PDA

View Full Version : Stepper strength



pminmo
11-22-2003, 01:17 AM
125ozin stepper 10 tpi ACME thread. What king of strength can I plan for?

motomitch1
11-22-2003, 04:18 AM
hi pminmo
That would work on a small machine I have a small mill with 200 ozin stepper motors and 16 TPI. I live in st.peters also if you want to stop by sometime and look at my set up.

umrk
11-22-2003, 09:48 AM
Acording to This Info (see Pg25) (http://www.nookindustries.com/pdf/powerac2.pdf) a Acme 1/2" x 10tpi using a plastic nut requires .039 in/lb (.624 in/oz) to lift one pound, so a 125oz/in stepper Should be able to lift 80 lbs. Of course that is going to drop as speed increases.

mike

balsaman
11-22-2003, 12:13 PM
I used that rod with 100 oz steppers. worked great.

Eric

pminmo
11-23-2003, 02:06 PM
Motomitch1, small world, check you email, I'd like to see your machine.

motomitch1
11-23-2003, 02:38 PM
Me check mail

snokid
11-23-2003, 04:06 PM
along the same lines.

if 10 tpi yeilds about 80# of force wouldn't 20 tpi give more force?

if that is the case why not use the threaded rod instead of the lead screw arrangement?

the only things I can come up with is amce rod would last longer and travel faster, but with a reduction in torque.

Bob

anoel
11-23-2003, 04:24 PM
Originally posted by snokid
along the same lines.

if 10 tpi yeilds about 80# of force wouldn't 20 tpi give more force?

Yes, 160# of force at half the speed. (in theory)


Originally posted by snokid

if that is the case why not use the threaded rod instead of the lead screw arrangement?


If you've got enough torque with the acme rod then you have a speed advantage.[/B][/QUOTE]

umrk
11-23-2003, 09:39 PM
With 125oz/in steppers you should have plenty of power for 10tpi, unless you have a very heavy carrage to move.

Its my understanding that Acme rods were designed for linear motion applications, lathes, mills... do use them, rather than typical V threads. It seems to me that the angle and shape of the threads would be more suited for motion and its forces rather than just fixed holding strength and ease to cut like wth a V thread.

V threads do work on smaller machines but I suspect that they will wear faster. I've not seen V threads used on larger machines.

Yes, by going with 1/4 x 20 rod you should get close to 2X the power (depending on their efficency), at 1/2 the speed. Your resolution will also double. IOW with a 200 step motor and 10 tpi you need 2000 full steps to move one inch, or .0005" (half a thousandths) per step. 20tpi would give .00025" per step

This sounds great for accuracy but it is basically wasted resolution due to backlash and flex, unless your building a Very strong (heavy) machine with zero backlash nuts and making Very light cuts. Even big lathes, mills... the cutter and what your cutting, will flex to some degree. Also the stepper motor can rotate a bit under load without missing a step, and those 200 steps may not be dead on accurate anyway. Then factor in wear on every component as you use it. It all adds up and real life accuracy suffers.

As I understand it 160lbs of force will accellerate 160lbs at 1G, or 16lbs at 10G. Do you really need that kind of force, while limiting your top speed by 1/2?

Acme rods aren't expensive, Enco (http://www.use-enco.com/CGI/INSRIT?PMAKA=408-0200&PMPXNO=941030&PARTPG=INLMK3) has them for $3.49 per 3' section.

mike

ger21
11-24-2003, 12:29 AM
Here's a question for you guys. If you look at just about any chart showing torque curves for steppers (at ~ 24V anyway), you'll see that the torque drops off rapidly as rpm's increase. So if you spin the motor twice as fast with the 1/4-20 to get twice the torque, don't you lose a lot of that 2x torque because the stepper is spinning twice as fast? This would mean that the 1/2-10 would be the better choice in most situations, unless you've got pretty small motors and / or are not planning on cutting very fast.


Gerry

fyffe555
11-24-2003, 12:20 PM
umrk

Re; '160lbs of force will accelerate 160lbs at 1G, or 16lbs at 10G'

I think you'll find that the reference you've given talks about the torque required to keep one lb in motion and not accelerate it at 1G which is a totally different order or magnitude.

As you suggest a 125oz/in stepper on 10tpi would produce about 80lbs force, (not power, hp=torquexspeed or work=forcexdistance for example). At say 1000rpm the stepper would give 100ipm.

The same stepper on 20tpi would produce about 160lbs force and 50ipm at 1000rpm. The acceleration is entirely a function of mass and inertia or weight of gantry and friction loads.

Some software controls acceleration too, below the mechanical max to avoid lost steps amongst other stuff.

For what its worth I'm using PacSci 116oz steppers on Acme1/2 10 and it works well with a 40lb gantry/router on linear bearings. Still learning but I'm happy at 60ipm rapid.

fyffe555
11-24-2003, 12:31 PM
Ger21,

I've replaced 3/8 16 allthread with 1/2 10 Acme on the same machine. I appear to get both more speed and power from the 1/2 10, can set up to take deeper cuts faster. Theres a noticable difference. I'm using the same pacsci 116oz which I think you gave me some help on a while ago(?) and turbocnc.

So - I think on my machine at least, at the ipm speed range I'm running at the 1/2 10 is better because the 'gearing' is better suited to the torque. The nut has a little less drag on 1/2 10 but its hard to tell. Obviously changed the turbo cnc setup to match. Tried running my drivers in 1/4step and its much quieter and smoother. Just need to build a bigger stronger stiffer machine now....

arvidb
11-24-2003, 04:45 PM
Originally posted by fyffe555
umrk

Re; '160lbs of force will accelerate 160lbs at 1G, or 16lbs at 10G'

I think you'll find that the reference you've given talks about the torque required to keep one lb in motion and not accelerate it at 1G which is a totally different order or magnitude.

Actually, no torque (or force) is needed to keep a moving mass moving. In a router, torque (or force) is only needed to

1) overcome friction
2) accelerate/decelerate (here's where the mass of the gantry/table/whatever comes into play)
3) overcome cutting forces (this is the part of the force you need that's difficult to calculate)

This is how force relates to acceleration and mass:

F=m*a,

where F = force [N], m = mass [kg], and a = acceleration [m/s^2].

To return to the above: '160lbs of force will accelerate 160lbs at 1G, or 16lbs at 10G':

1 G = 9.81 m/s^2
160 lbf (pound-force) = 712 N
160 lbm (pound-mass) = 72.5 kg

a = F/m = 712/72.5 = 9.82 = 1 G (the slight error of 0.01 m/s^2 is due to conversion errors between metric and imperial :-). That is, the statement is true if you disregard friction.

Remember that the Z axis needs to overcome gravity; it needs to "accelerate" at 1 G just to hold still.

Finally, this is how torque relates to force with a lead screw:

M = F * s/(2*pi) * 1/Eff, where

M = torque at lead screw [Nm]
F = linear force at nut [N]
s = lead of screw [meters/turn]
Eff = efficiency of screw, about 0.1-0.2 for all-thread, about 0.4-0.5 for ACME screw with Acetal nut, and about 0.9 for a ball screw.

Hope this helps!

Arvid

Mr.Chips
11-24-2003, 07:28 PM
I gotta ask.
What does the profile of Acme Threaded rod look like?
Is it a "V" shape or square?
Hager

balsaman
11-24-2003, 07:34 PM
not quite square, but much squarer than a threaded rod...

Like this \_/ sorta.... where threaded rod is like this /\/\

Eric

Mr.Chips
11-24-2003, 07:44 PM
Balsaman.
I see why these would be a better choice for a longer lasting machine that would also be more powerful. I saw one negative comment on this forum about Enco. Have you had any bad experiences? Their prices are tempting.
When I saw your custom CNC room, it hit me, there will be a lotta dust.

Hager

ger21
11-24-2003, 08:40 PM
Hager, you might want to look here for acme screws.

http://www.mscdirect.com

I've had pretty good service from them. One thing to consider is there are 2 different types of acme rod commonly available. The general purpose grade, and the precision leadscrew grade. The precision grade from MSC is only about $20-$25 for 6' of 1/2-10, so even though it's almost double the price of the general purpose grade, you might want to spend the little extra. For more information on acme threaded rod, look here:

http://www.nookindustries.com/pdf/powerac1.pdf

Gerry

balsaman
11-24-2003, 10:36 PM
here is dust for you...this is from my first cnc machine.

umrk
11-25-2003, 12:45 AM
Thanks for the corrections and explanations!

I've not bought much from Enco, but what I did buy was shipped fast and at a good price, buts its always best to shop around. The acme rods I got from them were better that the so called 'high quality' rods that came with my antibacklash nuts, but then I got them at a good price off of ebay, so no real supprise.

Are precision grade rods really needed for wood routers?
What kind of precision/resolution is really needed, all things considered?

As for dust, thats why most commercial CNC routers that I've seen came with a shop vac attachment around the cutter.

mike

chuckknigh
11-25-2003, 12:55 AM
It depends on what you intend to do.

If it's roughing-out plywood "yard art" signs for Christmas, you need almost no precision. Conversely, if you're making small wooden clock gears, or doing inlay work, then a few thousandths of an inch can become significant.

-- Chuck Knight