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Author Topic: Motion controllers - Take me to school  (Read 7215 times)

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Re: Motion controllers - Take me to school
« Reply #40 on: September 07, 2017, 04:33:11 PM »
Hi Ger21,
sounds like OP is talking about direct connection between the servo and lead screw.

Do you think that 160 oz.in is adequate in absence of that reduction?.

The reducing gearboxes on my steppers have spoiled me, I have 705 oz.in at low speed, more than sufficient.
The torque curve published by Vexta suggest that the torque has dropped to about 175 oz.in at rapids speed,
the Vexta drivers have 150V open circuit!

My suggestion to OP was 350 oz.in  ...do you think that's over the top?

Craig
My wife left with my best friend...
     and I miss him!

Offline ger21

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Re: Motion controllers - Take me to school
« Reply #41 on: September 07, 2017, 04:47:50 PM »
I would think that a 400w servo is overkill for his machine, even direct drive. Remember, peak torque is near 500oz. And I think those machines have 20tpi screws, so you have a large mechanical advantage.
Gerry

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Re: Motion controllers - Take me to school
« Reply #42 on: September 07, 2017, 04:53:35 PM »
Hi ger21,
yeah, 20tpi is mighty fine, should have any amount of thrust/accel.

I don't want to make a recommendation to him that would see him spend untold bucks without cause.

Craig
My wife left with my best friend...
     and I miss him!
Re: Motion controllers - Take me to school
« Reply #43 on: September 07, 2017, 08:38:19 PM »
Hi,
does anyone know what diameter the lead screws are. I know THK recommend no more than 2500 rpm for my 20mm
ground ballscrews with 300mm unsupported. All the manufacturers seem to have a similiar spec depending on diameter
and unsupported length.

I'm thinking if the Sherline lead screws are smallish in diameter Mike may not be able to spin them really fast anyway.

Craig
My wife left with my best friend...
     and I miss him!

Offline RICH

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Re: Motion controllers - Take me to school
« Reply #44 on: September 07, 2017, 10:32:48 PM »
Hey guys,

The "threaded" lead screws are 1/4-20.........

Check out  how expensive good small accurate ball screws are!

So I echo #5 from my post:
Is the current machine ( basic and all components) acceptable to do 1. to 4. above as is, with with steppers, with servos?

But #5 is useless without defining the first four steps. ;)

RICH  
Re: Motion controllers - Take me to school
« Reply #45 on: September 07, 2017, 11:14:53 PM »
Hi,
bloody hell, try to spin those things at speed and they'll whip themselves to shreds. Much more than 1 ft.lb
or 200 oz.in is likely to see them twist up like a carrot.

I think the OP may be on a hiding to nothing trying to get this machine to go fast.

Craig
My wife left with my best friend...
     and I miss him!

Offline sn96

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Re: Motion controllers - Take me to school
« Reply #46 on: September 09, 2017, 11:45:53 AM »
Hi Mike,
just been scanning around some of the other servo manufacturers and commonly a 400W servo has a rated torque of 1.27Nm which is equivalent to
160 oz.in. A 400W servo seems ridiculously large on Sherline mill.

My mill having gearboxes and so on is unusual and so my recommendation may likewise be way off as well. Do you have a specification for the torque of
the existing Sherline steppers? You know they work so you might say just double the torque and say 'now it'll work better'.

Craig

I believe these are the current specs. of my Sherline steppers:

Frame size: NEMA #23
Shaft: 1/4″ diameter, dual
shafts
Torque: 100+ oz. in.
Voltage: 3.2 V.
Current: 2 A
Step Angle: 1.8° (200
steps/rev)



________________
Mike

Sherline 5400 with Sherline steppers
Visual Mill 6.0 Standard
Mach3
Windows XP

Offline sn96

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Re: Motion controllers - Take me to school
« Reply #47 on: September 09, 2017, 12:15:29 PM »
Hi,
bloody hell, try to spin those things at speed and they'll whip themselves to shreds. Much more than 1 ft.lb
or 200 oz.in is likely to see them twist up like a carrot.

I think the OP may be on a hiding to nothing trying to get this machine to go fast.

Craig

No... just looking at different systems and yes more speed/performance for my machine is always welcome in my opinion. Is it practical or economical for a small beginner machine such as the Sherline? probably not, but if all I am ever going to make are small parts and if I want to spend any kind of money to improve my existing machine, why not look at other options? I don't understand the harm in that. I haven't spent a penny yet, and wont until I look at whats required, cost, and benefits (Things that are important to me). If a comprehensive servo solution is a few hundred dollars more than conventional steppers, then there may be something to consider... plausible.
________________
Mike

Sherline 5400 with Sherline steppers
Visual Mill 6.0 Standard
Mach3
Windows XP

Offline sn96

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Re: Motion controllers - Take me to school
« Reply #48 on: September 09, 2017, 12:45:10 PM »
Hey guys,

The "threaded" lead screws are 1/4-20.........

Check out  how expensive good small accurate ball screws are!

So I echo #5 from my post:
Is the current machine ( basic and all components) acceptable to do 1. to 4. above as is, with with steppers, with servos?

But #5 is useless without defining the first four steps. ;)

RICH 


Ding Ding! Schools in! I'm at my desk teacher Richy!

Shoots hand up in the air:

"Whats your opinion on closed loop steppers?" :D
« Last Edit: September 09, 2017, 12:48:56 PM by sn96 »
________________
Mike

Sherline 5400 with Sherline steppers
Visual Mill 6.0 Standard
Mach3
Windows XP
Re: Motion controllers - Take me to school
« Reply #49 on: September 09, 2017, 03:27:24 PM »
Hi Mike,
whether you use ordinary steppers, closed loop steppers or servos is not that important, any of these devices can do the job.

If ger21 is correct that your lead screws are 1/4 diameter then the speed at which you can rotate them before resonance is limited,
additionally they will flex torsionally for a loss of accuracy and if you up the torque the problem gets worse.

Look at your machine and see if you cant replace those lead screws with, ideally ground ball screws (really really expensive),or
rolled screws. Ebay has zillions of them, go for 16mm ones if you can get them to fit. They typically have a pitch of 4,5 or 6mm
so to spin them at 1000 rpm means rapids of 4m, 5m or 6m per minute, very useful.

If it proves impossible to replace your existing lead screws then significantly increasing the speed of your machine will be limited.
Under those circumstances better specified but otherwise standard steppers with decent drivers and a 68V+ power supply would easily
provide an increase in speed up to the resonance limit of the machine whilst conserving you budget. You could certainly achieve similar
results with servos say, they would cost a lot more and you will still only be able to turn up the wick to the limit imposed by the
mechanics of your machine not the capability of the servos.

Craig
My wife left with my best friend...
     and I miss him!