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  1. #1
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    Seatpost Scratches + CNC Lathe

    Would it be okay to put a ultegra 6600 branded seat post in a cnc lathe to take off .005-.01 inches, facing the material and getting rid of these nasty scratches? I was not intending to face the first 10cm that is inserted the frame, so clamping will not be a problem. I'm just wondering if taking .02 inches off the outer diameter over a 10-15cm span of the post will cause failure.

    In my opinion it won't cause any issues...but I don't know bike structural tolerances.

    Also I might note that while cnc lathing a seat post might seem far out there, I happen to be training on cnc machines and a little practice on my own stuff gives me something to do before I risk screwing up clients' stock.

  2. #2
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    Isn't easier just sand it and then polish it?

  3. #3
    SE Wis dedhed's Avatar
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    I'd chuck it in the lathe and run some emery cloth on it.
    '68 Raleigh Sprite, '02 Raleigh C500, '84 Raleigh Gran Prix, '91 Trek 400

  4. #4
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    Alright. Cool. Sanding it to me would be a mess in my dorm room.. but I do have to practice on a lathe anyways so why not. Other thing is that the scratches are the nastiest, deepest you can get. I really need to have my steel frame seat tube reamed.

  5. #5
    coprolite fietsbob's Avatar
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    traditional. no automation required, lathe .
    Chuck it up to the minimum insertion line so you can still see that .
    and so the post wont be too much undersized ..

    ream the seat tube too and the fit will get sloppy..

  6. #6
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    the shop I am training doesn't have a traditional lathe.. and quite frankly the ones they have are much safer and easier to use. I would touch off and keep track of how much I'm taking off fairly easily. Increments by thousandth of inch.

    no matter what I do I can't seem to get some little stub to go away in my seattube. I got out the file.. I know that it's near the back of the frame near he clamp crack.... can't fix it.

  7. #7
    coprolite fietsbob's Avatar
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    if there's a big capacity Drill press you can chuck it up in..
    & you just need to polish the parts you can see,
    and having it turn while holding
    a strip of progressively finer emery cloth around it will do fine..

    Or, are you going to machine a shim to take up the space you take off of the post and seat tube ?

    Or just buy a new post 0.2mm larger and ream the seat tube till it fits ?

    I went from 27.0 to 27.2 on my RB1 Bridgestone frame , with an adjustable reamer ...

  8. #8
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    No no. I wouldn't remove scratches below minimum insertion mark... no need for reaming unless I want to get rid of scratching burr. I tried getting it in the lathe but the chuck has a max of 2 inch diameter... end of seat post is more like 3 inches.

    Marcus, delete your post.

  9. #9
    coprolite fietsbob's Avatar
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    the chuck has a max of 2 inch diameter... end of seat post is more like 3 inches
    that's 77mm .. like small field artillery bores

  10. #10
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    You're over engineering this. If you want to chuck it into a lathe and use emery cloth or a Scotch Brite pad while it spins you can, but be warned the head will tend to snag things so it's a good way to break a hand or arm. If you do it, no long sleeves, and keep a light grip on anything so it'll slip from your hand if it snags.

    A safer way would be to fixture the post in a vise (you can use a collet block adapter if you have one) and polish out the post with emery cloth strips shoeshine style.

    How much you can afford to take off depends on the post, and how high you keep it. Modern posts have fairly thin walls and are engineered close to save weight. If you use one near it's max length it won't be forgiving, but if used at much less than it's max length, the torque is lessened and there's more safety margin.

    Also keep in mind that scratches aren't very deep so you won't be taking much off, but you will lose the anodizing which creates weathering issues.

    Summary - in your shoes, I'd leave bad enough alone, and try to camouflage the scratches with some wax. (but that's me and I take pride in my bikes that all show their heavy use).
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  11. #11
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    I honestly don't care much about cleaning it up.. but I wanted to see if it was possible to lathe it... which doesn't seem to be true in this machine when I test fitted today. however, polishing it in a chuck on very low rpm might be safe, but not worth my time. Good point about anodizing. It's a silver post.. I don't know much about anodizing but I just thought it was polished aluminum with no anodizing.

  12. #12
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    u and your lathe. Because you probably can do it doesn't mean is the right/easiest way to do it, like 50 or 70 % of the postings say, sand it and polish it, if you want to do it using a lathe go ahead, dunno why do u ask dude. Just go ahead and tell us if it worked because probably you can start a new business removing scratches from seatposts using a lathe. Or maybe taking pittings and scratches from forks and frames using lathe, that would be epic.

  13. #13
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    condescension.... not appreciated.

    I'm not shooting down anyone's suggestions. I would suspect that in your 3000 posts you've commented and chuckled at 500 people other people just as unacceptable as me. Gosh.. bikeforums is the only forum I've found people who are seemingly oversensitive about how OP's respond to recommendations. It's not like if I didn't go with someone's advice I did not get anything valuable out of the suggestion. If someone in this thread has hurt feelings about OPs not following through with 100% of the posters recommendation, then I suggest they find a more worthwhile outlet for giving technical bicycle input.

    I like being Socratic when I post threads. If I say something that sounds like I know the answer from the start, that is simply not the case. I apologize for coming off in cyberspace as jerk-off with a lathe.

    I have an open mind and I'm young enough not to know the in's and out's of cleaning up parts. If you were presented with the opportunity to use a lathe and you were curious about how facing a seat post with one might turn out, you might consult a bike forum.

    I had no intention of polishing the seatpost by hand because the scratches are so deep. Nor did I originally ever want to buy a bunch of products and do it by hand when I could do it for free and get a better finish.
    Last edited by hillzofvalp; 02-26-11 at 12:36 PM.

  14. #14
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    fietsbob: from center of post to outer edge of mount times 2.. not tube diameter.

  15. #15
    Senior Member skilsaw's Avatar
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    Don't argue with all the members who have answered your question, but not given you the answer you obviously want. Just go ahead and do it.
    The one who has the most bikes wins.

  16. #16
    Senior Member Commodus's Avatar
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    Quote Originally Posted by hillzofvalp View Post
    condescension.... not appreciated.

    I'm not shooting down anyone's suggestions. I would suspect that in your 3000 posts you've commented and chuckled at 500 people other people just as unacceptable than me. Gosh.. bikeforums is the only forum I've found people who are seemingly oversensitive about how OP's respond to recommendations. It's not like if I didn't go with someone's advice I did not get anything valuable out of the suggestion. If someone in this thread has hurt feelings about OPs not following through with 100% of the posters recommendation, then I suggest they find a more worthwhile outlet for giving technical bicycle input.

    I like being Socratic when I post threads. If I say something that sounds like I know the answer from the start, that is simply not the case. I apologize for coming off in cyberspace as jerk-off with a lathe.

    I have an open mind and I'm young enough not to know the in's and out's of cleaning up parts. If you were presented with the opportunity to use a lathe and you were curious about how facing a seat post with one might turn out, you might consult a bike forum.

    I had no intention of polishing the seatpost by hand because the scratches are so deep. Nor did I originally ever want to buy a bunch of products and do it by hand when I could do it for free and get a better finish.
    I'm a machinist for a living, CNC and otherwise. If I had this problem I would probably turn it down too. IF I could get it into the spindle bore, this whole op would take like 15 seconds. Polish it out? I think some of these posters just have too much time on their hands, although of course it would depend on the depth of the scratches. On one of mine, polishing could take days. Also I would be surprised if a silver post was anodized. I know mine aren't, but I suppose it could be. It's not a bad idea. Anyways, if your DOC is enough to take out the scratches, you'll get under the anodizing no problem. It's hard though, a little tough on your inserts, so make sure you get under it.

    Is it safe to remove material on a part like that though? NO idea.

    Whoa...just saw your DOC in your OP. .020? That seems like a lot, I can't imagine the scratches are that deep. I'd start with like .005, in case you're still planning to do this, maybe with some fancy setup.

  17. #17
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    Commodus, this is the newer version of the lathe available to me. It has a pneumatic chuck, but I'm not sure if it's the one listed on the linked machine which is 5.3". I would assume that it is a 5.3" chuck if that's the outer diameter. The issue is trying to get the seat post clamp between the the chuck clamping subdivisions (sorry for lack of better terminology).

    I would be taking off .020 MAX from the diameter.. not the radius. I would step it up by .001 at a time cause that's what this machine is capable of.

    I don't think it is possible to lathe it in this machine unless you go put a center hole on the clamp side of the post.. .and use the arm that comes in from the right of the lathe to hold the post so it doesn't vibrate. The spindle hole maxes out at 2 inches (1.38" on the site it says, actually), which is fine for the post... but putting the post clamp in the spindle would be very difficult from what an amateur operator told me. Of course, he never had done anything like that so maybe there is something to be learned here.

    Any suggestions? You have much more experience I'm sure of that. I might add that it is a post with some set back, so putting it in like any other tube will cause quite a bit of vibration I am thinking.

    Yes, everyone, this procedure wouldn't take more than a couple minutes and it would be perfectly finished. Also, wouldn't polishing afterwards or applying clear coat keep the aluminum from degrading?

    picture of the post model:


    Also have to consider clearances between the bit holder and the spinning post clamp... Though I would be happy to just go up enough to take of the ultegra lettering.
    Last edited by hillzofvalp; 02-26-11 at 01:03 PM.

  18. #18
    Senior Member BCRider's Avatar
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    Guys, he's not looking to ONLY flash up the seat post. He's also wanting to do this as a learning experience for the lathe itself. Learning to set up and program the lathe is as much or more the goal than just slicking up the seat post. You are all looking for the easy answer. He's not. He's looking to set himself a fairly involved maching problem and solve it as an educational exercise. It just happens to be about removing scratches from a seat post.

    hillzofvalp, the question was is it OK to lathe away the scratches by removing 5 to 10 thou from the surface. Well, that would depend on the wall thickness. If you've got an .090 or greater wall thickness I'd say you're good to go. If it's around .070 or even less like some premium posts then I'd say no. In between it would be a crap shoot. If it's a short road post then likely it would be fine. If it's a long MTB post that has lots of post exposure when you install it on your bike I'd suggest you don't want to machine anything away.

    And be aware that it's not a case of machining down to the minimum insertion point if your setup inserts the post below that on your bike. You want to only machine the portion above the clamping area as it sits when adjusted right on YOUR bike to YOUR height. And then the post will only be useable by someone that is your height or taller. You don't want to machine the part that sits below the clamping point.

    A lot of posts also end up bent slightly from use or possibly have a slight curve right from the factory from what I've seen. I've polished up a couple on my time and found that it's not uncommon to have enough run out to see it. Whether this occurs at the factory or from hard use I don't know. Just that it's rare to find a truly straight seat post where the post was not machined at the factory. Thompson is one such maker. The rest seem to be happy using the die drawn finish as it comes from the tubing supplier. Those are the ones that generally have a slight amount of curve. In any case setting it in the chuck may not be enough due to this curve.

    So first you'll need a chuck that can be set to zero any runout. Generally production chucks that only hold raw work pieces have a few thou of runout as there's no need for dead truness. So you either need a four jaw independent or a three or six jaw that has the centering feature. With the portion next to the chuck jaws indicated true to within a half thou you then use the indicator on the other end near the clamp to check that for true. If it's out by more than a thou or two then you won't be able to properly remove the material without creating a varying wall thickness. Now if your machine has zero backlash control over the cutter and the cross feed response is quick enough you MAY be able to set up the cutter to "trace" the runout so that it moves in and out a little as the post is turning and thus maintain a consistent wall thickness. Or if you'll be turning a length that is more than about 4 to 5 post diameters in length then you'll want to find a way to support the clamp end with a center so the post doesn't flex during the cutting. And if using a center you may be able to make up something to hold in the post clamp and then locate that in near the chuck, indicate the stub sticking out to true and then center drill the bit clamped in the post clamp to provide a near truly centered hole to use with the live center to support the post for the cut. The bonus would be that using the center would flex the post closer to a straight line over it's length.

    In the end all the prep will likely take longer than the actual cutting. But such is the normal course of life in any one off machining operation. Good luck with the job if you have the wall thickness to make it safe to proceed.
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  19. #19
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    Well, I don't have calipers on hand, but I know it's under .08 inches. I know this machine is capable of creating very very fine metric threads. I think if it would be able to trace it would need to have a special sensor.

  20. #20
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    I would recommend some inserts that are made for aluminum so you get a good surface finish. I just repaired a frame that was cracked from an undersized seat post, keep that in mind

  21. #21
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    When seat post is rolled on a flat surface there is no apparent indication that it's bent. The post was new 500 miles ago. I don't think I would need to take off more than 3-4 thou, and I would face it about 1 cm above the minimum insertion mark where I usually sit.

    I'm pretty sure it is a 3-jaw spindle, but there's a special collet holder that goes in as well.

    I wonder what's wrong with my seat tube. I am pretty sure all the scratching occurs near the very tip of the lug and some where in the empty gap near the rear. Is there a correct method to shaping it back? I tried filing with no improvement, though it may be the deformation of this post that is causing some extra friction.

    this is the type of seat lug..
    Last edited by hillzofvalp; 02-27-11 at 08:20 PM.

  22. #22
    Senior Member BCRider's Avatar
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    Quote Originally Posted by hillzofvalp View Post
    ....I'm pretty sure it is a 3-jaw spindle, but there's a special collet holder that goes in as well.
    Chuck, not spindle. If you're training on using the machines you may as well learn the right terms. The chuck mounts onto the headstock spindle or shaft so that may be where you've heard it wrong from someone.

    If by some miracle you have the right size collet, and that's highly unlikely given the radical range of post sizes, then by all means use the collet holder and proper collet. But as the guys with experience will tell you there's no leeway with a collet, or at least precious little. They either fit nicely or it's the wrong size. So if you are stuck using the 3 jaw be prepared to indicate the post by the jaws and shim the jaws as required to get the runout below a thou. A half thou would be nice if you're doing a serious job. You'll also soon find out just how truly round the post is as well. So a little reading between the lines to average out for center is often needed.

    Remember, you're doing this as a learning exercise. Being anal over getting it right is just learning for the time you're doing something REALLY critical. So take your time and learn all you can on the subject. It's a very simple job on the surface but when you begin looking at all the stuff that can go wrong if you don't do things right there's a bucket load of stuff to learn and try.
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  23. #23
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    Yeah, I definitely have work to do on the terminology. If 10-12cm of the 27cm post is in the 3 jaw spindle or appropriate collet (??) then I shouldn't have to worry about the post head causing vibrations, correct? I was only intending on turning 11cm or less of the remaining 15cm, just enough to go right before the ultegra logo starts or maybe after it ends if it's not vibrating too much or clearances allow

    I used the machine to shave down a stem shim that was 1 1/4 or so OD. I reckon they have a 1 1/8" collet if that's close enough I don't know (28.5 vs 27.2mm post). I'll check it out tomorrow.

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