G'Day Friends
I am searching for suggestions on my next build/conversion. I have a Bridgeport knee mill that I bought solely for this purpose that has being sitting on ice. I am now slowly getting thoughts together to purchase parts for the conversion.
I'm not really a newby, I built a router from the ground up and converted a lathe, both are Mach controlled and stepper run with gecko's as the drives, actually the router runs twin steppers on the X and Y, but its a big machine.

Now back to the mill.
I'm tossing up ideas, thoughts and parts. I definitely will be changing all lead screws to ball screws, preferable ground If I can get them at the right price. Motors and drives I'm still not sure.

On a mill of this size would you go Steppers or Servo,s?
Has anyone had any experience with the AC Stepper drivers advertised?
http://www.ebay.com.au/itm/CNC-Mill-High-Speed-Torque-Stepper-Motor-Driver-Controller-AC80-250V-8A-400KHz-/321170854171?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item4ac746511b
If we go steppers would it be better to go one bigger motor than two smaller on the same axis?
I have read a bit about stepper encoders, or is this just a waste of time?

Thanks for reading looking forward to your thoughts.

Cheers
Katoh

Personally I would use Servos but they are the more expensive option.
Steppers will work well just you will not get the same Velocity or Acceleration that you could expect from servos.
I did have steppers on my Bridgeport for a long time and it worked well, just a bit slow. They were 916 ozin and geared 2:1.

Regarding steppers with encoders, I think that is a waste of time.

Hood

Thanks Hood
I was thinking servo's, but I have never used them before. This could be a new experience.
With servo's you need the motor and controller as unit or do you look for motors and use something like a Gecko G340?
Looks like a lot of study coming up!

Cheers
Katoh

There are two (well actually 3) types of servos normally used with CNC machines.
Ac
DC Brushless
DC Brushed.
Of the 3 the DC Brushed is the easiest to mix and match drives and motors.
However AC will in my opinion get you the best performance.
It all depends on your budget though as DC will be cheaper, probably not much more expensive than steppers.
There are some nice second hand AC servos about however which can make them closer to DC servo prices, still not cheap however.

Servos need tuning which can be a bit daunting for people but a lot of the modern AC Servo drives do an excellent job of Auto Tuning and some even do constant tuning which means you never have to tune them and the drives automatically tune them constantly as they are running.

Hood

I've being looking at servo's on ebay, must say its getting a bit daunting.
I was under the impression AC servos are the best but you need to have matching drives and the drives need to be able to accept step and direction.
Looking at the DC motors and drives I can't seem to work out how much power I need. I know with steppers on the mill I would have started with 1200oz/inch motors as a min and worked up from there. but I can not make a comparison to servo's. What I thought would be large enough only seem to have 1/4 the torque and the prices are just way high.
Maybe I'm not reading this right.
Thanks
Katoh

Servos and steppers are two very different animals. Steppers produce their max torque at stanstill (Holding Torque) and it drops away with speed, quite rapidly after a few hundred RPM usually. A servo on the other hand normally is rated for constant torque and peak torque. The Constant torque is what the motor can produce constantly from zero RPM right up to Max rated RPM. In addition Servos have a big overhead for short periods, usually 2 to 3 times the constant torque.
 For  a Bridgeport sized machine I would think 750Watt AC servos would be more than adequate at 2:1 reduction, some people reckon even 400W would suffice although I am not so sure about that, suppose it depends what you will be doing and cutting  and what accel and Vel you are wanting.
 There are currently Samsung 750W drives, motors and cables on eBay, they are second hand obviously and they are asking, if I recall correctly, about $1800 for the three sets and 400W ones for about $900.

Hood

Thanks Hood

Again you have enlightened me. I do like the Ac servo's, and the Samsung ones do look mighty attractive, but with a price tag to suite. This definitely gives me a starting point to know what to look for.
I was also looking at those whopping big steppers, size 42's 4000 odd Oz/Inch beasts that you run with the big AC stepper drives, not as expensive as servos but still when you put 3 of them together and the drives and freight the servos really aren't that much more.
Well Ill have to start putty my penny's away, and keep an eye out for a bargain.

Many Thanks
Katoh

Servos are expensive compared to steppers but once you have used them, especially AC ones, then you will know the extra cost was worth it
Hood

In addition Servos have a big overhead for short periods, usually 2 to 3 times the constant torque.
 For  a Bridgeport sized machine I would think 750Watt AC servos would be more than adequate at 2:1 reduction, some people reckon even 400W would suffice although I am not so sure about that, suppose it depends what you will be doing and cutting  and what accel and Vel you are wanting.
 Hood

These days it is difficult to compare machines by 'size' so I suggest focusing on the weight (mass) of the load and not the size. For example, I just finished a servo conversion on the largest asian bench mill, namely the Industrial Hobbies mill with the 'oversize' table. While the table is close in length and width to the a bridgeport table, it is only half the weight. Another difference betewwn servos and steppers is that you can oversize a stepper without negative consequences. Oversizing a servo can result in all sorts of problems including making the system un-tunable, defeating built in auto tuining and/or having the motors break into uncontrolled self exciting harmonics (given an elastic connection to the load).

The IH conversion has 400 watt motors moving the 150lb table. An actual Bridgeport table is over 300lbs and, as you suggested, would require the 750watt motor to achieve similar performance.

On the IH conversion, the head was scratch built and weighed in at over 300lbs so the Z axis in fact has a 750 watt motor.

Mitsubishi publishes a software specifically for sizing their AC servo motors and that is what I use to spec the motor size for a particular application. It is a bit complicated and requires a lot of technical details about the application including things like the coeficient of friction of the sliding members and stuff like that, but one could also use a 'rule of thumb' approach and probably not get into too much trouble.  If one accepts that 400 watt is good for a 150lb table (with dovetails) and 750 watt is good for a 300 lb load (on ball slides), then it might be reasonable to extrapolate a rule of thumb like 1.25 to 1.35 watts per lb. of total mass to be moved (for a machine tool application).

Note that for calculating the horizontal movements of the table, the weight of the table (but not the mass) can be discounted. A 'live load' however shoudl be included since this is the actual typical operating condition. You might consider 1x the table weight as a reasonable WAG. So in our examples, the IH table would carry a 150lb load for a total mass of 300lbs to be moved. 400 watts / 300lbs = 1.33 watts per lb.  Similarly the 300lbs bridgeport table carries a 300 lbs live load (mill vice, 4th axis, fixtures, workpiece, etc) for a toal weight of 600lbs. Therefor 750watts /600lbs = 1.25 watts per lb.

The Z axis is a bit different in that ther is no live load . . other than the head itself . . which has 'weight' to be lifted in addition to the mass to be accellerated. So if the head is fully counterbalanced (air springs, aka struts, or equiv) Then just use the all up weight of the head as the load to be moved. If the head is not cojnterbalanced, then add the 'weight' of the head to the mass of the head and use that.

DISCLAIMER;   the proper calculation method for motor sizing are complicated and the 'rule of thunb' described above is not presented as the correct nor the preferred, nor accurate method to calculate servo motor size. It is a theoretical context without warranty express or implied  . . . use at your own risk . . .  batteries not included. 


   

Oversizing a servo can result in all sorts of problems including making the system un-tunable, defeating built in auto tuining and/or having the motors break into uncontrolled self exciting harmonics (given an elastic connection to the load).

Interesting, I found the exact opposite on my Bridgeport when I swapped it over to servos. I had some 1.5Kw motors and they were extremely easy to tune, in fact they were the first ones I have had any real success with the Auto Tuning in the Allen Bradley drives. Other setups (Computurn Lathe and Beaver Mill) the auto tuning has been only partially successful and needed a lot of tweaking to get to my liking.
Hood