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  1. #11
    You're right in that, I guess i'll have to try.

    SKF has published a huge pdf with info about bearings, setups etc, I spent a lot of time going through it. Some things are still a bit unclear to me though, and quite outside the spectrum of a hobby/home machinist in my opinion. I'll do my best to figure it out.
    Last edited by spresv; 01-01-2017 at 01:53 PM.

  2. #12
    Hi spresv

    Nice first machine, well done. I've sketched, deleted, pondered and wondered about doing an ATC spindle driven by my chinese spindle, along the lines you describe. Things I thought about:
    Running the spindle upside down (Like Deans says) - but didn't think about turning it the other way for direct machining!

    Limit to 12,000rpm to make the cassette bearing spec manageable (plus limit centrifugal forces in the the ATC mechanism)

    Tormach TTS system based on R8 spindle (but hadn't seen the LMS one, was thinking of making my own before you pointed that one out, thanks)
    In case you missed it they added the drawings - and they are metric:
    Click image for larger version. 

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    Gear down with at least 1 pulley set, maybe 2, to get the speed way down whilst multiplying the torque (hadn't decided on lower speed but maybe to 2000rpm ?)

    I also thought about the constant side loading but I was prepared to give it a try. Could rotate the spindle body around from time to time I suppose to load a different part of the outer race?

    The main thing stopping me doing the detailed work was that I should really finish my current machine (which is direct drive) - . . . although designing it in now before making the Y and Z parts would be better than a retro fit . . . . hum . . .
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  3. #13
    Thank you!

    12k rpm is asking much, Chris @ LMS says it's good for about 10k rpm. Angular bearings are rated at 12k rpm, if you double up they drop to about 9600rpm and if you decide to fit a radial shaft seal their limit is a surface speed of 18m/s. At 40mm diameter (the lower part of the LMS spindle) that's about 8400rpm.

    Still, my main concern is how much and by what means to preload the bearings. I hope I get to the bottom of this.

  4. #14
    Ok, I think I concluded to a design I'm comfortable enough with testing. It requires threading the 35mm part of the R8 spindle, so I can use a set of 7207BEGAP angular contact bearings. They are universally matchable, so the inner races are factory ground for a light preload, so fitting a KM7 nut will secure and preload them. Top bearing will be a standard deep groove 6206. The lower gap of the pair will have a flange that will secure the outer races and house a 40x62x8 radial shaft seal, series CRW1 or CRW1A.

    Click image for larger version. 

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    Also, (assuming the rest of the design works) the more I look at it, the more I don't like the design of the lever operated drawbar. The benefit is that you can use a small and cheap pneumatic cylinder, but the end result will look a bit bulky and amateurish to me, although fully functional. If I can spare the expense I might change the design to a 100mm bore 10mm stroke air cylinder. I think I'll need about 6mm of travel for 3 pairs of washers to release the collet, but I'll test as soon as i get them.

  5. #15
    Try and get those lower bearings as close to the end of the spindle nose as possible, it will reduce deflection.

    I think either operating method is good, both have merits, the lever and small cylinder will possibly be lighter.

  6. #16
    Yes, i've read that recommendation elsewhere, and it really makes sense to me, but it poses 2 problems. By far the bigger one is that I have no machinery to turn it down and grind it. The second problem is a design flaw that I couldn't predict. The end of the spindle head will be flush with the bottom of the Z-plate which is about 135mm from the table. That means that at the end of the z-travel, assuming a toolholder with an end mill with 20mm stickout, it will still need another 40mm or so to reach the table. If I get the bearings 20mm closer to the spindle nose, that becomes 60mm. I could maybe build longer toolholders, or use ones with er collets, or mount the head below flush from the z-plate, but wouldn't that be a problem?

  7. #17
    ER collet holders are very good, i have never liked the ones with a screw in the side to clamp the tool.

  8. #18
    They do have a cost advantage though, since you could build them yourself. If the rest goes to plan I'll get a tormach one, build some of my own and measure runout on both (including er ones) and report back.

  9. #19
    Hello guys

    Exams and a streak of bad luck kept me away. A couple of weeks ago I was building a set of carb mounts. 4 tools needed per side, and I can only mount one at a time, so a lot of tool changes. ER nut started to feel a bit notchy, no visible debris in the threads. I gave it an air blast, still notchy. Grabbed a pointy sharp tool and had a go at the nut threads, something did come out, so I did that again meticulously on both nut and shaft. Squeaky clean and all was well, no hint of blemish or damage on either threads. Last part, last side, roughing operation finishes, I go to unscrew the nut to change tool, and it just seized solid. I've never seen anything like it before, no excessive mechanical stress, no thermal stress, no nothing, and it just seized. When I say seized I mean I held the nut on a vice, I grabbed a 21mm spanner and a 1m long lever and it STILL wouldn't budge. Eventually with a lot of patience and even more swearing I managed to get it out. Bottom threads were absolutely destroyed, nut too, but the worst thing is that trying to get it free I must have applied force in a way I shouldn't have and I damaged the bearings, shaft wasn't turning true anymore.

    So, new bearings or new spindle? Two pairs of nachi bearings from bearingboys in the appropriate size were almost as much as a new spindle. So new spindle it is, it took almost 10 days to get here from Germany, ffs, but yesterday I got it. I mount it and straight away I noticed it was MUCH stiffer in rotation. Give it a spin by hand and it barely does one rotation. Hang on... Trusty Mitutoyo indicator doesn't lie, 0.025mm runout. Advertised as LESS than 0.005mm, that's at least 5 times more than advertised. Send the seller a message, hasn't come back to me yet, but I feel it's not going to end well.

    In other news, I decided to make a small fly cutter, I wanted to see if it could be usable in that spindle. I had some 7075 bar (don't freak out, it's just a proof of concept), so I turned a 12mm shaft with a 30mm body and a 8mm slot on a 10 degree angle to mount a turning tool with a polished carbide insert in. Now, I would swear I had a 95 degree left hand tool somewhere, turns out it was right hand... The one pictured on the fly cutter is the only thing I had that would mount and make a cut, even though it's the wrong tool to use.

    Click image for larger version. 

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    Turned out quite nice though. I tried slow cutting air, to get a feel for the speed. Around 2200 rpm vibration starts being noticable, 3000 and it's too much. I tried it on the lathe at about the same rpm and it only had a really slight sort of harmonic to it. My guess is that whatever imbalance it has is greatly accentuated by the spindle runout. Anyway, 6082 plate, 0.1mm DOC, ~45mm WOC, 2300rpm, 300mm feed et voila. Not bad at all, spindle could handle it, I guess if it was runout-free it would give me much more confidence. Not that I would go deeper, I don't care about material removal, just surface finish.

    Lastly, I decided to put the ATC project on hold for a while. I need to invest in other things first, an enclosure, a new cabinet for all the electronics, a half-decent hand-controller or panel etc. Most importantly I need a bigger, more reliable and robust method of workholding, so I could mount fixtures, jigs etc to be able to make multiple parts at once and minimize the frequency of tool changing. I got my eye on this, anyone tried it?

    Long post, that's all for now, cheers!

  10. #20
    Its a crap-shoot with these spindles, never know what you get, I have two and both totally different in feeling.

    Fly cutters are murder on the spindle, be careful and if possible, counter balance it a bit.

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