New cnc plasma build design specs?

[RyanS]

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]

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]

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]

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]

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]

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.

[gzm.pepe]

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.

Does this “rule” also apply for servomotors?

[plasmadog]

Hi guys
I rely like your approach Rodw with the stepper motors and keeping this simple especially for a plasma table , i your response to Ryans you have answered a few of my questions Thanks .

I am another guy building his 4x4 plasma table for cutting out 14 and 16 gauge mild steel. Looking at the Amazon stuff, but I am like a deer in the head lights when I get to this part. I am semi-retired and looking for a hobby making small signs and fascinated with motion travel.
I am at the part of building the steel table to cut only 4x4 sheets of metal due to the size of my small shop.
I am looking for advise on stepper motors , I have been racking my brain looking and reading for the past 6 months , I could purchase a ready built machine but the cost is way too much with shipping and duties and also the exchange rate here in Canada , so I love to build and weld stuff.
So here are a couple of things that I will be doing , taking in consideration keeping this simple and the weight of the axis down and not over building this project.

The frame for this table is 2x4 and 2x2 0.125 Steel except for the sides where it will be 0.188 thick to give be a little more meat for threads for the rails and slider. I am looking at Langmuirs Crossfire Pro approche to simple and efficient and ad a few personal touches.
The Y axis I trying to keep the weight down as mush as possible so my question here is that I am looking to purchase the steppers and drivers that will move this around there so many out there and I don’t want to over kill this build . Looking at the amazon stuff, butt I am like a deer in the head lights when I get to this part. I am semi-retired and looking for a hobby making small signs and fascinated with motion travel.
The Z axis I am purchasing already built here in Canada from a small company that just makes these
Here is a pic https://step-motion.com/products/z-a...ating-head-diy
I will be installing a THC control and I will be going with Hypertherm Max45 , unless there another machine that does just a good.
So once again advice for a complete kit steppers, drivers, power supplies, possibly 269 oz 2 on the X and one on the Y.

This Forum is awsome I will post some pics as go along .

[adbuch]

What control software do you plan to use? Centroid, Mach3/4, Linux, ...... etc. I have purchased and used nema 23 steppers from Amazon in the past and they worked well for my application.

https://www.amazon.com/gp/product/B074J ... =UTF8&th=1

Depending on the weight of your gantry you may wish to step up to a nema 34 for you Y axis motion.

You might consider reaching out to Tom (acourtjester). He has built at least 10 diy cutting tables and has a wealth of information to share.

David

https://www.plasmaspider.com/memberlist ... le&u=12881

tom.jpg

[acourtjester]

Yes sir David I will help who I can with their DIY table I will send a PM to Plasmadog

[plasmadog]

Hy Guys this is great news to know peaple out there that can help me out.
I am presently working on the table that will acomodate my shop and needs for my projects i will end up with a cutting surface of 50 x 38 inches .
I will post some pics as i go along guys . I am looking at Mach 3 with sheetcam . something simple and easy to learn. I have a question ? i am building all by hand exept for the Z axis there is a guy that buils them neatr toronto and it seams affordable and ready to go and for the price ! i would be all in .
heres the link Guys https://step-motion.com/products/copy-o ... sembly-thc

Thanks Again

[adbuch]

I think that is a pretty good deal. At $425, unless you have access to a machine shop to diy, then I would say "go for it".
David

magnetic breakaway floating mount.jpg

[plasmadog]

Ya thats what i say also , i dont have a machine shop and at this price for sure its looks well built and made right here . Not sure about the break away butt i will surely crash at some point they say lol.
Thumbs up

[adbuch]

Here is a nice simple design for a magnetic break-away mount, which was just pointed out by SegoMan DeSigns.
This one could be made by hand instead of cnc plasma cut, but of course you would still need the z mount assembly - floating or not.
I'm not sure what the advantage of the floating aspect is for plasma cutting, but I can see how it might be used for mounting a pneumatic engraver.

David


https://www.plasmaspider.com/viewtopic. ... away+torch

IMG_20171203_135424.jpg

[plasmadog]

Hey Thanks adbuch it looks awesome , and i am going with the floating head and THC control because i will be cutting 16 and 14 gauge steel.

[acourtjester]

If you have another free input you may look at a Ohmic sensor, very simple to wire and setup. There are a few cheap modules that can be used for a ohmic sensor that do the job very well. SheetCam has the change in the post processor to look for an ohmic sensor with the floating head switch as a safety backup.

[plasmadog]

ok I will look into that for sure. I am not there yet and I am not sure what to drive this table with yet as far , i know IL be using a least 3 289 onze nema 23 motors and a small stepper foth Z axis ,there are some packages that come with the three motors and drivers and the power suply 24 volts i believe and not sur if i can use thse board to add the Ohmic sensors on them ????
Lots of fum butt you know what ! step by step and as I go along with the build we will cross that bridge. hahahhaha.

Thank again
Brian

[acourtjester]

I normally buy the kits with 3 425 OZ motor and a 36 volt supply, not much difference in price but you get a little more reserve in the motors.
the BOB boards that come with them can add more inputs if you have a controller that will allow more than one BOB. there are many different controller electronic packages to choose from. I have built tables with 1 BOB and it make it tough as it limits the inputs to 4 or 5. The table I am using now uses the UCCNC UC 400 ETH which can have 2 BOBs hooked to it. The newer table I am working on has a LinuxCNC Mesa 7I76E which has many inputs and outputs. And has add-on boards for THC and Ohmic sensor so no BOB boards needed.

[plasmadog]

Oh OK so for me the mesa sounds like the best choice with the THC and limit switches yesss.

Taking notes here
Thanks