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irving2008
24-12-2009, 06:10 PM
A Tutorial on motor torque calculations

A question oft asked on MYCNCUK is "how big a motor do I need?". There is no simple answer to this, and the options are usually:
a) follow someone elses build and copy theirs;
b) take a guess and try again if you are wrong; or
c) work it out, which is the subject of this tutorial.

When choosing a motor you need to know:
a) what power and torque output is required at a given speed
b) what electrical characteristics are appropriate to acheive that

What I have tried to do here is the engineering approach, by showing the calculations needed to get some idea of power, which then dictates motor size. I am concerning myself with a stepper motor directly driving a leadscrew to move the gantry, table, etc. Similar calculations can, however, be done for belt drive or geared up/down with timing pulleys.

DISCLAIMER: This tutorial is to give you an insight into how to approach the selection of a motor. I take no responsibility for any consequences of following this tutorial and you alone are responsible for your choice and purchase of motors etc.

So lets start with assessing what torque might be needed. The basic properties we need to be concerned with start with the moving element, be it table, gantry, milling head or whatever, and that is its mass. We need to know this, either by actually weighing it, or by estimation based on the volume of material and the density of the material or by adding up the weights of the component parts.

A gantry for a router would be the weight of the slides (from manufacturer data) plus the weight of the aluminium parts (calculated using 2750 as the density) and the weight of the steppers, router, etc. Typically on a small router this would be in the order of 20kg, which we will use as our worked example - yours will be different. If you work out the motor needed for the heaviest element, then this is the worst case and the same motor will work for everything else (although you may chose to do the calculations for each axis in turn to see if there are saving to be made).

So, we know what our moving part's mass is. The motor has to make this component move, first by accelerating it and then maintaining that velocity. To do so it must first overcome the initial friction (stiction) and then maintain the drive against the friction of the moving parts and against any cutting forces. Minimising that friction is therefore crucial. For linear or rolling bearings the friction can be calculated and the stiction is generally very small. For dovetails (as on a mill) it is not easy to calculate and is best measured with a spring balance, firstly to determine what pull is required to get the table moving and then to maintain that movement. This might be as much as 15kgf initially, dropping to 5kgf.

The second aspect to accelerating the moving item is to overcome its inertia (the tendency of something to remain at rest) - this is true even if friction were zero.

The motor turns the leadscrew to convert rotational motion into linear motion. There is friction here too, expressed as the efficiency of the leadscrew. This is typically 80% for ballscrews and as low as 30% for trapezoidal screws (bronze or delrin nuts on steel) and inertia, as the screw itself has inertia which is dependent on its mass and its length.

Now we have all the elements we need.

So, considering the frictional component of the torque, this is given by:


Torque = F * p/(2pi * e)

[1]

where F is the force to be overcome in Newtons, p is the screw pitch in metres and e is efficiency.

For this example I shall assume a TR12x3 trapzoidal screw 12mm dia, 3mm pitch.

The force to be overcome is, as said above, either the stiction or the kinetic friction plus the cutting forces. For the purposes of simplicity
assume the cutting forces range from 5N for wood to 20N for alloy using the sort of spindles/cutters found in hobby sized machines up to 75N for steel on a mill.

The frictional forces are calculated from the mass of the load and the friction coefficient:

F = M * g * Fc

[2]

where g is gravity, which can be taken as 10

Typical static friction coefficients for common sliding mechanisms are:
0.003 for a ball slide,
0.01 for low-end ball races on aluminum channel,
0.05 for teflon on steel,
0.16 for bronze on steel
1.10 for cast iron on cast iron.

For most of these the kinetic frictional coefficient can be taken as the
same, although it is around 0.2 for greased cast iron to cast iron.

Assuming a low cost router using ball races and our 20kg load the frictional force (from equation 2) is 20 * 10 * .01 = 2N. Add to this the cutting force for wood at 5N and the force to be overcome is 7N, therefore the torque (from equation 1) is:

T = 7 * .003/(2pi * .3) = 0.01Nm

This doesn't sound a lot when motors are rated at 1 - 3Nm, but we haven't finished yet.

The second calculation is the inertia of the moving item, expressed in terms of the inertia seen by the motor. The symbol we use for this is J(load) and it is calculated thus:

J(load) = mass(load) * pitch^2/(2 * pi)^2

[3]
where mass in Kg, pitch in metres gives inertia in kg m^2
[note: ^2 means raise to the 2nd power, e.g. square it]

In our example we will use a trapezoidal TR12x3 single start screw to move this 20Kg gantry, so from equation 3, J(load) = 20 * 0.003^2/40 = 4.5 x 10E-06 kgm^2 (the 40 is a good approximation to 2pi squared). To this we add the inertia of the screw, which is given by:

J(screw) = 1/2 Mass * radius^2

[4]


where the mass is given by:

mass(screw) = pi * radius^2* length * density

[5]

In our worked example a 12mm screw 800mm long has a mass of 3.1416 * .006^2 * .8 * 7800 = 0.71kg and therefore an inertia of J(screw) = 1/2 * 0.71 * .006^2 = 1.28 x 10E-05, so the screw has a higher inertia than the load!

The total inertia to be overcome is the sum of J(load) and J(screw) = 1.72x10E-05 kgm^2. (Note the spreadsheet also adds in the rotor inertia of the motor)

Next we have to decide how fast we want the gantry to move under load. Typically for a wood router anything from 500 to 1000mm/min would be suitable, for cutting aluminium you might want to look at 1800mm/min or better when using small cutting tools. The maximum traverse speed is given by:

Smax = max motor rpm * screw pitch.

[6]

In many cases the speed will be determined by the available drivers and the motor. Few motors will give much torque above about 1000steps/sec on low voltages (24v being the typical supply used), so the maxium speed we could reasonably expect under load for a 200step motor is going to be 1000/200 * 60 * .003 = 0.9m/min or 900mm/min. At this speed the angular velocity of the screw will be:

w = 2 * pi * screw revs/sec

[7]
In our example this becomes 6.28 * 1000/200 = 31.4 rads/sec.

Note that the spreadsheet also shows whether the screw is likely to whip at the chosen speed depending on the type of fixing. For most basic systems fixed/free or supported/supported would be a typical configration, but this may need to be adjusted (or a bigger diameter screw chosen) for larger/faster designs.

Now we need to decide what acceleration we want. There is a correllation between the speed of movement and the ideal acceleration to avoid loosing steps but allow rapid direction changes for accuracy of cut. Obviously as the speed increases the acceleration needed to maintain cut accuracy is higher, however for rapids a lower acceleration can be tolerated. A typical router at around a 1000mm/min would need an acceleration on the order of 2300rads/sec^2. The torque required to achieve this acceleration against the inertial loads is

T = J * A

[8]
Which gives 1.72x10E-05 * 2300 = 0.04Nm. (the spreadsheet assumes rapids need ~1/3 the acceleration of that used for cutting).

Adding the two components of torque together we have a total torque requirement of 0.052Nm at the motor speed of 1000steps/sec (i.e. 5rps, 300rpm). The spreadsheet also adds in the detent torque (the torque needed to overcome the magnetic attraction between stator and rotor - this is what gives rise to the 'cogging' feel of a stepper motor when turned by hand.)

You can see that the torque required is very different to the 'torque rating' of the motor. It is important to note that the holding torque of a stepper motor is to some extent of little relevance. This is the physical torque required to overcome the electromagnetic forces holding the rotor stationary and is the torque the motor tends towards as speed drops to zero. In practice this torque is rarely available or used. While the size of a stepper motor generally dictates the low speed torque, the ability of the drive electronics to force current through the windings of the motor dictates the high speed torque. Remembering that a stepper motors torque ratings are based on sinusoidal drive current; running it on a square wave signal of a switched driver is at best an approximation at low revs and is progressively worse at higher revs unless there is sufficient voltage to force the current through the winding. A good rule of thumb, for best performance, is:

Vd = 32 * sqrt(L)

[9]
where Vd is operating voltage, and L is the motor inductance in mH. If your drivers are limited in voltage a low inductance motor is essential if you want any reasonable speeds.

The inductance of the windings and the drive voltage used dictates the corner speed of the motor. The calculations are too complex to describe here but the spreadsheet allows you to put in the motor parameters to get a go/no go view. In an ideal world you would want to run the motor just below its corner speed to get maximum power output and a torque that is essentially constant across a range of revs. Once you get past the corner speed the torque falls off rapidly. This is a consequence if you design for high power at cutting speeds (to minimise the likelhood of loosing steps) but then want fast rapids which take you over the corner speed - if the torque drops too low you will either lose steps or worse the motor will stall.

So, lets look at the motors available. Pick any website, such as Zapp Automation's, and look at the list of NEMA17 and NEMA23 motors. Here are the options:

Motor V A mH Nm Inertia
SY42STH47-1684B 2.80 1.6 2.8 0.44 68
SY57STH51-1008B 9.24 0.7 32.8 1.00 275
SY57STH51-3008B 3.10 2.1 3.6 1.00 275
SY57STH56-2008B 5.04 1.4 10.0 1.24 300
SY57STH56-3008B 3.15 2.1 4.4 1.24 300
SY57STH76-3008B 4.00 2.1 6.4 1.85 480

Plugging any of these into the spreadsheet gives similar results, so which to choose? Next calculate the ideal voltage for each (the spreadsheet shows this as the 'ideal voltage')

Motor V A mH Vd
SY42STH47-1684B 2.80 1.6 2.8 54
SY57STH51-1008B 9.24 0.7 32.8 183
SY57STH51-3008B 3.10 2.1 3.6 60
SY57STH56-2008B 5.04 1.4 10.0 101
SY57STH56-3008B 3.15 2.1 4.4 67
SY57STH76-3008B 4.00 2.1 6.4 81

Lets assume we want to use a low cost driver board, such as the System3 from DIYCNC which is OK to 2.5A but limited to 30v max, or the TBA6560 boards available on eBay. None of those are going to manage 60v, indeed 24v is the likely voltage, but the motors that are the lower ideal voltage will perform better with those drivers. So on this basis the SY42STH47-1684B or the SY57STH51-3008B would be contenders. I'd probably opt for the 1Nm NEMA23 motor over the 0.44Nm NEMA17 to give a bit more leeway and scope for upgrades. Anything bigger would be a waste of money and would perform no better (and usually worse - there is such a thing as too big a motor).

Below shows similar calculations repeated for a number of examples

25kg gantry 4' Rockcliffe oilite bronze on steel, TR12x3 1.2m long. 1000mm/min. Torque = 0.1Nm, power = 3W so a 1Nm - 1.5Nm motor.

35kg gantry 2m ballrace on channel, 16mm ballscrew 5mm pitch, 1.8m long, 2000mm/min. dense hardwood capable. Torque = 0.4Nm, power = 12W (typical 2Nm NEMA23 motor)

50kg dovetail table + 5kg workpiece + 5kg vice, 20mm ballscrew 5mm pitch, 900mm long, 1200mm/min, light alloy/steel cutting. Torque = 0.9Nm , power = 32W (8Nm NEMA34 motor)

50kg dovetail table + 10kg workpiece + 5kg vice, 25mm ballscrew 5mm pitch, 900mm long, 1800mm/min (with slightly reduced acceleration), heavy alloy/steel cutting. Torque = 1.1Nm , power = 64W (possible with 12Nm NEMA34 motor, but this is starting to get into servo motor territory to meet that speed/accel requirement)


The calculations are contained in a Excel spreadsheet in the attached zip file

tribbles
24-12-2009, 06:27 PM
Excellent advice!

jonm
24-12-2009, 09:54 PM
irvine:clap:
i read many posts on forums concerning stepper size
when i was looking to buy motors .. but could not find much information as in in , this motor + this driver + this load = ?
very informative read , sure this will help many of us noobies
with motor selection

:beer:

Wobblybootie
24-12-2009, 11:15 PM
Stunning Tutorial, I have been worried about just this problem. Many thanks indeed.

dickieto
26-12-2009, 02:38 AM
wow!:eek: thats a lot to take in, but very informative and usefull info for all us new builds out there keep up the great work:clap:i will need to dig out that old spring balance out the shed and get to work:beer:

Lee Roberts
26-12-2009, 03:23 AM
Clicking the "thanks" button wasnt enuff for me, great post and top marks irving....you the man !

Pengu
17-01-2010, 03:14 PM
Thanks Irving :yahoo:, just need to get my wizard hat out and start figure out what kinda motor's to get to my CNC conversion project :idea:

Thanks again

Mark

irving2008
06-02-2010, 11:20 PM
Tutorial updated with new easier to use spreadsheet that does critical speed calculations for the leadscrews as well as allowing you to plug in different motors to get a better view of the fit. Also does checks for performance at both 'cutting' and 'rapids' speeds.

muza
10-03-2010, 08:44 PM
Thanks for the great spreadsheet. So much key knowledge in one document!

massimomb
21-06-2010, 06:37 AM
Thanks a lot for the excelent advice,

I'm just concerned about the microstep selection, actually I've found the right balance in my cnc, I run 3 2M2280N coupled with 3 12NM nema 34 motors, I could run my cnc as fast as 10m/min in rapid and 2,5m/min while working (mach3, parallel port), but in a work of 4 hours it loosed the step and I had to trash the work out, so i reduced the microstep resolution to half step and it seem to work right, but the motors do a hell of a noise.

I've read different opinion on the web about microstep resolution, someone says that the more you set higher this value the lesser will be the torque, others says that in the modern drivers, microstep res. almost do not afflict the torque,

Who do I have to listen to?


Can you explain how this value can be taken in account?


Thanks a lot if you can reply.


M.


P.S. I'm wondering to move to servo with brushless motors,maybe this can be tha solution?

Robin Hewitt
21-06-2010, 09:14 AM
How long is the cable on the parallel port.

A TTL signal might get you a reliable 10 feet without handshaking at that speed, if you were lucky :smile:

irving2008
21-06-2010, 12:01 PM
I think its a fact that microstepping, where you are actually stopping at the microsteps (i.e. using them for resolution) definitely is less torque, simply because the motor cannot hold that position accurately against load. In a dynamic situation however, lots of factors come into play. For example, if the axis is in constant motion the only torque required is to overcome friction and cutting loads, there is little torque required to provide acceleration. Unless the motor is being operated close to its torque limit (at that speed/volt/current combo) then microstepping should have little impact. The general rule I have used is in the spreadsheet is that the motor should provide 3x required dynamic torque at the maximum speed.

Large motors have high inductance so the torque drops off very fast with speed - the corner speed of those motors is 240rpm. I don't know how big your axis are, but I'm guessing its going to be around 1 - 1.2m? With your 10mm pitch screws 2.5m/min = 250rpm, so that is close to optimal (and 1/8 stepping = 6664steps/sec) and it looks OK at cutting speeds, but its very marginal at 10m/min rapids and that is where you may have lost steps (=27000steps/sec). You need to reduce rapids to 7m/min but it should be OK at 1/4 or 1/8 stepping.

massimomb
21-06-2010, 12:23 PM
Thanks again, i will reduce the rapid to 5m/min and the working speed up to 2,2 m/min as top speed, setting the microsteps to 1/8.

M.

magno_grail
15-02-2013, 08:48 AM
I have a couple of questions about your motor calculation spread sheet. The motor inertia is input in gm-cm^2 (B32). The rotor inertia given in I27 must be Kg-m^2 since the total inertia is the sum of the screw inertia, load inertia and motor inertia, the others are in Kg-m^2. I27 = G35 which is a lookup to convert units of the motor inertia.
The problem is 1000 gm = 1 Kg and 100 cm = 1 m so gm-cm^2 should be divided by 1000*100^2 = 1*E7 not 1*E8 as in G35. The motor inertia in the sum is 10 times too small.
I cannot find the time used for the acceleration torque (I34). The equation is G32*I28. G32 is labeled as V but it is really the number of pulses/second of the stepper based upon a 200 pulse/revolution motor and the screw speed (which is the Max. linear speed divided by the screw pitch). From looking at the equations, the running torque is small compared to the acceleration torque unless there is a lot of friction in the system. I have not found any references as to what range of acceleration is needed for a CNC. Obviously, when a stepper system is tuned it will limit the acceleration so positioning steps are not dropped due to the loads. Too small motor torque will result in a very slow CNC due to acceleration limitations. At some point a larger motor torque results in small performance gains for the cost increases. Are there any guidelines for this? Thanks.

DRITECH
22-02-2014, 11:31 AM
Hi,

I am building a gantry robot and for the Y-axis I am using a 1610 ballscrew (16mm diameter, 10 pitch, 1300mm long). I estimated that the mass is 36Kg and for the speed I planned to have 1800mm/min.

Using these calculations I concluded that the total torque is 0.23Nm. From the Excel the estimated torque resulted to 0.67Nm.

Now I have the following questions:

1) Can I use there calculations for a ball-screw system?

2) What is the calculation to determine the estimated torque (0.67Nm) from the total torque (0.23Nm)?

3) Is 0.67Nm enough for such mass? I did some research for similar projects and they use a much higher torque stepper motor.

All kind of help will be appreciated and sorry for my bad English.

Thanks in advance.

Neale
09-06-2014, 11:08 PM
I'm trying to use the spreadsheet to check some parameters of my planned new router. This has thrown up a few questions:
1. Acceleration seems to be the largest contribution to torque required, but I can't find where load acceleration is built in. It must be in one of the formulae somewhere but I can't manage to unpick the formulae in the hidden columns to find it.
2. The spreadsheet recommends a motor torque based on an assumed safety margin of 3; if the safety margin isn't high enough with my intended motor, I presume that I could restrict the machine to a lower acceleration? But given that I don't know what acceleration is assumed, it's difficult to see what's happening. I do see that the acceleration torque number is linked to cutting speed.
3. For a twin ballscrew X axis, I assume that I can use the spreadsheet using actual ballscrew dimensions (single ballscrew) to give critical speed, but then use a double-length ballscrew in the spreadsheet to do the torque, etc, calculation.
4. If I go to a twin-motor setup, can I simply double the nominal motor torque? Or is it easier to halve the gantry mass and do the sums for a single motor/ballscrew?
To put some reality into this exercise, what would be sensible numbers to use for cutting speeds and accelerations? Bit of a "how long's a piece of string?" question, but my current router is so far out of the norm I don't feel that I can extrapolate from what I'm doing at the moment, and I don't know what kinds of numbers are reasonable. For starters, I would assume profile cutting, say, 9mm ply with a 6mm cutter in a couple of passes. Currently, with speed limited by machine rigidity, I would cut that at maybe 600mm/min.
Many thanks for any help available!

JAZZCNC
10-06-2014, 12:01 AM
I'll save you a lot of headache and just say use 3Nm motors run at 65-70Vdc on 75-80vdc drives and you won't have any problems cutting anything.

Cutting speeds will depend on many factors like spindle power and tool material but 4500-5000mm/min wouldn't be out the way with 2.2Kw spindle using carbide tooling.

IanParkin
10-06-2014, 08:34 AM
Can you suggest a supplier of a kit of parts to do as you have described Jazz? for a 4 axis machine?

Neale
10-06-2014, 10:09 AM
I'm planning to re-use a set of Zapp SY60 3nm motors, 68V linear power supply and analogue drivers which should do for starters, then, and I'll see how it goes. Can always add another motor if needed.
This whole design process is interesting: we are urged to study, analyse, carefully consider. I used the spreadsheet to get better insight into motor sizing but the answer, in practice, comes down to "SY23 because SY17 are too small, SY34 are too high inductance, and you might as well go for 3nm as they cost barely any more than the lower power motors in that frame size." Power supply sizing is similar.
Ian - you could look at the Zapp web site for a kit. The price (at least when I bought) is the same as buying the individual parts but at least someone has identified a bunch of bits that play well together.

IanParkin
10-06-2014, 11:43 AM
I have the much reviled ebay 4 axis kit all on one board which i bought to get me going

having said all that its been absolutely fine and reliable but now i'm using it more I thought i would get a proper set up with individual drivers so in the event of problems i can switch a faulty one out

I was looking at cnc4you's site but his drivers are only up to 50v with a 36 v power supply
I'll have a look at zapps site

Ian

IanParkin
10-06-2014, 12:04 PM
looking at zapps site I cant see any kits of parts but if i put together
4 dm856 drivers 4 nema 23 3 nm motors and one plc6x BOB and a 68v 6amp power supply
its 632 ish

Cnc4you's kit of parts is 356 plus vat 427 ish thats this kit ...Nema23 Stepper Motor Kit 4Nm x 4 Axis (http://www.cnc4you.co.uk/index.php?route=product/product&path=87_117&product_id=378)

will I notice any major difference with either of those over my cheapy ebay special ?

Clive S
10-06-2014, 12:29 PM
I was looking at cnc4you's site but his drivers are only up to 50v with a 36 v power supply
Ian
Cnc4you is the best place to buy the steppers 3.1nm low inductance that I know of. But not the drivers Ebay is your friend for them like this one :- Leadshine AM882 Digital Stepper Motor Drive 80VDC 0 1A 8 2A Protect Function | eBay (http://www.ebay.com/itm/Leadshine-AM882-Digital-Stepper-motor-Drive-80VDC-0-1A-8-2A-protect-function-/330782929553) although I think there is a newer model you might find a better price as well. ..Clive

JAZZCNC
10-06-2014, 12:34 PM
Don't go with the kits has they tend to be under spec'd on the PSU or have lower spec drives etc. You don't really save anything buying kits anyway if you shop around and to be honest if you build your own PSU you'll save more money.

Ian Check your PM's.

tzar
17-12-2014, 09:09 PM
The calculations are contained in a Excel spreadsheet in the attached zip file
Thank you very much for this! But I have question.
For example I have 2 screws for X axis and 2 motors as a result. How I should make calculations in this case? I just need to double "Holding Torque", "Detent Torque" and "Rotor Inertia"?

CNC Visions
05-05-2015, 03:37 PM
Thank you very much for a very informative explanation on motor selection.
I'm upgrading an older machine that had two motors driving X axis gantry. How do I calculate the motor size with two motors on the same axis? Is it a simple as dividing by 2?
Gecko recommends a 40% increase in motor size from base calculations, do you agree?
Thanks again
Mike

Boyan Silyavski
07-05-2015, 03:01 PM
Thank you very much for a very informative explanation on motor selection.
I'm upgrading an older machine that had two motors driving X axis gantry. How do I calculate the motor size with two motors on the same axis? Is it a simple as dividing by 2?
Gecko recommends a 40% increase in motor size from base calculations, do you agree?
Thanks again
Mike

Not so simple. First you need to know the moving mass. Then the desired speed and acceleration. Then you decide the motors and the ratios. Then you check if your BOB can cope with that. Then you check what your resolution will be. Some back and forth between all that and a final decision. At least that's my logic.

Clive S
07-05-2015, 03:35 PM
Then you check if your BOB can cope with that.
Do you mean the drivers?.
Mike But as Silyavaki has said there is more to it. How about posting some pics of the machine for a better answer. ..Clive

CNC Visions
07-05-2015, 03:58 PM
I did the calculation, 670 oz-in/4.6 NM is required to move the axis with one motor. I would normally add +30% for efficiency loss and use a motor in the 850/6 range. With all that, I have a pair of NEMA 34 frame motors that are 420/3. Will I be ok with these or should I go with something bigger? I'm sure that a frame size increase to 34 will slow down my Rapids.
My drives are Gecko 203V, I have several power supplies ranging from 36 to 70 volts available. I upgraded to a Ethernet Smooth Stepper with a high end BOB so I should be able to handle most anything.
Thanks again for your help.
Mike

Clive S
07-05-2015, 04:19 PM
Well are you using ball screws, pitch, size, direct drive, etc etc. Without full info nobody will be able to answer you. I take it you don't want to post any pics . Generally nema 23 are more than enough running at about 70V. ..Clive

CNC Visions
07-05-2015, 05:09 PM
Ball Screws, direct drive, .5"/rev, 48"X48" table, 2.2 KW water cooled spindle with 10" of travel. The machine is totally in pieces but I will post pictures as it goes back together. I'm replacing the rails and bearings so most of the machine is in tote boxes or stacked in a corner.

Clive S
07-05-2015, 05:32 PM
Ball Screws, direct drive, .5"/rev, 48"X48" table, 2.2 KW water cooled spindle with 10" of travel. The machine is totally in pieces but I will post pictures as it goes back together. I'm replacing the rails and bearings so most of the machine is in tote boxes or stacked in a corner.
Ok then I take it you have read this earlier in this post from Jazzcnc
I'll save you a lot of headache and just say use 3Nm motors run at 65-70Vdc on 75-80vdc drives and you won't have any problems cutting anything.

Cutting speeds will depend on many factors like spindle power and tool material but 4500-5000mm/min wouldn't be out the way with 2.2Kw spindle using carbide tooling. It might be better to keep away from direct drive and use belts and pulleys with a 2:1 reduction (that's what we tend to do this side of the pond) ..Clive

CNC Visions
07-05-2015, 11:26 PM
I apologize for not reading everything, Jazzcnc's post would have answered my question.
I'm looking into your advice about getting away from direct drive. It would be much easier to optimize stepper motor performance with the ability to change the drive ratio. I'm not sure why this side of the pond prefers direct drive and drives on the wrong side of the road.
One of the reasons I joined this group was to look into different ways to do things.
Thanks to all for your help
Mike

TheGoodGuy
25-05-2015, 08:19 PM
Hi!

http://www.ebay.com/itm/DC24V-Nema-23-Stepper-Motor-Single-Shaft-12-6Kgcm-1-8Degre-4Leads-56mm-4-Channel-/351294943128?pt=LH_DefaultDomain_0&hash=item51cacf6f98
http://www.zappautomation.co.uk/electrical-products/stepper-motors/nema-23-stepper-motors/sy57sth56-4004a-nema-23-stepper-motor.html

Which type should I choose? They both have the same holding torque.

JAZZCNC
25-05-2015, 08:28 PM
Hi!

http://www.ebay.com/itm/DC24V-Nema-23-Stepper-Motor-Single-Shaft-12-6Kgcm-1-8Degre-4Leads-56mm-4-Channel-/351294943128?pt=LH_DefaultDomain_0&hash=item51cacf6f98
http://www.zappautomation.co.uk/electrical-products/stepper-motors/nema-23-stepper-motors/sy57sth56-4004a-nema-23-stepper-motor.html

Which type should I choose? They both have the same holding torque.

Well out of the two the Zapp motors are the better motors with slightly lower inductance and I know Gary's motors are good quality.!. . BUT whether or not these are the correct motors you should use depends on what your connecting them too.?

TheGoodGuy
25-05-2015, 08:33 PM
I am going to use PoStep60-256 drivers and 48V PSU.

Boyan Silyavski
25-05-2015, 11:08 PM
I am going to use PoStep60-256 drivers and 48V PSU.

the motors that they sell at the same place http://www.poscope.com/nema23 are the same motors that zappautomation (http://www.zappautomation.co.uk/electrical-products/stepper-motors/nema-23-stepper-motors/sy60sth86-3008b-nema-23-stepper-motor.html) or cnc4pc (http://www.cnc4you.co.uk/Stepper-Motor-Plus/Nema23-3.1Nm) sells. That are the motors that most people drive at 70V with Leadshine drives.
I also am looking at that drive you say, plus they have new 8axis ethernet board. Just today wrote them a big letter asking them many things about the board and the drives.

What i am saying is that when looking at the pdf of the motors StepperMotorDimensionsNema23.pdf (http://www.poscope.com/index.php?route=product/product/download&download_id=108&product_id=85) the torque curve driven at 60V or 40V is very same till 3000rpm, hence 4 motors can be driven by 2x35VAC toroid 500w, which rectified will give ~49VDC, which is very near the drive capability /50VDC 6A/ . One of the questions i asked is if really this drive could drive the motor with this setup, i expect they will answer me tomorrow.

What i say is that will be better than the motor you say.

JAZZCNC
25-05-2015, 11:29 PM
I also am looking at that drive you say, plus they have new 8axis ethernet board. Just today wrote them a big letter asking them many things about the board and the drives.

I've just received one of these boards for testing so I'll let you know.! . . . . Board looks Ok but dissapointing that it doesn't come with any connectors of any kind. This makes connecting more difficult than other boards as it needs IDC connectors to connect to board and then Terminal blocks for wires.

Boyan Silyavski
25-05-2015, 11:41 PM
I've just received one of these boards for testing so I'll let you know.! . . . . Board looks Ok but dissapointing that it doesn't come with any connectors of any kind. This makes connecting more difficult than other boards as it needs IDC connectors to connect to board and then Terminal blocks for wires.

Great! given the price and the added capabilities like IO for custom panels i hope that works good. 125khz is also a good thing.

I agree for the IDC, but on the other hand i like that option , if their drives are good i mean, cause i could connect drives and BOB with cables. This drives should be DSP, yes? Its not mentioned at any place.

Honestly i realy have great hopes for that combo, cause i prefer EU made stuff than Leadshine for example in my machines. Especially given the size of the drives.

I was almost ready to buy a 4 motor drive and bob today. how good that you have it already.

My biggest questions are:
-any major drawbacks like definite inability to do something in mach3?
-4rth axis, gantry squaring and so on...


I will wait then to see what you say. I hope at the end somebody managed to make decent cheap Ethernet board with more than 4 axis.

toomast
16-07-2015, 11:49 AM
I've just received one of these boards for testing so I'll let you know.! . . . . Board looks Ok but dissapointing that it doesn't come with any connectors of any kind. This makes connecting more difficult than other boards as it needs IDC connectors to connect to board and then Terminal blocks for wires.
Would be interesting to read your opinion on these boards...

masinecc
21-07-2015, 02:11 PM
How can i calculate what motor size i need for my gantry if i use rack and gear on both sides. This example is for ballscrews. Looking forward for your answer.

Boyan Silyavski
21-07-2015, 05:22 PM
How can i calculate what motor size i need for my gantry if i use rack and gear on both sides. This example is for ballscrews. Looking forward for your answer.

Backwards. Choose the RP size, gearing so that you have as a result normal rapid speeds with acceptable resolution. Then figure how many RPM you need in reality so you have mostly that in mind. Then you go back to the gearing ratio and gantry weight and from there you decide Nema 23 or 34.

Obviously as the RPM is the important part here cause most possible it will be well geared, then you finish with low impedance motors driven by high voltage.

And knowing that nema 23 motor driven at 70VDC moves a gantry max 50-60kg with gear 1:1 without problem at 10m/min via ballscrew, you can figure the motor you need for your case.

Robin Hewitt
21-07-2015, 05:36 PM
Nobody works it out, they just assume that the biggest motor they can find has to be the best option. Then they buy a set of 3 because that saves even more thinking and they end up with a Z axis that could raise the Titanic and is so heavy the gantry has to be made from RSJ :hysterical:

Boyan Silyavski
21-07-2015, 07:51 PM
Nobody works it out, they just assume that the biggest motor they can find has to be the best option. Then they buy a set of 3 because that saves even more thinking and they end up with a Z axis that could raise the Titanic and is so heavy the gantry has to be made from RSJ :hysterical:


http://www.sherv.net/cm/emoticons/4th-july/smiley-waving-us-flag.gif

Zaryab
02-02-2016, 04:21 PM
Hi Sir,
I am making a small scale CNC. I just want to ask that how are you calculating the power in the end four examples of this article?

Regards,
Zaryab Shahid

RoyKO
01-04-2016, 09:47 PM
Thanks Irving for a great tutorial

driftspin
15-06-2017, 12:11 AM
Hi guys,


Can you please check if i am using this motorcalcs the right way?


(40kg moving gantry X+Z on hiwin type rails using 2* c7 type 2010 1500 mm ballscrews)
Really unsure about cutting forces (5N) and gantry weight distribution


Lets say i put this info in the sheet.
http://cnc4you.co.uk/resources/Stepper%20Motor%20Nema%2023%2060BYGH401-03%204Nm.pdf

INPUTS OUTPUTS Version 1.2 - 30 Jan 2010
Units Metric

Load Calculations

Mass of Load 40 kg Load inertia 1,013E-04 kgm^2
Friction co-efficient 0,1 Load torque 8,937E-02 Nm
Cutting forces 5 N


Leadscrew calculations

Screw pitch 10 mm Screw mass 3,676 kg
Screw dia 20 mm Screw inertia 1,850E-04 kgm^2
Screw length 1500 mm
Screw minor dia 16 mm
Screw efficiency 80% %
Screw Fixing Fixed-supported Critical Speed 1066 rpm


Max linear speed 2500 mm/min Motor revs 250 rpm
for: Cutting
Total Inertial load 2,890E-04 kgm^2
(inc rotor inertia)
Proposed Motor Acceleration torque 0,504 Nm
Rated current 4 A
Winding Inductance 3 mH Total Torque 0,594 Nm
Holding Torque 3 Nm at 833 steps/sec
Detent Torque 0,04 Nm
Rotor Inertia 275 gcm^2
Ideal Voltage 55 V
Motor Usage Esitmated Motor
Supply Voltage 68 V Corner Speed 1082 rpm
Set Phase Current 4 A Estimated Torque
(at required speed) 2,081 Nm
The motor & operating conditions chosen have a good
margin and should be OK at the required speed
(subject to driver circuitry)



So what i am looking for is the following.
My design is 1500 mm of travel on the X-Axis.
Gantry weight about 50-70kg
There will be 2 ballscrews.

I am planning on using a timing belt reduction 1:2 ballscrew:stepper

I am not sure how to input a reduction in the sheet.
Filling out 2005 type screw makes the screw rev by a factor of 2
Filling out 2010 type screw makes the motor spin to slow so torque is off by a factor??



Will 2500mm/min speed be plenty fast for a 2.2 kw chinese R20 spindle using carbide bits on aluminium?
What would be a material that needs a higher speed rates over 2500mm/min???
Planning on machining hard/soft wood, aluminum, plastics.

(i have no idea what speeds and feeds are in the resonable range )
I tried filling in some diameters/types cutters and number of flutes in a speed and feed calculator for wood and alu

Looked like 2500mm/min is max any one would need in the hobby range, am i right here?


I am not sure if i can just reduce the gantry weight to 50% if i plan on using 2 steppers. (slaved X axis setup)


Please point me in the right direction.
Any help is welcome.

Neale
15-06-2017, 10:18 AM
That spreadsheet is great, and incorporates a lot of theory. Two problems, really (and I don't want to take anything away from the work that went into it):

- it takes a bit of interpretation in some cases, as you have found. Two ballscrews, belt drive with step-down, etc. Not impossible to work around, but it's not obvious how at first glance.

- actually, there are only a few answers available although the spreadsheet results can help you tweak them.

I went through pretty much exactly the same process a couple of years ago. My machine is close to yours in size - 1500 long axis, two ballscrews, gantry around 50kg (guess). For motors, in reality, the choice is Nema 23 or 34. The larger motor has high inductance and a lot of inertia. When you do the sums, it gives poor performance. A friend recently swapped the 34 motor on a machine he bought second-hand for a 23 and significantly improved performance. So the answer is Nema 23. Make sure you get a low-inductance motor, and 3Nm or 4Nm will work fine. I use two 3Nm motors as 4N were not available widely at that time but I would probably go 4Nm today for the sake of a couple of quid. It is important to drive them properly, hence talk of 68V. Why this funny number? Because it's a safe value to use with a decent digital stepper driver (EM806 or AM882).

Speeds - I went through the same "what's a good speed" thinking. I'm using 2005 ballscrews which reach their critical speed at about the corner speed of the steppers (900-1000 RPM). That gives me 5000mm/min. Actually, I think I'm running at about 4500 at the moment to avoid any possibility of stalling one motor (which is also why you want modern digital drives with stall detection). This is not as fast as the machine could use although it works well. However, I seem to be doing quite a lot of fine cutting with a lot of 2d/3d detail and acceleration is more important than max speed in this case. Your 2010 with 1-1 belt drive would run faster than mine at the expense of resolution (but not enough to worry about).

Some thoughts to get you started, and i'm sure that others will be along with different views! However, my remarks are based on a similar machine that is up and running, not just theory.