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I'm here at the metal store, and I just bought a 1" peice of round bar that I was going to use as a frame slug. I have a cast steering neck with 1" holes for the downtubes, and the 1" bar fits well, but not incredibly snug. It's probably 5 thou shy of the tube od. I figured I may as well go back in and by the next size up and turn it down to as close as I can get it. Question is... How tight of a fit does it have to be and why? Some kind of a translation into thousandths would be nice, I still haven't totally figured out what snug, press, slip fits are definitavely yet.
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You don't need a lot of interference since the slug is basically an assembly alignment aid providing more weld surface than a butt weld. Five thou out is only half that per side and your welds do all the work. I'd use what you have but it can't hurt to turn a thicker bar down. If you can fit the slug in a (deburred) frame tube with a gentle push that's plenty.
Download this. Every machinist should have a copy and they change very little over time. It's a canonical fit reference.
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Why do people say to make slugs length at 3 times the width of the tubing? Still for alignment purposes?
Thanks for the download link.Comment
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The longer the slug the more accurately it can align two tubes and the rule of thumb wisely leaves plenty of room for weld. Weld repairs should be stronger than the parent metal so if the work is flawed it will still be more than sufficient. Compare weld surface area on a butt weld, a slug inside a butt weld (not much more, but the slug resists burn-through so ya can turn up the amps) and a long slug with slit or plug welds which add far more surface area. You could play on some scrap and cut it like a test coupon for a really accurate idea.Comment
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I think the three time thing is an engineer thing. I remember it being from the airplane crowd frame builders. But have been wrong on things multiple times. Plus if its long you can drill holes and plug weld it firstComment
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It's similar though aircraft repairs use tubing slugs ("inner sleeves") for lighter weight.
Begin around page 277 for acceptable aircraft practice. (I'm an AMT). The whole book is a very interesting read. Your taxes paid for it so enjoy!
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I thought someone on CC said don't use solid round stock but tubing. I could be wrong.
Here's one from the past -
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I think the 3 times diameter in length refers to maximum strength of the joint,
I know that when tapping a hole, you attain maximum strength at one & a half times the diameter of the bolt in thread depth,
any less and threads can strip when set at maximum recommended torque, and any more is pointless as no extra strength is gained.
I presume that at 3 times the diameter in depth for un-threaded inserts, like a welding insert gives it maximum strength.Last edited by Guest; 02-07-2020, 7:50 AM.Comment
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I'd want an engineering justification for that as solid stock repairs are quite common and the extra material cannot compromise a joint. Perhaps someone pressed or bashed a grossly oversize slug into a tube and cracked it but that's a fuckup. Chopper shit is 90-percent redneck backyard "slap it together and see what works" with random un-researched beliefs thrown in, yet joint failures are rare.
I don't do old wives tales and look for engineered examples where practical. If anything a solid slug is excess metal (see the aircraft tubing joints in the AC I linked) but choppers aren't aircraft and the weight doesn't matter. The (backyard) theory has been advanced that a solid slug might flex less than a hollow slug yet all sorts of heavy gussets (including official Harley frame repair gussets) have been used to reduce flex at weak joints and tubing sections. If weight didn't matter the strongest frame would be solid rod stock.
If in doubt, copy aircraft practice because everything less than aircraft practice receives lesser scrutiny, especially engineering evaluation and Non-Destructive Inspection.
We have a couple of CWIs here who might weigh in with example WPS for welded tube splices.
Here's the structural code for reference (interesting stuff, download a copy) but I've not time to dig in at the moment:
Per the AC, page 279 there is a directed minimum length but no maximum for the inner sleeves. Note their preferred tubing cuts are angled to prevent creating a stress raiser "ring" around the tubing. Ref. Figure 6-41:
Cut two sections of tubing from this inner-sleeve tube material, each of such a length that the ends of the inner sleeve will be a minimum distance of one and one-half tube diameters from the nearest end of the diagonal cut.Last edited by farmall; 02-07-2020, 9:05 AM.Comment
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Read stuff for both solid and tube slugs, some said tubing allows more flexing with less chance of cracking at the slug ends...tube allows for internal wiring if ya wanna go that route.If buildin' old school choppers was easy, anyone could do it... ain't nobody said it's gonna be easy...Comment
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Is there ever a concern about capping moisture up in a tube when you are doing this?
Should they be vented in some way, or an inert gas floated through when welding?Comment
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There's no need to purge carbon steel motorcycle frames (it's done on some aircraft tubing structures but they're considerably thinner) nor particular tendency to rust since welding drives off moisture. Stainless is normally purged with the same argon used to TIG it, ditto titanium. If you purge you need to make a hole for the purging gas then seal that or it's a moisture intrusion point.
Motorcycle frames are designed for mass production by wire welding. Their predecessors were acetylene welded or brazed. They are not purged during manufacture. They last fine but it can't possibly hurt anything done correctly.
Aircraft practice is (at owners discretion) to treat the inside of tubing and in some cases make holes for corrosion-preventive compound to flow through. Linseed oil is traditional and there are industrial compounds like CRC. If you treat with oil you are asking for trouble in future repairs because it's so difficult to clean inside tubing so I wouldn't do it on a bike.
Chromemoly is rust-prone but your motorcycle is unlikely to be made of it. Some were including (IIRC) Norton P11.
Unless your motorcycle gets immersed in salt water or similar I'd not worry about internal frame corrosion. It will be long dead of the usual causes many years before. The only bikes I've seen with frames rusted through were abandoned outdoors for many years, had shit paint jobs which were worn, and corroded from the outside inward.
Purging is harmless so if it pleases you have at it. Handiest way to set up a purging rig is two flowmeters on a tee so if you break one flowmeter you don't lose them both if you have them rebuilt or DIY with a seal kit (which is easy). All it costs is a tee, another flowmeter or spare regulator, green hose and a fitting (your local welding supply stocks the stuff or order online). That's how we set up our welding school booths. You can also purge with a Y-valve but separate flowmeters ensure precise control. Don't forget to shut off your gas when not welding.Comment
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all I would ad is chamfer both ends of your slugComment
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Good info, farmall.
Regarding Linseed oil and protectants:
We were trained to use weld through primers on structural components in automobiles. Unibody construction.
3M Makes a good product in a can for this purpose.
Haven't bought any in years. I think this is what it looks like now:
We were told it liquefies and travels in liquid form inside the rail when you weld, then dries and protects against rusting from the inside out.
Which happens because you burn off any coating in there during the weld and leave it naked.Comment
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