DIY CNC 2

Hi again, smart people! I was wondering if you could check over my shopping list and make sure I am purchasing everything I need and that everything I am purchasing will work with each other.

• Servos (SST57D3201) (came with the machine; they are quite old, but they should still work)
• Prox Sensors
• 7I96S
• Buck Converter for 7I96S
• PSU
• Stepper Drivers
• Shielded Wire

I have a question about my servos. From the data sheet, it says that each motor is 2 amps per phase. The data sheet doesn’t seem to mention whether or not this is 2A phase RMS or peak current. The stepper drivers I’ve picked out are 2.1 amps peak for the closest dip setting. Is this fine to drive these servos? I just want to make sure I’m not going to burn up my servos.

Nomenclature nitpick: Unless you've got encoders on there, those are stepper motors, not servos. And even steppers + encoders are typically referred to as "closed-loop steppers", not servos.


PSU: If you're going to use a switch-mode PSU, make sure you get a big (uF rating, voltage rating) capacitor to put on it. Motors dump energy into the PSU on deceleration, which can lead to voltage spikes and blown stepper drives and/or PSU. A toroidal/transformer based supply is often recommended instead for this reason.

This may also be useful: www.geckodrive.com/support/power-supply-basics/


Wire: You may want multi-conductor wire. If that's for the steppers, 4-conductor would be good. Running 4 separate wires to each motor seems unnecessarily obnoxious.


Missing from your list: buttons, switches, fans, terminal blocks, enclosure, mounting hardware, connectors. Specifically, you'll probably want to put your parts in a box of some kind, and at the very least an e-stop is a good idea. Generally you want that wired such that it hard-cuts power to the motors+spindle of the machine or otherwise renders it in a safe state.

Drawing out everything you'll use and the connections between them can help identify gaps- eg, making sure you have enough terminal blocks. It's shockingly easy to run out of those. Ditto for enclosure space- a little planning goes a long way there, and also getting a bigger enclosure than you think you need.


Finally: I'm not familiar with all the mesa offerings (I've got a 7i76e on my mill, IIRC), but try to think about what you might want in the future and then you can decide whether it's worth buying something with spare I/Os to support it now or accepting you may need to upgrade the card later to add things. Things you might consider:

-Coolant/air blast control
-MPG
-Spindle encoder
-Touch probe
-Toolsetter
-Hardware feed hold/single-step/pause/resume/etc. buttons
-Rotary axis

IIRC you've got a Taig, and IIRC those use ER collets or equivalent... my experience is re-zeroing tools with ER collets gets old fast. I more than once trashed a tool and/or workpiece forgetting to do so... a toolsetter IMO would be worth planning for.

Thanks for the info.

Nomenclature nitpick: Unless you've got encoders on there, those are stepper motors, not servos. And even steppers + encoders are typically referred to as "closed-loop steppers", not servos.

I meant to say steppers my mind just slipped I guess.

PSU: If you're going to use a switch-mode PSU, make sure you get a big (uF rating, voltage rating) capacitor to put on it. Motors dump energy into the PSU on deceleration, which can lead to voltage spikes and blown stepper drives and/or PSU. A toroidal/transformer based supply is often recommended instead for this reason.

Noted. The geckodrives are a bit too expensive for me to consider. If you have an recommendations for a good PSU I'm all ears. Additionally, regarding the drivers I have picked out, I assume it is RMS current right in that documentation?

Wire: You may want multi-conductor wire. If that's for the steppers, 4-conductor would be good. Running 4 separate wires to each motor seems unnecessarily obnoxious.

If I use a CAT6a cable will the shielding on it provide enough insulation to avoid being influenced by EMI?

-Coolant/air blast control
-MPG
-Spindle encoder
-Touch probe
-Toolsetter
-Hardware feed hold/single-step/pause/resume/etc. buttons
-Rotary axis

I'm probably just going to use a spray bottle with coolant for the mean time. Not sure what you mean by MPG, the machine I have has manual lead screw handles built in to jog it. I'm going to control the spindle with the 7I96S and hope it spins at the speeds I want while cutting. Touch probe's seem expensive unless there is a cheap one on amazon. Regarding a toolsetter, buttons, and rotary axis, all that seems quite far away atm. I'm going to worry about making it work first.
 

Logthor wrote:

I have a question about my servos. From the data sheet, it says that each motor is 2 amps per phase. The data sheet doesn’t seem to mention whether or not this is 2A phase RMS or peak current. The stepper drivers I’ve picked out are 2.1 amps peak for the closest dip setting. Is this fine to drive these servos? I just want to make sure I’m not going to burn up my servos.


 

Setting your drivers at 2.1 amps for your 2 amp motors sounds like a good "starting point" Then once you get it powered up, you can keep an eye on the motors to make sure they're not getting too hot as that is often the final rule whether an amp setting fully fits the motor/application. I don't see the rated operating temperature on the datasheet, as a general rule I like to keep them under 82C (180F), some are rated hotter. The cooler you run them the longer they last.

A stepper powered up and parked at 0 RPM has way more torque than it has while rotating. As a consequence, you can reduce its amps during 0 RPM, greatly reducing its heating. This reduces its holding torque some, so might take experimenting to find whether it is worth that trade-off.
I see your drives list function of sw4:OFF = half current, ON = full current. I suspect that is this feature, might be worth confirming.

@dbtayl on using a big enough enclosure: That is one detail I find too easy to get wrong. Countless times I think I started with a plenty big box. As the project evolves I find it is getting crowded.

I see, thanks for this info. I'm probably going to contact either Sherline or the manufacturer of the stepper (if they are still in business) to confirm the amps needed. Just to confirm though, those stepper drivers should work, right?

I plan on just mounting all the hardware on a piece of MDF so I don't think I really need to worry about space just yet. I really just need to get it working, then I can worry about bells and whistles.

Is there a cheap way of acquiring tools for the mill (e.g., cylindrical edge finders, collets, drill bits, end mills, etc)?

On your PSU: I have used a similar PSU on a similar project and it worked great. dbtayl's notes on PSU are correct, but another way to success is to use an oversized PSU. According to the specs you seem to be doing so.

CAT6A should do if you make sure it is multi-stranded. On a lot of cheap network cable each wire is solid = will break soon if it has to flex a lot. Breaking wires between your motors and drives can fry your drives. Just something to keep in mind.

Edit:
Amps and motor drives: I think your choice is just fine.

On your PSU: I have used a similar PSU on a similar project and it worked great. dbtayl's notes on PSU are correct, but another way to success is to use an oversized PSU. According to the specs you seem to be doing so.

So based upon what I'm using you don't think I need to swap out for a different PSU that has said protections?

CAT6A should do if you make sure it is multi-stranded. On a lot of cheap network cable each wire is solid = will break soon if it has to flex a lot. Breaking wires between your motors and drives can fry your drives. Just something to keep in mind.

I feel like this would do good.

Logthor wrote:

So based upon what I'm using you don't think I need to swap out for a different PSU that has said protections?
 

How many stepper drives? If 3, then we're looking at 4 amps per motor * 3 = 12 amps total. Your stepper drives will draw less than that. Now, cheap PSUs tend to be overly optimistic with their current rating. I think your PSUs are easily sized double what these specs require. Which I think is a good plan. Go for it. I would buy that.

Your twisted pair cable looks great. I was just coming back with a suggestion 18 Gauge 4 Conductor Shielded when I noticed your post.

How many stepper drives? If 3, then we're looking at 4 amps per motor * 3 = 12 amps total. Your stepper drives will draw less than that. Now, cheap PSUs tend to be overly optimistic with their current rating. I think your PSUs are easily sized double what these specs require. Which I think is a good plan. Go for it. I would buy that.
 
Are you sure it is 4 amps per motor? I thought that at most it'll draw 2 amps between the two windings (2.8 amps if it is peak). Sounds like a plan regardless. I was thinking I might just use a TL431 on the main bus and have it send all back voltage to a large resistor.

Logthor wrote:

Are you sure it is 4 amps per motor?

 

4 amps per motor... I was thinking worst case. In practice you won't hit that.

A TL431 Precision Programmable Reference by itself won't handle the current. It will need some form of power transistor to handle the current. My little home-built cnc mill I had for a while had simply a cheap switching PSU powering 3 stepper drives with no additional protections. If a person had to add something I would connect  a 35V 10,000 uF capacitor to the DC bus. This adds cushion to even out short duration voltage spikes from decelerating motors.