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New build design specs?

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RyanS
Posts: 11
Joined: Sun Sep 03, 2017 11:28 pm

New build design specs?

Postby RyanS » Thu Oct 05, 2017 1:54 am

My Powermax 45XP has arrived, so now the task of designing a 1200 x 1200mm (4' x 4') CNC table can begin.

I've noticed a few commercial tables are capable of 420 mm/s (1000 ipm). Is that to accommodate large plasma cutters that are perhaps capable of those cut speeds? In any case, the cut chart 45XP ranges from 2.6 mm/s (5 ipm) to 159 mm/s (320 ipm). Would 210 mm/s (500 ipm) be a respectable rapid rate? I may go higher, provided I don't have to add massively to the cost

What is the recommended maximum acceleration I should aim for?

I have no idea what torque or size steppers I should use, or what other variables apart from speed, acceleration and gantry weight I need to know in order to calculate that?

I've read steppers are sufficient for plasma cutters, but also that they're inefficient and run hot. Would I see a benefit from using closed loop steppers, or am I simply going to complicate my design?

tcaudle
Posts: 793
Joined: Tue Feb 28, 2012 6:47 pm

Re: New build design specs?

Postby tcaudle » Thu Oct 05, 2017 9:23 pm

On a rack and pinion machine ( or any other type) you trade Speed for torque....that's what transmissions do. You have rotary torque that converts to linear torque that move the load. Since steppers lose torque with RPM you are always chasing the curve. If you spin them fast and reduce use a transmission to increase the torque the trade off is less overallthan if you spin the motor at 50% of top RPM and use less step done. You can run the numbers anyway you want but the best ("golden" ) ratio is one that provides 1" of rev for 1 rev of the stepper motor. So lets take a typical rack and pinion setup. The pinion gear is 1" in diameter (DP) so it actually moves the load 3.1416" per rev. But we want 1" of linear movement so a belt reduciton back to the motor yeilds pretty close to that magic ratio. Now, Steppers typicaly if run at the optimum voltage for their inductance can hit around 1000 rpm with a light load. Probelm is that offers almost zero torque, Roll back to about 1/2 of that and you have 500 rpm where there is still about 1/2 the rated torque. (don't forget you also have to decelrate from a high speed and that takes an equal amount of torque. So if you have the magic ratio and you spin the motor at 500 RPM ....you get 500 IPM linaar speed with some torque reserve. You can push it higher for rapids but when you see a steepr system with 1000 IPM rapids that means its either geared wrong and has less torque or it prone to losing steps.

Ao what about torque? It equals acceleration. Because of Newtons Law that says objects with Mass have inertia and take more force to change their speed than to maintain their speed you need torque to accelerate / declerate a mass or chage its direction.

Mass on earth is the weight in Lbs divided by 32 . Force in the same units is in Ft-lbs (which is akso Oz-in) acceleration then is in Ft/sec/sec whichs conerts to inch/sec/sec. You calculate linear force from rotarty torque by takeing the torqe at the center of the shaft and dividing it by the Radius of the pinion gear. So if you ahve a 600 oz-in motor spining at 500 RPM you have about 300 oz-in / .5 in or about 600 oz-in (3.125 ft-lbs )

The basic formaul if you want to sit downwith a calculator is:

A = F/M X efficiency

Get everything into the same units and use and efficiency of .3 (30 % ) and you will be able to predict what a particular gantry will do with specific motors and linear componets. You goal is to be ablle to hit about 35 IPs/sec acceleration.

We can get into the whole stepper VS servo religious arguments if you want

RyanS
Posts: 11
Joined: Sun Sep 03, 2017 11:28 pm

Re: New build design specs?

Postby RyanS » Fri Oct 06, 2017 12:59 am

A religious argument in terms of are steppers or servos appropriate for the type of machines we build? Most industrial CNC machines use servos and I presume steppers just don't meet their design specifications.

I work in metric, but would using 1200oz-in steppers therefore yield double the acceleration, or is there a point where steppers, no matter what size can't accelerate as quickly as servos? Not that I think my machine needs more than 500 IPM. I've read (can't remember where) that it's better to use small pinions, but that seems like the steppers would have to spin faster to move the same linear distance

Where does the 30% efficiency come from? is that a rule of thumb for steppers, or steppers + rack and pinion and other efficiency losses? Does that take into account the friction of your linear rails?

Rodw
Posts: 87
Joined: Sun Aug 21, 2016 1:49 am

Re: New build design specs?

Postby Rodw » Fri Oct 06, 2017 6:56 am

Despite all of the advice you will receive on forums like this its ultimately down to you to analyse the feedback you get and make your own decisions at the end of the day. I was exactly where you are once when I decided to make my own plasma cutter. I'm no expert but I'll share my experience as I'm now close to the end of the journey and am confident I have a high performing stepper driven system.

For your first build, don't go with servos. You will have enough to do without adding the complexity that comes with servos.

Don't bother with closed loop steppers because steppers loose steps when they have run out of torque so they can't actually make up those steps as they don't have the torque to do it. So don't complicate the design and increase costs with something that will have no reward.

Industrial quality electronics (like the Mesa 7i76e I'm using) will blitz what can be achieved by a parallel port breakout board or Arduino. It also surprised me how much cooler the steppers ran. I refute any statement that steppers run hot. If they do its because of crappy electronics (and I've written some software on an Arduino that does run hot)

Hypertherm make a note on their cut charts that says something like their highest speeds were limited by the performance of their test environment. So why bother trying to exceed the maximum speeds they can achieve?

500 ipm (12.7 m/min) rapid speed is very conservative. I tested my hardware through to 21 m/min without loosing steps but backed off and settled on 18 m/min for X & Y rapids.

What acceleration to shoot for? Early in my journey I asked the same question elsewhere and an experienced plasma builder said around 700 mm/sec/sec was fine and thats what he used. Elsewhere, I've read 0.1g's (981 mm/sec/sec) was a good target for plasma. I settled on 1500 mm/sec/sec for my X & Y axis but believe I can push this higher through to 2000 mm/sec/sec or a bit higher.

What steppers am I using? For the X & Z, I am using 269 oz/in NEMA 23's. For the Y axis, I am using 1000 oz/in NEMA 34's. I originally was going to use NEMA 23's but I found these cool reduction drives that were designed for NEMA 34's that were so cheap it did not warrant my time to build my own for NEMA 23's.

What weights am I throwing around?
Well the torch weighs about 1 kg so the Z axis is probably < 2 kg
The total torch motion assembly ( X & Z) weighs about 6 kg
The total gantry weighs about 37 kg (23 kg for the complete gantry assembly rail plus 7 kg for each gantry end with motor).

In the end you have to make the decisions and build something or you will get bogged down in time wasting design.

I hope this helps.Let me know if I did not answer every question you raised.

RyanS
Posts: 11
Joined: Sun Sep 03, 2017 11:28 pm

Re: New build design specs?

Postby RyanS » Fri Oct 06, 2017 7:17 am

I'm probably more comfortable using steppers. Plenty of milling machines successfully use them and they'd place more demands on steppers than plasma would. I don't have any intention of building a second machine. I'm not sure servos are necessary even as an upgrade later?

Could I get away with a smaller stepper on the Z due to it having the least weight?

Rodw
Posts: 87
Joined: Sun Aug 21, 2016 1:49 am

Re: New build design specs?

Postby Rodw » Fri Oct 06, 2017 8:09 am

RyanS wrote:I'm probably more comfortable using steppers. Plenty of milling machines successfully use them and they'd place more demands on steppers than plasma would. I don't have any intention of building a second machine. I'm not sure servos are necessary even as an upgrade later?

Could I get away with a smaller stepper on the Z due to it having the least weight?


Ryan, I'm using this one. It should ship from Australia and they accept warranty returns to a Sydney address.
https://www.omc-stepperonline.com/hybri ... 2804s.html
They also have a stepper motor bracket I used to mount to the Z axis.

Cheers mate.


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