AM »
Chip,

I,ve seen lots of series 2 ,but none with the knee servo'd. Got a pic? Kneed to see one.
What ipm rapids do you get?
I,m not saying there not good. I'm saying you won't get the rapids you get on a quill. Can you get 100 ipm?
If you do lots of drilling, peck drilling, 50 ipm rapids just don't cut it.

Ed V
Ed VanEss