What size servo/stepper motors for Bridgeport?

[Ryobiguy]

Hello,
I've got a 1969 Series I J-head machine that's got ballscrews and used to be CNC. The old servos were not part of the deal (nor the refrigerator sized NC controller!) So right now it's a manual machine while I learn how milling works, but I want to get it back to CNC.

I understand enough about steppers that there's no problem piecing together a system, as it's pretty straightforward. Would steppers rated 900-1200 oz/in be good enough to drive the X/Y on 9x42 table with .200" lead ballscrews? I've looked at the motor torque curves and figured out the max power zone for various feed rates (ie, 900 oz-in model has lower inductance and does almost as well as 1200oz-in model, except at low speeds,) but I don't know how much torque is required.

But if I can afford it, I'd rather go with servo motors.
I see places like Keling have inexpensive DC brush servos (which seem easy to setup,) but I'm not sure if their largest DC servo is going to be powerful enough. It says 1125 peak oz/in @ 40Amps, but the Gecko drives they sell only do 20 Amps, so presumably the peak torque would be cut in half. Constant torque is listed at 226 oz/in, not sure if that's enough. I suppose servos would require belt gear-down to get the servos going faster into their high-power zone.

Then there are brushless/AC servo motors. I see these on E-Bay all the time, but the drives either require 3-phase 220, or have a serial interface rather than a simple step/direction interface that I can patch right into my parallel port. Another big turnoff is the AC servos I see on e-bay also have some connectors which are probably expensive and/or hard to find.
The drives usually have some funny connectors with some weird pinout with all these pin functions that are not clear to me. Some have encoder inputs with a whole lot of pins - are those for absolute/grey-code encoders? Probably I'd just need to dig into the tech manuals for each drive and figure it out - I've read through some, but they seem to assume that you know the basics, ie, they show just a pinout, not a thorough explanation of each signal. What do most of these drives need?
(BTW - I'm patient enough to wait for affordable parts to show up on e-bay, I'm willing to get an o-scope for PID tuning if needed, and given enough information I'm technically savvy enough to eventually figure out if a drive is well suited for a particular motor.)

I don't need insane feed rates, but I don't want to buy a whole lot of stuff and spend the time/effort hooking it up just to be disappointed with an underpowered setup.
I'd rather have servos that go all-out, instead of steppers which have to be conservatively held back so they have a huge safety margin on their non-linear torque curves, vibrate a lot, and potentially lose position.

So in each category (stepper, DC servo, AC servo,) what would be the recommended motor sizes, or power/torque ratings for a 9x42 bridgeport with ballscrews to get decent enough power/speeds to not be considered grossly underpowered?

-Matt

[HimyKabibble]

Matt,

I recently completed a conversion on my 9x49 BP clone. I used 850 oz-in servos with 500 line encoders from www.homeshopcnc.com. I have 2.5:1 GT2 belt reducers of my own design on X and Y, with 4-pitch ballscrews. I can get well over 330 IPM rapids (I haven't explored the true limits, but I'm sure it's over 400 IPM), and have enough power to snap off a 1/2" endmill without losing position. I CNC'd the knee with the same motor, a 4.8:1 XL belt reducer (same design as X/Y, jsut different pulleys and belt), retaining the 2:1 bevel gears, 5-pitch leadscrew, and added two 200# gas springs to carry some of the weight of the knee. This gives me 70 IPM rapids on Z. Drives are all Gecko G320s running on a 70V supply.
Keep in mind, with servos on a machine of this size and power, you NEED good limit switches to prevent a potential dangerous and expensive crash in the event of an encoder or servo driver failure.

Regards,
Ray L.

[CarbideBob]

It all depends on the gear (or belt) reduction.
You can pull a freight train with a washing machine motor if you gear it down enough.

You may want to decide on a controller first. Mach is an open loop system so it plays best with steppers.

Servos will cost you more and be harder to set up but provide better performance and speed.

When looking at servos look at continuous torque ratings. When looking at steppers look at the torque available at your highest running speed.
All the other numbers (ie: peak torque, holding torque) are worthless advertising specs.
Bob

[HimyKabibble]

Originally Posted by CarbideBob It all depends on the gear (or belt) reduction. You can pull a freight train with a washing machine motor if you gear it down enough. You may want to decide on a controller first. Mach is an open loop system so it plays best with steppers. Servos will cost you more and be harder to set up but provide better performance and speed. When looking at servos look at continuous torque ratings. When looking at steppers look at the torque available at your highest running speed. All the other numbers (ie: peak torque, holding torque) are worthless advertising specs. Bob

"Mach is an open loop system so it plays best with steppers." - That is simply not true. The interface between Mach and the motors is absolutely identical (Step/Dir) whether you're using steppers or servos, and Mach3 works identically with either.

"Servos will cost you more and be harder to set up" - Also no necessarily true. Servo prices have dropped precipitously in recent years, and the cost increment is now very small in this size range. Compared to the overal cost of a decent CNC conversion, it's lost in the noise. Setup is also not particularly more difficult, if at all. It's just different. And with servos, you have no problems with lost steps, and having to "derate" acceleration and top speed to avoid losing steps due to torque loss or mid-band resonance.

"All the other numbers (ie: peak torque, holding torque) are worthless advertising specs." - Peak torque is absolutely useful for determining what kind of peak forces you can generate when cutting. Likewise holding torque is important for determining how much force it will take to push a stopped axis off-position. If it's too low, cutting forces will push an axis off-position. The torque available at your maximum speed is often *not* a limiting factor, especially on a small machine.

Regards,
Ray L.

[cncnovice2]

Hi, Matt. I've got a bridgeport type bed mill (9"x42" table) with the head on the collumn. 5 pitch ballscrews, and 1:2 beltdrive reduction. I'm using 1200 Oz.In. Steppers on x and y. They easily do anything I need at 40 ipm feedrates. I'm sure they will go faster without losing any steps, but I wouldn't like to get clobberedby the cranks! The Z axis is the whole head and the ways are very tight. The 1780oz.in. motor is reliable at up to 30 ipm but it will stall at 35. I'm thinking of converting it to Servo with a Larken Viper 200 and a large motor. The power supply I'm using is a 800VA transformer which gives 70V. I'm using Gecko 212 drives and like them a lot! I hope this helps. Don

[glengeniii]

Hi,

I have a late 80's Bridgeport BOSS Series 1 that I retrofitted. I was real nervous about retro-fitting with steppers but I couldn't afford a servo system. I would like to say that I am very happy with the stepper set-up I used. I can maintain 150ipm rapids on all three axises, and have only ever lost steps on the z axis when doing heavy drilling operations. This machine is used daily and probably has over 1000 hrs. of operation on it.

Here is my retrofit setup: (I have CAD diagrams and pictures of how I wired and setup everything If anyone wants them, just email me at glengeniii@live.com)

~3 NEMA 34 1380 oz/in 6amp/phase 4 wire steppers for $175 off ebay. (I used shafts and couplers from Misumi and made adapter plates to mount the NEMA 34 bolt pattern to the bridgeports NEMA 42 mounts. I have the CAD data if you wanted.)

~3 Gecko 202 stepper drives $134 ea.

~PMDX-131 breakout / motherboard combo $183 (worth every red cent, if you use Gecko drives buy this board, it simplifies everything.)

~2 220vac/110vac 48vac/24vac 1000watt toroidal transformers wired for 48vac 2000watt output $147 ea. from Allied. (There are better deals out there.)

~PMDX-135-8020 Power preparation module $119 (powered with the two transformers above generates about 68-72vdc.)

~Original Bridgeport double pole anti-plugging switch with overload relay setup.

~Original Bridgeport 1:1 (yes, it's has 1:1 not 1:2) pulley ratio, and 5 pitch ball screws.

~Mach Motion CNC monitor enclosure with control panel $1395 (This was a later edition, it's a good investment but is somewhat costly.)

~Mach 3 cnc control software $159

Here is a little FYI that might be useful:

I tried using the factory Sigma and Superior NEMA 42 steppers, but they would not work properly. I tried wiring them every way possible, but could not get them to generate decent holding/turning torque. I don't know if it was because you can only get 7amps from the Gecko's, (there rated at 8.9 amps/phase) or if all three motors were bad. I also don't think if the motors were good, that 150ipm rapids would be possible with NEMA 42's using Gecko's because of mid-band instability problems, but I could be wrong.

I retained the stock 1:1 timing pulley ratio. (A lot of these machines came with 1:2 ratios.) Almost everyone that retrofits these machines using Gecko drives claim they can only achieve 60ipm rapids before losing steps. I maintain 150ipm rapids and believe it is because of the 1:1 drive ratio, and that I swapped the factory steppers for smaller NEMA 34 drives that the Gecko's are designed to handle. Another note definitely worth mentioning is that with the 1:1 drive ratio I can maintain higher cutting feed rates than with the 2:1 drive ratio. The reason is because a steppers torque output is exponentially dependent and inversely proportional to the motors rpm. It is a common misconception that an increase in drive ratios will always increase a motor's performance at specific feed rates. Sometimes it will, sometimes it won't it depends on the motor. With my set up, a 2:1 drive ratio would only produce 28% of the force a 1:1 ratio would produce at 40ipm. Optimal would be 1.11 ratio which would give about 13% more force. I wanted to increase my machines performance on the z-axis but 13% isn't worth machining new pulleys.

I would also like to add that both PMDX and Geckodrives produce inexpensive high qualities products, and are outstanding companies that offer excellent customer support.

[svenakela]

Great info glengeniii. I would like to see some images, could you add them to the thread?

[petextr]

Welcome all , i currently looking for some cnc kit to retrofit my bridgeport interact ,and i found 2 kits one with servo motors High-Torque Stepper Motor, Stepper Motor, Driver, Stepper Motor kit, DC Servo Motor, DC Servo Motor kit, Stepper Motor Power Supply, CNC Router, Spindle, and other Components. Automation Technology Inc
and one with stepper motors
High-Torque Stepper Motor, Stepper Motor, Driver, Stepper Motor kit, DC Servo Motor, DC Servo Motor kit, Stepper Motor Power Supply, CNC Router, Spindle, and other Components. Automation Technology Inc
But i still not sure with are better , might anybody can recommended any other kits , i want run my bridgeport by mach 3 software because old haidenhain tnc 150 is not very reliable can't feed program directly from computer and have not enough memory for my programs ,and is too old if some go wrong with old electronics , so i very interesting to convert to new servo or steppers ,also i have for sale old control tnc 150 ,all 3 servo motors sem and all electric from cabinet.

[brian257]

I am about to convert my BP also. I have been doing a lot of looking around and was very surprised to find that you can now get servo systems for the same price or less as a stepper system. Best I have found so far is from DMM at DMM Technology Corp., but just now also looking at the Automation Technologies servos. The DMM 900W motors are 1019 In Oz peak and 410 continuous when you convert from their Nm that they rate them in and 1,080 RPM. That will give 216 IPM on the X and Y if run direct drive. I was figuring that gives me a little over 700 pounds of force on the X and Y and 1,400 on the Z. I think that will do anything I ever need.

I will probably do a 1:1 pulley system just so I can tuck them under the table and not have such long appendeges sticking out. I would also like to retain the original handwheels since I will still do a good bit of manual machining.

[petextr]

what you think about nema 34 1805 oz hybrid steppers and micro stepping drives and run them on 11o v i think is strong enough to run BP

[mactec54]

Ryobiguy

If you are looking / wanting AC servos DMM have what you need, with very easy wiring & connection to Mach

DMM Technology Corp.

[lgalla]

petextr Nema 34,1805 steppers are a bad choice.They are too big.Large steppers are poor performers at RPM.A smaller servo would run circles around that stepper.Sorry I am not a very high tec guy but steppers over 640oz are a poor choice.Perhaps the tec guys will chime in and explain what I am refering to
Larry