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Drilling a Cast iron table saw top

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bp122

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Hi all

My axi table saw has a cast iron table top.
On its sides and back it has tapped m8 holes on the vertical edges, but the front side doesn't. Instead it has two blind tapped holes going upwards from underside to mount the fence rail.

This complicates the t square fence I am designing. It will simplify the fence a lot if there were tapped holes on the front face.

I think it measures about or just under 8mm in thickness on that face.

Would it be a good idea to drill and tap this face to attach a rather heavy steel fence rail? Would I need a special drill bit?
Of course, I will be using a drill guide bush to get it perpendicular to the surface.

My concern is only 8mm (or less) of thread engagement (unless I can put a nut in the back if I can reach in there)

Thanks in advance.
 

Rorschach

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Nut in the back would be best but 8mm for an M8 thread would give full strength. You need 3 threads full engaged for full strength, so for an M8 thread that is about 5mm of depth required.

Quick and dirty way to see if your material is thick enough is measure the thickness of a nut that matches the thread you want to use. If I recall an M8 nut is about 6mm thick.
 

AES

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It's quite understandable that when thinking about the properties of cast iron, people who haven't tried it before are a bit "nervous" about drilling and tapping it.

But the reality is generally speaking, MUCH better! CI is normally very easy to drill and tap and no special tools are needed, just standard metal drills (HHS preferred of course), ditto taps.

There CAN on occasion be a bit of a problem drilling/tapping into a new casting (there can be hard inclusions hiding inside the cast shape) but as your table saw top has undoubtedly already been machined (ground), the chances of you finding any such hard spot problems are very slim indeed.

Just make sure the drill and tap are both sharp and you'll actually find it easier than drilling, for example, MS plate. Just take it easy and you'll see little piles of "grey grit" (not curly swarf like with MS) building up around the hole. CI is very often self-lubricating, but if in any doubt/if your feel resistance building, especially while tapping, clear the "gritty swarf" off the tap and from the hole, then add a drop of ordinary light oil ("3 in 1" etc).

I can't quite picture your proposed new set for the fence (sorry, I don't know much about table saws and don't own one myself) but the only point you PERHAPS need to be careful of is if there are any rotary forces where you're bolting into that 8 mm web (IF I've understood you correctly) - that MAY need a bit of local stiffening, but not sure.

HTH. And just take it easy and when you've (successfully) done you'll wonder what you were worried about - 'onest!
 

bp122

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Thanks guys.
That puts my mind at ease.

It's just that I wanted to have the best information available before I wrecked something :lol:

I shall put a full blow by blow account of the fence design and build, once I am done.
 

TheTiddles

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Is it a TS200? I fitted a new fence system to mine years ago using in think M8. 1D of thread is plenty for low load especially if you are using several. Cast iron works beautifully, standard procedures (Centre punch, pilot, gradually increase etc) and you’ll be fine. I think you could also put a clearance in and fit a nut in the gap if it’s where I’m thinking of
Aidan
 

Spectric

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Hi all

Parafin is a good cutting lube for cast iron, remember having to drill and tap several 14mm holes in a compressor flange some years back.
 

AES

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As per my previous post on this thread, often I've found that no lubricant at all is required when drilling & tapping CI, especially when the CI has been previously machined, as would be the case with the OP's item.

However, I DO recommend a little lube when drilling - and especially when tapping - ESPECIALLY if undue resistance is felt. Apart from removing the tap fully to clear swarf, and then re-entering to continue the thread, just about any lube will do - paraffin, light oil, etc, etc.

The reason for both the clearing swarf carefully and for a drop of lube is that although CI is generally self-lubricating, the swarf you get from drilling and tapping in CI is not long "stringy curls" of metal (like you get with MS for example) but is a fine grey "powderish pile" of "sticky dust". This can (and does) often tend to block up the swarf removal spirals of both drills and especially taps, and that is more prevalent so in the smaller diameters (though 8 mm is not particularly small).

@the OP: Have you done this job yet? If so I bet you found it a lot easier than you thought, as I said in my earlier post in this thread. (Of course, IF you found it very difficult then I've moved!!) :cautious:
 

bp122

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@the OP: Have you done this job yet? If so I bet you found it a lot easier than you thought, as I said in my earlier post in this thread. (Of course, IF you found it very difficult then I've moved!!) :cautious:

Not yet. I was waiting for some of the metal bits for the fence, they must be in this week. SWMBO has tasked me with finishing the garden fence and the little one's nursery first, after that it is drilling time!
 

Cabinetman

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Hi, I’ve never done much metalwork, but I needed to attach something to the front of my Sedgwick spindle molder and that was about 8 mm thick cast-iron, absolute doddle of a job and successful as well, In that the bolts gripped really well.
Ian S
 

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