How To Select Motor Sizes

[robertspark]

yes those are aluminium but fabricated from sheet (folded and bent) and are not extrusions

That could be an advantage depending upon your intended rail arrangement and its moment of inertia. For a commercial table it would have advantages in mass production and lightness and rigidity .... but for a diy built table it would would be difficult to do if you do not access to a means to cut the profiles out cheaply. (they may have used an aluminium channel section looking at it again for the back and one upside down for the base [corners are too tight / sharp] .... big bit of 1/4" on the bottom though or is that the top / side??)

[EverydayDiesel]

I have been playing around with fusion 360 but I am no where near the level of skills I need to draw a table. I keep working on it....


I also found this on another site. All though this is for a mill cnc I am hoping that the forumlas work for my application

Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.

So, how do you figure the power you need?

Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but your'e not sure of what force you want at that speed.

Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. Multiply it by the IPM you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts so use a step motor.

Eq: Watts = IPM * Lbs / 531

Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feedrate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.

How about force? Not a clue? Use your machinist's experience on a manual machine. The handcrank is about 6" inches in diameter. How much force would you place on the handcranck before you figure you're not doing something right? I hear about 10 Lbs.

!0 Lbs is 160 oz, 160 oz on the end of a 3" moment-arm (6" diameter, remember?) is 480 in-oz (3 times 160) ot torque on the leadscrew.

The equation for rotary power is: Watts = in-oz * RPM / 1351

For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.

213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.

https://www.cnczone.com/forums/linear-a ... 0-cnc.html

[robertspark]

Another 2 things to consider (as I'm fiddling with my table now)
1) consider using RJ45's for sensors and low current wiring (it saves a whole load of time than soldering connections onto an 8-way (9/10 way) connectors ) .... crimp and move on!

2) cable chain helps to keep everything routed and tidy (I use 50x15), basicallly it is the same lenght as my axis as it is folded over in the middle so only 1/2 of it actually moves as its folded over in a "U" shape.

I'll look at numbers later.

fusion 360 >> youtube it is not that difficult... start off with something simple (3x3 X axis extruded 5') and add some end plates to it.

[robertspark]

you want to make a really fast machine.... (out of my price range!)
https://www.linearmotiontips.com/how-do ... nion-work/

[Rodw]

Your 3 x 3 will be perfect.

[EverydayDiesel]

The price seems to be around 3.5x the normal rack and pinion setup but it is really nice

at 1:15 they talk about the guides and I am interested in your take on what he is saying (roller vs linear guides)

[robertspark]

roller bearings in my opinion are better for the plasma environment, quite a few designs used v-groove bearings on v-track (I'm fairly certain the MechMates (used) them with big wood routers too).
https://www.ebay.co.uk/itm/VRD-2-54-5-8 ... SwUoNaEHrq

Linear guides such as the THK are precision guides and provide two axis guidance .... but they don't like fine dust in my opinion.... you can get them with wipers but in a workshop or fabrication enviroment in my opinion over time they won't last because the plasma dust is very hard (think case hardened) and makes a nice grinding paste..... but you could consider the rails as consumabes and replace them from time to time but its an expensive operation (hence my simple bearings running on 2x2 is a low cost problem solver).

Cheapo linear slide rails SBR and round rail from what I have seen have a lot of play in them (I've bought a few to look at)... they won't last long in my opinion..... but I have seen them used on various machines and z axis before.
https://i.ebayimg.com/images/g/lYQAAOSw ... -l1600.jpg

The sales guy is a little wrong in his presentation.... with a router you don't slow down (or you will end up with a tool rubbing), you lower the depth of cut (but I'm splitting hairs)... plus you don't need a powerful PC for sheetcam.... far from it!

[Rodw]

There are plenty of commercially built tables using linear guides. I use HGR25 and HGR15 on my machine.
THe SBR's require two rails where the rails only need 1.

Thats the trouble. Everybody has an opinion, its up to you to decide the right path!

[EverydayDiesel]

roller bearings in my opinion are better for the plasma environment, quite a few designs used v-groove bearings on v-track (I'm fairly certain the MechMates (used) them with big wood routers too).
https://www.ebay.co.uk/itm/VRD-2-54-5-8 ... SwUoNaEHrq

Linear guides such as the THK are precision guides and provide two axis guidance .... but they don't like fine dust in my opinion.... you can get them with wipers but in a workshop or fabrication enviroment in my opinion over time they won't last because the plasma dust is very hard (think case hardened) and makes a nice grinding paste..... but you could consider the rails as consumabes and replace them from time to time but its an expensive operation (hence my simple bearings running on 2x2 is a low cost problem solver).

Cheapo linear slide rails SBR and round rail from what I have seen have a lot of play in them (I've bought a few to look at)... they won't last long in my opinion..... but I have seen them used on various machines and z axis before.
https://i.ebayimg.com/images/g/lYQAAOSw ... -l1600.jpg

The sales guy is a little wrong in his presentation.... with a router you don't slow down (or you will end up with a tool rubbing), you lower the depth of cut (but I'm splitting hairs)... plus you don't need a powerful PC for sheetcam.... far from it!

plus the 'industrial computer' that he sells for several thousand dollars is actually a begal bone lol

so v-track rails is best for this environment. I thought about having two seperate rails 90 degrees out to help with any slop in the gantry (just extra insurance)

I wonder if some kind of brush for the rails would be a good idea. (stole this idea from the wood cnc boys)
Somehow put that brush along the rail to clean it as it moves
https://www.cncrouterparts.com/cdata/me ... G_7181.jpg

There are plenty of commercially built tables using linear guides. I use HGR25 and HGR15 on my machine.
THe SBR's require two rails where the rails only need 1.

Thats the trouble. Everybody has an opinion, its up to you to decide the right path!

I think you are right but there is a general consensus that linear rails will wear down over time and become sloppy

This guy complains about the precision plasma and the candcnc package

[Rodw]

Are you sure you are not getting the from the guys who don't use linear rails to justify why the don't?

[robertspark]

If you buy a quality rail (genuine THK or NSK.... i.e. non chinese copy!) then they will work way better than a chinese copy MGN or HGR or HGH etc profile .... which all seemed to have been well copied from the more expensive japanese made THK or NSK rails (in my opinion..... everyone has one and everyone's is different)

You can also design them to be incorporated in such a way that they are protected from any plasma pierce blowback and also use bellows to aide in keeping the muck out.

If you run a chinese copy from the (only) ones I have bought (cost effective and I can afford) they were either very tight and notchy on the rail (and the rail must be bought at the same time as the carriages from the same supplier !!!!!) or the carriages were very loose with a lot of play.

What I did find with my playing around with them..... as I took them off the rail (NOT ADVISED! as the balls fall out!) is that I measured the balls and bought replacement balls is they ran very smooth with the replacement balls and were very low friction with very little play.

As I lost some of the balls from the first one I took off the rail, the second one I took off the rail I did not make the same mistake so I washed everything in paraffin well thinking they may have had the wrong lube on them or dirt in them.

I found that when I lubed both carriages up and put them on the same rail (from the same manufacturer) the carriage that had the original balls in them was still very notchy, but the one with new replacement balls and the same lube (silicone) was very smooth and low friction and no play.

So if I were to buy cheap rails again, I'd do the same thing if they arrived notchy and bin the balls and replace with new once measuring their diameter, clean and re-lube all surfaces.

Again I decided not to use them because of the plasma dust but they were to be used for a Z-axis and floating head assembly, and am presently trying something else using (genuine) IGUS drylin polymer linear slides
https://www.igus.com/info/drylin-linear-guides-overview

Choose one idea, design and run with it....

If you spend say $1000 on linear rail and it lasts you 10 years before you have to replace it your annual cost is $100 / year + ten year down time to replace it + your periodic cleaning and lubing the rail time (5mins a day or once a week compressed air blast whatever you feel you need to do)

If you spend $100 on v-groove roller bearings and replace them every year + annual down time to replace them with lower cleaning / lube time

The operating cost will be the same....

If you sold them both at 10 years the depreciation would probably be the same given the capital cost of the replacement rail would knock a fair bit off the table and also the table with v-groove bearings would probably be worth less to a buyer

Many ways to skin that same cat and spit out the parts you require which is what it is all about the finished article in your hand.....

I spotted this.... probably not a bad deal for a big turn key table if you can transport it and save you all the agro of building one ... (what is your hourly rate... if its free / zero you are not valuing your time correctly [he says typing carp here! ].... and it'll probably give you a fair bit of years of use before selling it on given the rails are well protected ....
viewtopic.php?f=57&t=30424

not the same one as advertised, but seems to be the same manufacturer and size

[Rodw]

I also found this on another site. All though this is for a mill cnc I am hoping that the forumlas work for my application

According to our model, a 40kg (88lb) gantry on linear rails 5:1 reduction, 30mm pinion produces the following results

100 Watts 34878 mm/min (1373 in/min), 1,1 Gs, 1860 rpm
200 watts 49331 mm/min (1942 in/min), 1.5 Gs, 2630 rpm

But when reviewing the torque and current constraints
65 Watts 28000 mm/min (1102 in/min), 0.87 g's 1500 rpm is a more achievable rapid target with plenty of torque left.

In all cases, the time from 0 to rapid speed is 55 milliseconds.

So that really states the case that a well designed carefully matched stepper system is in its element pushing a lightweight gantry around in the absence of cutting forces.

[robertspark]

I think your wattage / mechanical power is a little low, and expecting a stepper motor to run at those RPM's a little high, you may be better to consider servos in my opinion, given they don't run out of puff
https://www.orientalmotor.com/stepper-m ... otors.html

I'd be interested to know the stepper motor voltage, amperage and inductance you were thinking of running with out of curiosity.

[Rodw]

TIme will tell. We are actually designing for 10240 mm/min which is the highest cut speed we could find and the rapids will be what they will be. We've every confidence in our model. Its just we think we have a carefully matched solution and people are unaccustomed to seeing that with stepper motor selection. 25-28 m/min is not that much of an ask as I'm getting 21 m/min now with 48 volts, cheap drives and crappy motors. I handed off some sheet metal today to make a case to hold my toroid power supplies but it will be a while before I get that finished off and powder coated.

[EverydayDiesel]

This was great feedback from someone who actually owns the precision plasma gantry
viewtopic.php?p=184367#p184367

He suggests planetary drive gear box instead of belts which is an interesting idea but I havent seen anyone do this.

Collectively the more I can learn from others mistakes the better table I can make. I like to build stuff once if at all possible

[Rodw]

Here is what one Aussie did with 7 arc min backlash gearboxes

GEARBOX TENSION.jpg

Ready to mount a NEMA 34 motor on top.

They are much more expensive than belts and you need to buy precision ones and check the backlash at the pinion to make sure they are within your tolerance spec.

[EverydayDiesel]

What are the disadvantages of belts? Regular maintenance doesnt really bother me

[BTA Plasma]

Belts tend to not be as sensitive to thermal changes, reduce vibration and if properly engineered will have a longer life than a gearbox. We build with Belt and Gearbox depending on the weight of the gantry. It is easy to armchair the opposing viewpoints but it is experience that will help you make an informed decision. For belts we have a bridgemill that moves a several thousand pound table with a 3" fast travel ball screw with a huge and powerful AC servo motor the size of a small refrigerator and it has belt reduction. It is engineered to minimize vibrational harmonics while giving it the longest life and serviceability. With a entry level or light industrial plasma table Chinese gearboxes are the norm. But there is no standard between Chinese mfgs. The better ones have a service life attached to them that will be better than other cheaper units that are not as well built. One of the challenges you will find in a gear drive system is that when its time to replace a gearbox on a dual drive system you are doing it in pairs. Gearboxes are a great way to move in precision ways but with light industrial and entry level machines there needs to be quite a bit of rigidity to the structure to get smooth corners at high speeds or a reduction in acceptable acceleration. Belts add a buffer between motor and drive system and believe it or not help at that critical moment just a few thousandths from a directional change. So plasma tables with medium duty structures can yield better results than their counterparts with heavy structures and gearboxes running at the same acceleration. It is very simple yet complex state of physics. Our machine we build is the Star Lab that does well with both styles of gear box and belt reductions and has a composite gantry. The gantry style was chosen as the strongest gantry with the weight exactly where it needs to be so the mass and thickness are a balance of proportional inertia elements where motor trajectory changes do not cause wobble in the cut. Tube and off the shelf extrusion are old methods of quickly building a gantry and are what are considered ground level. What you should really be looking for is a quality gearbox as the belt reduction units commercially available have very poor components.

[SegoMan DeSigns]

What are the disadvantages of belts? Regular maintenance doesnt really bother me

Lack of maintenance has led to the premature death of many machines (Plasma Tables included). My 5 y/o TM 2x2 Growth Series still cuts like the day I got it online and better than some $100,000 commercial machines because the operators don't give a crap about them.

Just keep in mind when you design / build your machine to allow scalability for upgrades in the future. If you can't afford the best parts now get it running with what you can and start a replacement parts fund with the profits it make.

[SeanP]

If you run a chinese copy from the (only) ones I have bought (cost effective and I can afford) they were either very tight and notchy on the rail (and the rail must be bought at the same time as the carriages from the same supplier !!!!!) or the carriages were very loose with a lot of play.

What I did find with my playing around with them..... as I took them off the rail (NOT ADVISED! as the balls fall out!) is that I measured the balls and bought replacement balls is they ran very smooth with the replacement balls and were very low friction with very little play.

As I lost some of the balls from the first one I took off the rail, the second one I took off the rail I did not make the same mistake so I washed everything in paraffin well thinking they may have had the wrong lube on them or dirt in them.

I found that when I lubed both carriages up and put them on the same rail (from the same manufacturer) the carriage that had the original balls in them was still very notchy, but the one with new replacement balls and the same lube (silicone) was very smooth and low friction and no play.

So if I were to buy cheap rails again, I'd do the same thing if they arrived notchy and bin the balls and replace with new once measuring their diameter, clean and re-lube all surfaces.

Again I decided not to use them because of the plasma dust but they were to be used for a Z-axis and floating head assembly, and am presently trying something else using (genuine) IGUS drylin polymer linear slides
https://www.igus.com/info/drylin-linear-guides-overview

Yes I went with chinese version as well LSK were the ones I got from Zapp automation, the y axis ones are still going great, but like you say I have those covered, the ones on the gantry I replaced the blocks a couple of years back, got a couple of spares (£25) as well as they were selling them off.
Mine are 30mm and I just have one rail on the gantry, it's ok most of the time but I feel it would be a lot tighter if I had another smaller rail on top at 90deg to the other, might do it someday!

I also was looking at those drylin parts yesterday, as my z axis leadscrew is slightly worn, I'm surprised it's lasted so long really, it's got a delrin anti backlash nut on now but is getting a bit tight on the lesser used areas, the screw is .5mm less in the middle that the ends, but the amount of sheets cut it's a wonder it's not more.

The drylin sounds ideal for dirty locations, especially not needing lube as that doesn't help but I feel it's the lesser of 2 evils!
I'm still running the original 25mm round linear slides on the Z, I do try and keep them well lubed with chain lube, and a clean down every now and then, again it's surprising how well they are lasting really, they were only chinese slides that velox fitted, but might be worth replacing seen as no lube needed.

Just modified the Velox Z a few days back by fitting enclosed sides and below the slider which should help a lot.

To everydaydiesel, I would say 1/4'' wall on the gantry is way to much, mine is 65'' wide and I only used 70mm (2.75'') box with a 3mm wall, you don't get any flex in that, ok bolting the rail it isn't thick enough, so I just slid a strip inside 1/4'' thick threaded to match, if you have a little clearance in the gantry holes that can be useful for adjustment as well.

I went with sprung tensioned pinion gear, I would do that again no problem it's very forgiving, you would think a set tension setup would give trouble as the rack wears more at one end than the other.

Have a look at my build if you want to see how I went about it, link in signature below.

[EverydayDiesel]

Thanks for the feedback, I will look at your build now.

When you say 1/4 wall is too thick are you talking about steel or t6 aluminum (or both)?

I might build something similar to JD2's gantry. I have a miller dynasty welder, but I have never welded aluminum before.


EDIT: Awesome build sir. That is very close to what I am looking at doing. Do you have any CAD drawings of the table? I am actively learning freeCAD (linux) and eventually I will be able to draw my table.


So many questions such a great table:
1. Who made your Z assembly?
2. Why did you put angle iron over the top of the (2x6?) Is the angle flatter then the rectangle tube? https://lh3.googleusercontent.com/pw/AC ... authuser=0
3. What size motors did you use for XYZ?
4. Would you do anything different?

[SeanP]

Sorry I wasn't keeping up with the posts there, thought you were still talking about steel box gantry.
The zaxis unit I bought from Velox cnc.
The angle does 2 jobs, well 3 really.
Traps the water tray sides in position, protects to rails from splashing and spatter and I also welded the slat fixing on there to slot into, all still working well.
You see many tables with the side rails level or not much higher than the slats, I took a different view, think mine is 3" taller, to me its ideal, helps to stop splashing, but positioning sheets with the use of a pry bar and tube under the sheet is a godsend, the tube runs across the slats parallel to the length of tbe sheet, then remove it by prying on the rail/angle sides and pulling tube out with a hook, can position a 8x4 sheet of 15mm onto stops easy enough that way, even with the forklift I still load this way.
I used a candcnc bladerunner kit for mine , steppers 620oz, parralell port system, I'm still happy enough with that, thc works great keeps up with anything I cut, 20g to 1" even cutting 2" holes in 4" tube with thc on!
Would I do anything different, well maybe the gantry, running with a single linear rail would be the main thing, having smaller double rail setup would have been better, might still add another smaller rail 90deg to that yet, gantry rack would have been better
facing away from the crap.
Gantry sides could have been stepped back more to make better use of length of table.
Appart from that no all good I think, and its had a good testing!
Hardly stopped since I built it, wish i had kept a track of the sheets, must be a stack 30' tall I reckon!
The drawings I have are in a mess to be honest, I was also learning Solidworks as well while designing.

[Rodw]

Sean If you check the torque specs of the rails, with the one rail you have, you are way over designed. I've got 2 x 15mm rails and was thinking if I did it again i'd just use one of them to save a bit more weight! (On a 40 x 80 mm ally RHS)

[EverydayDiesel]

Sean If you check the torque specs of the rails, with the one rail you have, you are way over designed. I've got 2 x 15mm rails and was thinking if I did it again i'd just use one of them to save a bit more weight! (On a 40 x 80 mm ally RHS)

How much do they weigh? Is it the actual weight or the parasitic drag that is an issue?

[SeanP]

I'd still sooner go with a double rail, it's not actual play I can see on mine it's more like give in it when you pull the top of the Z axis, it's only on very fast turns I sometimes see a bit of a rattle in the cut coming off a sharp turn, it does well really.
I'm not really sure of the weight, just looked at the lsk pdf and cant really make it out, can't find a set of scales to weight the spare I'll take a guess 18kg in total, sorry not more accurate.