Tap and Die wrought iron

I have a front door which has an elaborate cast iron/wrought iron frame. I really don't know which! It looks a bit like this
http://www.p-wholesale.com/upimg/21/844a1/wrought-iron-door-gt-new043-392.jpg
I need to drill into it and screw in a small screw - something like M3 or M4. I did manage to make a small pilot hole with an HSS drill, and I am fairly hopeful that I can enlarge that. Before doing so, I want to make sure that I can find a suitable tap for the M3 or M4 screw.
I tried Screwfix, and they sell whole sets for quite a lot of money, and none of them have good reviews. I just need one, so can anyone advise me what to get, please? And where?
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A good tool shop or engineering supplier. Of course you may not have one within reach. And they can be very expensive. Ebay is probably the easy option. Look for a known brand like Dormer.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Wednesday, September 5, 2012 2:39:43 PM UTC+1, GB wrote:

It's most probably mild steel unless it's quite old, in which case it *could* be wrought iron - but I'd not treat those very differently. It's unlikely to be cast iron, I'd say.
There are plenty of online suppliers if you don't have a good tool shop near you, or you could try eBay - e.g. <http://www.ebay.co.uk/itm/HSS-M3-METRIC-HAND-TAP-SET-3PCS-3-0MM-X-0-5MM-/250453022361?pt=UK_Home_Garden_PowerTools_SM&hash=item3a5029d699 You'd probably be looking to pay around £3 for an HSS tap (I wouldn't go for carbon steel) or maybe £5-6 for a set of three, plus around a fiver for a tap wrench. As long as you're tapping a through hole or one drilled significantly deeper than the screw you can probably get away with just a taper or second tap. (The traditional way is to start tapping using a taper tap, then when this has reached near the bottom of the hole do the same with the second and then the plug or bottoming tap) You can buy special lubricants for tapping, but for a one-off you could probably get away with 3-in1 or similar. I wouldn't try to tap mild steel or wrought iron without lubricant. Finally, remember that taps are quite brittle and an absolute PITA to remove if you break them in the hole, so take things steadily, make sure you start the tap straight, and don't try to take the tap hard down to the bottom of the hole (that part is much easier if you can arrange for it to be a through hole!) If you can use an M4 rather than M3 tap it probably will be a little stronger. Good luck!
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On 05/09/2012 16:04, snipped-for-privacy@gmail.com wrote:

be wrought iron - but I'd not treat those very differently. It's unlikely to be cast iron, I'd say.

you, or you could try eBay - e.g. <http://www.ebay.co.uk/itm/HSS-M3-METRIC-HAND-TAP-SET-3PCS-3-0MM-X-0-5MM-/250453022361?pt=UK_Home_Garden_PowerTools_SM&hash=item3a5029d699

carbon steel) or maybe £5-6 for a set of three, plus around a fiver for a tap wrench. As long as you're tapping a through hole or one drilled significantly deeper than the screw you can probably get away with just a taper or second tap. (The traditional way is to start tapping using a taper tap, then when this has reached near the bottom of the hole do the same with the second and then the plug or bottoming tap)

probably get away with 3-in1 or similar. I wouldn't try to tap mild steel or wrought iron without lubricant.

if you break them in the hole, so take things steadily, make sure you start the tap straight, and don't try to take the tap hard down to the bottom of the hole (that part is much easier if you can arrange for it to be a through hole!)

Thanks so much! I think that that answers all my questions, including the ones I have no asked! (I was all set to ask what the difference is between a taper, plug and bottoming tap.) It does need to be a blind hole, so I'll need all three.
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On 05/09/2012 14:39, GB wrote:

Would a self-tapping screw do the job?
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On 05/09/2012 16:05, Andrew May wrote:

It will need to be undone and done up again from time to time, so it's probably better to do it right from day one. Thanks
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On my old Rover, there are some self tapping bolts with a fine thread and flutes down them that go into mild steel plates. No idea what they're called or where you'd find them. The smallest are 1/4" diameter.
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wrote:

Taptite. They are available in much smaller sizes too
Ignore the photo on the link below as it isn't correct. They are properly threaded using standard metric threads and produce perfectly usable female thread forms in a wide range of substrates.
http://www.comdir.co.uk/Products.aspx/2164/Screws/TaptiteScrewsPanHeadPoziDrive /
(no connection etc)
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The Other Mike wrote:

Of which cast iron is not one.

http://www.comdir.co.uk/Products.aspx/2164/Screws/TaptiteScrewsPanHeadPoziDrive /
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wrote:

No, Taptite are for aluminium. They have a non-circular section, but no flutes, so they work by deforming a thread rather than cutting it. It's a bitlike a rolled thread, but internal. They work very well (especially on copper, which is otherwise a sod to tap), but only on ductile materials.
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Andrew May wrote:

very dodgy into cast iron.
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GB wrote:

MSC Industrial supply which used to be J&L.
www.mscdirect.co.uk/
Page 164. Metric HSS taps for about £4 each. Don't buy carbon steel, especially in small sizes. They snap like carrots. You won't need a bottoming tap (taper tap) for an easy cutting material like cast iron so a second (plug) tap will do most jobs with a bottoming only needed for blind holes. Cast iron taps best dry so no lubricant.
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Dave Baker



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On Wednesday, September 5, 2012 6:18:12 PM UTC+1, Dave Baker wrote: <snip>

We seem to have come to opposite conclusions about whether this is cast iron - I looked at the picture and reckoned that most of it would be mild steel unless it was old enouh to have been made out of wrought iron. I'm not sure what's the best test to suggest to the OP to confirm - for me I'd probably decide by the behaviour when drilling, as cast iron will produce small chips where mild steel would tend to produce spirals or helices of swarf, but for someone who has possibly not dealt with either it would be more difficult.
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If it's mild steel or a high quality cast iron casting, then you can buy a cheap set of carbon steel taps (all sizes). However this will probably be a set of just one tap for each size, approximately a second. So they'll tap through holes, but not blind holes. (If you're stuck, tap once, grind the taper off, then tap to the bottom again).
Carbon steel taps don't work on alloy steels, cheap cast iron or wrought iron (unless it's swords). Then you need to use HSS taps. These are a bit too expensive to buy a whole set, but they are worth buying as a pair: second and plug, of a useful size like M4 (or more useful M6). Taper (the first) taps are only usable in through holes, useful for experienced tappers, and only necessary in hard metals. Otherwise it's easier to just start with the second tap.
I wouldn't use anything smaller than M5 for this type of work, unless essential.
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Whatever tap(s) you use, NEVER drive the tap in until it's too hard to get it any farther, Take no more than one or two turns of the tap at a time, removing the tap completely and cleaning it (and blowing the hole out with an air gun if you have one) between each go. That way you're unlikely ever to suffer the disaster of a sheared tap.
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