You can't compare torques of steppers with Servos at all.
750w servo 2:1 would be fine I would think, 1 Kw definitely. Have heard people saying 400w but I am sceptical about that small.
They will be much more expensive than steppers but once you have had good servos on a machine you will probably not want steppers again
I put 1.3Kw Allen Bradley MPL servos on my Series 1 CNC after I took the steppers off and it was probably way more than needed but I had them sitting there anyway.

Hood

Hood,
This part of what you wrote is critical "They will be much more expensive than steppers"  Everyone compares undersized steppers to  properly sized servos.  Spend the same money on steppers as servos and the results will be much different.  In fact a stepper closer to the ideal driver for machinery than a servo is. A steppers torque is maximized at the bottom end of the speed range, as if you had  a gear box reduction.  A servos torque is lowest at the slow speeds where most of the cutting is done, so you need bigger servos to drive the same load that the steppers would do.  Properly sized steppers do not lose position.  Stalls during cutting are simply a result of steppers too small.  My home built Minimill has 960 in/oz steppers, 80 volt drives with a 1500 watt 68 volt linear power supply and the performance is really very good. I have 18" travels in all directions and easily do 300 ipm even cutting.

Gary, servo torque is near constant over its speed range, whereas a steppers torque quickly drops of as the speed increases.
The major advantage is the servo performance should be reasonably constant at all speeds, whereas stepper performance will decrease as speed increases.
A well designed stepper system can be as good as a servo system, however it's still an open loop system, unless you use closed loop steppers, but then you're into a similar price range as servos. I priced both options for my knee mill, and from what I remember, a closed loop stepper system was about 80% the price of similar torque servos, and would limit rapid speeds.

Dave, if you were to convert the bridgeport to be capable of those speeds, I'd be factoring in some form of auto lube system while the machine was apart. I wouldn't be wanting to risk slides running dry at those speeds, and even a manual auto lube system will make it far easier to keep things lubed. Arc Euro sell the needed bits at reasonable prices, although I'm sure there are other sources.

Yep, its got a lube system already but it would get a total overhaul if i did go ahead.

I'm getting plenty telling me to get an old VMV etc, but my main worries are that they all seem to have small beds, all seem to need too much power and all seem heavy.

While some these may be good traits for a machine, for me they make it hard to use, expensive to run and impossible to move

something like this...
http://www.aliexpress.com/store/product/New-Servo-motor-and-driver-set-2-4N-M-0-75KW-3000RPM-90ST-AC-Servo-Motor/314742_760999511.html

No details on the drive though.

The hardest part is the Z axis on a BP, no perfect solution just lots of workable ideas it seems.

Not much info there at all, price is very good, maybe too good.
I would see if I could get a manual first to see what the specs and options of the drives are.

Regarding the Z you could do the knee but would need a big motor to push it about unless you can set up some sort of counterbalance. Some of the Bridgeport CNCs used an air assist which supposedly worked well.

I was toying with the idea of doing the knee on my Beaver NC5  as well as the quill, would have been relatively easy as it already has an induction  motor on it for raising and lowering. The  Knee would have been for tool length offsets and quill for normal Z.
Got the Chiron before I got round to doing it so it never happened.  I think I have only used the Beaver twice since I got the Chiron as they were too big for the Chiron.

Downside of a knee mill is lack of toolchanger, some have them such as the Beaver VC5 (I think) and also mc's Matchmaker has one but most don't and even on the ones that do you  have to be careful with tool length selection due to limited quill travel.
 Another downside is mess (coolant and chips) Much worse than when it is a manual machine as you tend to be working it harder and faster.
A friend of mine did an Avon knee mill recently, built an enclosure for it and it works and looks great, heres a link to a couple of pics.

http://www.machsupport.com/forum/index.php/topic,29886.0.html

Hood

Quote from: Hood on May 12, 2016, 08:57:30 AM

You can't compare torques of steppers with Servos at all.
750w servo 2:1 would be fine I would think, 1 Kw definitely. Have heard people saying 400w but I am sceptical about that small.
They will be much more expensive than steppers but once you have had good servos on a machine you will probably not want steppers again
I put 1.3Kw Allen Bradley MPL servos on my Series 1 CNC after I took the steppers off and it was probably way more than needed but I had them sitting there anyway.

Hood

Hood,
This part of what you wrote is critical "They will be much more expensive than steppers"  Everyone compares undersized steppers to  properly sized servos.  Spend the same money on steppers as servos and the results will be much different.  In fact a stepper closer to the ideal driver for machinery than a servo is. A steppers torque is maximized at the bottom end of the speed range, as if you had  a gear box reduction.  A servos torque is lowest at the slow speeds where most of the cutting is done, so you need bigger servos to drive the same load that the steppers would do.  Properly sized steppers do not lose position.  Stalls during cutting are simply a result of steppers too small.  My home built Minimill has 960 in/oz steppers, 80 volt drives with a 1500 watt 68 volt linear power supply and the performance is really very good. I have 18" travels in all directions and easily do 300 ipm even cutting.

Gary, here is the torque curve of the motors I have on my Beaver NC5, as you can see the constant torque is, well, constant   add to that the peak torque you have in reserve for acceleration etc which is almost 3 times the continuous.


My Beaver mill is basically just a bigger heavier version of a Bridgeport and I have 8m/min rapids (I limited to that as 20m/min was possible) with I think about 800 or 1000mm/s/s accel, there is no way a stepper could get anywhere near that on that type of machine

Hood

Thanks, still far from sure what to do really.

Lots have suggested getting a donor, seems there are a few to choose from especially Bridgeport BOSS' and Interacts in various models plus loads of others.

Chances of finding a good mechanics/duff controls donor probably slim though, that still leaves transport and rebuild space etc.

I can see ballscrews setting me back £1500 so getting a donor built with them fitted seems a good idea if less money.

Tough nut to crack i think.

You do seem to have the same problem as me. Workshop size and power supply are limiting factors.
If I had a high enough roof, and a proper 3 phase supply, I'd have a VMC.

I mostly agree with what others have said about it not being worth converting a manual machine. It's a lot of work, and as has been highlighted over on mycnc, there are knee style CNC machines about for reasonable money.
If I hadn't bought the Matchmaker aka a Shizouka ST-N knee mill that was built from new as a CNC, I would of probably converted something SX3 size, although I did buy a Denford Novamill as an interim machine, which is partly why the Matchmaker retro is still not complete. However I did get the last of the relay holders for the toolchanger pneumatics wired up last week, inbetween trying to get caught up with orders after losing two days getting my lathe turret fixed 

Dave, i have the ball screws and motors in place so i am fortunate, but as i read this thread, its Hoods post #2 page 1 that sticks out to me.  a 3 axis 1200 oz-in kit runs $900 USD, keep it simple & drive what you have, double that figure maybe to make mounts, get pulleys and cables, computer etc, you'd have a cnc bridgeport with backlash.  like those folk i spoke of with the great big machines (the one i spoke of was a planer mill) youd have a machine that could do a lot, but would have limitations.

so for $2000 or less USD, would you have a machine that you'd consider worth that amount?  would its size capabilities make it worth it over your more precise less powerful mini-mill?  if you built it and loved it but hated the restrictions, the flaws could go away one small project at a time, 1st the sloppy x-y acme screws (the z is less a problem, the weight of the knee cancels out the backlash, the knee is always in the most downward position, pulled by gravity.)  then maybe x-y servos, giving you the encoder display (tho most seem to think proper steppers will not skip)

there still seems to be some driven knee doubts.  i can only say mine went up and down for 11+ years, the only motor fail was due to coolant leaking into the motor.  the motor was the same size as the other 2, but the x-y could rapid at 150 ipm, the knee was at 100 ipm.  never a problem.  it does have a rolled ballscrew, but hand cranking it, seems to provide about the same resistance as the bridgeport at work, which is acme screwed but 1 size smaller.

and i must ask for some forgiveness.  in a previous post i made the claim of a 1/8" drill deep drilling some cast iron slides.  isnt true, i looked at the drills which i still have in the shop today.  the 1/8 holes were connector holes to the holes i actually deep drilled which were .218" dia.  big difference, i will attribute it the the 3 olives vodka with ocean spray blueberry drinks i was sipping on.  my bad.

Thanks all, yes its all down to space and power I think as primary concerns.

I would not convert the BP without going ballscrew - the screws are knackered and I would want the freedom to climb mill at will without worrying about backlash grabbing the bit, I tried it manually and its a bit scary

I have no idea what I will do yet, this is a growing dilemma - each answer brings more issues.

Probably the only machine will be a BOSS or Interact but did they come with ballscrews??

Yes, all CNC Bridgeports had ballscrews and most, if not all, had chromed ways. X travel on the series 1 is only about 18", series 2 was a fair bit more.

Hood