[Darren]

Hi Guys,

I frequened this forum a couple years ago and collected quite a few parts for my planned machine. I have the following:

Thompson linear rails:
• 48" X 1" supported X
• 24" X 3/4" unsupported Y
• 18" X 3/4" supported Z

Other parts:
• 3 servo motors 360 oz/in with encoders
• Lots of plate aluminum and some extruded
• Computer
• Router

I'm tired of seeing all this equipment sitting in a pile, I either have to finish this project I started over a year ago or sell the parts. I'm intimidated by the power supply, drive boards, mounting the servos in the best configuration, and finding the correct linear screws. I'm no electrician but can assemble from plans and solder quite well. I'm suffering from information overload and procrastination. Could someone please show me the light? I'm trying to figure out how much it will cost me to get all the parts needed. From what I can remember I will need the following:

• 3 linear screws
• power supply
• drive boards
• software

I'd really appreciate pointers, suggestions, condolences... whatever you can offer that might get me out of this funk. I'd prefer to finish the project rather than sell the parts. Of course I'm kinda tight on funds as well so any cost savings ideas would be wonderful. I was considering Gecko drives back when I started collecting parts. Is there a less expensive alternative? I'm really trying to keep the costs down to finish but understand that might be futile.

Thanks.

Darren

[phillby]

Hi Daren,

I can't help with the linear Screws but here's a Link to Phils Electronics Site.

http://pminmo.com/

Its great for the electronics do it yourselfer.

Googling Turbocnc and Mach3 will point you to trial programs that can be used for setup and even small jobs.

Hope this helps.

[Darren]

Thanks for the link, unfortunately I didn't see anything for servo drivers Did I miss a link?

[Evodyne]

Darren,

Hi! I know how you feel! Where to start? It can make your head hurt. I think you can get a lot of help here, but it might be best if you can offer up a little more information. Do you know the specifications on your motors and encoders? That may point to certain drives. What kind of machining are you wanting to do? Metalworking? Wood? That will determine how much "oomph" you need. And is this to be a hobby machine, or a real shop tool? Knowing that, you can better pick out a drivetrain. I say drivetrain, as you might use belt drive or rack and pinion versus screws. Also, what about budget and your skills with building? That may also steer you one way or the other. In a nutshell, the more information you can provide the better. Look forward to hearing back from you...don't give up!

Sincerely

Evodyne

[Darren]

Ok, more info:

The servo's are Litton Clifton Precision Model# JDH-2250-BX-1C they are 360 oz/in and the encoders are US Digital E2-250-25-G, they have Ground, Index, Ch.A, +5v, and Ch.B pins. Please let me know if you need more info, I'll try and look it up on-line.

This will be a serious hobby machine. I will be working with wood mostly but would like the ability to cut aluminum, speed on aluminum isn't terribly important as long as it isn't prohibitively slow. I'm looking forward to any advice you can provide. I'm quite capable as a woodworker, have been remodeling homes and building furniture for many years. I'm mechanically inclined but don't know much about CNC to start with. I work in computers for a living so I'm adept with computers/software.

Budget, well... I'd like to keep it under $500 to complete it but I realize I might be dreaming. I'm just looking for the best bang for the buck to complete my machine. I'm a real DIY guy, I've assembled my own high end audio amplifiers and speakers from kits so I can follow schematics and am good with a soldering iron. I just don't know how to design stuff like power supplies etc. I'm great at duplicating proven designs with instruction.

I'd like to think I can build pretty much anything with proper guidance.

[phillby]

Sorry Darren,
I didn't notice the servos.
Alan is developing a servo controller but I don't know how advanced he is.

http://www.fromorbit.com/projects/picservo/

Cheers

[Evodyne]

Ok, more info:

The servo's are Litton Clifton Precision Model# JDH-2250-BX-1C they are 360 oz/in and the encoders are US Digital E2-250-25-G, they have Ground, Index, Ch.A, +5v, and Ch.B pins.

Do you have a datasheet or any thing on the motors? Stall current, rated voltage, rated rpm, that sort of stuff?

This will be a serious hobby machine. I will be working with wood mostly but would like the ability to cut aluminum, speed on aluminum isn't terribly important as long as it isn't prohibitively slow. I'm looking forward to any advice you can provide. I'm quite capable as a woodworker, have been remodeling homes and building furniture for many years. I'm mechanically inclined but don't know much about CNC to start with. I work in computers for a living so I'm adept with computers/software.

Hey, I'm like you-except for the capable, mechanically inclined part. See, you've already go the upper hand! I'll fess up-I've yet to start building my machine-though I have nearly all the parts.

Here's a short blurb on power supplies, but my time is limited tonight. O.K, servos are going to give you more upper end power and speed than steppers, but to take advantage of it you need those specs (voltages, currents, etc.) to match them with the right power supply or supplies. You want a supply that has a voltage at or just below the motors rated voltage. You might see values for continuous torque and stall torque. Your 360 oz-in rating is one of those two. The continuous torque is what the motor can deliver all day long and not overheat. It will have a corresponding current that will produce that torque when supplied with the rated voltage. But a servo will try it's best to give you what you want: load it down and it will produce more and more torque. It will produce the most just as it stalls out from too much load. That value is your stall torque. You can't run that way too long, as you'll heat up quickly. But it gives you acceleration and the abiltiy to push hard (for a short time) when needed. But to get it your power supply needs to deliver a lot of current. How much? The value given by the stall current value.

Example: I've got some big servos that will continuously put out 300+ oz-in when powered by 60V and drawing a few amps. But they will deliver up to 1500 oz-in when fully loaded. My supply has to deliver a stall current of 45 amps though for me to get that. For one motor!

So, basically, you want to get enough current from a supply to meet the stall current rating. I could go on, but I need to get off line. Get a pencil and paper and rough out what one motor would require. Believe it or not, you don't need three times that for three motors-you are very unlikely to simultaneously stall all three. Three motors will need exactly the same voltage. How much more current hopefully someone else can tell you. I didn't go this route-I'm building with three separate supplies. Ugh! I really need to go. Maybe more later. Bye!

Lance

[jjwl89]

Hi Darren,

It looks like those servos are the same "cheap servos" that can be found here. The ideal power supply for those is 36V @ 20 to 25A. You could construct a power supply fairly simply with a transformer, capacitor, and a rectifier. The 250 CPR encoders you have would work well with G320 Gecko Drive. Depending on the size and torque you want your router to have, you may want to go with a 4:1 or 2:1 belt reduction between the motor and screw (If you are going to use a lead screw), however, it all depends on what you are going to be cutting and the size. Also, you may want to consider 80/20 Aluminum Framing for the construction of your router. Hope this helps.

[Darren]

Yes, those are the servos. I bought them from Jeff Davis (HomeCNC) a while back. He modified them to accept the US Digital encoders and a NEMA 23 style mounting plate. He also tested them with a Gecko 320 drive with good results.

Here are the specs:
Use a 36VDC power supply.
No-load RPM at 36VDC = 3,130
(Ke = 11.5, KRPM = 36V / Ke)
80% of no-load RPM = 2,500 RPM
Leadscrew RPM @ 120 IPM, 5TPI = 600RPM
Reduction Ratio = 2,500 RPM / 600 RPM = 4.1666:1, Use 4:1
Kt = 1.351 Ke = 15.5 in-oz / amp
Maximum continuous current = 3 Amps
Maximum continuous torque = 3A * Kt = 46.5 in-oz
Torque on leadscrew = 46.5 in-oz * 4:1 reduction = 186 in-oz
Leadscrew thrust = pi/8 * TPI * in-oz = 365 lbs
Maximum speed at full load = 2500RPM /(ratio * TPI) = 125 IPM
Maximum speed, no-load = 3,130RPM / (ratio * TPI) = 156 IPM
Power delivered to load = RPM * in-oz / 1351 = 86 Watts

A link to the servos: http://www.automec-direct.netfirms.c..._dc_motor.html

Here is a quote from Jeff in the other thread:

Yes, I finished my testing and I have the full specs on these motors.

They are the next size smaller than the ones I get new from Poly-sci. They are 360 oz/in peak motors at a peak of 20 amps. This means they have an 18 oz/in per amp rating.

Testing results:

Made a bar that had a hole 1" from the center and placed a wire hook there to attach to a bucket. The only thing I had to add for weight was 45 cal bullets. I had my Gecko drives current limited to around 10 amps. I kept adding weight to the bucket and lifting the bucket with the servo motor. I got to a point where the motor would just lift the weight past horizontal and then the drive would fault and the bar would fall down vertical. I weighed the bucket and it was close to 11 pounds. So it lifted 11 in/lbs with my current limited to around 10 amps.

This is very close to what my Poly-Sci specs say. If you do the math, the spec says 18 oz/in per amp. So 18 * 10 amp is 180 0z/in. Convert that to pounds would be 180 / 16 = 11.25 lbs. I'm a 1/4 pound off. Maybe I don't have the current limit screw set to exactly 10 amps

The perfect power supply for these would be a 36VDC supply @ 20 amps. You could leave the Gecko drives current limit screw all the way up to 20 amps. This way if you needed the current for one axis that is working hard it can take what ever is available from the power supply.

[Evodyne]

Darren,

Hey-looks like he layed it out pretty well for you. So if you went that route you know pretty much the screw and supply parameters. Roton has a nice site with screw information, you might take a peek. jjwl89 was right, the Geckodrive G320s should work fine. Mariss has some nice products and other members have told good stories about his support when they "break" one of his units.

Lance

[ViperTX]

I would start building and by the time to get to the electronics....you'll have more buck available....

[Darren]

Well, it isn't a matter of obtaining the money, I have a set budget I don't want to exceed. I'm hoping to gather the remaining parts for $500 or less. I don't want to start building until I have everything, I really want to sit down with all the parts and work my design from there.

I was hoping there might be a less expensive option than the Gecko drives. That would make a huge difference in price but I suppose I could keep an eye out for used Gecko's. I guess I also need to look into belt drive type mounting for the servos and screws.

[Darren]

I got this power supply schematic from JavaDog (JavaDog, let me know if you didn't want this posted and I'll remove it). He has the exact same servo's I bought from Jeff. I'm considering using this design for my power supply. I don't know much about designing power supplies but it looks like two of the Gecko's will be driven by one side of the power supply and a single by the other. I'm assuming the Z axis would be one that is doubled up because of the minimal load it would see most of the time.

Schematic

Can someone suggest a good place to find large caps like the ones called out in the diagram? I'm having trouble locating some that size. Also, could I stray a little either direction on size? How much?

Thanks for any info. I just got a bonus at work so I'm getting ready to purchase 3 Gecko G320's and need a power supply next. Then on to screws and reduction and mounting motors....

Darren

[Evodyne]

Darren,

The supply uses electrolytic capacitors. These capacitors have two ratings: a working voltage rating (given in volts) and a capacitance rating (given in microfarads). They also have a polarity-one terminal is the "positive" and the other is the "negative".

Your supply will output about 24 VAC * 1.414 or 34 VDC. A volt or two less actually due to diode voltage drops in the bridge rectifiers. The capacitors voltage rating MUST exceed the 34 volts. They can be rated much higher (i.e. a 70 volt cap. is fine), but it can not be rated lower. You should be able to use 36 or 40 volt capacitors. I'd shoot a little higher just to be on the safe side.

The overall value of capacitance adds up for capacitors in parallel. So each pair of 37,000 uF caps act as a single 74,000 uF capacitor. More capacitance is better, but you can actually use quite a bit less and do just fine-even a single 4,700 uF capacitor would work in a pinch. Larger ones give less AC ripple and store more energy that can be drawn on for a short time if needed.

One final note: DON'T wire the capacitors in backwards-i.e. the negaitive voltage to the positive capacitor terminal. Bad things (like a small explosion) can happen!

Have fun!

Lance

[ViperTX]

Darren, I use the Sprague electrolytics at mouser.com.

[Evodyne]

Darren,

Thought of this after I posted the last one...

Your motors give a peak torque of 360 oz-in. Your ke is 15.5 oz-in/Amp, so you peak amperage will be 23.2 Amps. You want the supply to be able to handle this at 34 volts. If it can't it will bog down under load and you won't see torque anywhere near the 360 oz-in rating.

The diode bridges are rated for 25 Amps. They will be fine-you wont be pulling this kind of current too often. You can always simply buy bridges with higher ratings. If you buy bridges, make sure their voltage rating (PIV or Peak Inverse Voltage rating) exceeds your 34 volts. A lot of times they are already rated for 400 or 600 volts, so no problem there.

Similarly the transformer and fuse current ratings should be bumped up as well. 34 VDC * 23 Amps is 782, say 800 Watts. For reference, 746 Watts = 1 HP.

Your transformer should then have a 24 volt secondary (the center tap isn't needed-you're not using it) that can handle 800 VA. Currently (no pun intended) it is set for 10A * 24 VAC or 240 VA.

On the primary side it would draw 800 VA / 110 VAC or 7.3 amps. The 10 Amp breaker/switch should be O.K.

The slo-blo fuses would need to have their trip rating bumped up. As they are now they would trip you out at 5A. At 15.5 oz-in/Amp the best you would get is about 78 oz-in of torque. Not quite 360 oz-in, huh?

I would want to be able to push the 360 oz-in of torque briefly, but blow a fuse if at that level for too long. Why? The motor is going to get very, very hot at this level of drive. Blow a fuse or a motor? So I think I'd try to get a slo-blo at or just under the 23 amp value.

Hope this makes sense.

Lance

[ViperTX]

Evodyne....I think you strayed on the servos...he claims they are only rated at 3 amps max.....

[Darren]

Lance,

Thanks for all the great information. It will take me a bit of time to digest what you said, I'll study the diagram and read your post more closely tonight and see if I understand exactly what you are saying. It makes sense to me initially but I will need to look it over more.

[Darren]

Ouch, that size cap is spendy... $30.84 each and I need 4 for this design. Anyone have suggestions for a less costly route? Sorry, I'm really quite green in this area. I can assemble it, just can't design it

http://www.mouser.com/index.cfm?hand..._pcodeid=75007

I found these for half the price:

http://www.mouser.com/index.cfm?hand..._pcodeid=75007

And these:
http://www.mouser.com/index.cfm?hand..._pcodeid=75007


Good idea? Bad? Seems close to what I need.

[Evodyne]

Evodyne....I think you strayed on the servos...he claims they are only rated at 3 amps max.....

ViperTX....

Hi! The 3.0 Amp rating is for continuous running-running all day without overheating. He specifies the torque rating at 3.0 amps at like 45 oz-in, which makes sense as his ke rating is 15.5 oz-in/Amp. This is a different spec. than peak torque. If you look back he quotes "Jeff", who has the same motors, as saying the peak torque and current are 360 oz-in and 20 some amps. That's peak or stall current, when the rotor is nearly locked. In this state the full 34 volts of the supply sees only the winding resistance of 1.7 ohms, giving 20 amps. The motor in this case is dissipating all the power from the supply (800 Watts) as heat. Good for pushing through a tough spot-just not for long. This is the advantage of servos over steppers-plenty of reserve torque when needed. You don't have to build a supply to operate at this level, but it defeats the purpose of buying 360 oz-in motors if you don't.

Regards,

Lance :cheers: