Thread: Bridgeport Mill Spindle Bearing Max speed?

I have just finished doing several upgrades to my mid 70s Bridgeport J Head. One of the upgrades was a VFD, I set the step pully on the max speed of 2750 RPM?... I think thats right, not right in front of me. With this being said My VFD can run up to 90+ Hz putting my spindle speed @ aproximatly 33% faster than the max step pully listed speed. At the moment I only have the VFD set at a max of the standard 60 Hz as to not exceed the 2750 RPM. I ran the mill off and on for about 1/2 a day yesterday and toward the end of the day I noticed the quill was getting kinda stiff when retracted all the way to the top. I didnt notice anything getting hot or even warm but at the same time I wasnt really looking for it either. Today I was using the machine again and the quill was smooth and easy to move as ussual. Also FYI, There is no Spindle or bearing noise and The head and spindle was just oiled before using it this go round.

With all that said my questions are:
1) Could the stiff quill be due to bearings getting hot or worn?
2) With the VFD capable of spining the spindle faster than the max listed speed. What is the max speed the spindle bearings can handle?
3) If the someone thinks it isnt the spindle bearings causing the stiff quill, what do you thing could be another cause for this?

Thanks for your thoughts and advice,
Joe

ten yearrs ago, i put a vfd on a milltronic cnc to replace all of the varispeed pulleys etc for a timing belt and cogged pulleys. I called milltronic to get their recommendations on max speeds. The serial numbers told them that the max was supposed to be 3800rpm and thqat should only be run for 10 minutes or less. The newer ( at that time) spindles were rated at 4200. They were identical build and bearings.
WE finally set the spindle max speed at 6600 rpm. It ran most of the last ten years cutting graphite with 1/8 inch ball nose mills. It ran most times at the 5k area.
We recently changed purposes for this machine so we changed out pulleys for more torque. Max speed is now 2500.
It ran as cool at the higher speeds as it is running now.
If you have no real need for those high speeds, leave the parameters set so you have the normal original range of speeds. If you someday find a need for too much rpm. change the parameter to allow the higher frequecy.
If the motor is in good shape, it should be able to sustain twice the normal rpm or 3450. Anything over that and you have to expect the motor to fly apart , at the most inopportune moment.
ean out as much as you can
The stiff quill i dont think would be spindle bearings. gummede up bore in the head, gunk in the rack on he handle shaft and clthe back of the quill, stuck quill clamp would be my first guess. Lots of 0 or 10 weight oil on the quill OD.

The bearings can handle 10K + easy.. With proper lubrication.... Hi speed spindles often use a better lube system, than the stock total loss BP drip system...

As you speed up a spindle, the bearings will run hotter and expand... Quill fit is pretty tight in housing, so a few degrees can bind up quill a bit..

BP spindles are pretty slender, and you can get a lot of whipping, if you spin them too fast...

I have run my Step pulley at 120 hz for short run aluminum work.. ~5500 rpm at spindle.. I do not not recommend it for continual use...

The BP Vari speed drive, would not be happy long at 120 hz I'd bet...

There is not much to fly apart in a 3 phase motor (TEFC fan would be a weak point). I think Forest Addy has run a 1740 rpm 5 Hp motor up to 12k or so... Not much power up there though...

maybe nothing to some but i add some lubricating oil to BP head at least once a day and pull the way lubricator a 1/2 dozen times. if running above 4000 rpm it probably wouldn't hurt to oil it twice a day

As you are still using the original step head drive system the bearings in the pulley hub may be more of a limiting factor than the spindle itself.

BP used to offer a 5,500 RPM J head that had a 3,600 RPM motor. The M head had 3 motor options; 1,200 standard, with 1,800 & 3,600 RPM options. With the 3,600 RPM motor, the M head would run at 12,000 RPM.

If you run a J head over 4,000, you should be using Velocite 22 (ISO-22 spindle oil) instead of an ISO-32 hydraulic oil in the spindle.
JR

Originally Posted by Avrgjoe

My VFD can run up to 90+ Hz putting my spindle speed @ aproximatly 33% faster than the max step pully listed speed. At the moment I only have the VFD set at a max of the standard 60 Hz as to not exceed the 2750 RPM.

Just for your information and for your calculating...... The base for rating is 60 Hz, at 90 Hz, you will be at 50% more (not 33%), or 150% the rated speed of the motor. So, if your gearing gets you 2750 RPM at 60 Hz, you should be 4125 at 90 Hz.

I rebuilt the 2J head on an old Bridgeport BOSS machine a couple years ago, removing all the clapped out vari-speed components and substituting pulleys to give a 1:1.5 speed increase. Also replaced the stock 2HP motor with a new 3HP 1725 RPM motor, which makes up for the mechanical advantage lost with the use of the 1:1.5 pulley ratio. At 60hz the spindle speed is 2588 RPM. The back gear is still in place to increase torque for low speed work, although seldom used, as the primary purpose of the machine is small cutter work in graphite. Running the VFD to just under 120hz keeps the motor speed below 3450 RPM and yields 5K at the spindle. This machine often runs all day, with the spindle lubed with a feed from the Bijur system. I've measured the spindle temperature with a contact pyrometer on one of these day long runs; the spindle temperature went up to about 110-115 deg. F and stayed there. It's been running like this for a couple years now, with no signs of trouble.

Dennis

Spoke with a Baldor Motor / VFD rep. today about these questions, I posed the question about doubling the motor speed per the motor plate he said that was absolutly true but the torque curve will usualy suffer once you surpass the rated motor RPM range via a VFD. We also spoke about the likely hood of a new VFD on a nearly 40 year old motor causing the motor harm. He is of the very strong opinion that it will indeed cause the motor to fail more than likely sooner than later. The reasoning he has stated was pretty simple, motors back then were not made to deal with the pulsing DC current of todays VFDs, the windings insulation and the quality of the copper back then was not as good. These 2 areas alone will make the motor fail eventualy. His suggestion was to either replace the motor with a VFD rated motor or have my current motor rewound with VFD rated windings.

Just kind of curious about this line of thinking? he was very adimate about his opinions and seemed very well versed in what he was saying. I am not saying he is right or wrong, Just curious what others thoughts are about this subject.

Originally Posted by annoying

Just for your information and for your calculating...... The base for rating is 60 Hz, at 90 Hz, you will be at 50% more (not 33%), or 150% the rated speed of the motor. So, if your gearing gets you 2750 RPM at 60 Hz, you should be 4125 at 90 Hz.

I see my error, Thanks for the correction. I plan on making a spread sheet with all the Hz and corosponding RPM as it pertains to this machine in particular. Good to see the error of my thinking before I make the sheet.

Thanks,
Joe

Originally Posted by Avrgjoe

I see my error, Thanks for the correction. I plan on making a spread sheet with all the Hz and corosponding RPM as it pertains to this machine in particular. Good to see the error of my thinking before I make the sheet.

Thanks,
Joe

2 things...... to get around the annoyance of a spreadsheet.......

1) typically the VFD would have a display and often will display frequency when in use as a default setting. Look through your user manual on that. There should be a parameter you can change for "factor" or "multiplier" for the display. In your case would be 45.833 to display 2750 at 60 Hz. If posible, maybe you could mount the VFD in a box with a window and in a spot where you can at least kinda see it, to read RPM....

2) assuming you will use a potentiometer for adjustment, just print out a a label or sticker to place under the knob so that pointer can point out RPM at whatever intervals you can fit on said label.

3) OR... use a 4 digit display to tie into said potentiometer and calibrated to display correct RPM

I will have to look into my programing book for the VFD, I have not seen that it has that option but I would be very happy if it did. Ultimatly turning my VFD display into a speedo, that would be pretty sweet! As far as my set up.... its almost like you had seen it??

Originally Posted by Avrgjoe

Spoke with a Baldor Motor / VFD rep. today about these questions.. These 2 areas alone will make the motor fail eventually. His suggestion was to either replace the motor with a VFD rated motor or have my current motor rewound with VFD rated windings.

Just kind of curious about this line of thinking? he was very adamant about his opinions and seemed very well versed in what he was saying. I am not saying he is right or wrong, Just curious what others thoughts are about this subject.

You might get better answers to this over on the Transformers, Phase Converters, and VFD forum; some of the participants design these things for a living. The consensus of opinion over there seems to be that with the typical North American dual voltage motor on a Bridgeport, this could be a problem if running on the high voltage, 460 VAC. The reason is the high speed switching causes voltage spikes that are beyond what the insulation in older motors was designed for. Conversely, running on 230 VAC doesn't seem to be a problem, because the magnitude of the spikes is only half the voltage, and there are certainly enough Bridgeports running on VFD's to bear that out.

Dennis

both my step pulleys have non-stock motors. on one the spindle can be run up to 4,600 and the other can run a bit over 6,000. i use those speeds quite a bit and have never seen any quill stickage or hot bearings. both machines are 45 years old and the bearings may be original or may have been changed sometime in their life....either way they aren't new but are are still very good. both machines get ISO68 hydraulic oil in the spindles, same oil i use for the ways.

Originally Posted by JRIowa

BP used to offer a 5,500 RPM J head that had a 3,600 RPM motor. The M head had 3 motor options; 1,200 standard, with 1,800 & 3,600 RPM options. With the 3,600 RPM motor, the M head would run at 12,000 RPM.

If you run a J head over 4,000, you should be using Velocite 22 (ISO-22 spindle oil) instead of an ISO-32 hydraulic oil in the spindle.
JR

YOu think that 6500 rpm with existing spindle bearings will be safe? I see only max of 10 minute cycles with tools requiring these speeds. Now I have a BP CNC with automatic lube that takes care of the ways and spindle from teh same reservoir. Is velocite 22 good for the ways?

We ran a used ( 20 yrs old?) BP at 5500 RPM constantly 8-16 hrs a day for around 7 years routering glass filled plastic before the bearings failed because nobody oiled it for around a year. Replaced them and it was still going strong when I left. We would fill the stock oilers once a day.

I run my 1966 step pully J-head at 150hz all the time giving me 6880 spindle RPM on the highest pulley. The spindle bearings get just a little warm but not hot. I do keep it well oiled and most of my runs at that speed have not been longer that about 20 minutes. My motor is a GE replacement and so far its been happy running at 150 hz.

I believe it's harder on the motor running it slower than 60 hz than faster. The cooling blades don't push much air at lower speeds. The motor sees the same voltage on the terminals no matter what frequency you are running it's only the pulse width that changes. The average voltage will be lower over one cycle of the PWM frequency depending on the pulse with ratio but when the pulse is on it is about 310 volts .

Craig

I am still having a fair amount of stiffness in my quill at the very top, it seems to tighten up after I have been using it for a while. It is still relativly loose in the middle of the quill travel. I have yet to run my spindle speed over the rated speed, (60 hz). With this being said I am still hesitant to run the VFD past 60hz in fear that it will get worse and / or damage something.

Any ideas why the quill down feed is tight at the top of the travel (almost like I have it locked)?

Thanks,
Joe

Avrgjoe. I don't think that your quill is overheating at teh regular rated speed lest your bearings are beyond shot and then you would feel it. Did you ever pull out that quill completely and mic it? Maybe you have dirt in that part of the rack? My Chinese BP clone goes to 4800 rpm and when I run that for say about 7 minutes at a time between tunring teh spindle on and off and run it like that for say 4 hrs It barely gets warm to the touch on teh upper head. It seems liek a very strong concensus says that it will work and it has worked.

How tight have you done up the head rotating - pivot locking bolts, theres a torque spec in the manual. Over tight will cause quill movement issues! As the castings get distorted.

As to rewinding a working motor before VFD use. it would cost just as much as rewinding it after a insulation fault. The VFD if set right will protect its self + motor in these circumstances. The only thing getting it rewound first gives you is a choice of timings (remember its not going to fail when its not being used!). Personally getting the existing motor insulation tested would be a far better option. If its good use it. If its suspect getting it rewound now would be prudent! and avoid a inconvenient break down.