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I found a deal on a Boss for a retrofit, and it looks to need spindle bearings. Has anybody ever spun a Bridgeport spindle to 6000 RPMs with better bearings? The machine would be dedicated to cutting aluminum with small tools (1/8"-3/8" endmills). It sure would be nice to have the extra speed.
I have heard of it being done using stock bearings on a couple of series one machines. I think you are going to want a VFD to run up the RPMs on the motor. People will tell you that this is bad, but in practice with light cutting like you are describing I would think it to be OK. My two cents.
John Delaney
www.rwicooking.com
I am planning to use a VFD to do the retrofit, so running-up the motor RPM's should not be the problem. I will not need the low RPM grunt either, since it would be dedicated to aluminum. My primary concern is the dissipation of heat from the spindle when I do turn-up the speed. Researching for comparison, I noticed that Tree made quite a few knee-type mills (Journeyman Series) with 6000 RPM spindles. I wonder if they had any type of extra cooling provisions (oil spray, heat sinks, e.t.c.)?
You didnt mention the model of your machine, but i think it should be possible. We have bridgeports that are capable of 6.000 rpm, and one that can do 10.000 rpm Keep in mind that your spindle will heat up, even when you replace the bearings. The VMC560, VMC1000 both with TNC370 controlls (built in 1996) can do 6.000 rpm standard, and i measured differences in the datum when running those rpm's for prolonged periods.
We do alot of 3D-milling, and sometimes the difference can run up to 0.05mm or 0.002 inch in the Z-axis.
Thanks FiXeL!! I'm assuming the .002" Z-axis difference is due to thermal expansion of the spindle components? As for VMC's with higher speed spindles, they usually have extra cooling provisions to dissipate heat from the bearings during operation, such as through the spindle coolant or spindle bearing oil-mist/sprays; so I'm not sure that is a fair comparison to a 4200 RPM knee-mill. I'm not even sure what the duty cycle is on a knee-mill at 4200 RPM's, since I have never ran one that speed for very long. I currently have a benchtop CNC mill that runs 5000-6000 RPM's for long durations (45 minutes at at time), and the spindle bearings only lasted 1-year; but that may have been due to cheap/low-grade bearings. I put Timkin bearings in it this time, so hopefully they will last longer. Maybe I should just keep looking for an older VMC with the spindle speed range I need. Anybody else have experienced feedback on spinning Bridgeport knee-mills to 6K?
The OEM Bridgeport spindle bearings were DB mounted with preload for position accuracy and stiffntes, had contact seals for contamination resistance, grease lubed for simplicity and were rated for 6300 RPM max. The exact same bearing with a non-contact seal and grease lube were rated for 9500. WIth a proper oil bath lube (regardless of seal), the same bearing was rated for 11000 rpm.
Why the difference??? Viscous drag from the grease and contact friction for the contacting seal.
If you installed a true machine tool grade 7200 series angular contact bearing of an equivalent size, you would theoretically have a limit speed of 10K for grease and 14K for oil lube in the same spinde. Reason: proper bearing preloading plus plastic cage which guides balls better/truer at high speed.
BUT, the most common cause of bearing failures is traceable to a lube failure. It is either burned up due to overheating or contaminated from dirt or the like. The OEM BPT spindle bearings don't have provisions for an external seal - they rely on the seal in the bearing to keep out contamination. Thus, installing machine tool grade 7200 series bearings is impractical unless external sealing provisions are made and that is tough due to packaging constraints.
Meanwhile, you only have so many fatigue cycles in a bearing. Thus, the faster you run at a given loading, the sooner you'll use up the fatigue life and so so in proportion to the speed. Thus, at a constant 3000rpm you may last 25 hours which means you'll only last 12.5 at 6000 - these are for illustration and do not represent true life calcs/expectancy.
BUT, the grease will tend to deteriorate faster due to thermal oxidation at higher speeds (viscous drag). Thus, you're grease life may still be OK at 3000 for 25 hours, but yuu only may see a grease life of 10 hours at 6000. This is why you might expect 12.5 hours but get "cheated" out of 2.5 or more at the higher speed. Add some contamination or dilution and you'd be lucky to see 6 hours.
This is why the super high speed spindles are oil air mist lubed. The oil mist provides the lube needed to prevent siezure and the air jet provides cooling all the while minimizing viscous drag. Plus, clean filtered oil is provided for in the lube system as opposed to fill and forget grease....
Hope this helps explain why you simply can't just spin the daylights out of a spindle and/or don't get the life you expect even if you don't spin the daylights out of it....
Great enlightment!! Brings back memories from my fluid mechanics and machine design classes!! LOL. What are your thoughts on ceramic bearings in a Bridgeport? From my experience with ceramic bearings in similar rotating applications, the heat build-up is much less than steel bearings. This would in-turn "save" the lube in the bearings from overheating, and also minimize spindle growth due to larger temperature gradients. I'm just not so sure as to how well ceramic bearings will handle shock-loading due to interrupted cuts or an accidental tool crash. Your thoughts?
Great discussion going here!! I appreciate the feedback.
Ceramic balls were created for ultra high speed use. Yes they are hard but they are also not as tough as steel (little to no ductility as I recall but this knowledge is a bit dated).
What hasn't changed are the physics of the system. At high speeds, the outer raceway sees whatever radial loads are applied PLUS the centrifugal loading of the balls. Thus, ceramic balls (having less mass than a comparably sized steel ball) will generate less centrifugal loading of the raceway. At the astronomical speeds the bearings were asked to run at, large light balls (ceramic) or many more small diameter balls (BNT and BNC style bearings) were the only way this could be achieved.
At 5-6k (within the design window of a typical 2J BPT spindle bearing) ceramic balled bearings will tear a bigger hole in your pocket but not necessarily provide any better value over a traditional bearing. At one time, they were NOT as tolerant of abuse (interupted cuts could cause them to shatter - after all, they are "pottery" to some extent).
Keep in mind that high speed spindles are typically NOT going to see grunt type cutting. They cut with MANY cutting blades rapidly cutting into the material at shallow to moderate cutting depths.
Grunt type spindles have either duplexed, triplex or quad ball bearings at the work end to share the load. In real grunt use, they use tapered rollers or cylindrical rollers to absorb the radial loads from radial milling. A ball thrust deals with axial loads from drilling. Again, this is usually done at lower speeds and heavier loads.
Milling machines can be compared to diesel trucks and F1 engines. The diesels handle the hard grunt work and chug along at low speeds for long periods with little maintenance. F1 needs no explanation.
Bottom line: $peed co$t$ money and time (for maintenance). How fa$t do you want to go and for how long???? Unless your machine was designed from the get go to run at speeds that necessitate, you can bet that retrofitting to ceramics probably won't do that much for you. But they are great to have for the "bling" factor.
And yes, bearing design is a fastidious application of mundane mechanical dynamics, thermodynamics, metallurgy and lubrication engineering.
Has anyone ever tried using a spindle speeder in a Bridgeport? I have an R2E3 CNC that I would like to get up around 10K-15K rpm but I can't find a speeder that will fit an NMTB 30.
I'm curious if your using the stock spindle bearings or something else when spinning up to 6K or more? We have two BTC's and one needs bearings in the spindle. I've herd of some people putting four lower bearings in place ov the two that are normal, are you doing this? just curious thanks for any info, Jon P.
RE: spindle speed-up of CNC
I am not familiar with the bearings in the latest inquiry regarding speeding up of the spindle. Regardless, the prior information posted pertains as much now if not more so.
Simply put, there is MUCH more to making a high speed spindle than simply putting in/on different gearing and/or a higher speed motor.
First you have to see if the bearing package contained in the mill is suitable/capable for the intended speed. (see prior post)
IF not, then you have to use high speed bearings (IE: BNC/BNT's or ceramic balled replacements). (See prior post)
Then, you have to construct/modify the lube system so that it will support the speed potential. (see prior post)
Then you have to redo the preloading so that you don't burn up the parts before you even start to cut. (do it again)
Finally, you need to know how to properly reassemble the spindle (not a simple R&R sort of deal) and regrind the ID to make the tool run coaxially with the new bearing C/L's.
Don't forget to re-balance everyting. Tolerable imbalance at low speeds becomes astronomical at speeds 2-3 times that of the OEM design intent.
If all this sounds confusing or too complicated, the task is probably not one that you may want to tackle as a DIY project. You're gonna need some professional help.
Some older machines (and tooling) were never designed or intended to run at 6 digit speeds. The technology that existed when they were created necessary to run at those speeds didn't exist or the capability was never intended.
Should you chose to press on regardless, BE CAREFUL.Things that fail at high speed can do real damage to surrounding hardware and terminal damage to human tissue if it parts company.
I think Jadaan27 was meaning using something like THIS it would indeed be good to find one with the QC30 taper..
Hood
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