I finally have all my linear motion rails/bearings and have all the aluminum I need to begin building my CNC Router. I think I've talked myself into going with servos instead of steppers. I'll be using this machine for more than I had originally anticipated so I want it to mill aluminum without struggling.

My dillema? I know nothing about servo motors or driver boards etc... I'm a quick study and have no problems assembling electronic equipment, I just need some advice from those with experience.

What size servo's would you recommend? I'm assuming I'll be buying Gecko's driver boards but am not sure which model... of course I know it will depend on the servos etc...

I've found the servo's below, what do you think?

Below are some random statments from the seller's description:
>>>
Electro-craft DC brush type servo motors Model E540. Shaft is 5/16 dia with a small pulley at the end. Remove the pulley and the shaft has been machined down to 1/4 inch for about 5/8 inch. Encoders are BEI MOD56-551 and look just like the HP Heds series. unable to determine encoder resolution but I believe them to be at least 250 CPR. I've been running a set with Gecko G320 drives and they perform great. 240 in oz peak torque. Quadrature operation, 5 vdc 35 ma. 24-28 vdc 3770 RPM.
<<<

240oz/in isn't terribly strong for a servo from what I've seen but more than twice the torque of the steppers I was considering.

I appreciate everyone's help on this. I intend to begin my build over my xmas vacation and will definately take pics and document my experience for those who are interested.

Any recommendations? I'm trying to keep the servos and driver boards within a $500 budget. That would put my machine costs at around $750 since I got all my aluminum free. It would also keep my wife happy :D I know, that's gonna be tight. The Gecko's alone will be more than $350... doesn't leave much for Servos. Oh, is there good free software that will run servo's well? I noticed on the Gecko website they say with the G320 you can run the servo's with stepper software.

Darren

Here are some pics of my rails/bearings acquired from ebay. I got everything I needed for less than $250 shipped. If ya keep on those auctions you sure can get some good stuff for a steal!


Z axis (3/4" X 18" supported) (http://thomaswoodcraft.com/39_1.jpg)

Z axis 2 (http://thomaswoodcraft.com/5e_1.jpg)

Z axis 3 (http://thomaswoodcraft.com/66_1.jpg)

Y axis (double 1/2" rail x 36") (http://thomaswoodcraft.com/5d_1.jpg)

y axis 2 (http://thomaswoodcraft.com/6a_1.jpg)

x axis (3/4" X 24") (http://thomaswoodcraft.com/thomspb12_3_1.jpg)

I think Jeff (HomeCnc) had some nice servo's with encoders for $75.00 each in the for sale forum.

Turbocnc will run your servo's fine at $20.00 to register.

Originally posted by balsaman
I think Jeff (HomeCnc) had some nice servo's with encoders for $75.00 each in the for sale forum.

Turbocnc will run your servo's fine at $20.00 to register.

That sounds perfect. I'll look into that. Turbocnc sounds perfect. I'm trying to get it working for as little money as possible but don't want to skimp... probalby what 90% of the people are doing I suppose :) I'll check those out.

[EDIT]

Just checked them out and think I'm going to buy three. Which acme thread pitch would you recommend with those motors? 10 tpi?

Thanks again.

Darren

I'm running 350 oz/in servos on my ali machine, with a 1:1.5 reduction, they have awesome power. Good choice.
My screws are about 5.5Tpi (poor metric conversion, sorry), perhaps 10tpi is a little fine for those servos?

Darren, don't forget about the power supply, fan, heatsinks, enclosure box, wiring, switches, etc. in your budgeting.

Originally posted by buscht
Darren, don't forget about the power supply, fan, heatsinks, enclosure box, wiring, switches, etc. in your budgeting.

Yup, I'm not counting the smaller ticket items as I can slip purchases like that by the wife without raising an eyebrow :) I have a case and plenty of wiring left over from computer parts and fans. I'll still need to get heatsinks, switches, power supply, and a few other things though :)

Thanks for the info. I'm still not sure of absolutely everything I'll need... I'm sure there will be a surprise or two. Hey, is there an article or anything I can read to figure out EVERYTHING I'll be needing? That would be great. I still don't know all the smaller misc. items I'll be needing.

Darren

Making a servo controller box is not that cheap! Be careful about this. Look at my website, I have a picture of a servo controller box that I built for someone on the Zone. It was about $800 just for all the parts. Geckos were part of that cost.

Originally posted by HomeCNC
Making a servo controller box is not that cheap! Be careful about this. Look at my website, I have a picture of a servo controller box that I built for someone on the Zone. It was about $800 just for all the parts. Geckos were part of that cost.

OUCH!!! I guess I'm a bit naive about the cost of this. I'm looking at the box you built. I see a torroid, cap, 3 gecko's, and another board hooked to the gecko's. What is that board? A break-out? I'm still learning. I can't be spending $800 on a servo control box. :(

Darren, some other things that you haven't mentioned are the lead screws that you want to use, I'm guessing that these could cost anywhere from $25 to $100 per axis. Framework could run another $200

Originally posted by buscht
Darren, some other things that you haven't mentioned are the lead screws that you want to use, I'm guessing that these could cost anywhere from $25 to $100 per axis. Framework could run another $200

I have all the aluminum I could ever want and I figure my lead screws being acme screws will be no more than $20 per axis. I plan to make my own anti-backlash nuts etc...

Why does the servo control box cost $800 to build? What am I missing???

3 Gecko's = $350ish
Power Supply = $50?
Heat sync's = ??
Misc. Cables = next to nothing

What else?

I would think a good power supply is more like $150-$200. Breakout board = $125
Decent cables can be a lot more than nothing.
That box on Jeff's site has a pretty nice case, more $$.

Gerry

It does not have to cost that much if you just want the thing to work...bells and whistles can come later.
Mine was around $500
$125 for power supply (toroid, cap, rectifier, resistors)
Three Gecko 320s $350.
Printer cable hooked to terminal strips $10?
No need for a breakout board....
Three push button switches from Radioshack for Gecko reset $2
Main power switch with box $2 (15 amp wall switch and handy box)
Cut up 12 ga. extension cord for servo power cords
cat5 wire for encoders
My Geckos barely get warm, no heat sink, just spaced with washers above the mounting board.
The rectifier's "heat sink" is a piece of thin aluminum (soda can) which steps it above the board and it does not get warm either.
Yeah, if money were not an issue these beautiful products that are shared here at the Zone would be real nice, but meanwhile back on Earth there is tuition for the kid and insurance and........

Originally posted by sol
It does not have to cost that much if you just want the thing to work...bells and whistles can come later.
Mine was around $500
$125 for power supply (toroid, cap, rectifier, resistors)
Three Gecko 320s $350.
Printer cable hooked to terminal strips $10?
No need for a breakout board....
Three push button switches from Radioshack for Gecko reset $2
Main power switch with box $2 (15 amp wall switch and handy box)
Cut up 12 ga. extension cord for servo power cords
cat5 wire for encoders
My Geckos barely get warm, no heat sink, just spaced with washers above the mounting board.
The rectifier's "heat sink" is a piece of thin aluminum (soda can) which steps it above the board and it does not get warm either.
Yeah, if money were not an issue these beautiful products that are shared here at the Zone would be real nice, but meanwhile back on Earth there is tuition for the kid and insurance and........


There we go. This is more like it. I have a case, no need for heat syncs if you have aluminum of your own...I can make some easy. Cables are cheap and the break out board is only for limit switches right? No need right away for that. I can always do those later.

Phew, thought I was going insane there for a minute. We went from DIY to retail.

One question. Why are people using expensive toroids for their power supplies?

Darren

Higher amperage
Plitron's toroids have two sets of outputs; that is they are essentially two transformers in one; wire the outputs in parallel and double the amperage...they run around $80, but shipping takes another $20...they're a heavy spool of copper. :)

Ah, makes perfect sense. I've built 3 power supplies with the toroids but it was for a home theater amplifier that I assembled. I don't know much about electronics but I can assemble with the best of em :)

Thanks for the info.

Darren

Darren

I also am trying to build a controler box to power the same motors
that Jeff is selling.
I also need to keep cost's down.
Have you located the toroids yet? I am tring to find them.
Please continue to post your progress on building this box
I think it could help alot of us.
Sol thanks a ton for the help

Bill

The toroid was purchased from www.plitron.com It is in fact two transformers in one. This is one reason why I like them. If you purchase the 10 amp 25VAC output it will convert to a nice 36VDC 20 amp supply. If you get the 15 amp toroid you can have a 30 amp supply. Remember, the larger the amps you go, the larger the Cap must be. (more money!)

There are places to cut costs, but be careful. On my first servo controller box I made my own encoder cables. One cable must have been not quite up-to-par. Everything worked fine for a while. All of a sudden one connection on the encoder must have disconnected. When this happens the servo motor turns back into a DC motor, and you have the full DC volts applied to it. It starting running at FULL RPM. I had to hit the power switch before my table slammed into the stop. I now purchase proper encoder cables from USDigital.

Like someone said. It's all in the bells and whistles (or quality) you can afford.

Jeff! I can imagine your panic motivated leap to cut the power!
Have you heard of any other stories about home machines with servos running away? I have a shoptask metal lathe fitted with servos and intend on installing limit switches, but I worry about the inertia carrying the force through and doing damage.

Doing damage to what? The limit switches or the lathe?

Install the limits so the axis can travel by them without damage. Make sure there is room after the limits for things to stop without crashing into something.

Eric

Just so you know. Limit switches or any software E-stop will NOT stop a run away servo that has lost encoder connection. This is so with the Gecko drives. (Other drives, I don't have experience with).

Limit switches and software e-stops are only good to stop steps from being sent to the driver. A run away servo must have the power cut! Or in the Gecko's case I was able to hit my "disengage" switch, which faults the drives.

This is true. Estops by law has to be HARD WIRED to cut the power to the machine. Another contact on the estop switch can be used to notify the computer that something has gone teribly wrong.

Eric

Ah! How about adding switches directly in series with the motor power cables. I don't want a multitude of wires traveling about the machine, but if this will work for preventing that "runaway", should it ever occur, maybe they can be discreetly hidden. Are there any other 'cons' to doing this?
While on the subject, are there circuits that could be utilized that would not only cut the power, but instead, brake the motor?

Balsaman:
Above you asked "damage to what?"......
Mainly damage to the machine...or body parts. Dan at Camtronics noted that servo motors are "less forgiving". I think this is what he meant.

---------------------------
A paranoid limit student
John

Use regular limit switches in series to the computer. make sure you hardwire a e-stop pushbutton that cuts all AC power.

Eric

My entire e-stop circuit is low voltage. I use a solid state relay to power the PSU, with the ground connected to the e-stop button. If you check Balsamans router thread, you will see his err/res manmager circuit. I have used this as the e-stop input to the computer, and have a 5v line which is switched on with the PSU that goes into this circuit as a "ghost" gecko drive if that makes sense. Again, the ground from that 5v PSU goes to the e-stop button, so when it is pressed, the PC knows it is in e-stop. Also, if any of the drives fault, the software knows about it. The third thing I have implemented is Mariss' idea of using a large resistor to "suck the PSU caps dry" which is levered across the PSU terminals in the event of an e-stop. If you look at the CAD_CAM_EDM_DRO boards file section, you will see it in there as "e-stop.pdf". Of course, I had to alter it a bit to use a low voltage system. This will aid in braking the motors quicker coz the cap isn't still supplying them with power. Hope it helps, but I think I'm rambling a bit! If you check that estop circuit, you will also see how the "latched relay" is used to turn on the power, but cuts out if the power is lost. This will prevent your motors coming back on when you release the e-stop button.

I think I understand the use of limit switches through the software, but whats going to stop the motors if communication is lost and I'm not within reach of any mechanical cutoff. I'm sure that during an operation I might run to get a beer or something and if it(the malfunction) occurs during this time, well.....a metal machine can really tear things up.

-------------
a guy without 20/20...or that trees and forest thing.
John

Thanks Kong, I was typing my post while yours was being entered. this answers a lot of questions.

Breakout board sounds more expensive than it needs to be, is this optically isolated? Here's one for $5
http://www.embeddedtronics.com/breakout.html

Originally posted by ger21
I would think a good power supply is more like $150-$200. Breakout board = $125
Decent cables can be a lot more than nothing.
That box on Jeff's site has a pretty nice case, more $$.

Gerry