An abrasive question- Mtbr.com
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  1. #1
    will rant for food
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    An abrasive question

    I'm looking for ways to reduce my surface finishing time while also avoiding the risk of a Repetitive Stress Injury (RSI).

    Obviously carbon fiber is way softer than metals, but there must be some sanding principles that many here would find relatable.

    So I have an solution to my problem already in mind, and I was just going to go ahead an implement it, but I wanted to ask first if there is any known prior art.

    It is worth noting that every power sander I've tried is inappropriate for my current routinely built object, a curvy handlebar. Random orbital sanders have a large flat area that is only useful for the straight sections of tubing, so about half the bar. On the convex curves, the contact area is so thin (22.2mm tubing OD) it's like scribbling with a thin marker. On the concave curves, the edges of the sander leave gouges (CF, again, being very soft).

    The fastest and most consistent approach I've conjured with manual sanding is the "Nobody f@#&s with the Jesus" from the movie The Big Lebowski. The part where Jesus (the insufferable rival bowler) and his buddy are polishing their bowling balls in such a way as to suggest they polish their testicles on a daily basis and this somehow makes them superior at bowling.

    (What? Was that not the conclusion you drew from the scene? Or was it really that they were just jerks?)

    Anyway, I use a 12" x 1" strip, and I timed my arm motions to some electronic music at 128 beats a minute. I am planning on making a simple device that can imitate my arm movements, more or less like a set of windshield wipers but with the arms oriented differently than normal wipers. The ends of the arms would have little clamps. In this way, low cost sheets of sandpaper could be used and there would be no dedication to a fixed brand or size in terms of consumables.

    So I guess the abrasive surface would be like a small portable belt sander, but inverted and dependent on tension.

    I'm looking and looking, and ... no one has built this already?

    Really? I've built things that are more absurd (rotocaster) and it worked out, so I'm not scared to do it, just, really there's no prior art here?
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  2. #2
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    How about something like a violin bow with the 1 paper clamped at each end. You could adjust the length of stroke and tension. Could potentially use a wider strip on the flatter parts and narrower on the complex curves.

    Have you looked at sanding sponges (or sponge sanding backer blocks) to conform to the contours?

  3. #3
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    Why not some kind of Dynafile?


  4. #4
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    You could buy a tube polisher?

  5. #5
    will rant for food
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    See, this is why I complained: in short order, I have learned a few things.

    High five, folks. F@#$*in high five.

    @bme107: I sampled some SIA brand foam backed abrasives (via Express Composites right close to me, how handy). They're good sand papers, dare I say superior to 3M offerings (and I usually think 3M makes the best of anything), very durable, they can keep a soapy lather going for way longer than conventional sandpaper, all of which is plenty good for plain clothes carbon abrasion. But it is still a vector for an RSI type injury. I have significant neuropathy in my index finger (it both hurts and feels numb at the same time) from a previous career, so this concern is at the front of my brain.
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  6. #6
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    Quote Originally Posted by cerebroside View Post
    You could buy a tube polisher?
    Another vote for using a tube polisher.

  7. #7
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    I would consider building a machine that used 2x72 inch belts. They are prevalent in the knife making community. You could leave the platen out of the design and use a light spring in a tensioner to allow the belt to conform to your tubing.

    Basically a stationary tube polisher, with abrasives available just about anywhere.

  8. #8
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    Do you have any pictures of the parts needing sanding, or a video of the process? I can't help but think that a belt sander would make short work of tearing right through CF.

    I wrote my previous response on mobile device while traveling on subway. Back at my desktop I googled sanding bow, for the hell of it. There are tons of cheap DIY solutions that use abrasive strip/roll or belts to do round or complex curved projects like you're doing.

    An off the shelf solution for $20 https://www.rockler.com/rockler-sanding-bow

  9. #9
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    Did i miss something? what are you actually sanding? Carbon?

  10. #10
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    @bme107: I have some bow sanders courtesy of Velo Zephyr (Steven H). They're good in terms of reaching interior contours, but they are not fast and they are all manual. I'm convinced that the 180 degree wrap effect shown by the tube polishers is the fast way to go.

    Consider handlebars as what they are to a lot of people: get it it to fit exactly right and not be totally ugly, otherwise don't make me think about it and make it cost something at least somewhat reasonable.

    If I'm spending five hours on sand and polish, "reasonable" cost goes out the window. I need something that can be done in minutes per grit. I'm convinced that requires a power output beyond what my meat and bone appendages can do.

    @plummet: yes, carbon
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  11. #11
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    Quote Originally Posted by cerebroside View Post
    You could buy a tube polisher?
    it that like a fleshlight?


  12. #12
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    Give me more detail of what you are sanding and why.

    If you want to get away with less sanding then more mould prep is the go. If you prep your mould well enough you may and up with a part almost ready for a quick buff and clear coat.

    Otherwise depending on the surface you can use a variety of tools.
    Torture board for flat surfaces,
    A long length of Emery sanding paper for rounded and surfaces.
    Palm sander.

    Grit from 36-2000 depending on the finish.

    Can for crazy removal of stuff super fast. Angle grinder with 36 grit wheel.

    That is stupid fast. But be carefull you might grind the too much off in a heart beat.

  13. #13
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    @plummet

    Maybe these images might (maybe?) help. My main problem is related to my method of resin placement. It is injected using VARTM. I have a 1.5mm thick area around the flashing zone that accomplishes a similar task to MTI tubing, where the resin approaches from all angles and seeks the shortest possible path to full injection. My molds are twin / opposed closed solid molds. There is no *room* for MTI tubing, hence the nearest-neighbor approach.

    The natural consequence here is that I have some flashing with carbon in it - ideally just veil carbon. The surface finish of all the rest of the part is good straight outta the mold. The issue that arises is in sanding off the flashing, some small portion of the molded surface becomes blemished and is impractical to blend with the very small machining scallops. I don't currently have a good example of this, unfortunately. Maybe I can produce one and get back at ya.

    I can assure you that my parts already have such a good release character that they almost *fall* out of the mold when said mold is opened. The scalloping is asymptotic; at some point my CNC just vibrates too much no matter how many close passes I design into the mill pathing.

    As a result, I feel the need to sand and polish (or at least just sand) the entire thing. Getting this done really fast involves a thin strip of abrasion wrapped around as much small diameter tube area as possible. I think this adequately describes the 3-pulley tube polishers that can cover a 180+ degree circumference in one go.

    First pic is the raw flashing outta the mold, second and third show a quick hack job of sanding.

    You're exactly right about sanding off too much material too fast - it is way, way too easy to gouge, even down 0.2mm to the structural material.

    An abrasive question-20180919_161444.jpg
    An abrasive question-20180919_161458.jpg
    An abrasive question-20180919_161501.jpg
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  14. #14
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    Hmmm. After removing the flange I dont know if if be keen it be hitting it will much more than wet sanding in varying high grade grits.


    It will be way to easy to remove structural carbon.


    Perhaps if you go down the tube polisher route you could consider scotch bright or maybe even polish wheels with grit style polish.

  15. #15
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    I don't do carbon bike stuff, but I do a lot of composites work (kayak paddles) and I can't imagine that stuff taking that long to manually sand off. I would get a board with some good 36 grit silicon carbide sandpaper onto a block and take 90% off real fast. The other option I would try is a sharp knife. Get a proper carving knife and if you know how to sharpen it, you could probably slice off that stuff, especially when its fresh from the mold and the epoxy is still green.

    If you want to buy a machine, I would by some kind of stationary drum sander and just carefully run it on top of the drum with some good paper and a slightly higher grit, maybe 120?, so you don't take it off too fast. You can also get soft drums that are inflatable and you could maybe put a super fine grit on it for finishing

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