help with servo motors Please.

Once upon a time I thought I had the drive system worked out.i had intended Using stepper Motors.

Then one of my friends said why not use servos you can get a faster working speed.
machine bed is 2635mm long and 1220 wide.
so after some calculations came up with two 400 watt to position the R&P x
one to position the Y and one to position the Z. which is above the calculations of 2 x 230 watt for the X
I wish to run the machine faster than 7.2 metres per minute.

Ive looked at loads of profi Machines they are R&P 90% run on steppers. Usually somewhere about 12NM.

After endless searching on the NET for suppliers in the UK I am finding the choice very difficult and of course,
Know very little about servo Drives.

Could someone explain what a multiaxis drive is and can I use one to drive 4 motors on 3 axis.
and are there any other suppliers in the UK that sell larger servo motors than Remote control Aeroplanes.
the only one ive come across is ZAPP

If your using R&P then chances are you'll have a high pitch so lack of speed won't be an issue even with steppers.
Steppers are a lot easier to setup and get working and they are much cheaper. They are just as accurate as Servo's if tuned correctly with far less hassles.

The Only reason I would consider going to the expense and complexitys of servo's is if the Gantry was heavy or I need very high speeds. Speeds well above 7.5Mtr min are very easily achived with steppers and depending on Gantry weight they most likely won't need to be 12NM either.!

What do you plan to cut at above 7.5mtrs/min.?

[QUOTE=JAZZCNC;54875]If your using R&P then chances are you'll have a high pitch so lack of speed won't be an issue even with steppers.
Steppers are a lot easier to setup and get working and they are much cheaper. They are just as accurate as Servo's if tuned correctly with far less hassles.

The Only reason I would consider going to the expense and complexitys of servo's is if the Gantry was heavy or I need very high speeds. Speeds well above 7.5Mtr min are very easily achived with steppers and depending on Gantry weight they most likely won't need to be 12NM either.!

What do you plan to cut at above 7.5mtrs/min.?[/QUOTE

the Gantry will be 101 KG all kit and attached, .
also in the Future if things go well I was expecting to increase the Length of the machine.
For the most part it will be cutting limestone and sandstone.

in the beginning I thought 1600oz steppers which comes around the 12NM mark direct drive
then
What I did was work out the HP/Watts I needed then added some extra to cover any eventual loss.
I thought to drive a Cutting head through the stone Extra Torque is needed. and so higher Watt which also means Faster machine,
Profi stone machines cut 300mm per minute cutting granite shapes and cutting straight lines with High head 6 metres per minute.
im looking for comparable speeds to use the machine in a working environment cutting limestone which is about 2/3s the force needed .

Steppers work out about a third of the price.
but continually seeing more favouring the Servo.

Hi ramsbury

Is there a specific reason why speed is important when cutting/milling stone. most of the techniques i have considered, seem to show slower deeper cuts make more sense than fast low surface cuts. BUT I DON'T KNOW< JUST ADDING AN OPINION. I think it was the chip load calculations for the bit, that i hit my limits with

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Originally Posted by ramsbury

What I did was work out the HP/Watts I needed then added some extra to cover any eventual loss.
I thought to drive a Cutting head through the stone Extra Torque is needed. and so higher Watt which also means Faster machine,
Profi stone machines cut 300mm per minute cutting granite shapes and cutting straight lines with High head 6 metres per minute.
im looking for comparable speeds to use the machine in a working environment cutting limestone which is about 2/3s the force needed

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I'm not into cutting stone so don't know the speeds and feeds but what I can tell you is you'll need one hell of powerful spindle to cut Stone at 6mtr/min.!!

There's a huge differance between 300mm/min and 6000mm/min and I don't know of any cutting situation or material where you'd go so Much faster for straight cuts than you do for curved cuts other than slowing down for corners.!. . . . Are you sure they cut at these feed rates.?

Also if you running R&P I doubt you'll want to run it direct drive because the pitch will be high and resolution lower.? Chances are the Servo or larger stepper will have shaft size around 16-19mm so looking at my Ondrives catalogue this will mean Pinion size around 29T Minimum.
This will give Diameter of 43.5mm x 3.141=136mm. This will be the pitch so with 3000Rpm servo direct drive you'll have rapid speeds around 408mtr/min.!!!! . . . . . With a 2500PPR Encoder the resolution will be 136mm/2500=0.05mm
Even with Steppers you'll be at 136mtr/min (1000rpm x 136mm=136) and with 2000micro steps resolution will be 0.07mm.

To be honest I'm not sure 400W servos with heavy gantry with direct drive will be enough anyway so you'll want some ratio to increase torque.
Another thing you should be aware of with steppers is that higher Power(watts) motors doesn't always mean higher speeds, often larger motors spin slower than smaller motors. They will carry the torque higher up the rpm curve before stalling but will spin slower.

Now the other problem with servo's comes from the Pulse rate needed to get full speed is far higher than the parallel port can handle. So then your into expensive Motion control boards and Breakout boards that can handle the high pulse rates needed.
It's for this reason I feel steppers are better because the Cost doesn't just stop at buying the servos/drives. Then you have all the extra tuning setup etc that goes with them, where steppers are pretty much Fit with quick setup and forget.

For simplicity and reliabilty I'd go with Steppers and High Voltage drives with a 2 or 3:1 ratio using R&P. This setup will still give high rapid speeds and good resolution with good torque where it matters which is when cutting.

THat is a Very big thank you,
its something Ive been struggling with because these manufacturers are from China and I took the Dims and speeds directly from their machine sites.

Just been convinced to go with Steppers.

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Originally Posted by ramsbury

THat is a Very big thank you,
its something Ive been struggling with because these manufacturers are from China and I took the Dims and speeds directly from their machine sites.

Just been convinced to go with Steppers.

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Your welcome and please Don't get me wrong I'm not saying Steppers are better than Servo's as they clearly are not, but I am saying in this case I don't think Servo's will be the best for this machine given it's task.

I'm sure the cutting Feeds you mentioned will be un-obtainable in stone without a very very Expensive and powerful spindle which makes the the Whole Servo/Stepper debate mute unless your prepared to part with 3-5K for just a spindle.!!

In Semi Industrial/DIY use then Steppers will more than match the rest of the machine.

Contacted Marchant Dice asked for a quote saying gantry was 120KG, I was quite surprised they suggested a nema 23 small nm with a 12:1 gearbox. I thought that rather weak.

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Originally Posted by ramsbury

Contacted Marchant Dice asked for a quote saying gantry was 120KG, I was quite surprised they suggested a nema 23 small nm with a 12:1 gearbox. I thought that rather weak.

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Well yes and no the 12:1 ratio increases torque approx 12 fold. I presume they where working on using R&P which if using figures like I quoted further down would give a speed around 11mtr/min and a pitch around 11-12mm which is ok. Working on a conservative 800 micro steps and 12mm pitch then you'll have resolution in the 0.01mm range.

The only problem I see with this setup is higher backlash, because you'll have backlash in the gearbox and backlash from the R&P. Thou For cutting stone I don't think this will be an issue.!

I don't think it would either I just thought a nema 23 stepper with 12:1 would be working too much on a 120kg gantry.

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Originally Posted by ramsbury

I don't think it would either I just thought a nema 23 stepper with 12:1 would be working too much on a 120kg gantry.

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No the gearbox is doing the work and it's designed to do that work. So long has you choose a low backlash box it will be fine. Something like this but the 5' arc minute version.
RE55 Planetary Gearbox