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Z Axis Help Appreciated!!!!!
« on: January 21, 2012, 11:34:16 AM »
Hey Guys,

I want to get your thoughts one more time if I might!!!!  Its been a little over a year since I tackled building my own cnc wood router.  For the most part have been been extremely pleased, and everything has done exceptionally well.  I did run into an issue with z axis motor slippage right out of the gate.  It didn't really supprise me at first because I had concerns that this might happen.  I build and design equipment at work and alot of my design was based around some spare parts we had from older equipment that had been decomissioned.  LM guide and things of that nature that really elevate the cost of building one of these.  As I began calculating the cost of this machine I opted to make the spindle lead screw out of a precision acme screw with an acetal anti backlash nut.  Once assembled I realized that there was a large amount of friction which in turn would lead to loss of motor torque.  I didn't really worry about it much, as I decided that eventually I would end up with an old servo that I could rob the ball screw out and retrofit into my application.  When I started running the machine I started loosing steps immediately on the z axis.  I changed my timing pulleys to give larger gear reduction and then decreased the resolution on my stepper driver.  The stepper motor is a 425 oz. in. in case any of you are wondering with a one and five twist 1/2 precision acme screw.( I believe that's correct on the acme screw, its been awhile!!!!)  Once the motor was retuned and calibrated that took care of most of my problems.  I turned down my rapid speeds in my cam program and haven't had many more issues.  I had no intentions of making what I actually have been on this machine.  Funny how things work out.  Gunstocks seem to be pretty lucrative to me and fun.  My initial thought were name plates and maybe some small relief work.  Now that I have bored you all to death I will get to the point!!!!  When finishing one of these units there is an extreme amount of time with start and stop points and moving to them.  So when you combine this with slow rapid movements that really creates a long run time.  Now to avoid alot of sanding you only finish with ball so fast, but rapid can be as fast as the machine can stand.  X and Y can rapid at 200-300 IPM with no slippage, but the best I can do on Z is about 90.  Now in my cam program I can only set the rapids overall, not by individual axis.  So in turn this slows everything down.  I have checked every possible potential failure that could cause the z to slip and cant find anything it could be except the motor and that there is just to much mass to be shifting very quickling with the additional friction of the lead screw nut.  That ball screw I was waiting for hasn't happened yet so I have decided to go with a larger stepper with its own power supply.  I thought about isolating the Z axis with its own power supply and trying that first.  I haven't put an amp clamp on it to see what its pulling or if its getting the power it needs.  I bought a four axis kit from Keling and all steppers are using the same power supply.  Y gantry are two steppers slaved together  and X is just one stepper.  I am enclosing a picture of my z axis assembly so you all can see and maybe shed some light.  I am certain I have over killed the replacement stepper motor.(1600 oz. in)  It want to increase the resolution a little more than on the previous and just want to be sure I have enough power.  Everyone's continued  help on this forum is greatly appreciated!!!!
Re: Z Axis Help Appreciated!!!!!
« Reply #1 on: January 21, 2012, 11:39:16 AM »
Sorry,

I'm having trouble getting the picture uploaded.

Offline Hood

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Re: Z Axis Help Appreciated!!!!!
« Reply #2 on: January 21, 2012, 02:30:56 PM »
Not sure why you are having to reduce the rapids in CAM, in fact I cant see how you can as a rapid is a G0 and that can only be set in motor tuning.  Just reduce the Velocity in motor tuning for Z and that will restrict  it to that velocity.
Hood

Offline RICH

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Re: Z Axis Help Appreciated!!!!!
« Reply #3 on: January 21, 2012, 03:57:36 PM »
Quote
I will get to the point!!!!
 
Me too  :D

You started off with  425 oz. in stepper which provided a Z IPM of 90.
Now you bought a 1600 in oz stepper to replace it.
You have a four axis kit from Keling and all steppers are using the same power supply.

Quote
every possible potential failure that could cause the z to slip and cant find anything it could be except the motor

For 3D stuff I needed  to change my Z motor on the mill. The CAM programs produce very small moves and the motor
must be able to handle the accell & velocity called upon it for those moves. The initial motor never faulted once in a number of years for 2D stuff.
The hard part was that it skipped randomly and I could not reproduce it ...sometimes line 50 other times line 10,000....drove me bonkers.

That said....

Please answer the following:

What rapid feedrate do you want the Z to have?

What is the amp rating of your power supply and also the voltage output?

What is the amp and voltage rating for your drives?

What is the z axis weight? ie total weight of what goes up and down....

How manny amps are used for the other two motors? X&Y

Can you supply info / specs on each of the stepper motors you have?
Motor curve would be nice.

Can you use a in oz torque wrench and measure the torque required to just start lifting the Z axis?

The power supply needs to have available current  for all the motors you will use. Increase the voltage and you will increase the speed.
You can increase efficiency of the screw by going to a ball screw. You can increase axis speed via reduction or using multi start screws.
Not sure what you kernel speed is but increasing it will increase avaliable velocity for a given steps per but the other compopnenets will need to
be adequate to achieve increased velocity. Connsider conterbalancing your Z so it doesn't take as much torque to lift it up.  Just a few thoughts.

Now if you want to skip all of the above maybe have a look at equivilant machine and see what is being used for the Z axis.

RICH

Offline ger21

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Re: Z Axis Help Appreciated!!!!!
« Reply #4 on: January 21, 2012, 04:37:49 PM »
Quote
I am certain I have over killed the replacement stepper motor.(1600 oz. in)

I wouldn't count on it. Especially if you're using a gear reduction with it. My guess is that the gear reduction may have been the problem in the first place.
A gear reduction on a direct drive acme screw will often reduce performance, as the steppers lost torque as rpm's increase. The reduction is making it spin 2-3 times faster, and could actually be reducing your torque.

A 1600 oz motor is serious overkill for that application, and that's generally a bad thing. I've seen a lot of people buying those motors lately, and in most cases they perform worse than properly sized motors. The main reason is that they tend to have very high inductance, which causes the torque to drop off very rapidly.

Quote
It want to increase the resolution a little more than on the previous and just want to be sure I have enough power

Are you adjusting the microstepping to change resolution. Reducing microstepping can cause motors to run rougher and suffer from resonance, which can lead to lost steps.

What kind of drives are you using, and what voltage are you running at?

Is that a 5 start acme screw? If it is, I'd try running it 1:1, with the microstepping set to 1/8 or higher, and see if you get better performance.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Z Axis Help Appreciated!!!!!
« Reply #5 on: January 21, 2012, 05:53:42 PM »
Wow!!!

Thanks for all of the input so far!!!  I am going to try to cover all aspects and replies, so bare with me this is not my area of expertise!!!  I am running a 48v 12.5 amp power supply.  I looked up my motors, but could not find a torque curve chart.  Yes I am aware and new from the start that a ball screw would be way more efficient, but like most I am on a limited budget and was sure by now something at work would have croaked and I would have had one.  It is a five start acme screw.  Originally I tried the one to one ratio and when the machine hit rapid it made a crazy noise and slipped..  Of course all of these test were done basically cutting air.  The weight of the spindle assembly if I had to guess would be in the area of 10 to 12 pounds.  I think the amp max on my drives are 3.0.  They are KL-4030's.  Hood, I understand what your are saying so I am tweaking on that as well.  My hats off to all you guys, as you make me look like a charity case in comparison!!!LOL(That's why I'm Here!!) By the way the amp max on the drives are 3.0 amps I believe. That's what they are set at.  Everybody enlighten me here if I am wrong.  If you increase resolution you loose motor torque?  That was my thought process for going to the larger drive and motor.  This motor slippage that I have seen has been non predictable.  When I thought it would slip it hasn't.  When I thought it shouldn't it has.  Out of fifty runs its only done it maybe three times.  Am I pushing the envelope here?  Should i just deal with it when happens and go on?  I would put a torque wrench on it, but I don't have anything that small.  For the most part I have been thrilled with the machine overall, but every once in awhile it drives me bonkers.  The guy that takes care of the electrical side of things that I work with is telling quit jacking around and put servos with encoders on it.  I really don't think my machine is that bad and am just trying to fine tune it a little.  Please guys, feel free to add more input.  Don't worry about offending me as I realize that I am not an expert or I wouldn't be here.   All of your help is appreciated!!!

Offline ger21

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Re: Z Axis Help Appreciated!!!!!
« Reply #6 on: January 21, 2012, 06:13:09 PM »
What is your Z axis acceleration set too? It may need to be decreased.

Those 4030 drives are only rated for 40V, so I'm surprised you haven't fried them at 48V.

I'd set the microstepping to 1/8 or 1/16, and reduce the current to 2.7 amps, as the motors are only rated for 2.8 amps. Supplying too much current can result in rough running motors.

Those motors would perform much better at 72 volts with a Gecko G201. With their high inductance, you're only getting about half the speed they're capable of.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline RICH

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Re: Z Axis Help Appreciated!!!!!
« Reply #7 on: January 21, 2012, 09:15:14 PM »
If you have the time, we can have some fun learning and the replies will be meaningfull.

You didn't answer the first question, namely, how fast do you want to go or what IPM is desired? Reliably!

RICH

Re: Z Axis Help Appreciated!!!!!
« Reply #8 on: January 21, 2012, 10:23:20 PM »
If I could get them running a 200 ipm I would be thrilled.  Dont think its an issue on x and y just maybe z.  Probably wouldn't be an issue then if I would go ahead and put a ball screw in.
Re: Z Axis Help Appreciated!!!!!
« Reply #9 on: January 21, 2012, 10:24:41 PM »
Gerry,

What am I looking at cost wise in swithching to the Gecko