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    Default BF30 Spindle

    Hi Everyone,

    Well I am busy working on an enclosure for my new electronics, I started thinking about the spindle in the Mill. I am going to replace the standard BF30 spindle motor with a DMM tech servo using a belt drive. However I am concerned about using the bearings that came with this spindle, I want to have a speed range of around 6000 rpm, 10000 would be great however probably not that useful. I did think about buying a Tormach spindle at a cost of $580 then there would be shipping and taxes. Until the idea came that replacing the bearings may not be so bad. I hear a lot of people talk about ABEC 3's and ABEC 7's which I cant justify....why not use taper roller bearings? I read somewhere that’s what the machines come with. Even SKF Taper roller bearings have a standard rating of 7000rpm....and cost alot less than angular contact bearings! Not sure what size I would need.
    Is this an option...or am I missing something critical?

    Has any one taken apart the spindle on the BF30? If so do you know what the bearing sizes are? Was it easy to replace them? Is it worth it or am I worrying about nothing

    Many thanks,

    Andy

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    They are nowhere near as precisely made as A/C bearings, although there are new precision variants being made. They get hot at higher speeds, too, due to the fact that there is a large contact area on each roller and that the roller is turning and sliding at the same time. The bearings may do 7k rpm, but they will get very hot and you'll end up with issues like toolholders sticking, etc. due to the heating of the spindle. Tapered roller bearings are awesome as ballscrew support bearings, though. It saves a ton of money to use them for that.



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    -deleted- duplicate post.



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    Not sure I see how tapered rollers would save "a ton of money" on the ballscrews. If you go to VXB.com, you can get very nice A/C bearing sets suitable for ballscrews for very little money. Despite their low cost (under $15 for a pair), I've found the quality to be excellent. I've used them on all the Bridgeport quill drives I've built, and they've all had no measurable backlash. Even Nachi's are under $25 for a pair.

    Regards,
    Ray L.



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    Quote Originally Posted by Winnfield View Post
    They get hot at higher speeds, too, due to the fact that there is a large contact area on each roller and that the roller is turning and sliding at the same time.
    Where does the sliding come from



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    Quote Originally Posted by johnsattuk View Post
    Where does the sliding come from
    Any modern tapered roller bearing will have tapered rollers, specifically so they DON'T slide. Sliding would result in considerable heat build-up, and greatly accelerated wear.

    Regards,
    Ray L.



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    Quote Originally Posted by HimyKabibble View Post
    Any modern tapered roller bearing will have tapered rollers, specifically so they DON'T slide. Sliding would result in considerable heat build-up, and greatly accelerated wear.

    Regards,
    Ray L.
    Precisely the point I was making



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    When I upgraded my BF30 with a belt drive I ran some tests on bearing temperature as a function of time and speed. This was done at the factory bearing preload setting which I haven't touched. I settled on 4000 RPM max with the following graph:



    55+ degrees feels very hot to the touch. I would not feel comfortable running hotter than this for any length of time - take a tool holder out and you can't touch the shank for more than a second or so. I could also measure significant spindle length growth based on the increased temperature. I think I looked at an NSK taper roller bearing catalogue and for bearings of this size I seem to remember the continuous RPM rating was around the 5000 RPM mark. No doubt the stock bearings are not up to NSK quality either so should probably de-rate a bit from that. So if I was in your position I would get AC bearings all the way. The BF30 bearings are pretty big. If you could get to 10,000 that would be a very useful speed. You could fly through aluminium with that, with small 2-3mm cutters it would make a huge improvement. With my set up I am pretty happy because I can cut steel and aluminium with various size cutters, and still not have to change the belt ratio (just the VFD speed), but there is no doubt that it would be even better if there was more speed available.

    LongRat
    www.fulloption.co.uk


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    A 50C temperature rise in 30 min. strongly suggests significantly too much pre-load, or poor quality bearings. I can run my machine all day at 8200 RPM, and it never gets close to that kind of temperature rise. It would be interesting to back off on the pre-load a bit, and repeat the test.

    Regards,
    Ray L.



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    Quote Originally Posted by johnsattuk View Post
    Where does the sliding come from
    Some sliding happens typically at startup and at high speeds, hence their inappropriate use when needing spindle speeds near the rated speed limits. The thing that keeps the sliding from happening is the preload, which, to keep the rollers from slipping at high speeds and startup has to be very high - so high that the bearings get very hot. They are not *supposed* to slide, but they do without an enormous amount of preload.

    The sliding while turning I was referring to is the retention system or cage that keeps the rollers from sliding out at high speeds. It isn't like the bearing spacer in most bearings. The rollers have lots of contact with the cage or a rim that keeps the rollers in the bearing.

    Regarding the cost of the bearings vs. A/C on ballscrews: Thet do represent a huge savings over p4 bearings, and as you already know, the preload on the tapered roller bearings is a simple affair and makes eliminating any possibility of backlash very easy. There is a Master's thesis floating around hre somewhere that goes into it in detail. Some kid from MIT studied it.



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    Quote Originally Posted by HimyKabibble View Post
    A 50C temperature rise in 30 min. strongly suggests significantly too much pre-load, or poor quality bearings. I can run my machine all day at 8200 RPM, and it never gets close to that kind of temperature rise. It would be interesting to back off on the pre-load a bit, and repeat the test.

    Regards,
    Ray L.
    Bit of both I imagine. A good year later and the spindle doesn't get anywhere near that hot any more. Still got good runout, low noise and decent finish on my parts so I still haven't touched the preload! If and when the bearings give up, I'll definitely be going the AC route. And probably up the speed too.

    LongRat
    www.fulloption.co.uk


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    Thanks for the reply, so AC's it is then....anyone know the dimensions or is this a take it appart and check? Just measured backlash on my system after installing the servos...0.02mm on X and Y with a 0.012mm on the z. Am wondering if double nuts would be a benifit or making my own bearing blocks for rathe than using linear motions. This was I can use the taper roller bearings.

    Perhaps finding that paper would be usefull.

    Thanks,
    Andy



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    For single nuts those backlash numbers are excellent. I don't know what parts you are looking to make but if I were you I would save the hassle and extra space that is needed for double nuts and simply stick with what you have. It would be very difficult to make parts significantly more accurate than ±20µm on these machines even if you had no backlash at all, I would say your backlash is well matched to the rest of the machine and leave it there.
    I found Linearmotion's bearing blocks to be of pretty low quality, and only once modded were fit for purpose. I have no doubt that home made ones could be made better, but the Linearmotion ones do still represent decent value for money even after factoring in the time to fix them.

    Sorry I don't know the bearing dimensions for the BF30 spindle.

    LongRat
    www.fulloption.co.uk


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    if my math is right, that's under 0.001" backlash on each axis.. Forget the double nuts, that's pretty damn good.



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