Attaching extension to cast iron table saw top?


I've got a Unisaw that I want to attach an extension table to the rear of. There's two holes (approx 3/8" diameter) already drilled in the rear flange of the cast iron top (it came from the factory that way), but they're not tapped; just through-drilled. They're a little too small for clearance of a 3/8" bolt; it would be nice if they were whatever the tap drill diameter for a 3/8 UNC is supposed to be, but I haven't measured them exactly, so I'm not sure.
I suspect the easiest way to attach something would be to tap those two holes and bolt on a piece aluminum angle, to which my plywood table would get attached. How difficult will it be to tap these holes? I've tapped holes before, but never in cast iron. Does it go easy? Is there any risk that I'll damage the top? What if I end up having to drill some additional holes in the flange?
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My guess you have the required tap. 3/8-16 tap drill is 5/16. If not, why not use a bolt & nut that will pass? May need to go metric for a better fit

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Because it's danged near impossible to get to the back side of the flange to hold a nut in place. The sheet metal cabinet is in the way. I tried reaching in from the throat opening, but I'm not enough of a contortionist to get my arm where it needs to go.
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Mine (a 1987 model) is tapped to accept an M10-1.0 bolt, which is what was used to attach a Jet-Lock fence rail.
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in order: 5/16-18 NC is F (.2570"/6.528mm) 8-1.25mm is H (.2660/6.756mm) 5/16-24 NF is I (.2720"/6.909mm) 8-1.00mm is J (.2770"/7.036mm) 5/16-32 NEF is 9/32" drill hole size 9-1.25mm is 5/16" 3/8-16 NC is 5/16" 9-1.00mm is O (.3160"/8.026mm) 9-0.75mm is 21/64" 3/8-24 NF is Q (.3320"/8.433mm) 10-1.50mm is R (.3390"/8.611mm) 10-1.25mm is S (.3480"/8.839mm) 10-1.00mm is 23/64"
from kbctools.com catalogue, not double checked/verified. Letter equiv's from Machinery's Handbook. If you want to know if, consult it. If you need to know how to make the tool to make the tool, consult it.
If you screw up you can use a heli-coil, etc.
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Cast iron drills, taps and machines great. When tapping, back out often. It does tend to clog.

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Cast iron is one of the easier metals to both drill and tap.
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Thanks, that's what I was hoping to hear.
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Just make sure you go slow, with lots of oil and backing out frequently to remove the waste. Cast iron likes to crack under the wrong conditions.
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Check them again!! there tapped. 3/8" - 24. If they are not tapped and you have a recent Unisaw, call your dealer and make him replace the table.
Frank
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Cast iron taps fairly easy, but it clogs the tap, so back it out to clear the chips, then tap anouther 1/4 - 1/2 turn or so. DAGS comes up with this site for drill sizes: http://www.newmantools.com/tapdrill.htm
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Those are designed "exactly" for that purpose. Those holes match the Biesemeyer back rail, which an outfeed table can be attached.
The Biesmeyer rail is attached with thru bolts and nuts.
Go here for detailed instructions on how to build the outfeed and bolt sizes....
http://www.biesemeyer.com/print/comm_fence/Commercial%20%20Fence%20Manual.pdf
No drilling required......
Roy Smith wrote:

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I don't know where you get your Unisaws, but where I get mine those holes are tapped.
Frank
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Roy Smith wrote:

Why not just use a 1/4" or 5/16" bolt with a nut on the back side?
Chris
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Maybe I'm not understanding this question very well. You want to attach an extension to the BACK of your Unisaw? For use as an outfeed? If you do, the miter slots will be useless, unless you have a way to cut them through. Either way, drilling and tapping holes in cast iron is easy and explained in other replies.
Dave
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Yes, exactly.

I figure I'd just cut two dados in the top to line up with the slots.
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wrote:

back of your saw! I re-read the original post again and realized you did not say what I thought it did.
Dave
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