Drilling holes in cast-iron table saw extension

I just got two nice new solid-surface cast-iron table saw extensions to replace the webbed ones that came with my old Craftsman table saw.
The old webbed extensions had holes on either end but the new ones have holes on only one end.
I would like to still use the old webbed extensions to further extend the new extensions but to do so it seems like I would need to drill 4 oversize 5/16" screw holes (oversized to allow fine adjustment) to line up with the holes on the webbed extension.
The holes would have to go throuh about 3/8" of cast iron.
How hard would it be to drill such holes? - Any special bits? - Or should I take this to a machine shop or something to get it done?
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Cast iron generally cuts like butter. Slow speed and start with a pilot hole. Any regular drill bit will do just fine.
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Cast iron drills easily enough - nothing special needed except maybe a bit of oil or some WD40 to lubricate the bit. Drill a pilot hole first which will keep you larger bit from wandering.
Can't say I'm crazy about your idea of attaching the solid extensions to the web extensions but maybe the other way around would be better. Use the lighter web extensions furthest out and make sure to use some big washers on the bolts that hold the extensions together. Be sure you have some metal shimming material so when you assemble the extensions, you can shim them so the top surfaces are flat across and in alignment with the main table.
You didn't mention how wide or heavy these extensions are but consider that you may have to add bracing to prevent warping of the extensions over time or breaking the cast iron edges because of to much stress.
Bob S.
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BobS wrote:

A few years ago I added a router wing to my contractor saw outboard of the pre-existing webbed wing (although it was designed to be a replacement wing; not an additional one. I ended up building an additional outboard wing for the other side of the saw out of oak and melamine to better balance the load and to increase the saw's ability to handle sheet goods. I have had no problems with warpage or sagging since then and have not added bracing.
Drilling holes was pretty much a non-event. Just be sure to do it in places that leave you room to get a wrench around both the bolt head and the nut so you can tighten the hell out of it as required.
--
Mortimer Schnerd, RN
mschnerdatcarolina.rr.com
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I guess I was unclear -- I was planning on mounting the lighter web extensions to the inner solid extensions (both from a weight/balance perspective and because I want the nice smooth solid surface extensions closer to the blade which in fact is the primary reason that I bought the solid extensions to replace the old web ones).
Second, I shouldn't need to use shims if I use the same size holes as in the original extensions since they leave enough room to move the extensions up/down to align the surfaces..

The extensions are each 24" wide. I was planning on adding legs at the far ends either made of 2x4's or angle iron.
Thanks for the comments and feedback!!!! Much appreciated.
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If I'm understanding correctly, shims generally aren't for vertical (up/down) alignment. They are used to correct angular misalignment. For example, after bolt-up, you find the outside edges of the extension are lower than the inside (bolt side) edge so that the extension slopes downward. Shims on the lower side of the adjoining surface, below the bolts, will kick the outer edges up so that the extension is co-planar with the table.
Tom Veatch Wichita, KS USA
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<Tom Veatch> wrote in message wrote:

Correct and thanks for stating that procedure so succinctly....
Bob S.
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No problem drilling in cast iron. 1) center punch - with regular bits, 2) drill small pilot hole (approx 1/8") 3) drill larger hole.
I use "cutting oil" (normally for cutting pipe threads and readily available) for a lubricant when drilling iron or steel.
That's all there is to it.
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Not hard to do. Centerpunch and pilot with a small diameter or use one of the "Pilot Point" bits I've seen from DeWalt. Best to use a cutting oil for lube and cooling. I usually use the same cutting oil sold for cutting pipe threads. Drill at a much slower RPM than you'd use for wood. If you can jig it up to a drill press, that'd be my choice.

Not really except the brad point bits used for wood would be a poor choice. There are two general "standard" twist bits, those with tips ground at 118* and others at 135*. 118* are very common and should work fine. If you have 135* bits, they may work better in cast iron, but if you don't have them, it wouldn't be worth going out of your way to get bits with the shallower angle for no more than you're talking about.

You could do that, but it's simple enough to do yourself.
Tom Veatch Wichita, KS USA
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The only thing I can add to other's remarks is to be cautious as the drill bit starts to break thru, If you are using a powerful 1/2" drill by hand the bit can grab when you least likely expect it to as it breaks thru. Leave the trigger lock off when it gets close.
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Snip

Drill cast iron dry, no lubricant is necessary, use ordinary twist bit.
--
http://doit101.com

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