stepper Horsepower Equivalents

[wjfiles]

I am building my first Mill/Router and would like to find out what size steppers I need
I cannot related holding torque to Horsepower.
I know that I would need the equivalent of A Half Horse Power
as I will be milling Alluminium.
Can anyone advise what size Steppers I would need.

The More I learn The Less I Seem To Know
BFiles

[Al_The_Man]

Horse power is seldom used in sizing servo motors, due to the fact that rate of movement (e.g RPM) is a component of HP.
Servo's. typical operate from zero to their maximum speed so the available torque factor at any point of its speed range is of more use.
There are many factors involved in sizing servo's typicall one of the highest load conditions exist when accelerating/decelerating a load (inertia).
And there is a recommended maximum of motor to load inertia to be observed of approx. 5:1 or less.
To get an idea of different scenarios for different machine designs, download one of the free graphic sizing packages from Kollmorgen (Motion village) etc most servo/stepper manufactures will have these for download.
Al.

[NC Cams]

Sometimes you're dealing with apples and oranges and pickles and you can't do direct comparisons but here are some basics:

HP = T x RPM/5252

Where
HP = HP
T= torque in ft-lb
RPM = Rev per minute.

and

1HP = 745 watts

1 watt = 1 volt x 1 amp.

thus if you have a 1hp motor and it is runing on 12 volts, at peak power it consumes about 62 amps. If it were 1hp and ran on 120 volts, you'd be looking at 6.21.

Thus, a DC motor makes absolute peak torque at stall but NO horsepower (do the math)

However, the amount of torque is a function of how much current flows thru the windings of the armature times the number or turns of wire. Suffice it to say that 20 turns of #22 wire will NOT make as much current as a comparable armature with 20 turns of #16 wire, assuming it will fit and was running at same RPM (or, oops stall)....

[wjfiles]

Thanks for your responses,
I am a little wiser.
I guess I will have to start with the maximum power requirements of the cutters at the speed and feeds I will use and factor in machine induced loads.
I have a feeling that if I do not use a big enough stepper I will have to sacrifice
rapid traverse speeds for more cutting power by gearing down motor lead screw ratios.
I still have some study to do to get a handle on this.
Thanks again
Bfiles

[CJL5585]

Originally Posted by wjfiles I am building my first Mill/Router and would like to find out what size steppers I need I cannot related holding torque to Horsepower. I know that I would need the equivalent of A Half Horse Power as I will be milling Alluminium. Can anyone advise what size Steppers I would need. The More I learn The Less I Seem To Know BFiles

You might want to start a new thread titled something like "What size steppers for use in machining steel and Aluminum?" under the milling and machining forum.

The reason for this would be to get input from the guys who do machining on a daily basis, or are in business.

Sorry you did not get any answers to your question regarding the size of steppers needed. I will give you an estimated starting point, which is an estimated guess, and that is about 1200 oz/inches. The guys on this site can tell you about the reduction ratios to get more torque for milling, and more bang for the buck.

I only work in wood.

Jerry

[NC Cams]

Some things to consider as they pertain to the original question:

The basic equation that governs all dynamic motion is F=m*a where:
F = force (needed/exerted)
M = mass of object to be moved
A = desired rate of acceleration.

The force needed to cut is basically a function of the per toot load that is need and the speed of the cutter. This is then multiplied or divided by the gear ration depending on if gearing is done to amplify speed or torque.

Therefore, the TOTAL force needed to establish the power required to "cut steel and aluminum" (real generic all outs) is basically:

Ftot = Fmass + Fcut

If you don't know or cant specify the mass or desired acceleration, you can't compute the Fmass that will be needed.

If you don't know or don't specify the speed of cutter, nor the tooth load of the material to be cut, you can't calculate the Fcut.

If you don't know the torque or speed multipication factor of the drive, you also can't come up with the force needed to move the cutting spindle.

Ftotal in this case = Fmass (idunno) + Fcut (idunno)

hence, Ftotal = I dunno or it is anybody's guess...

It becomes pretty hard to recommend a motor when so much is unknown or unspecified.

The options are at this point are:

a. Eliminate/measure the unknowns and calculate the loads
b. Guess with wild abandon
c. Guess with a SWAG based upon experience
d. Choose based upon a known group of estimates.
e. Choose based upon a blend of D and A or C above.

Under the circumsances, the member seems to have come to the correct and proper realization that E is the best alternative under the circumstances.

With all due respect, the motor selection for ANY CNC project is done using one of the methods outlined previously. The more you know and/or can calculate, the less of a SWAG you'll have to make.

[wjfiles]

Hi Guys,
Thanks for the responses, your comments and advice taken on board.
The machine I am building is a beefed up version of the machine in the gallery by
"djastram".
I am using 25 mm Hardened Ground shaft and linear bearing on the X, and 20mm shaft on the Y & Z axis.
Length of X = 600mm
y = 400mm
z = 250 mm
The reason for the 25 mm X is because I am useing a single rail each side.
The main frame is mostly 16mm ALI. Machined from Plate and Flat Bar
Bearings on the bottom of the gantry span 160 mm to give some vertical stability.
I will probably make a spindle with RC collet chuck. I don't particularly like routers because of the noise factor. ( my neighbors pollution level starts at about 2.5 decibals)
Lead screws are undefined at this time , main unknown factor at this time is $$
The ultimate objective is a machine with enough capacity to replicate itself. for which one attempt may not be enough.
Thanks again.
Bfiles

[fpworks]

Don't forget to account for losses due to leadscrew efficiency.

With ballscrews, expect to lose only 7-10% of motor torque converted to linear thrust. If you use acme screws...I would use 40-50% to be safe.

Your losses from your way system will probably be negligable, so call it zero.

Steppers will lose torque as their speed increases, so ideally you want to operate (CUT and RAPID) at slow SHAFT speeds. This doesn't mean linear movements have to be slow. However, this will require high pitch lead screws that will reduce your resolution. (smallest step linear movement)

You really need to specify a system, not just steppers. What I mean by a system:
-steppers
-leadscrews
-power supply
-stepper drives
-PC/drive computer

For instance, if you go pick out the first set of ballscrews that fit in your machine off ebay, chances are that you will have a compromised system and you'll wish you did things differently. Work out the numbers of the ENTIRE SYSTEM (electrical and mechanical) and you'll see what I mean.

I don't believe that you will have to guess very much to specifiy a stepper system. But you will need to get as much information as possible:
-performance charts for the steppers you are considering
-capabililities of stepper drives you are considering
-what is your minimum required resolution
-how much are you willing to spend? (DON'T forget about the power supply/driver system...this will probably cost far more than your steppers)
-establish some performance requirements and a budget
-consider your spindle performance as well.

[wjfiles]

Hi fpworks,
Your comments are valuable food for thought.
I am at the stage where fore thought is still a possibility. It is cheaper than after thought.
At this time I have concentrated on the structural design of the frame etc.

I do have a StepperWorld FET-4 drver board.
It has Dip switches for HiTorque,Half Step settings. From appearance I would say the components are not over sized by any means.

I am considering two options for the power supply. 1 Build one from scratch to available plans and two modify a computer 400 watt supply. I have some expertise in this area and have the resouces of a prototyping plant for circuit boards.
One concern I have is that steppers, if under sized can skip steps . This to my mind would be Ideal for manufacturing scrap ??
Can the risk of this be eliminated by the use of encoders for feedback.
Some of the engravings I plan to do may take 12 hours to do, so skips would be totally unacceptable.

As for the lead screws I am 90% on ball screws at this time.
Patience and $$$$ are the criteria, which ever runs out first.

Bfiles

[fpworks]

You might have some trouble using a switching power supply that is commonly used in personal computers. You will probably need a linear power supply (of course, these cost a lot more)

Encoder feedback will run your price up pretty fast, not to mention that you are vastly increasing your system complexity and could be opening up a big can of tuning problems/troubleshooting. I suggest using that extra money to make your non-encoder stepper system overkill, then you will enjoy better and more reliable performance.

Let me give you a rundown of a system I recently spec'ed:
-6mm pitch ground ballscrews (93% efficient, from manufacturer)
-200 step/revolution stepper motors (-34 sized, I want to cut at maximum 50 ipm, the steppers provide 350 oz*in torque at my desired cutting speed...from the stepper's performance chart...NOTE THIS IS NOT THE SPECIFIED STALL TORQUE-that is nearly useless)
-Gecko 201 driver cards with 10x microstepping (this gives me 2000 steps/revolution, I had to consider that in the above step)
-25 amp, 36 volt power supply that runs all three axes (I can reconfigure it to run at up to 50-something volts, but I don't because the steppers approach their max temperature at the higher voltage)
-Linear bearing ways (assume way loss is zero)

This above combination provides 0.003mm linear resolution.
350 oz*in torque will provide 270 lbs of linear thrust at my cutting speed in this application. As the speed decreases, this value will go up substantially since the steppers make more torque at lower speeds. Just make sure your driver cards and power supply can support it.

350oz*in*3.1415in/6mm*.93 = 270 lbf (note that I didn't include the unit conversions)

If I want to look at my max rapid speeds, the Gecko drives switch to full steps at some point, then I have to reference the performance chart again, consider acceleration/deceleration forces, etc. and pick a safe speed, then verify with some empirical testing.

There is a lot that I left out here...there are lots of people who know this stuff really well. It would be best if you called a vendor and talked to an applications engineer.

Justin