Re: [CR] Snapped ajustment screw

(Example: Racing:Beryl Burton)

Date: Tue, 16 Mar 2010 01:42:56 -0800
From: "verktyg" <verktyg@aol.com>
To: minneman@onomy.com, Classicrendezvous@bikelist.org
References: <9103102d1003151544l265e9b4bh3568ecd45ce77a5d@mail.gmail.com> <9327C3B25BD3C34A8DBC26145D88A90717318E@hippy.home.here> <00dc01cac498$161ec9b0$425c5d10$@com> <472674.65452.qm@web111012.mail.gq1.yahoo.com> <1577.99.157.205.104.1268720924.squirrel@www.onomy.com>
In-Reply-To: <1577.99.157.205.104.1268720924.squirrel@www.onomy.com>
Subject: Re: [CR] Snapped ajustment screw


Scott,

Here's a link to a type of EDM machine called a "tap buster" that a lot of larger machine shops used to have on hand. The going price to remove a broken tap used to be ~$45-50+ per tap which could be cheap considering the alternative on a large complex part.

They require no immersion.

http://www.electroarc.com/tabletop.cfm

The biggest expense is set up time to make sure that the electrode is properly aligned with the broken tap or in this case screw.

Many times I've seen broken adjuster screws that were severely bent before braking. The steel used in many dropouts is soft enough that the screw can slightly embed itself into the dropout.

When I run across a bent adjuster screw, I screw it out a little bit before trying to straighten it. That way if it breaks I can file off the bent threads and unscrew it. I usually replace them once they have been bent.

Recently a frame builder friend had a problem trying to drill out a broken screw. He discovered that he was trying to drill into a broken drill that someone had had broke off in the screw!

Chas. Colerich Oakland, CA USA


> John,
>
> I'm not sure exactly what you're saying. My note was reporting that,
> having used extreme care and caution and years of experience (I'm a
> mechanical engineer who is very comfortable in a machine shop and I've
> done this kind of "drill it out" process with some regularity over the
> years), it's doable and it's not that hard. In particular, I have removed
> broken adjuster screws from bike frames more than once, without damage to
> any of the frames I've done it to.
>
> I agree that if you're in a situation where you simply *cannot* mess up a
> frame, then there might be a professional you'd want to turn to...that's
> why I was asking Jack for more details. I'd certainly want to *know* that
> the professional was going to use at least as much care as I would, and
> I'd want to know they were more skilled at the sort of operation being
> considered than I am.
>
> Furthermore, some of the techniques being bandied about here are simply
> not sensible for anything but the most valuable frames. Wire EDM doesn't
> really make a whole lot of sense for this kind of problem, and
> plunge/sinker EDM will work, if you want to pay for it. EDM requires that
> the part be submerged in an insulating fluid (like oil), and the rigging
> needed to get the tail end of a frame aligned with the business axis of an
> EDM machine (and submerged) isn't pretty. Those techniques make sense for
> removing taps or drills (very hardened materials) in a situation where the
> value of the thing it broke in makes the cost worthwhile.
>
> I keep a good machinist in the background, too. This is simply not an
> instance where I'd trot him out, unless it was a frame where the slim
> chance of damage is simply unthinkable.
>
> Ok. Loads of disclaimers and all of that, again. If the chance of
> damage, for you, isn't slim, and you're likely to mess up the
> frame...don't try it!
>
> I'm also the sort who might decide that *building* an EDM rig from scratch
> for such a purpose might be a fun distraction.
>
> There are a bunch of frame builders and restorers on this list. Speak up,
> please -- how do you approach this? I can pretty much guarantee that few
> of you have an EDM rig in their shop.
>
> Cheers,
>
> Scott
>
> Scott Minneman
> San Francisco, CA, USA