I'm building a 4' x8' plasma table , the gantry is about 100 kilos, can I drive it with the 495 oz motors and the 4030 drivers from kelling? I 'll use two directly (no pulleys reduction) to gear racks.
Thanks for help
Luis

The pinion PCD will determine your trade off between speed and acceleration.

I calculate your theoretical max acceleration will be 18 ft/s/s divided by the pinion PCD in inches.

Ok, but the question is: will these motors and drivers work for this weigth?
Luis

You didn't mention rolling resistance so I assume you have that well covered.

If so, anything will move it.

OTOH it is extremely heavy so the question seems to be how quickly you can get it up to speed. 100kg is about 220 lbs which is a lot of metal to throw around. Is this a router/plasma bed?

I also assume that's pull in torque rather than holding torque for the motors, if it's holding torque you will lose a lot of acceleration.

Thank you Robin, It is only for plasma and torch operation,no contact. I think I can lower the weigth of the gantry.
Luis

I wouldnt bother lowering the weight until you do the math.

My gantry is big and heavy (see www.ubcustom.com/cnc) and on top of this im turning two 6foot long 1" shafts. I barely have any friction from the 8 OPN pillow block bearings (4 each side) but i even accounted for that. My I think 400 or 600 oz in (i forget) motor can turn that no problem. Its all about the reduction and acceleration that you need.

Here try this.... find out how much HP your motor is rated at. If you have a chart that came with your motor look at the torgue vs speed curves. Power = torgue times rotational speed (i.e. rpms). Check your units!!!! Be wary of the torgue vs speed curves. If you find the maximum (rated HP) by looking at the curve you are forgetting that your motor will probably not even operate at this speed hence not give your the torgue u want. Now that you have determined the power (or HP) number you are comfortable carrying around in your pocket move to step 2. This is your motor HP.

Step 2 is calculating the linear horsepower you will require.

power = work / time = force x distance / time = acceleration x mass x (distance / time) = acceleration x mass x (average velocity)

Now say you want to pep your machine up to a velocity of 1000ipms in 4 seconds (no laughing please) you get these numbers....

acceleration = (1000ipm) / 4 sec = _____
mass = 220lbs/32.2
average velocity = (1000ipm - 0ipm) / 2 <<EDIT THANKS ROBIN>>

Chance your velocities from ipms to feet per second
do the math and your answer will be in lbf/sec
Use the unit converter and find out your required HP

If your required HP is more than your motor HP you are pretty much screwed. Either change your acceleration or reduce your weight. Keep repeating this step until your motor will work. Or get a bigger motor :-p

Someone please check my theory. I ALWAYS make mistakes!

power = acceleration x mass x velocity

Perhaps a rather complicated way to do the sum?

1 hp = 550 ft.lbf/s so easier to work in ft/s

final velocity = 1.388 ft/s
but it's accelerating so you need average velocity to make this work...
av. velocity = 1.388/2 = 0.694 ft/s
acceleration = 1.388 /4 = 0.347 ft/s/s
mass = 220lbs/32.2 = 6.83 slg

.347 x 6.83 x 0.694 / 550 = 0.003 hp

Check that by doing it another way...

ft.lbf = half mv squared
0.5 * 6.83 * 1.388 * 1.388 = 6.579 ft.lbf
divide by 4 seconds = 1.644 ft.lbf/s
divide by 550 for hp = 0.003 hp

Hardly any force is required

f=ma 6.83 slg * 0.347 ft/s/s = 2.37 lbf

A horse would have little trouble providing this acceleration :D

lamicron let us know what you come up with. Your motor should have no problems turning that heavy beast of a gantry.

Post your calculations with explanations and we can check them for you. Sketches help too. For instance draw your whole x axis setup with the motor, gear reducer, shafts, rack and pinion whatever, showing all diameters, weights, acceleration, velocities etc and get one of us to calculate it. If something is missingf from the equation somoene will speak up and say oh we need to know whatever. This will help boost your confidence as you go along and build your machine. Its a great way to catch mistakes and gather opinions. I mean look at the mistake I made by using final velocity instead of average velocity.

I wish I posted up all my calculations on cnczone and had the guys here to criticize the hell outta them before I spent all that money on pulleys, belts, gearboxes, motors etc. It all worked out at the end so whew i guess. My machine moves 5 times faster than it needs to and i can even hold onto my gantry and go for a ride.

Erik

Thanks for the help, I know you people are ready to help,
Here the first pics of my "monster" .

I mean look at the mistake I made by using final velocity instead of average velocity.

Hi Erik

On reflection I think I'm wrong and you are right, it all depends on how you look a it.

I've done the sum for a motor that will acheve the 1000 ipm then run out of steam.

You did the sum for a motor that will deliver 2.37 lbf at 1000 ipm which is what you would need for a linear acceleration.

I think your .006 hp is the better answer :confused:

Apologies for that, I'll leave you to do the maths in future :D

Robin

Here the first pics of my "monster" .

It must be wonderful to have that kind of space to spare :D

Robin im glad i met you on this board here. Hopefully we can check eachothers calculations. Do you go as far as calculating the inertia ratio of the servos? Apparently from a robotics point of view (especially with systems that are used for critical applications) it is required or highly recommended. I did this with my table. I didnt know the inertia of my servo so I did some research and used an educated guess. I know the inertia ratio has to be between 3 and 5? How about tuning the servos? Do you go as far as using an oscilloscope to tune your servos? I know they are expensive and no one has one in their garage haha. I know for these home built machines using youre ears to tune your servo is probably good enoug eh?

Hi Erik

I'm much too lazy for that. I usually try and keep the transmission as light as possible, avoid all rubbing components and then ignore it :)

However, you are probably thinking about that chunky screw shaft you have winding the gantry along. Must take some torque to accelerate that shaft. Some head scratching required here, but think I have it. How does this look?

f = ma = mv/t

First, define the units...

The force (lbf) required is the mass of the shaft (slugs) times the final velocity (radians/second) divided by the time taken to achieve the change in velocity (seconds).

Converting revs/second to radians/second is the same as converting radius to circumference, multiply by 2pi

To convert that force to a torque means multiplying by a distance.

If all the mass of the shaft was concentrated at one point, how far off centre would it be? This is the radius of gyration. A solid shaft is effectively a disc and the formula for a disc is radius squared divided by 2

Measuring the acme screw radius from centre to half way up the thread sounds like a pretty good approximation to me. Measure in inches and you'll get pound inches, let me stick with feet...

Multiply the radius of gyration by the force for a torque in ft.lbf

If I combine the radius of gyration formula with the force calculation, I get...

Torque = mv r squared / 2t

Divide again by the time for power in ft.lbf/s

Divide by 550 and if I haven't blooped, there's the answer in your beloved horse power :D

Robin

OR, in CNC speak...

Required torque in oz.in = shaft weight in lbs x revs/second x shaft radius squared in inches x 0.1302 all divided by time allowed to get up to speed in seconds.

That's...

/ 32.16 for slugs
x 2 x pi for rps
x 16 x 12 for oz inches
/144 for radius squared in inches
/2 from the bottom line

Erik, please check before anyone uses it :D

F = force
m = mass
a = acceleration
T = torque
w = rotational acceleration
I = inertia
r = radius of shaft

F = ma
T = wI

I = 1/2mr^2

Therefore the torgue required to turn that screw shaft alone would be

T = w I = 1/2 x w x m x r x r

[EDIT] so that means
T = 1/2 x rotational acceleration x mass x radius squared
T = 1/2 x v/rt x mass x radius squared
T = 1/2 x v/t x mass x radius
T = 1/2 x final velocity x mass x radius and divide that all by time required to get to that final velocity
T = (1/2 x v x m x r ) / t

Please note radius shown is radius of shaft NOT of pulley turning shaft
[END OF EDIT]

Now you are talking about radius of gyration. I am not sure what you are using off-axis. Do you have something sitting on the screw off the axis of the screw shaft? Im probably just calcualting this a different way so moving on....

Now that you have the torgue required to turn that shaft, you can divide it by the radius of the pulley or whatever is turning the shaft to get the force required to turn it.

As for the amount of torgue required to push the object that the screw shaft is moving can probably be done by a simple free body diagram knowing the thread count per inch and transferring the force. Even easier use the energy method, find out how much power is required to move that object and transfer that into the torgue equation as calculated above?

There are fantastic and excellent worm gear and screw-type power transmisison products formulas in mechanical engineering textbooks and probably online. That would be your most confident answer in terms of moving that object on top of the screw shaft.

Sorry I didnt check your calculations. i started to get confused and just wrote out some stuff. HOpe this helps you at least verify some of your calcs.

Please check my calcs too and see if they make sense?!?!?!!?

Erik

Please check my calcs too and see if they make sense?!?!?!!?

T = 1/2 x rotational acceleration x mass x radius squared
T = 1/2 x v/rt x mass x radius squared
T = 1/2 x v/t x mass x radius

An extra r suddenly appears? Apart from that we agree :)

The formula already includes the radius of gyration for a round shaft :confused:

Hope your text book defines the units, otherwise it might be more useful in the bin :D

Just for the record, I bougth a cnc system to Keling last week, I received today , fine service from John, and USPS priority mail,
Thanks John
Luis

I received the cnc package from Kelling, 495 oz motors , only thing I'm not really happy with the 3/8" shaft, it looks to tiny for the gantry, but I think there is enough power to move it.
I have alredy weld the slat supports, and put the levellers.

UGhh! forgot the photos...

I get now the gantry with bearings, it slides very easy,smooth, the weigth by now is about 40 kilos, hope to have it painted by weekend..

I tried to buy the gear rack and spurs from Macmaster, they refuse to sell me because of my location( Colombia) I told them that I wanted the product to be shipped to MIAMI to my agent, but they say NO, WE DONT SELL TO YOU!
What is that about???
Luis

Wich version of MACH is the best for plasma and oxy torch? I dont know what to download.
will it work with corel files saved as dfx or any? I ask this because in the laser machine I only use one softwre, import from corel and run it in the lasercut software.

Wich version of MACH is the best for plasma and oxy torch? I dont know what to download.
will it work with corel files saved as dfx or any? I ask this because in the laser machine I only use one softwre, import from corel and run it in the lasercut software.

I would think you would want Mach 3 (lock down version). Are you going to use a THC for your plasma?

Not yet , but surely for the next table , I'll be building it next month. Now i'm waiting for the spurs, Skip a laser cnc friend from the forum helped me to buy it from Mc master.
In the weekend will mount the y and z axis. The frame is going to be painted also.

today photos, not much progress but hope to have it painted for the weekend.
Still waitng for the gear racks and spurs .....
The gantry weigths about 80 kilos but you can move it with one finger, only thing I worry about is the inercia force..
Tomorrow i'll start to mount the motors

Wow, thanks guys for that quick class, I am just at that point of running numbers for the motors of my table. I'm planning on driving 3/4"-10 threaded rod with the motors & have a coupling on the gantries to move them. I'll post some pics of the machine & the #'s once I run them in the next few days. I posted that pic in another thread a little earlier, but that's where it is at this point.

Thanks again.

I get now the gantry with bearings, it slides very easy,smooth, the weigth by now is about 40 kilos, hope to have it painted by weekend..

I tried to buy the gear rack and spurs from Macmaster, they refuse to sell me because of my location( Colombia) I told them that I wanted the product to be shipped to MIAMI to my agent, but they say NO, WE DONT SELL TO YOU!
What is that about???
Luis

Looks pretty good!!! Very,very sturdy... :) (Keep up the good work!!!)

How is the bearings preforming under the weight?

The weigth is abput 80 kilos , it runs smooth, and very easy , you can push it with one finger, bearings are ok no sound at all, waiting to look for the innercia forces.

The weigth is abput 80 kilos , it runs smooth, and very easy , you can push it with one finger, bearings are ok no sound at all, waiting to look for the innercia forces.

Yes.. There are only two ways to work around those forces.

1. Go slower...
2. Get stronger steppers.

What kind of steel will you be cutting with this machine?

I think about 1 " steel plate. plasma and torch, maybe up to 3" with the oxytorch we'll see. . I have to change some parts at my second table, like motors....

I think about 1 " steel plate. plasma and torch, maybe up to 3" with the oxytorch we'll see. . I have to change some parts at my second table, like motors....

Will the 495oz motors be to weak for this table?

I have two for the gantry, with 3:1 pulleys, anyway I will change them for 600 or 800 oz.

I have two for the gantry, with 3:1 pulleys, anyway I will change them for 600 or 800 oz.

May be a good idea, might not.

If these are stepper motors you don't want to trade a motor with enough power and speed for a slower motor with too much power.

If everyone becomes fixated on oz.in as the only meaningful bench mark when buying stepper motors, we must expect our suppliers to quote holding torque rather than pull in torque. This doesn't actually tell you very much about what the motor can do, but the big numbers sure look impressive.

For understanding this you need two things

A graph of torque against speed for the motor.

How far the gantry moves with one turn of the motor.

Then you can guesstimate how fast the motor can drive it before it runs out of steam and how quickly you can get up to that speed.

I use these 495 oz. steppers, because i have seen it in other similar projets. I guess i have to try it to know.
I'm using 20 pitch 20 teeth gears.
As I'm building this cnc to sell it i'll need to know from you the experienced people, wich will be the best choice in step motors.
Thanks for the help

May be a good idea, might not.

If these are stepper motors you don't want to trade a motor with enough power and speed for a slower motor with too much power.

If everyone becomes fixated on oz.in as the only meaningful bench mark when buying stepper motors, we must expect our suppliers to quote holding torque rather than pull in torque. This doesn't actually tell you very much about what the motor can do, but the big numbers sure look impressive.

For understanding this you need two things

A graph of torque against speed for the motor.

How far the gantry moves with one turn of the motor.

Then you can guesstimate how fast the motor can drive it before it runs out of steam and how quickly you can get up to that speed.

I think he might benefit from stronger motors... The table is a very heavy design and for the size steel he said he will be cutting a slower but stronger stepper might be better.

But in saying that,switching from to Nema 34 might not always be the best for thin metal that needs plenty of speed... :)

Might just want to try with one axis to see if it is worth it..

Now I received the machine from the paint shop, start to build it...

How will I protect the hardware and the pc in the machine? Any fuse or breaker, ??
I was thinking about to use one of that used for pc's for 1000 watts? Will it work for the motors and pC???

I'm having troubles with the X and A axis, the A is rotating as the x and is twisting the gantry, I conected it as slave with the same step numbers, i dont know how it started to work in the correct direction but after a few trials it started again to twist the gantry.
Also have the y and Z axis stuck in the limits, they hit the limits but don't reverse.please help,
Thanks
Luis

Luis,
I see in another post you want a machine part g-code file to experiment with.

The attached is a cultivator point CAD drawing I've done & the g-code file for it.
To use the G-code file, you will have to change the file extention from DOC, to TAP & then load into Mach.
Approximate settings for the cuts were: 3/16" steel, at 60 IPM, 35 amps, 75 PSI air.

Have fun with it!:)

Your cable chain is looking good.
WT

Thanks WT, i'll ry that file tomorrow.
Luis

Where do I have to conect the plasma , to the output or to the input pins? And what do I have to enable? I'll use a 5 volts relay to conect the plasma, is it safe, its only signal or is there any power in that conection?
Luis