Stepper motor upgrade NEMA 34 or NEMA 42?

I have a nice old Bridgeport BOSS 5 CNC mill that I’ve converted to Gecko drives and Mach 3. Everything works fine, but the machine is not very nimble. I still have the stock old monster NEMA 42 stepper motors. With the acceleration rates that I have set for these motors I sometimes cannot achieve my target feed rates for example when cutting a helix. Several years ago at the CNC workshop I was advised by the guys at the Automation Technology booth that I should change the machine to smaller NEMA 34 motors as the new 34’s have more power than the old 42’s and they would accelerate much quicker. At the time they did not sell NEMA 42’s now they do. I was wondering if a modern new 34 is really better than a modern new 42. Yes the 42 has more rotational inertia, but it also has gobs more power to fight that. The price is irrelevant as the 42 is only a few bucks more and should be a direct fit where the 34 I’ll need to adapt it. Another issue is the Gecko 203V drivers only put out 7 Amps. The 42 only needs 6 amps, but the 34 needs 8.8 amps for max power. As I only have 7 amps to give it, the 34 motor will not be operating at full power.

Again the goal here is acceleration should I go with a NEMA 34 like this one?
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Or a NEMA 42 like this one?
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Or leave it alone?

The other concern is that with all of this extra power if I accidentally crash the machine will I seriously damage it? I don’t really need more power, just acceleration.
Thanks!

I'm currently in the processing of doing a retrofit on my Series 1 with stock Superior Steppers, however i'm using the Leadshine drivers. There has been several posts on this over the years that i've come across. Nema 34's will give you the faster acceleration and easier stopping, and are much more powerful than the stock Superiors. Those motors you linked are not a direct fit, the shaft lengths and sizes are different. A rigid shaft extension would be necessary with some sort of locking mechanism better than a keyshaft. A key with a set screw just won't hold up. The stock pulleys use a taper lock bushing adapter on an H series pulley. A direct replacement Nema 42 would be a KollMorgen stepper, which surely is considerably more.

On a friend's recommendation I changed the setting in Mach 3 to "CV Mode" and I doubled the speed on the helix. It still can't go more than 16 ipm on the Helix, but it's better than the 8 ipm I was getting on the helix. There is still a lot of room for improvement, but that was a free boost. I'm also considering changing to Servo motors instead of the steppers. I have a pair of Larken Viper 95 Servo drivers collecting dust from another project that went another direction. I would just need to buy motors and fit them.
Perhaps this motor?
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If you're going to go the stepper route, I think getting a factory matched motor and drive is a lot easier on you. Sometimes tuning a mis-matched motor/driver combo can be very trying. Look at Automationdirect for good prices on matched servos and drives.

We have some users for this kit, NEMA42 kit with USB connection

it works great

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Your fundamental problem is not the existing NEMA 42 motors, rather the driver (Gecko) which only runs off 80V DC max. I don't know how you have it wired now, but essentially you don't have the necessary voltage across the windings if they are put in series, or the necessary current across the windings if they are hooked in parallel to get full performance out of the motor. You may find a size 34 motor that is better matched to the voltage and current limits of the Gecko, but then you will still be effectively limited to about 225 watts of mechanical output considering typical stepper efficiencies. The most straight forward way is to get yourself some proper industrial drives (I use old superior electric SS2000D6 - leadshine offers modern varients) that run off a170 volt DC bus (rectified line current). They will be able to hit the windings of the stock motor with enough voltage to overcome the high inductance and keep up torque at higher speed. You need to wire it in series and so can run full current output (~6 amps) output, provided the driver is set back to idle currents of 1/2 that value. 10 microsteps is smooth, more does nothing useful, but don't expect greatly improved resolution as the step increments and torque output through the microstepping are not uniform enough to be very useful on the old stock motors. Your effective resolution will remain 0.0005" regardless. These 42's are very robust however, and the stock mechanical drive setup is excellent. Study some of my old posts, I've explained it in great detail previously to others. You'll be good for about 400 wats of mechanical power, generally enough. If you are skeptic, change the drive on one axis and compare, then talk.

Thanks BOSS5 for your reply. You are correct. I did already replace my motors with the new NEMA 42's while I picked up a bit of performance, it was probably not worth the effort and cost. While these are nice smooth quiet motors. They fit easily and are a good replacement option, they are not a big upgrade from the stock 42's. To fit them I just replaced the hubs on the taper lock timing gears with 3/4" bore. Perhaps in the future I'll change the drivers and up the voltage. Thanks