So I bought a unused/used "buildyourcnc"

All things related to the Centroid Acorn CNC Controller

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metalman2
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Re: So I bought a unused/used "buildyourcnc"

Post by metalman2 »

so I've got new dm542 drives and a db-25 board in the cart! The power supply that I have is a 36 volt...Is that sufficient? Do the stepper motors or drives determine what power supply you need? One more question, for now!! My computer guy says he can turn off all the power saving features on a laptop. Is it still a bad idea? Thanks
Richards
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Re: So I bought a unused/used "buildyourcnc"

Post by Richards »

Let's back up just one step. I use DM542 drives because they are a good match to the Oriental Motor stepper motors that I have on hand. Your motors determine the drive. My motors draw much less than the 4.2A maximum of the DM542. They can run at 50VDC, so the DM542 is a good match.

The maximum voltage that you can feed your motors depends on the impedance of the motor. That number is shown as mH in the motor's spec sheet. You find the square root of the mH value and then multiply that number by 32. That is the maximum voltage for your motor. You can use less voltage, but the motors won't run as fast as they could if you used full voltage.

I use the Oriental Motor PK268-02A motor because those are motors that I removed from some Kodak printers that I converted. They have 3.6 mH of inductance so they can handle voltage up to 60VDC. The DM542 is rated at 50VDC. I use a 48VDC power supply. If I were to buy a different power supply, I would buy a 36VDC model. They motors would run a little cooler and I don't need full speed. But, as you can see, there are tradeoffs. Your power supply would be excellent with my motors.

As to your question about using a laptop with all of the power saving features turned off. I can't answer that. When the manufacturer recommends to NOT use a laptop, I draw on their experience and use what they recommend. That has always worked for me.
-Mike Richards
ShawnM
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Re: So I bought a unused/used "buildyourcnc"

Post by ShawnM »

Yes, it's a bad idea to use a laptop. When you start to have issues you'll get limited support if you are using a laptop, it's not recommended by Centroid for a reason. Buy the proper PC, they're very inexpensive.

As for your power supply, read this page by Gecko Drive, it'll help you understand what amount of power you need for your motors.

https://www.geckodrive.com/support/step ... asics.html
metalman2
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Re: So I bought a unused/used "buildyourcnc"

Post by metalman2 »

Is impedance and inductance the same thing? The spec sheet I was provided, if it's actually for the motors I have, show a 7.5mH/2.8A value if wired parallel. With my math that's something like 87V, is that correct? Seems pretty high.. Should I just order new motors to be sure? My laptop questions were just because I stand all day at work and thought it'd be nice to be able to sit and throw a program together instead of standing more. If it's not recommended then I won't do it.. Sure appreciate the info!!
Richards
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Re: So I bought a unused/used "buildyourcnc"

Post by Richards »

Describing impedance and inductance would take awhile. Google can help better than I can. The only thing that you really have to know is what units the motor manufacturer uses. All of my stepper motors use inductance.

87 Volts MAXIMUM for 7.5mH is the proper number. Remember that the formula gives you the MAXIMUM voltage. In a tightly controlled system where you know all the limits and you want the highest possible production, running everything to the MAX will produce more parts per day. It will also wear everything out much faster.

Maximum voltage means maximum heat. Steppers get hot, sometimes really, really hot. Using less than maximum voltage helps keep the temperature down. Using maximum voltage means that the motors will respond as quickly as they are able. You might not need them to ramp up that quickly. Other things to consider would be the physical construction of your mill or lathe. How much of the motors potential torque is required to overcome the friction of the machine? My old manual Harbor Freight mill needs a lot of grease to make up for its shoddy construction. It's a manual lathe, so I can easily feel the force necessary to make cuts. My Taig mill with decent ballscrews and tighter tolerances runs well with much less mechanical resistance; however, the design of the mill limits the cuts due to flexing. There are always trade-offs.

If I were building a controller with your parts, I would try to use your existing motors. As mentioned before, I would replace the stepper drivers due to the 50kH limit of your existing drive. Without knowing a lot more about your machine, I would only be guessing about the proper stepper driver and whether you should replace the motors. Leadshine has several models of stepper drives. They have some drives that can easily handle 80V, but they cost a lot more than the DM542 drives that I use. Sometimes it is less expensive to replace both the drive and the motor instead of buying a drive that matches your existing motor.

The final choice depends on what you expect your CNC machine to do. If the machine has low friction so that most of your motors' torque can be used for cutting and if you can take multiple light cuts, then the choice of motor/driver is much less critical than if your machine has a lot of friction and/or you need to take maximum cuts.

Starting with what you have, replacing only those parts and pieces that clearly won't work properly with the Acorn, is where I would start. Build your control box. Call it Version 1. See how it works. If Version 1 needs tweaking, replace whatever needs to be replaces and call it Version 2. Experiment some more. You'll soon find what works best for your needs and your budget. I sometimes get carried away thinking how much a $150,000 Haas could do. Reality quickly set in when I remind myself that making five or six simple parts per month doesn't require that Haas. My little Taig handles my needs. I don't even need to use the ClearPath servos that I thought I would need on the Taig. It works fine with the cast-off steppers from my used-parts drawer.
-Mike Richards
metalman2
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Re: So I bought a unused/used "buildyourcnc"

Post by metalman2 »

Wouldn’t the 36v power supply limit the motors to only that? This machine came as a kit so I’d think it would function properly. Can I just look for drives with less than a 36v requirement that would be in the 100khz range?
Richards
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Location: South Jordan, UT

Re: So I bought a unused/used "buildyourcnc"

Post by Richards »

Drives are rated at the MAXIMUM voltage that they can handle. For instance, the DM542 can handle voltages up to 50V, so a 36V power supply would work fine with that drive. Most drives also have a minimum voltage, so check that spec if you plan on using 24V or less to power your steppers (not a likely situation). You'll note that many stepper motors have a voltage spec of 2.5V or similar. Don't worry about that spec. The stepper driver handles the voltage requirements of the motor. For instance, I'm using a 48VDC power supply. The drive may give a very brief 48V "surge" to the motor and then "chop" the voltage down to a very low level. Because of the inductance of the motor, it never really sees that 48V surge. By the time the incoming voltage overcomes the motor's inductance, the drive has dropped the voltage. That's how motors with low inductance respond more rapidly than motors with high inductance.
-Mike Richards
metalman2
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Re: So I bought a unused/used "buildyourcnc"

Post by metalman2 »

So here's where I'm at. I've got the new PC up and running, software installed and communicating with the Acorn. I was hoping to connect it to my existing drives and motors but can't find anything on here that that pertains to a CW230 drive. What would be my next step?
drdennis
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Re: So I bought a unused/used "buildyourcnc"

Post by drdennis »

@metalman2 I am not sure how or why the DB25 interface was suggested to you. You don't need it; you can wire the motor drivers directly to the Acorn terminals. The DB25 connector carries duplicates signals found on Acorn terminals (with the exception of having different input voltage characteristics).
Unless you have a need to interface with 5V logic you can safely ignore the DB25 connector.
Cheers
drdennis
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Re: So I bought a unused/used "buildyourcnc"

Post by drdennis »

Regarding the CW230, you can as well play with them. I disagree with the 50kHz being such obvious limitation. To give you an idea how to use that number -- the stepper motor needs certain number of pulses to make a single revolution. Nominally 200, sometimes 400. The CW230 lets you program how many pulses you want per revolution -- seven configurations ranging from 200 to 12800. You can do the math to see how fast the motor will turn given the 50000 pulses per second max and the selected range. Of course the other number you have to pay attention to is the desired resolution which is obviously going to change depending on how many steps you have per revolution.
Last but not least, as was pointed out, you will need sufficient torque (i.e., current) to make the motor move without losing steps. It gets complex quickly; my point is that given that you are starting with this stuff, you should play and slowly acquire the requisite knowledge. All you have to do is pay attention to how hot the motor is (they can get hot, steppers are quite inefficient by design) and whether or not you are losing steps (losing steps means some pulses didn't make the motor move as they should have).
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