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Thread: Opti BF46 Build

  1. #13
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    Default Re: Opti BF46 Build

    Time for another update.

    I have nearly all the mechanical design done for the servo and ball screw supports.



    Arriving at this bearing arrangement was a bit of a contrived process. I pulled one of the existing Optimum mounts off, which they tell you to re-use with their ball screw kit (which is all 25 mm, 5 mm pitch Hiwin screws; very happy with those). Their thrust bearing arrangement on X and Y is crappy at best - a pair of Chinese 6004 ball bearings with the outer rings separated by a spacer, and the inner ring preload simply set by squeezing them together with a jam nut arrangement (no positive stop). Yuck!

    I came across automation4less.com and decided to use their flanged ball screw supports. The thought process being they were real AC bearings, and using an axially piloted bore would make them easy to remove later on for inspection and re-greasing. They showed up with some Japanese brand bearing I'd never heard of, KYK, which I figured was better than some Chinese brand bearing I'd never heard of.

    After a bunch more forum reading, I convinced myself to further upgrade these supports by putting some side shield NTN 6204ZZ bearings on the floating end, and a pair of the much vaunted NSK TAC bearings on the fixed end (20TAC47CSUHPN7C). I think I found a pretty good deal on them for $57 ea on eBay. The outer rings are 1 mm wider than the bearings the supports came with, so I will need to surface grind ~2mm off the end cap's projecting boss to get the outer ring preload set back correctly.


    After all that, it would have made more sense to just modify the Optimum bearing housings, but I already own all the parts, so I'll solider on with the original (now modified) plan.

    For belts and pulleys, I'm going with a 5 mm pitch, 15 mm wide, Gates PowerGrip GT profile. 34 tooth on the servo, 68 tooth on the ball screw. The large diameter is to keep the running belt loads and installed tension reasonable, and to stay within the servo's max shaft load spec. DMM says 300 N max radial load for the 880-DST servos I'm using. My installed belt tension will be just about 150 N, which takes the servo radial load right up to 300 N, max, under any operating condition.

    Popular think on the Zone, right now, is to use CarbonGT belts, as they are very stiff and don't stretch much after break-in. I'm pretty sure Gates doesn't actually make a 5 mm CarbonGT belt. I found one catalog that showed part numbers for 5 mm carbon belts, however if you look up those part numbers on Gates PartView, the specs say Aramid fiber, not carbon, and calls them PolyChain GT. If you spec PolyChain GT in Gates design software, it says the minimum installed tension for a 15 mm wide is 174 N, which would have been well over my servo's design limit. So, I recommend to anyone considering the PolyChain to download Gate's Design iQ software, and check their system design before going down that path.


    The pulley diameter choice was also driven by the small side pulley diameter. I wanted to use tapered bushings to mount all pulleys to their shafts, and the servos have a rather large 19 mm shaft. 19 mm, plus room for the bushing, then the pulley over that limits your minimum diameter. Why not just use set screws and keys, you ask? As a mechanical engineer, I abhor using keys and/or set screws for power transmission; even more so in a system with precision control requirements. Leave the keys and set screws to lawn mowers!

    I ordered my pulleys from cmtco.com, as I really like their tapered bushing hub design. They are able to package their tapered hub inside the small 34 tooth pulley and still fit in a 19 mm bore. Also, they offered pulley sizes I wanted in aluminum. The big 68 tooth pulley would have had an unmanageable inertia in steel. The inertia issue was something I never considered, which this forum led me to the light on (thanks, mactec54, and others!). The only downside to these timing pulleys is their rather extravagant cost ($235 per axis!).

    With three days off next week, I'm trying to get all the parts gathered up that I can, with hopes to push this thing to near completion next week. The biggest remaining unknown is the electrical enclosure. I need to get that laid out tomorrow in CAD and get an enclosure ordered. I wish I could find a how-to guide for dummies on wiring CNC controls. I've got some ideas on the right way to get from the outlet on my wall and out to the servos, but I'm sure I'll trip over a few things like where I should be putting in contactors or relays to let the motion control board to control stuff, and the right way to do the e-stop cascade. At least with wires, changes are easy, unlike aluminum channel that you've already made a bunch of chips on.

    -Steve

    Attached Thumbnails Attached Thumbnails Opti BF46 Build-x-axis-ball-mount-screw-2-jpg   Opti BF46 Build-x-axis-ball-mount-screw-1-jpg   Opti BF46 Build-bf46-mill-assembly-2-jpg   Opti BF46 Build-bf46-mill-assembly-1-jpg  

    Opti BF46 Build-polychain-warning-jpg   Opti BF46 Build-img_20160318_205104-jpg  
    Last edited by Windscreen; 03-19-2016 at 02:22 PM.


  2. #14
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    Default Re: Opti BF46 Build

    nice to see. I am currently going through the same process



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