My machine keeps stall on me

[wizard]

Hi there
Today I had been busy doing all different kind of experiment on my machine.
And I noticed that I could easily stop my X axis from moving or causing it to lose steps, and I am quite sure that this shouldn’t be the case, If I can cause it to lose steps so easily for sure it will happen during actual cutting. I think my X axis has a design flaw; (The way that I am attaching the pinion and the motor to the rack is not rigid enough.) I am attaching some pictures of it.

That is a rather flimsy looking bracket. I doubt that it has anything at all to do with your problems unless the pinion is springing out of engagement.
[quote]
Any Ideas on how I can make it more rigid will be greatly appreciated.
[quote]
Completely scrap this design and build a new one that is inherently more rigid. Cantilevered sheet metal is not the way to go here.

By the way proper engagement of the pinion to the rack is very important. Generally you only have a thousands or so, often less to get the center distances right in a rack and pinion setup. An additional concern that pops up in your design is that the pinion be parallel with the rack. Screw up either of these and you get rapid wear on the gear teeth.

I should mention that my Y axis is in much better shape. And the Z is ball Screw and again is in good shape. One thing that I learned from all these is that for my future machine if any I will defiantly go Ball Screw all the way and will never bother with Rack and Pinion.



Regards
Johnansaro

There is literally no difference in the engineering effort to do a proper rack and piñon axis as opposed to a leadscrew driven axis. The only advantage is that there are a lot of canned solutions about the net for functional leadscrew driven solutions. You still have too make all the same engineering calculations up front too make sure you will have enough torque for a given axis, that you get acceptable resolution and acceleration performance.

[louieatienza]

There are two great formulae that Mariss from GeckoDrive has in his website for calculating linear and rotary power. For linear it's Power (Watts) = IPM * Weight (pounds) / 531. For rotary it's Power (Watts) = Torque (oz-in) * RPM / 1351. From these two equations you can get a good idea what's going on, even though we're not taking into account inertia or friction or efficiency.

So let's assume your entire gantry, including bearing blocks, all the LM components of the gantry and Z, spindle, steppers, etc... weigh in at about 100lbs. We'll do an easy one to start: say you want to rapid at 300IPM. That ends up being about 54W of power needed, using the linear equation. So what about cutting at a somewhat normal speed, like around 50IPM? Let's figure out how much torque we'd need by working backwards.

You have a 13 tooth pinion. According to some online specs it has a pitch diameter of 26mm, or about 3.215" pitch circumference. Meaning, every 360deg. rotation of the stepper moves the gantry (if it did move) 3.215". So to travel at 50IPM, the stepper must rotate at about 15.55rpm. We need to know about how much power we need to move 100lbs at 50IPM. That would be about 9.416W. So the torque we need from the stepper would be:

Torque (oz-in) * RPM / 1351 = 9.416
Torque * (15.55/1351) = 9.416
.0115T = 9.416
T = 9.416 / .0115 = 818oz-in!

It should be obvious that using even a 2:1 gear reduction would work. This doesn't leave much "safety margin" since the actual torque output of the 640oz-in stepper is a bit lower... I looked up a 640oz-in stepper and derived some rough power numbers from the torque/speed chart:

100rpm, 475oz-in, 35W
200rpm, 450oz-in, 66W
300rpm, 380oz-in, 84W
400rpm, 320oz-in, 94W
500rpm, 270oz-in, 99W
600rpm, 240oz-in, 106W
700rpm, 200oz-in, 103W
800rpm, 175oz-in, 103W
900rpm, 165oz-in, 109W

Again, these are rough numbers, but it illustrates a few things. One, the peak power is around 600rpm. Two, the power output grows linearly from zero to about 300rpm. Three, anything about 600rpm would be useless to utilize because any gain is speed is negated by decreased torque. Four, you'd need a higher gear reduction to utilize the power output of the particular motor. With a 2:1 belt reduction, you could potentially utilize up to about 250rpm and still have enough torque for 400IPM rapids. With a 3:1 reduction you could potentially go up to 500IPM for about 536IPM rapids. 4:1 starts the point of diminishing returns as you'd max out at just below 700rpm, for 550+IPM rapids.

[NIC 77]

Hi awerby and louieatienza
Thanks for the reply.
regarding the microstepping, please take a look at the following:

To calculate the resolution information, we need two values:

– Steps per revolution: Most steppers use 200 steps per revolution, but you should see what your manufacturer says. For servos, it is a function of how many steps per revolution the encoder has.

– Microsteps: While your stepper drive may accomodate very fine degrees of microstepping, it’s wise to remember that you lose a lot of torque the more microsteps you use. Consider the following when you select how many microsteps you can actually count on:

Microsteps/full step Holding Torque/Microstep
1 100.00%
2 70.71%
4 38.27%
8 19.51%
16 9.80%
32 4.91%
64 2.45%
128 1.23%
256 0.61%
As you can see, that 256 microstep resolution is an illusion–there’s only 0.61% of holding torque available at that resolution.

I can show you many more supporting Documents on the issue. Also the site micromo.com explains all this.

Regards
johnansaro

I have been confused by this as well in the past. What I've come to realize is that whoever put this information out on the internet had no clue what they were talking about. I've even made my own post here about this subject to try and clarify. Without needing to understand all the technical jargon of how stepper motors and drivers work, which I personally don't care about as long as it works as advertised, this information is in fact, not true.

To say that you'll only have 20% or so torque available with 10x microstepping that you would with no microstepping, when I look at actual torque vs speed graphs for different motors that use microstepping drivers, I can tell you that this isn't true.

What you need is a 2:1 or 3:1 gear reduction and some way to spring load the pinion into the gear.

[louieatienza]

I have been confused by this as well in the past. What I've come to realize is that whoever put this information out on the internet had no clue what they were talking about. I've even made my own post here about this subject to try and clarify. Without needing to understand all the technical jargon of how stepper motors and drivers work, which I personally don't care about as long as it works as advertised, this information is in fact, not true.

To say that you'll only have 20% or so torque available with 10x microstepping that you would with no microstepping, when I look at actual torque vs speed graphs for different motors that use microstepping drivers, I can tell you that this isn't true.

What you need is a 2:1 or 3:1 gear reduction and some way to spring load the pinion into the gear.

MicroMo makes precision micro stepper and servo motors and have been around a long time... so I think they know a thing or two.

This is probably the most misunderstood thing... but it has been explained below.

The thing to take from this is not to rely on using microateps to gain back resolution from a system where the mechanical resolution is very low (or the gear reduction is low or you have a net gear increaser) because you'd eventually reach a point where it may take several commanded steps for an axis to move.

[NIC 77]

MicroMo makes precision micro stepper and servo motors and have been around a long time... so I think they know a thing or two.

This is probably the most misunderstood thing... but it has been explained below.

The thing to take from this is not to rely on using microateps to gain back resolution from a system where the mechanical resolution is very low (or the gear reduction is low or you have a net gear increaser) because you'd eventually reach a point where it may take several commanded steps for an axis to move.

I don't know what the people at micromo were actually trying to say in regards to microstepping. I think they were trying to show that accuracy doesn't necessarily increase with more microsteps, even though resolution does. In retrospect it looks like the whole basis of this paper was in regards to accuracy, and not torque output.

http://static.micromo.com/media/wysi...pping%20WP.pdf

What I do know is that when the average lay person, such as myself, or johnansaro, who is only interested in the practical ramifications to machine design, sees what they've published, it very much looks like they are saying that you will have 20% of the available torque using 8 microsteps vs no microsteps, which is of course, not true.

After reading this paper, johnansaro thought, oh, all I need to do to get more torque so my motors don't stall is to reduce the microsteps. And that is the conclusion that most people come to after reading this paper. So even if the paper is correct, it still leads people to the wrong conclusion!

I take back what I said about them not knowing what they are talking about, and instead replace it with, they have written a paper that is so full of technical jargon that the average person can not understand, that even if true is misleading, so that the average user of stepper motors will be mislead into believing that microstepping drastically reduces available torque.

[louieatienza]

I don't know what the people at micromo were actually trying to say in regards to microstepping. I think they were trying to show that accuracy doesn't necessarily increase with more microsteps, even though resolution does. In retrospect it looks like the whole basis of this paper was in regards to accuracy, and not torque output.

http://static.micromo.com/media/wysi...pping%20WP.pdf

What I do know is that when the average lay person, such as myself, or johnansaro, who is only interested in the practical ramifications to machine design, sees what they've published, it very much looks like they are saying that you will have 20% of the available torque using 8 microsteps vs no microsteps, which is of course, not true.

After reading this paper, johnansaro thought, oh, all I need to do to get more torque so my motors don't stall is to reduce the microsteps. And that is the conclusion that most people come to after reading this paper. So even if the paper is correct, it still leads people to the wrong conclusion!

I take back what I said about them not knowing what they are talking about, and instead replace it with, they have written a paper that is so full of technical jargon that the average person can not understand, that even if true is misleading, so that the average user of stepper motors will be mislead into believing that microstepping drastically reduces available torque.

Then that's the problem with the reader. I don't think that white paper was intendes for the average Joe either, but for OEM engineers. But I didn't interpret it the same way you or John did. The intent was to show that one cannot simply rely on microstepping to gain resolution. And just because one has such resolution doea not mean you achieve the positioning you intend.

[johnansaro]

Hi There
I would like to thank all the people who replied to my post my particular thanks to Louieatienza
And , Torchhead for your in depth and detail reply. Keep up the good work and many thanks again.
I am sorry for my late reply. I was busy implanting and installing your recommendation and suggestion.
I should say that All is fine now, and I am not missing any steps or line anymore, and machine is working great all by Spring Loading my motors plus adding the 3:1 pulley reduction and my Microstep is set
At 2 .

Now I have a question on my Vacuum Table. I may have to start a new thread.
But here is my question through my Calculation the volume of all my Profile cutting is about 8 litters
And all the Hoses (20 mm diameter Hose or pipe) is about 6 litters but off course if I use 30 mm Diameter pipe or Hose it will jump to 14 Litters.
Now my question is as follow If I connect all these to a collector and then a Vacuum pump. What size Collector would be the right selection? If I am using a 1 meter long 160 mm Diameter Pipe as my Collector the volume would be about 20 Litter, is this enough for a Proper vacuum?

[louieatienza]

Great to hear! And you're welcome. Been a great community here, as even though I worked with CNCs for about 12 years prior to building my own and finding this forum, it has been a huge help.

Would be great to see pics of your modifications, so that others with a similar issue can see before-and-after images...

[johnansaro]

Hi Louieatienza

I am sorry for delay to reply.
attached are some pictures of the modifications that I made to my X and Y axis.

On the issue of spring loading the pinions, I have some questions

How tight is tight enough for the spring loading the pinions?
and on the Y axis, knowing that I have two motors, It would be difficult to spring load the Y pinions with same force or tightness. Is there going to be
any Problem. Off course at the moment I have no problems and the machine is working perfectly.

as you can see my design is some what crude. But after my design and during building process, i searched the net and I noticed that I could Install the motors on one 3 Cm thick plat rather than two plates that I am doing.

Regards
Johnansaro

[johnansaro]

Hi Louieatienza

How are you?

What do you think about my brackets? do you have any recommendation in order to make it better?

regards
johnansaro

[louieatienza]

I think you're on the right direction there. I'm not against your belt reduction assembly, as it's better to have that output shaft supported with two bearings as you have it. It would be better to have as few pieces bolted together as possible, so as a future upgrade you may consider finding some rectangular tube of proper dimensions to construct that assembly.

But I would say, as the machine is running to your expectations, to leave it that way for now, and as your experience and needs grow, you'll be able to redesign those components.