First Proper CNC Router

[Davek0974]

I use Aluminium warehouse, i have heard various stories about them but i still cant find anyone better/cheaper

[JAZZCNC]

I use Aluminium warehouse, i have heard various stories about them but i still cant find anyone better/cheaper

ASC metals in lincoln usually knock spots off AW
http://www.ascmetals.com/

[cropwell]

ASC metals in lincoln usually knock spots off AW
http://www.ascmetals.com/

I have used Forward Metals for small purchases of Ali, and they seem good. https://www.forwardmetals.co.uk

[diycnc]

So after 6 months my CAD models are starting to turn into a CNC!... Slowly....

The design has changed a bit, but the concept is mostly the same.



I went with aluminium flat bar rather then cast plate in the end. Very quickly learned that was a bad idea! Flat bar is very far from flat!
Managed to get everything moving freely so far despite the not flat bar using plenty of shims to level things up. First project when this thing is up an running will be new cast aluminium plates.

So far I have the gantry carriage and ball screw all made and assembled. Made my own ball screw end supports from 50x50 aluminium bar with angular contact bearings, and bored out and threaded the HTD pulley to replace the lock screw. Everything moves freely and feels rock solid, so very happy with the result.

Sick of drilling and tapping holes now.

[diycnc]

As I slowly edge closer to the electronics step, I was wondering why we have to over spec a switch mode power supply.

The stepper drivers we use limit the current to the motors, so why do we need to make sure we choose a power supply that can supply say +20% more current? Surely a driver set to say 2A will only draw 2A from the PSU?

I'm sure there is a very logic explanation.

[driftspin]

As I slowly edge closer to the electronics step, I was wondering why we have to over spec a switch mode power supply.

The stepper drivers we use limit the current to the motors, so why do we need to make sure we choose a power supply that can supply say +20% more current? Surely a driver set to say 2A will only draw 2A from the PSU?

I'm sure there is a very logic explanation.

Hi diycnc,

Why not build one yourself.

It is only 5 orso electrical parts.

Grtz Bert.

Verstuurd vanaf mijn SM-A320FL met Tapatalk

[Neale]

As I slowly edge closer to the electronics step, I was wondering why we have to over spec a switch mode power supply.

The stepper drivers we use limit the current to the motors, so why do we need to make sure we choose a power supply that can supply say +20% more current? Surely a driver set to say 2A will only draw 2A from the PSU?

I'm sure there is a very logic explanation.

Well, there's a very technical explanation! Actually, the stepper driver does not quite limit current to the nominal value. In fact, it's often unclear what that nominal value means anyway - peak current? Average current? So the value that you set the stepper driver to will be somewhere near the maximum current drawn from the PSU but not necessarily exactly the value you expect. The other problem is that the stepper driver doesn't actually supply a steady current over the whole pulse, but chops it alternately on/off so that the average is around the desired value. So peak current draw might be higher than expected during the pulses, even if the average is around what you would expect. The SMPS must be able to cope with these short-duration pulses without going into "overload" when it might shut down altogether for a brief period, until the "overload" has gone away. This can be pretty catastrophic for the machine overall and losing steps might be the least of your problems. Hence the need to over-spec a SMPS just to make sure that it never goes into current-limiting or overload mode. I would be worrying if the SMPS is only capable of supplying 20% over nominal current - doesn't seem like much headroom to me. That's why the usual advice (even from people like Leadshine who should understand these things) is to use a "linear" power supply which is very tolerant of pulse current overloads.

This is a bit of an over-simplification, but the fact is that the current draw is very "peaky" and not steady, you are adding together the loads of multiple drivers without any idea of whether their pulse loads will occur simultaneously or not, and you have a power supply that is not capable of handling overloads gracefully. A linear PSU will be heavier, probably larger, more expensive - but more reliable. And they're easy to build - there are several "how-to" threads on the forum. If you go SMPS, over-spec the rating to make sure!

[Voicecoil]

The other option might be to add a decent size capacitor (maybe 4700...15,000uF) on the output of the SMPS to deliver short term current pulses without tripping the protection. I say "might" because some wimpy SMPS's are just so wimpy that they find it difficult to start up into a large capacitive load, so you'd be b******d from the start. However some do have a "soft start" provision that avoids this and such an approach has got me out of a hole once or twice before.

[diycnc]

Thanks for the explanation.

If you were going to use a SMPS, how much extra over the nominal current would be recommended?