Stripped threads on tablesaw arbor bolt holes. Repair how?

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I have a General 50-185M1 table saw. The arbor is held to the top by 4 5/16 bolts, two in front and two in rear. Both of the rear bolts have stripped the threads from the cast iron table top. One of the bolts had obviously been cross threaded when the saw was manufactured. Saw is out of warranty.
Does anyone here have experience with repairing something like this? I would appreciate some advice on how to go about getting this fixed. I have no experience with metal working.
Ray
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If there's enough "meat" left in the cast iron, you could drill out to the next larger bolt, then tap new threads. Another possibility might be Helicoil inserts, rather expensive, available at your local auto parts store. HTH. Tom
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Wed, May 2, 2007, 1:50pm (EDT-3) snipped-for-privacy@msn.com (tom) doth sayeth: If there's enough "meat" left in the cast iron, you could drill out to the next larger bolt, then tap new threads. Another possibility might be Helicoil inserts, rather expensive, available at your local auto parts store.
Helicoil, I couldn't remember the name. I think they're available at real hardware stoers too. Should be able to get them on-line.
Might even be able to just tap new threads without drilling first, depends.
Might be able to weld it, drill a hole, and tap new threads. But for something that you're talking a professional doing the work and expensive. I'd save this option for last.
Or, maybe you could just buy a new, or used, part.
JOAT What is life without challenge and a constant stream of new humiliations? - Peter Egan
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On Wed, 2 May 2007 17:26:15 -0400, snipped-for-privacy@webtv.net (J T) wrote:

If it vis a through hole what would be wrong with a longer bolt with a nut? jesse
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The nut would probably get in the way of running a board through the table saw. I would just as soon not have a hole drilled through the top of mine. You do know that the arbor brackets are fastened to the top!
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Thu, May 3, 2007, 2:28am (EDT+4) snipped-for-privacy@telus.net (Jesse)doth query: If it vis a through hole what would be wrong with a longer bolt with a nut?
You asking me? Could well be, but being as I'm not really familiar with his machine, or there to look at it, can't say for sure.
JOAT What is life without challenge and a constant stream of new humiliations? - Peter Egan
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Ray Miller wrote:

This is one reason I've always been a big fan of studs vs. bolts .. .. screw in a stud with Lock-tite and it's there forever. Cast iron tends to shed it's threads if over-torqued a few times.
That said, I would try to drill & tap for a bushing .. the size you drill/tap will depend on what brand/type of bushing you use. Heli-coil, keensert, several others to choose from. Unless you are unusually adept at drilling/tapping perfectly straight holes in confined spaces, it would probably be best to remove the top and perform these tasks in a drill press.
If the through-holes in the trunnion are big enough, you may be able to jump up a size or two, drill/tap and be done with it.
I would definitely not weld up the hole and try to drill/tap back to the original size. Cast iron does not like to be welded .. .. cracks and warping will happen if not done correctly.
Again .. once you get the bad hole repaired, I'd replace all the bolts with studs, LocTite'd in place. If you ever need to make adjustments in the future (and you will) the bolts take the punishment, not the casting.
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Ray, I agree that one approach is to use the helicoil. But be aware that you will likely need a special tap for the helicoil. Some helicoils, not all need a special tool to insert them. And you still have to drill the hole out. Typically the helicoil approach needs more side wall material that can be sacrificed to make the hole big enough to fit the helicoil in.
For the "other" approach, yes you will need enough side wall material to tap to the next size. Just make sure you use the right size drill. Not everyone has drill sets with the correct size. When you drill the hole out make sure that you drill as straight on as you can. If you have the access to a drill press, take the part off and use the drill press. The results will be better. Also use a drilling lubricate such as "Tap magic". Holes like this have been known to have the drill bit grab and then break. The same goes for the taping of the hole. If the hole is not a through hole then you will need a "bottoming" tap. This tap has a squared off head so you can thread as close to the bottom of the hole as possible.
What every the approach used take your time. Do not drill at high speed, less chance of the drill breaking. I figger you have one shot at fixing this. Good luck
Paul

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goaway wrote: Also use a drilling lubricate such as "Tap magic".
Cast iron = kerosene
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Sorry to hear about your troubles. Good advice from everyone especially Paul.
IIRC Helicoils aren't that difficult to install but I've only installed helicoils in Al heads for spark plugs. Kit came with a tap that didn't require drilling (Al head) and an installing tool for screwing in the coil. This approach, as was said, will depend on how much meat is left around each hole.
Whatever you try GO SLOW, once something breaks off in the hole you have cut you chances of a good fix drastically.
Also keep in mind that you can find a contractor saw on Craigslist for around $100.
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Ray Miller wrote:

<snip>
As has been mentioned by others, Helicols.
I would not try to do this repair with hand tools.
Knock the saw down so you can get the table in a drill press, better yet, a universal milling machine, to install the Helicols.
You get one shot at this one, IMHO, do it right.
Have fun.
Lew
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I would. An old hand-cranked flywheel bench pillar drill is the _perfect_ tool for carefully and accurately recovering and tapping holes in cast iron. A suitably modified baseplate and shortened column allows you to "drill through its feet".
If you're regularly involved in resurrecting old iron (typically old steam engines), then it's well worth setting up such a machine for this task. They're not that hard to find (eBay, plant auctions etc.) and the extra control of hand-crank over power is a better way to do it.
Also remember that not all "helicoils" are genuine Helicoils. Helicoils are a coil of shaped spring wire, most of the "spark-plug recoverers" are solid bushes. It's well worth paying the money for a real Helicoil compared to a cheapy, even if thise means paying for the guy who already owns the magic Helicoil tap to do it, rather than doing it yourself from a kit with a disposable tap. Wire spiral inserts lose a lot less radius than bushes do. Helicoil's shaped wire gives a much stronger installation into cast iron than a simpler wire profile that uses a standard tapping thread.
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Thanks to all responders. Looks like heli-coils are the way to go. I called a lical machine shop and they also agreed that was the best bet and they are willing to do it for about $30. Since they have the tools and experience with this product I will let them do it. They also can recover more easily if something breaks.
Ray
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Ray Miller wrote: > Thanks to all responders. Looks like heli-coils are the way to go. I > called a lical machine shop and they also agreed that was the best bet > and they are willing to do it for about $30.
I call that a VERY good deal.
Lew
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snipped-for-privacy@earthlink.net says...

(piggyback)
What nobody mentioned--probably because it likely would not work--is to chase the thread with a same-size tap. More than once, I have thought threads ruined and chasing brought them back to life. Anyway, the machine shop must be owned by your brother; I can't imagine them doing anything for $30. :-)
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says...

Actually, this is about what we'd charge at the shop I work at. It is a pretty quick fix for people that do it on a regular basis. If you brought just the top in and it was ready to be fixed, it would likely be done before you could go to McDonald's and get a Big Mac.
Mike
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The Davenport's wrote:

It's a lunch-time fix but I would probably charge him $10 a hole. Bridgeport. Clamps. Indicator. Gage pin(s) to locate the holes. Drill. Tap. Helicoil. Needing to eat my baloney sandwich when the boss wasn't looking. Yeah ... about $10 a hole. ;-)
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wrote:

Good decision!!
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Great deal! Let us know how it works out.
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Take the part to an auto repair shop or lawnmower mechanic and he will heli-coil it which is a permanent fix. Or you can buy the kit and do it your self. It involves drilling and tapping. If you don't have any experience with this don't try it unless you practice on something else. Too bad you're not close to Norfolk Ne. I'd do it for you cheap.

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