CNC machines

[Ned__]

According to my limited understanding, - if I match these steppers up with and 80 volt power supply, and drives that are rated for 80 volts, I could achieve high speeds?

I don't use thin metal, ever. 3mm is the thinnest metal I use (Arc welding everything these days)
So really high speeds are not important for this build.

 

[MattH]

According to my limited understanding, - if I match these steppers up with and 80 volt power supply, and drives that are rated for 80 volts, I could achieve high speeds?

I don't use thin metal, ever. 3mm is the thinnest metal I use (Arc welding everything these days)
So really high speeds are not important for this build.

View attachment 306370

TBH it's not a simple question. The speed you can achieve with a certain voltage will depend on the inductance which you would need to know.
The trouble is that torque drops on steppers as speed rises, usually to an unacceptable level. This is why large steppers are used but geared down so much, they are very slow but product huge torque, at high speed they have no torque.
There are Stepper speed calculators online but you would need to know all the parameters of your motors.

A lot will depend on your gantry weight but in my mind those motors are way to small.

Without a lot of planning all you can really do is suck it and see.

 

[MattH]

This is a fairly standard setup for a Stepper driven gantry machine (the cheap Chinese CNC routers us this), large stepper (either standard or better like the below hybrid) driving through a large reduction gearbox to a pinion that rides a rack.
The rack it very highly geared i.e. one turn of the pinion is about 100mm of travel therefore even a slow stepper needs gearing down through the gearbox:

 

[Ned__]

I see, I understand now, thank you for explaining that, it makes sense.

Well, as you say, suck it and see. Steppers are reasonably priced, so, I can always change if I find its not working out. My machine is belt drive. In fact its really an upsized version of the Openbuilds Ox cnc, and I've seen those things fly with similar steppers and gearing to mine

Ill be using a 20 tooth GT3 pulley

 

[Ned__]

This one is really fast, uses rack and pinion like you mentioned Matt

 

[Hood]

Hood what servos were you looking at? Was looking at servos last night.

There were several ones available from AliExpress etc, not sure if I have a link still.

They're not. Rapids are fine, easily 15M/min but the acceleration is to low, anything above 0.9G introduces to much jerk and are 0.9G the direction changes are way to slow (for 1.5mm) and cause overburn and arc wander.
On 5mm thick and upwards they are great but thin needs loads of speed.

That may depend on the weight of the gantry or possibly it is the type of servos, what were the ones you had?
On my X axis I think it is a fairly small motor but not sure if it is as small as 400w.

You can get 750w Delta motors and drives for about £350, so maybe that would be where I should look if thinking of getting some as spares.

 

[Hood]

This one is really fast, uses rack and pinion like you mentioned Matt

The rapids in that vid are reasonably fast but acceleration is slow, that is something you will likely have to live with using steppers.

 

[MattH]

There were several ones available from AliExpress etc, not sure if I have a link still.



That may depend on the weight of the gantry or possibly it is the type of servos, what were the ones you had?
On my X axis I think it is a fairly small motor but not sure if it is as small as 400w.

You can get 750w Delta motors and drives for about £350, so maybe that would be where I should look if thinking of getting some as spares.

if I was building from scratch I would go down the R&P with hybrid steppers route.
You can get the acceleration from steppers because of the high starting torque.

Yes 750W+ servos are great but you are looking at over £1k just for the motors.
Servos are great when there are cutting forces as you need more torque as speed increases. For Plasma the torque requirement doesn’t increase with speed as there is no force on the head

 

[Hood]

if I was building from scratch I would go down the R&P with hybrid steppers route.
You can get the acceleration from steppers because of the high starting torque.

Yes 750W+ servos are great but you are looking at over £1k just for the motors.
Servos are great when there are cutting forces as you need more torque as speed increases. For Plasma the torque requirement doesn’t increase with speed as there is no force on the head

You can get the Delta ones way cheaper than that, drive/motor/3m cables for £300 incl post but customs charges will apply I suppose.

750W Delta B2 Servo Motor Drive Kit 220VAC 0.75KW Delta Servo Kit Brake 2.39NM 3000rpm ASD-B2-0721-B ECMA-C20807RS/ECMA-C20807SS - AliExpress 13

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

 

[MattH]

You can get the Delta ones way cheaper than that, drive/motor/3m cables for £300 incl post but customs charges will apply I suppose.

750W Delta B2 Servo Motor Drive Kit 220VAC 0.75KW Delta Servo Kit Brake 2.39NM 3000rpm ASD-B2-0721-B ECMA-C20807RS/ECMA-C20807SS - AliExpress 13

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

£300 each isn’t far off my £1K considering a dual drive x axis. With import that’s probably over £1200 for 3.

I’ve bought leadshine servos which have a bit better documentation and software. They are a bit more money.
I’ve imported 2 sets of 400w units and a set of 750w over the years and they work great.

For a cheaper Plasma build I’d still go with hybrid steppers.
Considering what you can get for £10k:

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[Hood]

£300 each isn’t far off my £1K considering a dual drive x axis. With import that’s probably over £1200 for 3.

I’ve bought leadshine servos which have a bit better documentation and software. They are a bit more money.
I’ve imported 2 sets of 400w units and a set of 750w over the years and they work great.

For a cheaper Plasma build I’d still go with hybrid steppers.
Considering what you can get for £10k:

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Find many great new & used options and get the best deals for EXDEMO FABERTEC PL2 1325 PLASMA CUTTER with HyperTherm 45XP £10,000 With VAT at the best online prices at eBay! Free delivery for many products.

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Ah ok, thought you were meaning for one motor as you had said.

Yes 750W+ servos are great but you are looking at over £1k just for the motors.

 

[MattH]

Motors

 

[Hood]

Motors

Yes, motors, no mention of drives

 

[MattH]

Yes, motors, no mention of drives

Ok ok, drives included. But motors, plural, not one!

Anyway, sorry for digressing.

 

[henry Kadzielski]

My two cents worth. I have the NEMA 34 850 in/oz servo motors on all my drives with 15:1 planetary reductions and 29 tooth pinions on all the gear racks. Now the problem is with the weight of my gantry now (with all the bits hanging off) is about 200kg+. Moving this mass at 23m/min wasn't a problem and looked impressive, but the direction change would rattle my table which is almost 7m long. This sudden change in direction (23m/min and back in the opposite direction 23m/min) would trip the motors on overload. Not good when you are cutting something. So I reduced my rapid speed to 19.5m/min to reduce this. Now cutting I normally cut thicker stuff, up to 1" plate (mainly 6mm and 10mm ). I have been doing the 6mm at 45 amps no problems and cut speed as has been stated before is then slower which is easier on the machine. I had also cut some 3mm steel for small BBQ and found that when cutting my business name out the 'S' seems to be a problem with the machine doing the back and forth in the three segments to cut the 'S' at about 4m/min and shaking the **** out of the table. So my machine is now not so good for thinner stuff. I may need to switch to the 'fine cut ' consumables for the thinner stuff, however there has been no need. Horses for courses, it seems that you cannot have everything

Now I got servos as I wanted speed and wanted the torque to do routing and as such got the biggest that the supplier had. Two drawdacks with servos 1. cost and 2. setup and not to mention when there is a problem to locate that problem and get it resolved. At times I could have torn my hair out and poured petrol on the machine, however when it goes it goes very well and i would not change a thing

 

[Ned__]

Heya Henry ) cheers for the info, and Hood and Matt, great reading this morning, appreciated.
Hood I remember you showing me those deltas before, ages ago lol.

Henry I weighed my gantry a while back, without motors (4) and without wires and without anything attached to the z axis

It weighed 15 kilo

The motors I bought are 1.1 kilo each, 4 of those so lets say another 5 kilo.
I'm going to make the z-axis as light as possible, lets say another 5 kilo (wont even be that)
and wiring another 5 kilo and I'm at 30 kilo total

 

[ned]

I found a gantry that I'm going to copy.

This guy built three plasma tables and what you are looking at below is the evolution that the gantry plates underwent.
I doubt he is reading this, but if he is, imitation is sincere flattery so please accept my apology.

I got the second rail on last night so am going to start designing the gantry plates today

 

[ned]

I know a nema 23 is 56.4mm wide, so I photoshopped a row of motors and deduced that his plate is 4.5 nema motors wide, which is around-abouts 250mm

I did actually try to engage the guy on the net but he doesn't answer questions, so that was that.

 

[ned]

Cutting a wooden beam for practice, to mock up a gantry to estimate what it should look like

I photoshopped a drawing of it, might be too big,

It's 350mm wide to the guys 250mm. This size gives me 210 mm clearance between the steel and the bottom point of the Z axis.
I will make an MDF version tomo and see what that looks like.

You can see the outline of the plates I have now in the MDF drawing below

The saw likes wood!

I need to do more work to make the saw square.
The aluminum gantry beam I bought is 150mm too long, it needs to be cut to length.

I'll practice on the extra material before making the final cut for the gantry beam.

 

[Hood]

I cut a bit of 1.5mm 316 last night and it was at 10m/min with 3000mm/s/s acceleration.
Vid here

As can be seen there is still some corner rounding 1/16" or about 1.6mm radius, not really and issue for me but could be for some. If using steppers, even if you could reach 10m/min I suspect you would get nowhere near the acceleration, certainly the videos you posted earlier of other tables are no where near that. You could of course use Exact stop mode but that might not be great on a plasma and may actually cause more issues as the arc would stop for a fraction of a second.

One thing I must say is a water table is the way to go especially if going to be cutting stainless. This edge would have been totally black with oxidisation on my last table but with this it is fairly clean only the very top edge being black and that is a fraction of a mm thick if you consider the complete edge is only 1.5mm.