[endgrainguy]

I"m having massive problems with my new shop built CNC. I'm getting inconsistant results from day to day, hour to hour. I "discoverered" yesterday that my x axis screw, (a THK 20 MM d. 20 MM lead, four start, bought used in apparently good shape) was apparently mapped incorrectly, as determined by measuring a 24" run called for in MIDI screen. I had assumed it was in fact a 20 mm lead, as labeled. It appeared to be off by alot --about 2.7 percent. I did the math, adjusted the steps in motor tuning, tested it a dozen times with good results, and figured I had solved some mysterious issues i was having. Ran my first set of parts, with lousy results. Tool paths seemed inconsistant -a second program for a finish pass calced from same drawing didn't match up, and I was left pretty confused. All individual tool paths were fine, producing the desired size pocket. The problem was in initial referencing. I rereferenced beween programs, which may have been a mistake, but I should be able to do that, right? Today I checked the x axis again, and results were again way off -this time in the other direction, and I got it back to going the distance asked for by undoing most , but not all ,of the change I made yesterday. So now it seems I've had three different values work, and I"m sure this will stop working as well.
Another issue is that the reff X axis behaviour changes radically with speed settings of the axis. At higher speeds (200 IPM) it doesn't do its return trip -just clicks off and stops, at what would be an obviously very different position than if it were traveling slower. This I"ve found to be related to trying a higher debounce setting, which I did because my limit switches were tripping at moments of high resistance to the cutter.
Could I have a corrupted version of MAch (it came on a refurbished PC sold by an Ebay CNC stuff vender). Could the PC itself be the problem? I'm so frustrated and confused, and I have no experience with this stuff beyond the misery I've put myself through these past couple months trying to build a functional machine.
If anyone who is expert in these issues lives near Greenfield , Massacusetts, (I"m actually a few minutes away from there, in Colrain, I"d be happy to pay for a consultation to try to figure out my problems. I'm a full time woodworker, and all other work has ceased as I"m consumed with getting this beast producing for me. Thanks!

[MrWild]

Sounds like your lead screw is floating. Moving in the bearings, bearings moving in the housing, or your nut is floating.

[rgoldner]

It's possible that you do not have your motors tuned correctly and are missing steps. It could also be noise adding steps.

[andy_ck87028]

Endgrainguy

Being in Australia, I'm a long way from you.

Are there any men's forums in your neck of the woods?

Rather than CNC expertize, your problems might well be solvable with the help of a bloke with a solid background in general engineering problems.

Being in the company of someone who you can share the problem solving with will take a lot of the emotional intensity away.

Take good care of Endgrain and the CNC problems will get solved.

Andy

[ger21]

How are you homing your machine at 200ipm??

Homing must be done at slow speeds for good repeatability, and preferably without using debounce. Debounce causes a delay in switch activation, which can cause inconsistency as well.

From what you've described, though, I'd also guess that you have a mechanical sloppiness somewhere.

[Gir]

There's probably nothing wrong with your PC. Double check to make sure the lead screws are moving back and forth, and don't run your machine so fast that it misses steps. Try running between 100-150 instead of 200. Also, to see if it's an alignment issue that's making you miss steps, decouple the motors from the screw and try moving the axis back forth. If you feel it dragging along in certain areas, your screw is a bit screwed up (budump, cha), and if it's at the ends it's an alignment issue.

Worst comes to worst, maybe you should try buying a new screw and nut. Sorry to hear about your problems, I'm sure you'll get it figured out soon!

[endgrainguy]

I guess it seems more likely to be electronic than mechanical. If it were mechanical, then somehow the effective lead of the screw was changing every so often, but was very stable until changed again, or at least stable for awhile. That doesn't seem possible.I'm running the axis way slower than its capable of, jogging it at around 100 IPM, when it runs smoothly at up to arround 300, at least -- (with a 31" travel there's not much use for high speeds). I am stepping at 1/25th microstep, because it's such a coarse lead (20 mm). I'm using Keling drivers and steppers and BOB, the 640 OZ/inch motors and KL-6050 driver on x and y, with the KL-8078 on Z.
Now noise, which I have no real understanding of, seems more likely. It's as if the pulse rate is slowly, or periodically changing by some definite percentage of the expected rate, around 2 or 3 percent. I did use sheilded cable for limit and home switches and for steppers though, and the x axis stepper doesn't run in a cable carrier at all. Any suggestions on how to diagnose "noise' issues? If it were lost steps, again, I don't see how I could get consistant results at times. Lost steps is more random, right? My profile tool paths meet up nicely, and I can send out a dozen sets of GOX0, GOX21, commands, and it gives perfect results on both ends. And then hours later, they're way off. I'll check my other axises. The Y has a funky used ball screw I'm replacing anyway, but Z is very nice fine pitch ball screw.

[Gir]

Noise is rarely a problem, but since you're using shielded cable It's extremely unlikely. Working for a while and then suddenly being off by a good bit seems like missed steps, especially since you described the clicking noise (those are missed steps). Try running it at 100 IPM for a while and see if you still get skewed results.

[endgrainguy]

Thanks for suggestions. I'm not in practice homing at high speeds , but I was trying to determine if the distance traveled in response to a command was changing in response to setting the axis speed higher or lower in motor tuning, and i homed the axis to start each check, which is how I noticed the erratic hopming at high speed with debounce up high (2000). I can understand why that would be the case. I ramped up that debounce setting to try to stop the limit switches from tripping when I was cutting, which they did every few minutes, finally disenabling them, and i forgot that i had experimented with debounce. Doesn't address my unstable travel issues though. You guys all seem to think it's mechanical, and I guess if y and Z are stable over time that would sem to point that way, but I still don't understand how a screw can demonstrate to have an effective lead of about 19.6 MM, and respond perfectly to a dozen 21" long travel commands, and then a few hours later, start behaving as if it were a 20.05 mm lead (these numbers just approximations, but close). Is it possible for missed steps to be so consistant?

[mcarvey]

I've only used servo's before so I can't say how common/consistent lost steps are. It is a little tough to diagnose your system since there are many places where things can go wrong. You need to systematically find out what is working correctly and only procede to the next step if you're 100% sure that it is working the way that it is supposed to. He is how I would procede if I had a similar problem.
1. Check the mechanicals. Mount a dial indicator on some place solid, and clamp a pair of vice grips (being careful not to chew up the surface) on the end of the ballscrew (or motor if you can do that) Make sure that one revolution of the ballscrew corresponds to a certain linear distance. Test it with a load on it. If there is any backlash it is coming from either the nut/screw combination, the coupling between the motor and the ballscrew (if you've clamped onto the motor), the AC bearing between the screw and your table, or a combination of some of the above. Once you are convinced that rotation of the motor can be consistantly be mapped into a linear travel you can cross this off as a source of your problems.
2. Make sure the electrical system is doing what it is supposed to. This is a little more difficult to do because you are testing the interaction between mach, the motor drivers, and the motor (and its loading conditions) Most people don't have a convenient way of making sure that mach is correctly outputting pulses so you may want to think about just wiping the computer, installing new OS, install fresh version of Mach. If you computer is not doing the correct thing, it will probably take more than 2 hours to diagnose and be very frustrating. Better to start fresh and not have to worry about that.
Once you have a fresh install, make the stage go back and forth at a slow speed (like 10 ipm or so). If it doesnt' work consistently at 10ipm, it probably won't work consistantly at 200. If it does work, start increasing the ipm to 20, then 30, etc. If it works consistantly at 60ipm, but not at 75ipm you'll have a much better idea of you're problem.
How have you grounded the shielded cable by the way?

Hope this helps
-Matt

[cut more]

Matt,
There are a number of guys in southern N.H. who have experience with Mach.
In fact there is going to be a meeting sunday I think. I will e-mail the head of the group to post the time under the mentor forum.
If you r machine is vibrating enough while cutting to trip your limit switches then either you have very weak springs on those switches or you are working the machine pretty hard, which could lead to lost steps. I can't explain the varying inaccuracy over time unless, over time something heats up and causes binding in the way/guides. You didn't say what type of machine it is (moving gantry/moving table) but does everything move freely? how about after the machine has been running for a while?

Just a couple thoughts,
Cutmore

[mactec54]

Hi endgrainguy
You said the limit switches are tripping how did you ground the sheild for your wiring if
they are tripping then you have a wiring problem they should not trip if done right
once a limit is tripped you should rehome the machine

[endgrainguy]

Switches are wired in series, normally closed. I used 2 conducter shielded cable (22g), twisting the conducters together so they act as a single conducter. The bare shield grounding wire is twisted and soldered together at each switch (though open to air for an inch or so there) and then in the controller box, on one end only, twisted together, along with the shield wires from the steppers and homing switches and grounded to the "green" grounding leg in the electircal box from which my 110 power comes from. The switches are cheap roller switches, and so vibration at intense cutting moments could easily be causing the trips. And the whole machine is mounted on a cabinet base which is on casters, contributing to that problem. The switch trips didn't occur with finish pass cuts. The cuts that caused the trips were at 40 IPM with .5 inch spiral bit , about .3" deep in cherry- definitely a heavy cut from my perspective. I eventually disenabled them to finish the job. I did "run from here' when they tripped without rehoming.

[ger21]

When a limit trips, you lose position. Run from here after a limit trip will put you in a different spot than when it stopped.

Do some research on star grounding. All the grounds should run to a single point, not in a long chain.

What kind of spiral bit are you using? Was it sharp? Cherry should cut like butter at that shallow depth.

[mactec54]

Hi endgrainguy

Your limit switches sheild can not be connected like you have them each switch
needs to have it own sheild to ground & only one end to be connected & nothing
floating in the air at each switch

Are you using 3 wire 110 AC or 2

[endgrainguy]

Thanks for all your input. The manual scew mapping was a great idea. I did that, and indeed my lead in very close to its stated 20mm. Turning it 30 revolutions by hand and measuring the resulting 23.6 odd inches of travel with a steel rule gave me a number a few thousandths of an inch shy of 20 mm. So the step number I've been using to get very close to the travel I'm asking for is apparently inflated to compensate for the missing steps, which you all probably knew all along. I'm still learning here. I am amazed at the machines ability to miss steps in such a nearly consistant manner.
I then switched drivers on x and the perfectly performing z, which have different drivers. Same results though, making me think perhaps x stepper was the problem. I switched steppers, and same results. So I"m left to conclude that despite the smooth feel to me, the ball screw is causing missing steps.I should say that in motor tuning the axis sounds horrible, especially at very slow speeds. The smoothest travel came from using highmicrostep values, 1/25 or 1/32, but it's still usually a bit grumbly, and inconsistant in its smoothness, which I guess is a clue. It's weird, because it feels smooth when turned by hand.
I have an embarassing history with this ball screw. I initially planned to build not a CNC machine, but a manual XYZ template following joint cutting machine , a giant JDS Multirouter. When I started looking for the components to build it from I discovered CNCZone, which introduced me to linear bearings, and my first Ebay purchase, my big Ebay bargain, was a two axis table with THK rails on two axises, 39 inches of x travel, and a cross table with 11 inches or so-- all for $200. The X had a ball screw on it, which I knew nothing about, and it was in the way of what was going to be a lever pushed and pulled travel. In taking it apart, without even knowing it was a ball screw, I spilled the bearings. I did my best to pick them all up, but didn't understand the value and precision of the thing.
Then months later my CNC plan developed, and I went to put what I now realized was a very valuable ball screw back together. It's a THK BLK 2020E KX-OI27, 20 mm diameter, 20 mm lead, four starts, with both its THK end mounts. I saw I was missing bearings. I bought the smallest size in that category from the Ebay guy who sells them, .1237, as I measured mine at .124, and thought I would be OK if they were a little smaller. I only have a cheap, thousandths-measuring caliper, which i don't feel adept at using. I just measured the smaller balls and they seem closer to a full thousnadth smaller than the ones that were in the screw, so I guess the original balls must be closer to.1245 or so. I thought the smaller balls (about 10 of them)wouldn't create a problem, but perhaps because they ended up pretty much grouped together they are causing roughness and missed steps? Or maybe I need better lubrication? I put a little grease in when I reassembled, but its pretty dry. Its also possible I put too many bearings in. I basically put in as many as I could. I just disassembled it, and they're a total of 215. Since it's a four start screw, one would expect a multiple of four, right? It seemed smooth, but snug. You could hold the 39" long screw by the nut and it will hesitantly at first, then faster, spin itself down by gravity.I seem to remember before I took it apart is was even smoother. Also, is there a left and right to the nut? Does it matter which way the cylinder to receive the attachment block faces on the screw? Thanks for all your help. I wish I had just bought a working CNC machine and altered it for my needs, but it's way too late now.As ever, since I"m asking so much here, if anyone would rather talk than type, you can call me at 423 624-3048, or send me a time and number to call. Thanks, Al

[endgrainguy]

Well it wasn't the ball screw. And towards the end of the four hour procedure of taking the x and z axis sufficiently apart to disect the ball nut, sort the balls, and reinstall them methodically sorting and spacing the 15 of 212 that are smaller, and then putting it all back together, I realized there was a much easier test. I should have simply decoupled the motor from the screw, and used a tick mark in the outboard motor shaft to see if it was still losing steps even without the motor connected to axis. This would have revealed, to me ,as it did,that the problem was not the ball screw, or anything in the axis, but electronic or settings. So, wasted time, but knowledge gained.
So the answer is : Motor Tuning, Step and Dir Pulses, If they're at 0 set them to 5. It also smoothed things out a bit to enable "Sherline 1/2 pulse mode" in ports and pins.
I still have annoying resonance issues at mid speed, I guess the notorious "midband resonance." It's not really a problem on Z, where it expresses itself dramatically. With its 2 mm lead, it runs very smoothly from 1-38 IPM or so, and then stalls completely up to about 50, but I"d not want to go faster than 50 anyway on Z, and a reliable 35 IPM is actually fine speed, even for the joint cutting work with router horizontal I'll be doing.
On X, with its 20 mm lead, I get best results microstepping to 1/16 th, and then it runs smoothly up to about 60 inches minute, and smoothly again at about 90, and can go to 300 IPM fine, even at 1/16th mictrostep.
Y, with a .5 lead two start "no name" screw with a little roughness in the screw in a couple spots is good to about 18 IPM, and then bad to about 35 IPM, really atrocious in the middle of that range. Then it runs better, smoothing out beautifully at about 50, and can go to at least 120 IPM, far faster than I'd expect to use it.
I guess this means I have to be careful to avoid these speeds in programs, which will be tricky , and probably means simply limiting feed speed for operations like inlay cutting to 18 IPM, which might be totally appropriate anyway. Most joint cutting operations don't use Y at all anyway, which is the worst offender. I had bought a replacement ball screw for the Y, since I attributed its roughness to some actual rough spots in the screw, and the Ebay guy has been two weeks in sending it out -hasn't yet in fact. When I e-mailed him a few days ago asking if he'd shipped he responded twice, once through direct e-mail, and once through E-bay. Here are the two responses:

"Hi.
Hasn't shipped yet.
My grandma died and I have been out of town for the last week and will be out of town for the next several days still.
I will get to it as soon as I get back.
Sorry again for the delay."



"Hi.
I am currently out of town due to a family emergency (my step-father died). I will be back in town next week and I will ship your item them and send you a tracking # ASAP.
I apologize for the delay and promise to get it out as quickly as possible when I return. "


I asked him if he wasn't sure his dog hadn't eaten the paperwork!

That screw is NSK ground single start 20 MM dia. 20 MM lead,(about $85 including shipping) which should fit my existing mounts with no machining except free end bearing journal, so it might be worth trying to see if I get less pronounced resonance, and smoother action with it.
Is it likely that the funny resonance damper devices people have made to increase their workable feed speeds will help with the resonance problems I've described above? Thanks again for everyone's help!

[guru_florida]

When I started with Kcam (Kellycam) I was getting a lot of missed steps. I tried emc2 live cd next and never went back or considered anything else. I hear good things about Mach too, but you should try emc2 to see if it makes any difference. I had bad audible noise with Kcam and the motion was really slow. With emc2 my screw ran soooo smooth and fast. The sound was like music to my ears!

emc2 has a new live cd out. The computer boots into linux, it doesnt modify the hard drive at all. You will need to run the StepConf Wizard and enter in your wiring configuration for the parallel port - step, dir, etc for each axis, and the speeds. Its pretty easy setup. Save the generated config file and config dir to your hard drive of flash drive. Then run emc and try the sample.

Did you check your stepper wiring to see if it was correct? What sort of stepper amp/controller do you have? (I am suprised no one has suspected that yet.) Also, what are the accell/decell settings you have for Mach? It may be missing steps as you accellerate or decellerate to max feed speed.

Perhaps the axii you bought were giving the seller trouble too and thats why it ended up on ebay. But dont fret, It sounds like the parts were good quality so it probably just needs some proper tweeking and maybe a little reengineering.

C

[ger21]

Just a note. Using "Sherline Mode" in Mach3 basically cuts your step rate in half.

[endgrainguy]

Well it sounded odd to me too. The idea came from guys on Mach support forum . http://www.machsupport.com/forum/ind...c,6837.10.html
I've tried it both ways, and the difference is actually subtle if at all. Thanks for pointing out the dampeners. I made a simple bandsaw and drillpress made damper and solved the resonance problem! I know, I should have made it with the router, but this was quicker for me where I"m at. Made from 1 1/2thick jatoba (hard, heavy Brazilian wood, rough bandsawn to 4 1/2" cylinder, with eight 1" diameter holes 1/4 from its outer edge,drilled 1 1/8 deep. Made plugs from the same hard wood with 7/8" plug cutter, just a 64th or so shorter than depth of holes. Covered with a plate of 1/4" plywood, and screwed it to an aluminum hub I had hanging around, attached to the shaft with a setscrew. Pretty much completely eliminated the resonance I was having on the Y axis, which otherwise screamed like a banshee at 20-30 IPM, pretty useful speeds . This was very cool!
i double checked the Sherline mode issue, and it didn't effect anything I could tell. Certainly didn't cut my travel to commands in half. registers and moves correctly either way. And I"m restarting software, and sometimes PC, after changimng it. There may be issues related to this and "Check that your Dir & Step "Low Actives" are All Checked, Axis reversing/direction changes should be done in Home & Limits page, "Reversed" box,Your Keling boards may not operate properly if not set this way. " issues perhaps specific to Keling drivers, also from Mach support forum. ?
which I've also done now, with no apparent effect.

The resonance problems happen at such low RPMS --like 35 to 50 or so. The dampener is like a miracle! I'll make another for X, using the one I made for Y. Now its just dust collection, work holding provisions, and I"m back to work, making stuff with a CNC (I hope!).