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07-27-2012, 10:39 PM #1
1000-oaks to the white courtesy phone: technical question
I have a machining question and you always know everything about random technical shit, so here goes:
I've had this burr on the inside of my seat tube for a long time, but it was pretty stable and never did anything more than gently scratch my seatpost so i never gave a shit. it was in the middle of the front of the seat tube, so i'm sure it was welding slag or some other such buisness that survived the initial reaming. Lately it's gotten completely out of fucking control, has shredded my seatpost, and it takes some serious gusto to pull out the post. So, on top of the initial burr (now long-since filed away), everything is gouged to hell and scraping everything else. badly. I went after both surfaces with a sander and flap-sander wheel respectively, which temporized it pretty well, but after a few rides it's almost seized after being slammed down and the thompson looks like a badger got a hold of it. Clearly, flap-sanding is not going to fix my problem.
I'm going to tee up a new post to replace the shredded thompson, but i've got to get the seat tube cleaned up or i'll just kill another one. I thought about reaming it, but I obviously don't want to change the inner diameter, and that just seems like overkill. So, i'm thinking ball-hone now, but im not sure what kind of grit/finish range im looking for. I'll probably grab a pretty course Al oxide (60) one to knock down the uber gouges and then finish with something in the fine-medium (200?) range, but the fucking things come in like 20 different grits and i don't even have a guess where to start. Any ideas?No, the real point is, I don't give a damn
07-27-2012, 11:27 PM #2
Not sure what kind of ball-hone you have in mind; are you talking a single aluminum oxide ball stone on a shaft? Or a cluster of small balls on a wire "tree", which is used for engine cylinders? (Can't imagine they make one small enough for your use.)
I'd try a car brake master cylinder hone on an electric drill. The three long slender stones would probably work well in a deep narrow bore like a seat tube. Don't take too much off; just knock off all the high points, use slowish speed & lots of cutting oil or WD-40 to help keep the stone from packing up with aluminum dust. Could probably even use water as a lubricant, maybe with some degreaser mixed in for added viscosity. Do it with the frame upside down and use plenty of your lubricant so you don't end up with aluminum paste in there. Soon as the stone pores gets clogged with aluminum it'll stop working, so keep flushing it.
Then go back in with aluminum polish or rubbing compound using one of those little foam balls on the end of a drill shaft, like they use for getting in tight spots in aluminum car wheels. (Hopefully you can find one small enough.) Polish it up real nice and then use a new seatpost, as you found out once the metal starts "galling" (tearing against itself) you're screwed. The new seatpost should have an anodized surface, which will be harder than the inside of the seat tube and should stop the galling because only the inside of the seat tube will wear, instead of both surfaces tearing against each other. I'm not a fan of greasy seatposts, but you might try some dry chain lube in there to help keep it from happening again. Probably won't end up perfect, but should be good enough for government work.
You're lucky, the seatpost could have been locked in there permanently. Gummy metals like stainless steel are horrible for this sort of thing, which is why you see bronze nuts on stainless bolts sometimes.
Last edited by 1000-oaks; 08-01-2012 at 03:21 PM.
07-27-2012, 11:53 PM #3
Also, might want to use a seatpost other than a Thomson. They make the seatpost wall thicker at the front and back by squashing a round heavy-wall tube (or possibly extruding it in that shape, didn't look too close), then turning the OD in a lathe to make it round again. That leaves a ton of machining grooves in the OD, which carries in dust and isn't as smooth as other posts.
07-28-2012, 06:09 AM #4COWHAMPSHIRE PARADISE
- Join Date
- Dec 2009
holy fucking shit........
rogSKI THE EAST
cuz it ain't fucking cool
07-28-2012, 06:24 AM #5
Nice response! Ditto on the thompson, one of them seized in my frame and the results were not pretty.
07-28-2012, 09:09 AM #6
I can get this in Al oxide - think 120 is course enough for the first pass?No, the real point is, I don't give a damn
07-28-2012, 02:03 PM #7
120 might be too coarse. I'd rather spend more time working a finer hone in order to spend less time polishing away the scratches added by a coarse hone.
07-28-2012, 02:13 PM #8
This guy makes a good point about using the speed of the drill to control the pressure of the stones on the walls of the brake master cylinder. Faster = more pressure, so take it slow. The hone is designed for steel, so aluminum will cut much more easily. It'll be hard to keep the stone from packing up with aluminum, so keep dipping it in lubricant every few seconds and maybe wire brushing it off.
07-28-2012, 02:35 PM #9
Just occurred to me the relief slot for the seat tube clamp it going to prevent you from using the hone in the top inch or so of the seat tube. How good is the frame? If you're willing to put some work into it, you could prep for the hone job by putting duct tape on the inside of the seat tube, across the slot. Then fill the slot with JB Weld and build it up to either side of the slot a bit on the outside of the tube for stability. Let the plug set overnight, wrap a bunch fiberglass filament packing tape around the top of the seat tube tightly to lock your JB weld plug in place, remove the internal duct tape, smooth out the JB weld a bit with a Dremel if you need to, then do your hone job. Carefully chip off the JB weld when you're sure you're done honing & polishing. Hopefully the JB weld will keep the hone from catching in the slot.
07-28-2012, 05:11 PM #10
Grit wise, I think i've settled on this plan: 1: http://www.brushresearch.com/brushes.php?c1=1 in a 180 to start and then in a 320 to finish it. i'll just get their honing oil and plan to use a bunch of it. 2: http://papa-johns-toolbox.amazonwebs...B000R2XNKA.htm to buff/polish.
sound reasonable?No, the real point is, I don't give a damn
07-28-2012, 05:51 PM #11
Don't worry about the relief for the seat clamp - use a slow drill speed to help sink the hone into the seat tube, get 'er up to speed, wiggle it around in there a bit, and keep the hone spinning as you come up and out the top of the seat tube. I kind of make a vertical sawing type of motion when I'm doing this. Total time for the whole procedure on new bikes, typically between 3-4 seconds and 8-10 seconds, depending on how gnarled things feel.
07-28-2012, 07:09 PM #12
That polishing kit looks perfect. You really probably don't really need to polish the inside of the seat tube, but it can only help and it sounds like you want to do a kick-ass job to make sure the problem doesn't happen again. The Canfield is worth it.
willmtbike4food actually has experience with this, but I'd use this "3-Stone Hone, Fits Cylinders to 2" (21.4-50.8mm)". Lisle makes good stuff:
Mfg info: http://www.lislecorp.com/divisions/p...n=1&category=1
For an engine cylinder I'd use the ball hone, but for this job I really think the long stones will give you better results since you just need to knock off high spots. The ball hone will attack the metal EVERYWHERE; instead of just the high spots. You don't want to be taking material off the low spots, or you'll end up with a loose seatpost.
The ball hone is better for engine cylinders because the cylinders can wear to a taper, and 3-stone hones can make the taper problem worse by putting MORE stone pressure on the larger ID end. The ball hone puts LESS pressure on the larger ID end.
But your seat tube isn't tapered, so use the $10 3-stone hone and just take off the high spots. Best tool for the job IMHO; don't lose any sleep just because the tool is cheap.
Last edited by 1000-oaks; 07-28-2012 at 10:49 PM.
07-28-2012, 07:24 PM #13
Before hitting the top section, if you have a Dremel with stones, you could use it to put a little ramp or radius on the leading edge of each of the stones on your hone. That might make the stones less likely to catch on the far side of the slot. If you have a tiny file, you could also knock off the sharp corner on the inside far edge of the slot so the stones are less likely to catch.
Let us know how it turns out, and maybe change the thread title to "how to repair a gouged seat tube" or something for future maggot reference.
07-28-2012, 08:01 PM #14COWHAMPSHIRE PARADISE
SKI THE EAST
- Join Date
- Dec 2009
cuz it ain't fucking cool
07-28-2012, 10:31 PM #15No, the real point is, I don't give a damn
07-31-2012, 10:40 PM #16
auto zone + sears + new seat post = fixed.
i'll throw up a "how to" in a separate thread in the next few daysNo, the real point is, I don't give a damn