Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: edvaness on November 07, 2008, 07:00:26 PM
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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
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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
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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.
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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
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;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
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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.
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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
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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.
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Hi, Ed V
Search for Bridge Port Series 2 CNC, I have 2 of them, There Air Assist Ball Screw Knee.
Chip
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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
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Ed not sure what rapids the Series II had on the knee but I would imagine it would be as fast if not faster than 100IPM. They use ballscrews and air assist which basically means the knee has very little weight. If I remember correctly you can adjust the air pressure for the workpiece load as well but maybe Chip can give you more reliable answers.
Hood
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I converted a Jet knee mill and did the knee instead of the quill. I used a 1200 in-oz stepper and put it directly on the crank position. It has worked for a year, but I recently tried to cut a mold in aluminum that ran a couple hours and I had some loss of Z position. The funny part was it actually GAINED in Z, so I wound up deeper that it was supposed to be. I think it was because the Zup moves were G00, while the down moves were at a slow feed rate.
I had an AC servo on hand so I decided to try that. I removed the gears and was able to mount the motor inside the knee casting with a short belt to the top of the screw. But I can only get a 2:1 belt drive in there, so the servo cannot quite handle it. I am waiting for a gearbox that fits on the top of the servo.
I like having the quill free to do delicate drilling or tapping operations. I also like it for tool change- I use the Tormach tool holders, and I put a heavy spring under my drawbar. I built a simple bridge above the drawbar. To change tools I simply push the quill up to compress the spring and eject the collet.
There are some photos of my Jet conversion on my website plsntcov.8m.com
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Interesting website Ron, very informative.
Thanks for the link,
RC
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Maybe the true reality of the choice to cnc the quill or knee is that most of us are retro fitting manual machines and the quill was not designed (or not close tolerance enough) for milling and would need substantial rework to fit the purpose. This could also be true of manufacturing machines manual or cnc that have had their hay-day and are now on the second-hand market.
I have a Chinese mill (which was all I could afford at the time) which is similar in size to a small Bridgeport. I'm in the stages of converting it to cnc and have 3 axis working now, I chose the knee to get moving first over the quill as the machine has essentially a mill/drill geared head and the instruction manual clearly states to: Lock the quill before milling.
Also the knee is an easier prospect. I fitted it with an air assist ram 100mm dia (4") with an inlet that would let me fit 1/2" compression fittings I coupled this to a precision air regulator, the only one I could find (on eBay) had larger inlets and outlets so I had to fit reducers to fit 1/2" and left the exhaust without a reducer. It finely balances the knee @ approx 50psi.
It runs a 24v 375w dc motor/ Gecko driver and is geared 4-1 to a 16mm pitch 25mm dia ballscrew I had to fit a brake which is on the top end of the ballscrew. Encoder is currently 500 cpr but I'm looking to raise this to 1000 cpr to get the resolution closer to the other axis. Backlash is currently 0.08mm and I'm hoping this is all in the second-hand ballscrew, I have designed all the nuts on the axis's to accept another ballscrew nut to reduce any backlash. With Mach3 backlash setup, The knee has good repeatability.
Once up and running, with my inexperience at tuning Gecko's I was concerned about machine making a noisy vibration like the bass note of a cello which seemed to fill the whole workshop when moving the knee in the downward direction. This was cured by much head scratching and then retuning the gecko drive which cured the problem. The servo drive G320 is very tunable on this axis and I could see the springiness of the system on a dial-gauge as I tuned it in.
After many hours fiddling I was satisfied that all was good. It now achieves 4000mm per min (157 ipm) at 150mm/sec/sec without faulting the drive. I expect more fiddling and tuning could raise that some but I'm happy.
The main important thing with all this sliding up and down of the knee is lubrication and plenty of it. I've made a air assisted oiler for all the slides and x-y ballscrew the z ballscrew is greased.
So for most of us cnc'ing the knee is the way to go.
John
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I'm with Hood. I think the smart way to handle Z travel is as he proposes. Use the knee for TLO's.
My Series II knee has huge box ways and an air assisted knee. I am sure I could get the
knee to some serious rapids. I have seen knee's driven with 40 in-lb servo's (read huge), and
the servo's were driven by Gecko's. These knees were able to drill holes in 3/4" steel plate
all day with perfect reliability.
I think driving the knee is going to wear it out prematurely. I have heard other state that this
is the case. Looking at the wear on some older machines that never had a power knee
seems to indicate that this is indeed the case. The tremendous weight on the dovetails
makes them prone to wear just under normal (handcranking) use.
My series 1 cnc is a bit restrictive with the 5" of quill travel, but as Ed says, it is certainly still
capable of doing most jobs.
Scott
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Looking at the wear on some older machines that never had a power knee
seems to indicate that this is indeed the case.
Most likely because of the manual labor that is involved in cranking those tables, they only get used when absolutely necessary. That's how it is in my case anyhow, but I'm a bit on the lazy side. ;D
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Hi, Ed V
Mine are Stepper Motor, Haven't used them in quite some time, No pic.'s, Yes the air assist is adjustable, Up to 1,000 lbs part wt.
Sorry no real speed reports, I do like the independent Z quill feature.
The servo powered one's went back to Z axis spindle and I'd think there faster in Z axis.
Chip
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I just got done re-biasing the optos on my BOB, and my knee rapid speed has gone from 25 IPM, BOB-limited, to 60 IPM, motor limited. Not bad!
Regards,
Ray L.
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Driving the knee is a waste of time, rapids far too slow, much better to spend £15k on a proper VMC ::)
(My BP with a 8Nm stepper driven knee conversion works fine, it won't win any speed competitions (neither will my car), but does juuuust fine).
When it does eventualy wear out I will buy another mill, pick the best bits from both machines and rebuild, but by then "star trek replicators will be available and I'll go from CAD to part without all the messy swarf. ;)
Derek
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Gee, we seem to have an awful lot of very happy knee-drive folk here, considering what a bad idea it is! :-)
Now that I'm getting 60+IPM out of my knee, and 300+ IPM on X & Y, I'm a VERY happy camper!
Regards,
Ray L.
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Ray,
Glad ;D your happy, Just make sure that knee Gib is properly adjusted. with the knee clamp off , and well lubricated. with 20w way oil.
If your happy with the Z rapids , That's a good thing, But I'm sure others will tell you it will wear after time. OIL-OIL- OIL.
Best of Luck
Ed
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Hey Ed, Britt will be giving you stick about this when she sees it as her brother Brett has his Frankenstein using the knee. ;D
Hood
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Hey Brett
Lets hear about it.
Britt
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Isn't Britt short for BrittKnee ?
You better hold on to something Ed.
RC
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Isn't Britt short for BrittKnee ?
You better hold on to something Ed.
RC
Yes I believe it is RC, I also think she has two middle names as is common with rednecks, if I remember correctly they are inda and edstones.
Hood
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Interesting thread.
I considered driving the knee on my Atlas but chose the quill.
This is a 38 year old machine converted to CNC. Like echoed before, one of the pains of driving the quill
is mounting everything to head and the backlash must be dealt with. I used the sensitive feed adjustment
which happened to be activated by pulling out the handle thus i still have manual control if desired.
The knee like the x & y has a digital readout so setting position manually is rather easy if required.
But that's my nickle,
RICH
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Ed, the Knee is painfully slow on mine, 30IPM. Considering what the mill is, it really isn't an issue except I'm impatient. As Hood said earlier, I'm planning on useing the quill on the hurco for the z and the knee for tool height compensation but I hopeing for 600 - 800 IPM rapids on all axis on it. The place a slow axis will kill you is doing 3d work. If all need to move equal amounts in the same amount of time, the slowest limits all. Actually that is true for any combo move.
By now, you should know about Britts knee. Its the first bit of her leg that you see after she breaks her foot off in your *&%&*^%. You know you can't denie that happens quite often. ;D
Brett
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LOL :D
600-800 ipm.? That's about the speed Britts leg was at when she put her foot up me arse.
But as far as seeing her knee goes. NO . It's somewhere under all the hair. ;D
But, as far as using the knee for tool length comp, that's a whole different story.
Ed
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Ron G,
Very interesting web site you have. I like seeing neat workmanship, and its there.
I have plans for a 9 cyl radial engine, and some others I picked up at a model engine show in Detroit a few years ago,
but theres just not enough hours in a day. Need to get the Hyster done first.
Ed
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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.
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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.
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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.
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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.
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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.
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"
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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
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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.
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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.
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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.
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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.
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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.
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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.
Where did you get the gears? I'm actually very pleased with how mine works, I just want to make it quieter, and more durable. The gears, I think, are a significant source of noise. Other than that, it works great as-is - less than 0.0001" backlash, and excellent repearability.
Regards,
Ray L.
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The gears are from MSC, they are off the shelf Boston Gear units. I had to modify them of course. Does yours have straight cut or spiral cut gears? I don't think these gears are much quieter than the original ones but i don't get the cogging noise from the old acme screw. The gears sound better at 150ipm than they do at 100ipm.....figure that one out. Soon we'll see how they sound at 180ipm.
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The gears are from MSC, they are off the shelf Boston Gear units. I had to modify them of course. Does yours have straight cut or spiral cut gears? I don't think these gears are much quieter than the original ones but i don't get the cogging noise from the old acme screw. The gears sound better at 150ipm than they do at 100ipm.....figure that one out. Soon we'll see how they sound at 180ipm.
Mine are straight-cut. What kind of drive are you using to get 150 IPM?
Regards,
Ray L.
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I'm using a gecko 320. I have run it at 150ipm but the pulse rate is too high to be reliable so i keep it around 100ipm. It's only got a 250cpr encoder on it now but it's going to be a 120cpr encoder. The belt drive thru the original crank shaft is 2-1 reduction, then the gears are 2-1 reduction also.
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I'm using a gecko 320. I have run it at 150ipm but the pulse rate is too high to be reliable so i keep it around 100ipm. It's only got a 250cpr encoder on it now but it's going to be a 120cpr encoder. The belt drive thru the original crank shaft is 2-1 reduction, then the gears are 2-1 reduction also.
What kind of motor? My 850 oz-in peak servo will drive mine to 70+IPM, running through a 4.8:1 reducer. I'm sure it will go somewhat faster (I currently have my SS limited to 128kHz step rate), but I doubt I'd make much over 100 at best.
Regards,
Ray L.
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I'm running the same hopeshopcnc motor as you. Now that i think about it, i may have more than 2-1 gear reduction on the belt drive side. Might be 3-1?