View Full Version : Servo/stepper size for Bridgeport w/ballscrews ?

11-30-2006, 12:02 AM
What size servo/stepper is required to run the X-Y axis on a Bridgeport type machine with ballscrews. I believe the table is 9X46 and the spindle is 2 hp. I have searched and have not found any general recommendations. I would like to go with servos but cost has to be taken into consideration. I will use Geckos for which ever I decide.

Any input is appreciated.

11-30-2006, 03:16 AM
I used 916 oz/in steppers at 2:1 reduction on the manual Bridgeport I converted to CNC and then used the same motors at 2.5:1 on my Interact retrofit. Both work great with no problem, I get 2500mm/min on the Interact, the manual mill was a bit faster as it had less reduction.

11-30-2006, 04:19 AM
what about the size for servos? I found pulleys today for 1:1, 2:1, or 3:1 but I really don't want to go any further than that if at all possible, and I would like to drive the knee.......the X & Y, I have a pretty good idea, but what will it take to drive that knee? Also, mine is a little heavier with a 10x54 table

NC Cams
11-30-2006, 09:19 AM
Any REASONABLY sized motor witll RUN the machine axis when you DIY retrofit them - thus, a SWAG motor selection could end up to be quite adequate.

HOWEVER, when it comes to fast rapids and abruptly quick accelerations, you need to consider how fast you ultimately want to move it in order to properly engineer the motor sizing and/or the gearing needed to connect them.

A more scientific way to SWAG the motor size is to match the motor sizing already being used on a machine with comparable performance that you're trying to achieve.

As far as Bridgeports go, the classic mill was fitted with fairly healthy servo motors when they did CNC variants that became Eztraks and/or V2XT's - I have one of each but don't know the motor sizes off hand. Can't speak for the stepper variants used on the machines that preceded them.

The following thread may also be of assisance to approximate motor size calculations:


Specifically post #15

12-07-2006, 08:18 PM
Does an adapter plate need to be made for a BP Series1 Boss5 if I want to mount modern servos or is there a "bolt-on" option? I have read about needing a 5/8" shaft and that a Nema 42 is probably the best fit but not quite clear on whether this is a simple matter of taking off the 'old' and installing the 'new'.

Dennis Storzek
12-08-2006, 12:36 PM
Bridgeport supplied 29 in./lbs. steppers with the Series I Boss machines, which have 2HP spindle motors. Centroid supplies 29 in./lbs. servos with their conversion kits, with 40 in./lbs. as an option. I'll leave the Imperial to metric conversion to you.


12-08-2006, 12:49 PM
I am not talking about power. I was asking about the actual mounting plate sizes on the housings matching/bolting to the mounts on the mill.

12-08-2006, 01:11 PM

These are the 29 and 40 in./lbs. motors mentioned above and yes they are nema 42. I have bought them from Ajax to use on other equipment also. They are made by a well known English servo manufacturer.


12-08-2006, 01:34 PM
So you are saying "yes" these will bolt right onto a BP Series I CNC mill with no mounting plate needed? Just unbolt the old and bolt on the new?

12-08-2006, 01:54 PM
The boss 5 bridgeport with a ajax retrofit works great, the only thing you have to be prepared for is to get the keways on the timing belt pulleys that fit on the motors need to be opened up to 3/16" key as the origonal is a 3/32" woodroffe key. Also the motor mount on the Z axis needs a 1/8" spacer plate on the face as the bore in the casting is not big enough bore to accept the motor register boss. Apart from these points which are not mentioned in the instructions it is a relatively simple upgrade.

I found this post.


12-08-2006, 02:01 PM
Thanks that really helps. So I am guessing from that the bolt patterns are the same. Does anyone know for sure?

12-08-2006, 02:15 PM
This should verify it.


12-10-2006, 12:33 PM
For those interested, I found the found a site for English to metric conversions. Try the following link.


12-10-2006, 01:15 PM
Thanks that really helps. So I am guessing from that the bolt patterns are the same. Does anyone know for sure?

Yes the 42 size motors are a standard flange dimension but like I said in a previous post if your motors have a .125 deep mounting bosswhich is larger in diameter than the bore in the casting for mounting the Z axis drive then I had to fit a spacer plate in between motor and mounting face and the face of the motors. The pulleys on the origonal boss stepper motors had a 3/32" wide woodruff key in them and like the AJAX servo motors all had 3/16" wide feather key which means you either get the keyways in the pulleys opened out to 3/16" or step the keys from 3/16" to 3/32" I did latter on mine as I could'nt at the time get access to a 3/16" keyway broach and I've had no problems in two years use, if using servo's be sure to follow the instructions from the motor supplier regarding care not to put to much end pressure ( knocking them on with a hammer) on the shafts which could damage the encoders if proper fitted they should slide on smoothly and be locked onto the shaft with the taper locks on the X & Y axis and the 1/4" inch grub screw on the Z axis, hope this info clarifies the queries Cheers Colin

12-11-2006, 03:28 PM
I love it when a community comes together! Thanks for all the insight.

NC Cams
12-11-2006, 04:55 PM
Another tid-bit for you with respect to (WRT) motor sizing:

The Bridgeport Eztrak 2D/3D CNC mill used 3100rpm, 19lb-in motors on the various axis.

The Bridgeport V2XT variant of a mill based VMC 3D CNC used 4000rpm, 26lb-in motors on the various axis.

BOTH motors used 2:1 timing belt drives between the motors and ball screws. Again, English-to-metric conversions are left up to the final user...

04-13-2007, 08:33 AM
NC Cams,

the torque values you gave for the Bridgeport servos are they the rated torque or the max torque ?



NC Cams
04-13-2007, 10:15 AM
The values are off of the labels on the motors and it does not indicate if it is rated or stall/max torque.

YOu could look on/at the SEM servo website for more info on the SEM servo motors which is what Bridgeport used.

04-13-2007, 11:03 AM
I know this thread was recently brought back from the grave, but for what it's worth -

For the X and Y - For X and Y I used size 42 steppers through a 1.5:1 ratio with 5TPI ballscrews on my BP retrofit and I can reach 135+ IPM using 48 volts (I have never measured the actual Amps) to the steppers, but I de-tuned the motors to achieve around 90 IPM to guarantee there were no lost steps during acceleration and rapids. Even with the gib locks snugged down, this arrangement is powerful enough to move the table and saddle. If the gib locks are really cranked down, the motors will stall out. 90 IPM is fast enough for me for the table and saddle. I am not 100% sure, but I believe the steppers I used are around 800 - 900 OzIn and again, I went through a 1.5:1 reduction.

For Z axis - I chose to drive the quill through the original, manual handle axle. There is a fair amount of backlash but I have found ways to get around it. I am using a small, NEMA 23 stepper (276 OzIn I believe) on the quill drive through two, 6:1 reductions - one on each side of the head to provide a resultant 36:1 drive to the original quill handle which has a slightly defeating ratio. This screwed up arrangement proved to be ridiculously fast - I had to de-tune that also. It is now more than fast enough for peck drilling and such, but it does not have enough power to push a 3/8" drill through aluminum at a normal feedrate. I am likely going to replace the small stepper or increase the reduction further. In fact, I am headed to Automation Direct right now!

I think driving the knee is not a really good idea for a Z axis on a BP retrofit. The knee was not really intended for that purpose - that's why the quill has 5" of stroke. I evaluated it when I was designing my retrofit and eventaully decided against it - even counterweighted, it is cumbersome. Driving the quill, in the opinion of many, is a better solution and there are several ways to accomplish this.