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  1. #1
    Join Date
    Feb 2006
    Posts
    1187

    lets talk bearings (Mini Mill x2)

    Ok I am trying to figure out if I can use two bearings in place of one bearing.

    the two bearings are half the load rating of the original. Will using two at half the load rating equal the one? or be close?

    This is for the X2 mini mill head r8 spindle. I am probably making a new head. I plan to use the spindle and I want to go from a 62mm O.D. to around 47 to 50mm. To do this I will have to go to a bearing that has half the load and static specs.
    The reason for going smaller is so that I can fit the spindle and bearings into a quill housing

    So anyone who can chip in, I would appreciate it.

  2. #2
    Join Date
    Mar 2006
    Posts
    2712
    The bearing manufacturers have that info in the tech /engineering section of their (online) catalogs. Usually, two bearings of half the capacity do not equal the capacity of one at twice the capacity of the lesser pair. Short answer to your question, no.

    You might find bearings in the ID & OD sizes you want, but they are probably wider. If you are designing/building your own quill type spindle, you should be able to match up what you need.

    Since you haven't given bearing type or spec.s, We can't suggest replacements.

    Dick Z
    DZASTR

  3. #3
    Join Date
    Sep 2006
    Posts
    6463
    Hi, it all comes down to your balls....if you have bearings with smaller balls that means for every rotation of the bearing inner race the balls will rotate faster and so increase the wear rate, and adding more balls to carry the load does not alter this.

    Smaller balls also have a more point contact area than a ball of larger diam, and at a certain speed will skid on the tracks instead of rolling......higher speed causes more friction and more heat.

    This is why a bearing can fail from high speed applications.

    In a bearing configuration where you have two angular contacs (or tapered roller bearings) back to back, (cups facing OUTWARDS), only one will be fully under load, usually the one at the top, the other bearing behind it (at the bottom) is the back up bearing that imparts the preload......although both bearings initially are equally loaded, when the cutter is doing it's work the lower bearing has reduced loading applied to it and the upper bearing does all the work.

    End mills (and drills and slot drills) have a spiral fluting and rotate in a clockwise direction and this imparts a down pull to the spindle, which applies load to the upper bearing, (cup facing upwards) even though most of the loading for end mills is a side force from the feed rate.

    This down pull can lead to the cutter being pulled out of the collet, except in Autolock collets with threaded cutter shanks that will tighten the grip of the collet as the load increases.

    Only in drilling, where the quill presses the spindle down into the lower bearing cup would the bottom backup bearing be loaded to the same degree as the top bearing.

    It would pay to have a bearing pack of the bottom two bearings rotated, that is the upper bearing moved to the bottom and vice a versa, to extend the life of the bearings, provided the bearings were an easy fit on the spindle, but seeing as in good design the bearings would be held together by a nut in intimate contact with preload, this is quite easy, but does mean a removal of the spindle etc.

    Once the upper bearing becomes "noisy", indicating track wear and ball deterioration, both bearings will need replacing for another matched pair, even though the lower one is still in good condition.

    Using a shim to take up the "play" that occurs from wear is false economy, when all the time and effort is in the quill removal and/or head housing strip down.

    With end milling it would seem that the bearing configuration with the cups facing INWARDS would be the better design as this would place the lower bearing (doing all the work) closer to the moment of inertia and leverage from the end mill which is the work type where most accuracy is needed, as opposed to drilling where end loading does not impart so much side deflection forces, and the upper bearing of a bearing set with the cups facing INWARDS would just have to resist the down forces of the quill agains the spindle and drill bit.

    I think the design with the cups facing OUTWARDS is the most popular configuration, but for what reason I cannot guess why, when it is logical to use the cups facing INWARDS.

    Cups facing INWARDS means the bearing preload would be applied by the bearing retainer cap, whereas with the cups facing OUTWARDS, the preload is applied by the retainer nut on the spindle.
    Ian.

  4. #4
    Join Date
    Feb 2006
    Posts
    1187
    Thanx Rich and Ian

    I was hoping to be able to squeeze a drill quill into the x2 mill head but it looks like I will need to build another head. I will be heading to the metal surplus here in Florida tomorrow, so I am trying to figure out what my shopping list will consist of.

    Now I know every one talks of angular contact bearings but mine has radial bearings on the spindle and has worked fine for over 5 years strong with some heavy milling in steel involved. And they are still fine. But for the new head I will buy new angular bearings in the configuration you speak of Ian. I will probably end up machining a quill, unless I find something at the surplus yard.

    Can't wait , I am like a lil kid at Christmas !!

  5. #5
    Join Date
    Sep 2006
    Posts
    6463
    Hi, sounds like a complete makeover....once you start redesigning you can't stop....LOL.....BTDT.

    Are you going to fabricate the head or rework a casting, or even make a patten and casting from scratch?

    Does anyone know what the "ideal" configuration is for a spindle bearing grouping with twin angular contacts at the bottom and a radial at the top....IE, is the best way with cups facing inwards or outwards?

    I've always been under the impression that the bearing layout works best with bearing cups pointing out, but i'm considering on a build I'm doing to now have them pointing inwards.
    Ian.

  6. #6
    Join Date
    Feb 2006
    Posts
    1187
    Yeah alot depends what I can find in their(surplus yard) parts pile.

    And yes a complete new head is looking what I will have to build to be able to put a drilling capable quill in it. I was hoping to be able to put smaller O.D. bearings so that I could squeeze a quill into the original mini mill casting.

    I might have to mill my own quill which the more I think about it, the more I am wanting to do that. I will just add a rack to it.

    And as far as bearings go, I will probably use angular on the bottom and radial on the top

    I also found this guy's blog for his X3 mill and the way he did his spindle bearings http://www.home-machine-shop.com/X3_...uce_Murray.pdf

  7. #7
    Join Date
    Mar 2006
    Posts
    2712
    FYI; http://timken.com/en-us/products/Doc...ing-Manual.pdf

    It takes a while to load.LOL

    Dick Z
    DZASTR

  8. #8
    Join Date
    Sep 2006
    Posts
    6463
    Hi Zip, you might want to consider a solution I'll have to adopt.

    About 1980 I bought a small jig borer, which is like a mini mill but a bit bigger....the only problem was it had a #2 morse taper in the spindle and no draw bar, so I set about designing a quill mod to accept a new spindle with larger bearings at the bottom end and R8 in the spindle end but changed this to ISO 30 taper for an ISO 30 chuck and ER32 collets.

    To enable bigger bearings to be fitted I shrunk on a larger nose cap, ala Bridgeport design, to the end of the quill and bored it out to 72mm for two 35mm bore angular contact bearings I had, giving me a 35mm diam for the spindle.

    The bore of the quill was 45mm diam so the bearing inner race diam at 35mm was the only bearing that would work.

    The 35mm spindle diam did not allow an R8 collet in the spindle, so the ISO30 taper was decided on as it meant the biggest part of the dumpy taper would be within the larger diam of the spindle end that stuck out beyond the bottom bearing diam, and the small end of the ISO taper would be within the 35mm spindle diam.

    Fitting a nose cap to the quill end meant I lost about 20mm of the quill overall length and travel, but as I had 163 mm of available quill length left it was OK.

    One of the biggest problems with any quill spindle is the draw bar hole, especially as the quill is driven from a longl splined or keyed shaft at the top end, and drilling the hole all the way through the spindle without breaking the drill off is another headache, even when you drill from both ends.

    There are lots of complications in the drive when a quill is planned, one being that you have to have a pulley with its own bearings fixed in the head with a splined or keyed shaft passing through the pulley to transmit the drive to the spindle as it racks up and down.

    There is a significent pull to the side from the drive belts and this imparts a sideways pull to the spindle end, leading to all kinds of problems if the drive pulley is just a sliding fit on the splined shaft.....high speed is not an option.

    The spindle drive end that originally was in the mill had two keyways instead of a splined shaft, and also a seperate bearing housing for the pulley drive in the head, which only gave me a 13mm core in the spindle end to get a drawbar through.......9mm being the max drawbar size practical, so it was a no go situation.

    The ISO 30 collet chuck with ER32 collets I bought on Ebay has a hole all the way through it, so I intend to fit a long cap screw screw in place of a draw bar, through the chuck from the bottom end and only into the end of the spindle bore taper, there being clearance behind the ER32 collet to allow an allen cap screw head to secure the chuck.

    This does mean that for other tooling like drill chucks, boring head, sitting saw holder, fly cutters etc, I would probably have to go to 3/4"parallel shanks, ala Tormach design, and have the ISO 30 chuck with a 3/4"collet in it when not doing any milling.

    The most logical way would be to go to ER40 in the spindle end (to give maximum tooling diam capacity) with retainer screwed cap and have all tooling with a 3/4" parallel shank in place of the ISO 30 taper as originally planned.....but that does mean a hardened and ground spindle taper

    So now we have a number of options for a spindle, and on the strength of it I would not fit a quill where a complete Z head assembly on linear rails, or dovetails if already fitted, with counter balance could be fitted, even if it did mean a heavier Z counterbalanced drive.

    It's true that a quill gives a lighter sensitive drive to smaller tooling, but as the head is counter balanced, the only problem would be the inertia in the Z drive when fast pecking a small drill.

    The simplicity of the complete head with motor and poly vee belt drive via a VFD is a lot to be considered when the design of a quill is compared, and if the head casting was in alluminium the weight factor would be largely reduced.

    Making a quill a neat sliding fit is the first headache, driving it is the next, and the length of trave is the last but one....the stick out at maximum length is not as stable as the head at lowest level.

    The quill also has to be driven up and down, and the option is for rack and pinion, spring compensted, or offset ballscrew drive........quills suck!
    Ian.

  9. #9
    Join Date
    Feb 2006
    Posts
    1187
    Well made it to the surplus yard. Didn't get everything I needed but got some stuff. They had no quills laying around, all the racks and pinions were too huge. They had tons of bearings but not organized. But got enough steel to rebuild the head.

    Yeah Ian quite a project you had !! My idea is basically this Click image for larger version. 

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    So currently the mini mill head is sandwiched between two steel plates. It will get removed and I will build a steel housing for the quill. The quill will be 3 " O.D. steel 1/2 wall. With races milled out for the bearings. It will have slots on the side to so that they act as a tract to allow it to slide up and down. A third slot will be in the back that will house the rack.

    And yes you are right the top of the spindle is going to be challenging, for one on the mini mill r8 spindle the top has a reversed thread so that a locking nut can go on. Normally on a mini mill this is no problem but now that I am adding a slotted 3 inch tall sleeve on it I have to worry about spinning off. I am thinking that I will assemble the sleeve and the spindle and then tighten the nut down and the put a mark on the nut and spindle.
    I will then make a slot on the nut and spindle so that a matching custom lock washer will sit on top and then be held in the lock position when the drawbar is tightened.

    So just waiting on parts , I ordered the r8 spindle, spindle nut and spacer from LMS. Next week I will order everything else I will need to complete the project.

  10. #10
    Join Date
    Sep 2006
    Posts
    6463
    Hi, you did say the "the third slot to house the rack"........rack and pinion are for quill travel on manual mills.......ballscrews are for CNC in the Z drive surely?

    If you drive the quill with a rack and pinion you'll get backlash problems....it's just not the best way to do the Z axis drive.

    You will also have resolution problems in matching the rack/pinion pitch to the stepper motor, whereas the ballscrew Z axis is a direct drive.

    I may have overlooked current practice in this quarter, but I would go to offset ballscrew Z drive any day, even where a rack and pinion was already in place.....I don't think having an offset ballscrew drive bracket on the end of the quill is bad design.
    Ian.

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