Hope this doesn't over flame your noobness - but.... can I ask a simple question? what is your rig designed to do? I ask because I'm thinking 1.5inch ballscrews 36inches long - 125 lb gantry - this is for serious high speed deep milling of carbon steel - yes?

I will be doing some high speed machineing on this machine center.  I am not sure how much though.  I plan to do mostly aluminums and woods, but am designing to be a sturdy workhorse capable of handling steel and stainless as well.  That is why I am going with a 7k rpm servo as a spindle motor to give me the option for rigid tapping and other options.  I will be using a 2" wide timing belt to run the drive shaft on a 2:1 ratio.  I have a air release cat 40 taper to automatically release and grab tools.  Also I am hopeing to get 10k to 12k rpm comfortably at the tool.  This machine center is in the "from scratch" stages.  I have all the machine frame designed, and most of the parts fabricated.  I have only ran into the problem of the electronic parts (what all pieces needed to drive servos), but thanks to some good responses here, I will be moving forward shortly.  I had to stop fabrication due to motor mounts, and the electrical enclosure needed.

The gantry weight of 125# is just the gantry FRAME, not the drives rails, motors, etc.  I expect it to come in around 160 - 180 pounds when complete.  That is why I was looking at 1/3 hp dc motors for the axis drives and retrofitting encoders to the output shaft or drive shafts, to turn them to servos.  I would prefer to ac motors and encode them to work as servos, but I have not found much information about ac drives.  There are a ton of them out there, but no good explanation of how they work, hookups, etc.  I prefer ac over dc for a hp and economincal reason, more power for less juice, and motors are far cheaper 1/3 the price.

I have the machine designed in SolidWorks, and this weekend I can generate some 3D models and post them if you like.  You would be able to open them in Internet Explorer and rotate in 3D measure, etc.  As a quick background note, I am the Engineering Manager at a medium sized fabrication plant.  We have just about every cnc fabrication tool you can think of, from plasma's, laser's, shear's, turret's, brake's, mill's, lathe's, and on , and on, and on.  I am good friends with the owner, so I have the benefit of using anything I need to build my machine (mainly cause he wants one too , which is a project for later)

My plan is thus:  I want to finish this machine and use the experience (and hopefully revenue) to build a larger 5'x10' machine mainly for wood routing, and extremely light machining on soft metals. I would like to incorporate a 4th axis and tool changer onto my bigger machine, but at the moment it is just wishful thinking. 

I'll make a couple of observations that may or may not help. You say you're happy to use steppers if you can use feedback/encoders because you're concerned about lost steps. Steppers only lose steps if they are under-spec'ed for the job or they are incorrectly driven. Adding encoders does absolutely nothing to change this. If motor x loses steps without encoders - motor x will lose steps with encoders.

But guess what - an under-spec'ed servo will also lose steps/position regardless of the fact it has encoders. Conclusion - encoders will do nothing whatsoever to stop either motor from losing position if it isn't powerful enough for the application.

Your spindle motor.... 1/3HP - 7000 revs - I think you need to reconsider this. Considering the massive construction of your machine this seems to be waaaaaaay too puny - 1/3HP is ~250Watts - that's about dremel power!!! you'll be lucky to cut plastic. 7000rpm - not for wood at any sort of feedrate - you'll need way more.

Just my 2c

Please forgive me if I am wrong, I thought that in the closed loop system, ie. an encoded motor, if it were to lose steps, then encoder would provide the feedback, and the system could do error correction to get back into position.

In regards to your concern about the spindle servo, you are correct, I had not thought about that.  I was sizing everything to run off the standard off the shelf gecko drive, but I guess I will need to seriously upgrade the spindle servo.  I will have to do some research to see what a good size spindle servo would be.

Please forgive me if I am wrong, I thought that in the closed loop system, ie. an encoded motor, if it were to lose steps, then encoder would provide the feedback, and the system could do error correction to get back into position.

OK a fair thought - and a very common one. Let's try to answer by asking a couple of questions...

Question: Why does a motor (stepper or servo) lose steps/position?

Answer: Because it doesn't have sufficient power to overcome the resistance it's being subjected to.

Question: So how many steps does it lose?

Answer: all of them for ever and a day. (or maybe it vibrates the obstruction sufficiently to eventually get going again - this is typical for example of a "sticky" leadscrew)

Here's a pretty good read: http://www.torchmate.com/motors/electronics_selection.htm

Very, very interesting article.  I was under the impression that steppers were notorious for losing steps on a milling application (from an article I read somewhere).  That is why I was heavily leaning towards a servo application.  1200 - 1500 oz steppers would be a more economical situation.  I was not being smart or crass in my last response, I am working hard to try to understand the electronics part of the motors and their drives.

http://www.anaheimautomation.com/manuals/L010165%20-%2034Y%20Series%20Spec%20Sheet.pdf

That motor for instance is only approx $175 new. (1700 oz one)

Hey - no problem. If I had a quid for everyone that thinks steppers lose the occasional step here and there I'd be a rich man. The question to ask is why would they do that? Maybe they loose concentration and forget what they're doing or something

Seriously though - I'm not dissing servos - far from it. For a start I don't know an awful lot about them - but I do know steppers, and I know there's lots of cr*p talked about them. Steppers generally don't live up to expectations because they're either not sized or driven properly or both. As a rule of thumb (FWIW) I use this: if the application demands more power than you can get from a (properly driven and properly sized) stepper then use a servo. But first - know the power you're going to need. So far I've never had the need for a servo.

Cheers

Ian

1700 oz would be Kong size to me . My steppers are only around 700-800 oz-in, and the first thing to wimp out is my weak porter cable router. I weigh in at 200 lbs, and I can't stop my gantry pushing against it with all my weight, and thats at a speed thats faster than my cutting speed. Steppers have more power the slower the speed, so If I were to really slow it down and run in reverse, I think the earth would start to slow down. As Stirling stated, a properly driven system is where all your power potential comes in to play. For instance, I can grab the ball screws or motor shaft and make the motor loose steps with little effort. It would be impossible for me to make the motor loose steps by hand from putting pressure against the ball nut on that same screw (or whatever the ball nut is attached to. Spindle, gantry, etc...) Same goes for my belt driven axis. By the time the power is transfered through the different pulleys, lost steps are of little concern. Most likely you won't need your machine to go 1000 inches/minute, so a properly built machine matched to do what you NEED it to do is fairly easy, even with small steppers. However, if somebody was to say, "hey, I'll set your machine up with this nice servo setup in trade for your steppers" I would gladly swap them. 1700 oz/inch?? Yeah, thats pretty big looking through my eyes. Wish mine were that powerful, just for the bragging rights. 

I would rather spend the extra money on the front end and get more than I think I would ever need, than to reflect back, and wish I had got more.  I have never heard anyone complain their car has too much horsepower   !!  I want to only by at my upper limit using 75% of available power.  I have to seriously upgrade my spindle.  I hadn't given it much thought, until Stirling mentioned how puny it was (my manhood was threatened   )   Anyways, my project goal is to be able to comfortably run a 1" (25.4 mm) 4 flute endmill through carbon steel at a reasonable clip.

I am having serious fun with this project, and really can't wait to complete it!