View Full Version : Mystery Problem

07-27-2006, 07:46 PM
I have my X axis up and running. I have a rack and pinion system ala ShopBot. I have 640oz motors which are spring loaded into the rack. I have Geckos and Mach3. And I've got the worst kind of problem. And when you have a problem with a home-built CNC you've really got a problem because there's no one to call. I can go to my basement right now, load in some g-code and watch the gantry run back and forth flawlessly until the program is over. Then I can do it again and again even going as far as to shut down the PC and reboot. But-- If I go down there in the morning, turn everything on, reboot the computer, load the code, etc., it will run into the program for a few seconds, maybe more, then stop for maybe a full second. Then it'll make the most god-awful metallic clanking sound you ever heard. You just know it's broken the rack or the pinion, or the cog has slipped on the motor shaft. And then it'll start traveling again as if nothing was wrong. Then it'll run any number of cycles without incident. The rack looks fine, the gear looks fine, the motor shaft looks fine. When I finally get my Y and Z built and start cutting real projects I could have a real catastrophe. Please help me with any suggestions. Thanks

07-27-2006, 07:48 PM
Im not a morning person...I know how your machine feels.....;)

Sorry, Im a noob with no suggestions....but that sounds like one seriously weird problem. Im sure someone will have a useful suggestion.

07-27-2006, 08:17 PM
I'm no expert, but that sounds alot like stepper motor timing!
My 1st router did the same noise until I figured out how to tweak all the motor speed and frequency settings in Mach3!

Don't let this stop you!

If memory serves me correctly, the motor is stepping way too fast, and is missing steps!


07-27-2006, 09:50 PM
If memory serves me correctly, the motor is stepping way too fast, and is missing steps! Eric
Thanks. I thought there might be an electronics factor. Can you recall anything about which settings you adjusted? I'm incredibly green to the software.

Jason Marsha
07-28-2006, 12:33 AM
Config > Motor Tuning. Adjust the velocity numbers from here.

So we can get an idea post the velocity , steps per and Acceleration values.


07-28-2006, 06:06 AM
Run the Mach3 Drivertest and see if it looks OK (flat line). You might have something interfering with Mach3.

07-28-2006, 09:15 PM
Config > Motor Tuning. Adjust the velocity numbers from here.

So we can get an idea post the velocity , steps per and Acceleration values.


Sorry for the time lag in responding. That "work thing" you know. Just got home.
Couldn't remember how to run the drivertest, but when I was setting things up a few weeks ago the line appeared to be pretty clean. My motor settings are as follows: velocity-- 60, steps per-- 8000, acceleration-- 1.246875. I believe those are default. Wouldn't even know what to change them to. If it matters my Profile is SoundLogicM3.

Jason Marsha
07-29-2006, 08:59 AM
We also need to know the specs on the rack and pinion to get the correct setup values especially for the steps per box.

As Gerry said you may have something interfering with Mach3. Before I cut anything on my machine I do a Ctrl + Alt + Del and remove any unecessary processes or those that may affect mach3 such as qtask.


07-29-2006, 09:38 AM
We also need to know the specs on the rack and pinion to get the correct setup values especially for the steps per box.

That may reveal something REALLY interesting because I didn't plug in anything for those values. Here they are.

14-1/2 Deg Spur Gear 12 Pitch, 12 Teeth, 1" Pitch Diameter; 14-1/2 Deg Pressure Angle Gear Rack 12 Pitch, 3/4" Face Width, 3/4" H.

Doing this from work so my responses may be delayed. Thanks for the help.

07-29-2006, 11:26 AM
That grinding sound (as bad as it sounds) is the classic "overdrive" sound steppers give out when they are asked to perform outside their speed or torque capabilities. The issue is probabaly not software related otherwise it would show up at other times and under running conditions. Your velocity and accel numbers are not outrageous for the size motors you have.

So what is different in the morning than later in the day? It would point to something that is mechanical. While it's not unheard of for electronics to misbehave because of heat, typically stuff runs better when it's cool than hot. Do you have the current limit resistor on the Gecko calculated right and in place tightly? Are all your connetions to the motor nice and tight including the wires at the Gecko's? If the current limit is set to low then it's possible the motors would work okay after they warm up or when the mechanics of the table are warmer. Before you fire up the power in the morning grab the axis and move it manually letting the motors freewheel (easy to do on rack and pinion). How does it feel? Does it bind or have greater resistance in any spot in the travel? If the Gecko's are limiting the current (due to the wrong resistor value) too low then any increased friction or a small amount of binding might be enough to cause the problem to show up.

If all the values and mechanical things check out then swap the X Gecko with another and try it again. If it persists swap the motor. Troubleshooting will be frustrating because it is intermittent and seems to be right on the edge of some parameter.

One thing you need to do is profile each axis and KNOW where the edges are. Once you know you have the Steps per unit (i.e. you command 1" of movement and you get 1" at the tool) then you need to do some testing. If your motors are directly connected to the pinion and the pinon/gear ratio moves you 1" with one rotation of the gear then than means it takes 200 pulses at the motor to move 1" and with 10X microstepping of the Gecko's it would indicate 2000 computer pulses to move 1". To get to 8000 there has to a 4X added stepdown in the mix somewhere.(?)

If you issue a 1" move at the software and the table is really moving 4" in the same time it would force the motor to move/ accel 4 times faster because it's going to try and move 4" in the same time interval. Thus the move (to the motor) is "give me the RPM to move 4" in one second" at a velocity of 60 that makes it 240 IPM rate.

Once you have the distance (step per unit) set correctly then start testing the axis using slightly increasing numbers. Issue manual commands through the MDI window like: G00 X15 The G00 says "move as fast as you can out to 15 inches from 0". Type in the command and hit Enter to execute. Keep increasing the velocity until you can make it skip steps (the grinding sound). Now back the velocity off by 25% and start increasing the accel the same way. You want to find the optimum combination for each axis. Know where the stall points are (like finding the stall speed on an airplane). Steppers stall at specific velocity and accel numbers. Change either one and the other moves on you. The rule of thumb is you should sacrifice some velocity for faster accel since it's accel that will give you nice sharp corners and angles.

Side note: The power supply voltage gets involved here too. The higher the voltage the faster you can spin the motor (up to a point). It's a factor of the motor impedence (fancy electronic speek for the AC resistance) and other things but a general rule of thumb on steppers is 10 to 25 times the nameplate rated voltage. As long as you keep the current limited by the motor drive (which is set on the Gecko) then you will be inside the operational parameters of the motor. Common stepper supply voltages (for Gecko drives with an 80V upper limit) are 36, 48 and 72. It doesn't have to be exact but in the ballpark based on the motors. Once again the motor nameplate voltage is the basis. That is why lower voltage ratings on motors are good since you can use a lower Motor DC for the same performance (at probabaly a higher averate current).

While there have been a lot of direct drive rack and pinion designed (easy interface) it forces you to sacrifice torque for speed. If you can move 1" inch for one turn of the motor that means you should be able to move 500 IPM at 500 RPM! The Unless you are cutting 24 ga steel with a 40A plasma then those speeds are wasted. It's like having a car that only has third gear. It will run fast but you have to accelerate carefully! Honestly a better design is to have some form of belt reduction so you have lots of torque and use all of the speed (speed you can't/don't use is wasted). Since you don't have a transmittion to change gears (like on your car) you have to pick a ratio that is a compromise. While it's true steppers develop their best torque at low RPM it forces some pretty low accel numbers if the gearing is wrong.

The down side of building your own CNC table is that there is no factory to call (that may also be the upside) but once you get things worked out you are your own best line of support. This list and the MACH specific ones are excellent sources. And the response is often faster and better than most factory support.

Hope this helps in your project. One last setting to check in MACH3: Make sure Enhanced Pulsing is enabled (checked).

Tom Caudle

07-29-2006, 09:00 PM
Responding to Tom Caudle's post: Sad to say most of what you said is over my head. But-- that's what this hobby is all about- at least to me: going where you've never been and trying to figure it out. I don't even speak the language. But I'm determined to get it done. I had thought about swapping out components. I have no qualms about that whatsoever. Not a bit too proud to diagnose via process of elimination. I'm proceeding at a great disadvantage since I have no idea what different tweaks even mean. But I've learned from your post that G00 X15 translates to 15" of gantry travel (once I get the 1:1 thing established). At least, I think that's what you were saying. That gives me something I can relate to. Bob Campbell calculated and supplied the limit resistors. I'll print out your generous post and take it to the basement. I'll post my results. May be a few days. Thanks so much.