Are my stepper motor expectations too high?

[hoople2]

Why is my CNC slow?


I built my CNC machine from scratch a while back, but I'm just now beginning to use it, and I'm a little disappointed in it's speed. Having no experience with CNC machines, I'm not sure if my expectations are realistic. I've seen a range of speeds on this forum and see that others are happy with similar speeds, yet I just have a feeling I'm doing something wrong and don't want to live with a slow weak machine and then find out after hours of slow results that I have something simple wrong.


I can't even go 40ipm with no load without the stepper motors crapping out. By crapping out, I mean they stop turning and just sit there buzzing. I can run 20ipm with a 1/2 bit cutting 1/2 inches of MDF, but that's so slow it burns the wood (that's partly because the bit is getting dull at this point). I'd like to mill aluminum but any missed steps will waste a lot of time.

I know it's not because of a mechanical misalignments in my machine, because even without the machine bolted up I can stop the screws by hand (when I have Mach3 set to 40ipm).


I've tried the whole range of micro-step settings as well as the current settings on the drivers, but haven't found a sweet spot that gets the performance that I expected. The only thing I can think of is maybe getting a higher voltage supply but this one should be fine as far as I can tell.

Any advice is appreciated!


Here's my setup:
36v 10A power supply
Ethernet SmoothStepper
DB25-1205 breakout
DM542 Fully Digital Stepper Drive
wantai Stepper Motor 57BYGH627 Nema23 270oz-in 3A
Mach3

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[Jim Dawson]

More voltage would help, would allow the steppers to turn faster. More importantly, what is the pitch of your lead screws? Any reduction in the drive system?

[109jb]

We really need to know more about your machine, but I can start off by saying you should get a 48V power supply. The motors ate 3.5mH inductance, so really want 60V, but your drivers are limited to 50V.

Back to the machine itself. Please describe it. For example, how big is it, what kind of guides on the axes, ACME screws, ball screws, belts???

[Fastest1]

Does it move the amount of distance requested accurately?


Sent from my iPhone using Tapatalk Pro

[louieatienza]

What pitch screws are you using?

[hoople2]

Thanks for your replies...

To start off, I've gone ahead and ordered a new 48V supply.

As for my machine, I drive both the Y/A and X axis with 24" 1/2-10, single start screws. And yes I see that there is where I can make improvements. This build was last winters inside project and I forgot that I used single start screws until you asked about them. Now that I think about it I remember choosing accuracy over speed. But after watching it run I would have done that differently. Now I want to replace them. I'm thinking about getting 3-start screws. I also realized that I need to finish some details like adding limit switches. I was using the soft-limit switches in Mach3 but in order to be sure exactly how hard I can push the machine I'll need to know where it starts missing steps and the best way I can see to do that is to use Mach3 verify mode while making cuts (I didn't even know there was a verify mode until I started searching this morning for a way to detect missing steps).

(and yes, the distances are calibrated properly).

Do you have thoughts or related threads that you think rise above the rest on how to select the start-count for my new screws (maybe 4-start screws would be better?)?

[louieatienza]

Thanks for your replies...

To start off, I've gone ahead and ordered a new 48V supply.

As for my machine, I drive both the Y/A and X axis with 24" 1/2-10, single start screws. And yes I see that there is where I can make improvements. This build was last winters inside project and I forgot that I used single start screws until you asked about them. Now that I think about it I remember choosing accuracy over speed. But after watching it run I would have done that differently. Now I want to replace them. I'm thinking about getting 3-start screws. I also realized that I need to finish some details like adding limit switches. I was using the soft-limit switches in Mach3 but in order to be sure exactly how hard I can push the machine I'll need to know where it starts missing steps and the best way I can see to do that is to use Mach3 verify mode while making cuts (I didn't even know there was a verify mode until I started searching this morning for a way to detect missing steps).

(and yes, the distances are calibrated properly).

Do you have thoughts or related threads that you think rise above the rest on how to select the start-count for my new screws (maybe 4-start screws would be better?)?

4 start would be just fine, 1/2"-8, 2 start (.250" lead) would work nicely, even 4-start (.500" lead) works well too. You'll still have plenty mechanical resolution, .0025" with 4-start, and your steppers' torque will be better utilized.

The thing with steppers is that they lose torque proportional to speed. The other thing is that as you speed up, you're heating your leadnut, and it can get hot enough where it can expand and bind. The muti-start leadscrews are more efficient at converting rotary motion to linear motion. There are many on the forum that saw an increase in speed and performance using multi-start leadscrews.

[wizard]

My concern is that you said you can stop the screws by hand. Not sure what that means but it is a bad idea to grab a moving leadscrew. In any event the motors you indicate should be rather difficult to stop especially at lower speeds.

This brings me to the drive, are you sure you have that drive set up correctly? You need to set the current correctly and make sure all other parameters are right if any. Id also make sure your power supply is able to handle the current demand from all axises. Also it is worth checking how well the steppers lock when the axis isn't moving. The drive should be able hold the stepper against moderate effort to turn the shaft if not it goes back to current settings. However the feel of the motor can depend on the driver and its ability to put a motor in low power mode during idle.

You will want to disassemble the machine (well disconnect the motors) and check each axis for binding. Also are these faults focused on one axis or all? I ask because if you can get any of the axises to fault there is less of a chance of a binding condition. There is the issue of the nuts binding after heating up mentioned above but cold the nuts should only have moderate resistance.

In any event what I'm trying to say is that rushing into a rebuild here might not give you the results you want. It sounds more like a lack of torque or binding causing a motor overload.

[ger21]

What do you have the drive current set to?

And have you oiled or lubricated your screws? That can make a big difference. What kind of nuts are you using? You should be able to get close to 75ipm with what you currently have.

[Toinvd]

For the current, Your motors should get warm in about 5 minutes use. If not the current is below the allowed settings .

[shooter64738]

I run an identical driver and similar motor configuration, however I used NEMA 24 motors. I have used the DM542T drivers, but I went with a power supply per axis. They are 60 volt 6 amp supplies adjusted to 49.9 volts. As Jb109 pointed out the DM542 driver is maxed at 50 volts so don't go over that. I selected 60 volt supplies so I could go to a higher voltage motor without having to replace the supplies. They all cost about $25 regardless of voltage. Also if your supply is adjustable, check the voltage setting on it.
You could be losing a lot of energy in a binding axis, but your power supply is under powered for sure. One easy way to tell, is to run one axis, then 2 axis's together. Do something like X50, then do X-50Y50. (assuming millimeters) you will probably notice a rather significant difference in speed when 2 are running vs one.

I can run my steppers at 1321 rpm. With a 5mm lead ball screw, I can get 250 ipm, with no load. obviously with load it will lose steps and stutter at that speed, but your motors are definitely capable of more.

[coherent]

There are so many variables that can result in a slow machine it's nearly impossible to give you a helpful answer. To start with, if the steppers are disconnected from your screws, does each axis move freely up and down the full length of each axis? No binding and shouldn't be too hard to push. Screws turn freely in the nut/ballnut? Are you using a proper lube. If no issues, are your motor shafts and screws aligned together and the couplers aligned/fit well and there is no binding or misalignment there? Next check all of your wiring and connections. When nothing is connected to a stepper shaft, it should be nearly impossible to stop it with your fingers by grabbing the shaft when turning. If all these are ok, you could have the motor settings wrong in Mach. Do some research and make sure there are no issues there. There are a number of possible incorrect motor tuning/settings that can cause issues. Are your micro steps and steps per set to the correct and most efficient settings. The larger power supply may help. Bigger steppers may help, but bigger isn't necessarily faster. If what you have is not stalling due to binding or friction then it's not a torque issue. If everything else is ok, you could have a driver problem. But first check all the more likely areas before you start buying more electronics or hardware. Binding and friction is the first most likely culprit on home made machines. Then the other areas mentioned. Good luck.

[hoople2]

Thanks to all for your input! I've implemented some of your advice and now I'm very pleased with the performance.

I started by upgrading the power supply to 48V and replacing my 1-start screws with 5-start screws. I was actually looking for 4-start, but the 5-starts were easier to get so I went with those. The screws allowed me to run the motors slower and still get higher feed rate and I can't see any difference in accuracy. I also picked up some stronger motors but only replaced the one on the x-axis (though I think I could have left that as it was). Now the machine seems quite peppy cutting at 45ipm with no missing steps. Now it's running as good (or better) than I had hoped. I'm very happy with it. So much so, I replaced my old router with a 2.2kw water cooled spindle. That was a very worthwhile upgrade. Now I can actually stay in the shop while it's running (without ear defenders even). The extra weight was a little too much for my wimpy z-axis though and I soon lost the bearings in it. I replaced the z-axis with a more robust assembly and now things seem very balanced. I can even whip through aluminum at 20 ipm with a 3/8" end mill (~3/16" depth). Not sure how much faster I can go with that but I don't want to do a destructive test so I'll probably never try to go faster with that. I still have enough accuracy to cut very fine hardwood inlays too.

I'm not quite done yet though. Now I'm in the process of moving the x-axis screw to the front side of the gantry so it's closer to the z-axis. The distance between the z-axis assembly and the x-axis screw was about 4 inches and I was able to move the spindle just a touch more than I'd like by pushing on it side to side. I couldn't really tell by looking at my cuts, but I know it must have had an effect in deeper/wider cuts and the new z-axis gives me a little more room where I need it so I decided to move it.

Anyway, it's been a lot of fun building this thing, and the fun of using it is just beginning. Now I'm looking at converting my bench-top metal mill. So much fun, so little time.