Attaching extension to cast iron table saw top?

Question

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?

Rick S. Carusi · Accepted Answer

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

Roy Smith · Answer

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.

Chuck Taylor · Answer

Mine (a 1987 model) is tapped to accept an M10-1.0 bolt, which is whatwas used to attach a Jet-Lock fence rail.

bent · Answer

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 size9-1.25mm 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  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.

CW · Answer

Cast iron drills, taps and machines great. When tapping, back out often. Itdoes tend to clog.

cselby · Answer

Cast iron is one of the easier metals to both drill and tap.

Roy Smith · Answer

Thanks, that's what I was hoping to hear.

Upscale · Answer

Just make sure you go slow, with lots of oil and backing out frequently toremove the waste. Cast iron likes to crack under the wrong conditions.

WoodWizzard · Answer

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:

Pat Barber · Answer

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....

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drilling required......

Roy Smith wrote:

Frank Boettcher · Answer

I don't know where you get your Unisaws, but where I get mine thoseholes are tapped.Frank

Attaching extension to cast iron table saw top?

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