tapping a hole

reevesmaconOctober 3, 2004

I'm looking for some advice on drilling and tapping a hole in cast iron. What materials and equipment do i need? Specific info please. I'm a woodworker, so i have a good drill press. Also sometimes i will need a hole where the DP can't go. Where can i get the tapping equipment at a good price? Would a used set on Ebay be OK, or does this type of tool need to be new? If this is the wrong forum for this type of question, please let me know. Thanks, Chris

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You can't beat E-bay for the price of large tap&die sets... Check it out...

    Bookmark   October 3, 2004 at 7:14PM
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E-bay is a good source as "kbeitz" says but you might want to try a new set of Craftsman, Taps and Dies from Sears. It is inevitable that you will break one or the other as time goes on and all you have to do with the Cratsmans is take them into Sears for a free replacement - no questions asked.

    Bookmark   October 5, 2004 at 11:52AM
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But you can't buy a set from sears for $20.00... I got a new very large gold coated set for that price off E-bay...
There is so many to chose from... Look for the ones that have the most bids... Other people know what to buy...

    Bookmark   October 5, 2004 at 8:07PM
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I haven't bought one of those eBay sets, but as far as I'm concerned the jury's still out. I've talked to folks who have bought import taps and drill bits, and had them bend, break, and in one amazing case, a drill bit's flutes _straighten out_.

    Bookmark   October 5, 2004 at 10:16PM
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Stay with quality stuff like Sears or similar. And since cast iron can be hard to drill and tap, go with premium materials, cobalt or better.

John's info is correct. Imports from China, India, and similar countries can be really poor. I bought a set of HS drill bits to 1/2 inch from Northern Hyd. a few years ago. I figured the $14 low price was an import, but I needed some extra bits and I couldn't afford American quality then. It turned out that the steel was so poor, that every bit in that set chipped the flutes, just drilling soft steel, within a hole or two.


    Bookmark   October 5, 2004 at 11:51PM
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I bought a set of Asian taps and dies. They are great for cleaning up threads, but they couldnt ever really cut new ones. YMMV.


    Bookmark   October 6, 2004 at 3:29AM
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E-bay has the good one to... They even have snap-on if you got the bucks...

    Bookmark   October 6, 2004 at 7:35PM
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If you are not looking for an entire set try Manhattan Supply (mscdirect) and buy the bits and taps for the job you need. The 'import' stuff they have will at least do the job, as opposed to the butter soft bits and taps found at other places.
Quality cast iron is relatively easy to drill, even with HSS bits. ?No name? bits are a bad idea. They will likely result in an oversize hole and cause problems with tapping (the thread depth will be to shallow). ?Tap magic? or a good tapping lubricant should be used. Be sure to get the tap started straight. Clamping the tap in the drill press and rotating by hand while gently pressing on the quill feed is one good method. Do not attempt to use the drill motor!
Once the tap is started solidly you can loosen the chuck and use a tap wrench to finish up the thread. About every 1-2 turns you should remove the tap completely, clean it and the hole with some air (for cast iron ?Gun Scrubber? works well also for cleaning), apply more cutting fluid, and run the tap back in.
There are tables giving drill sizes to match taps available.

    Bookmark   October 8, 2004 at 6:53PM
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When tapping holes in cast iron it is best to start the threads with a starter tap, then finish with a running tap.

    Bookmark   October 9, 2004 at 5:19PM
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"...starter tap, then finish with a running tap."

No such thing. Taps are available in taper, plug, and bottoming.

    Bookmark   October 10, 2004 at 10:36AM
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Here in Pa some people call them Starter and finish taps...
We know what they are trying to say...

    Bookmark   October 10, 2004 at 8:51PM
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Since this is a national forum, would it be so difficult to use the correct terms?
I can just see someone starting out and trying to use a local term and look up tools.
Life is difficult enough without making it harder.

    Bookmark   October 18, 2004 at 10:39AM
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you forgot to mention hand machine and a multitude of other special ones (some to spiral the cuttings up out of the hole) so whatever you use use only a very sharp tap and for cast iron kerosene or wd40 works wonders but not on steels or alum. ss is another beast best not attempted by the feint of heart.

    Bookmark   October 19, 2004 at 10:26AM
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Since the original post was about tapping a hole in cast iron, I just got the October metalworking flyer from MSC, and they have a TiN coated HSS spiral point gun tap that is recommended for cast iron. In case you want to check it out, their part number for a 1/4-20 is: LU88116009.


Here is a link that might be useful: MSC

    Bookmark   October 19, 2004 at 9:27PM
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Hi everyone A trick that my Dad showed me, is to drill a hole in a block of wood that will just clear the tap. Run the tap through the hole and into the part that you are tapping. This will make the threads square, and there is less chance of breaking the tap.

    Bookmark   November 12, 2004 at 7:51PM
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On my old tractors, I use Helicoil inserts in the castings as necessary and have found that those taps really work better with Marvel Mystery oil as a lubricant.

    Bookmark   November 13, 2004 at 5:23PM
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You can get good imported taps, but you need to make sure they are marked "HS" or "HSS" for "high speed steel".

Cheaper, unmarked ones are probably just high carbon steel and not likely to be as hard or as durable as HSS taps.

Get yourself a tap/drill chart. Make sure you drill the correct size hole before you try to tap it. Too small a hole will break the tap. Too large a hole and you'll have reduced thread engagement.

Brickeye is right, no such thing as a starter tap, but that's what people tend to call taper taps.

Here's the generally agreed standards: bottom taps, 1-3 threads chamfered. Plug taps, 3-5 threads chamfered. Taper taps, more than 5 threads chamfered.

A handly tapping aid is a spring-loaded tap guide. This goes in the drill press or lathe chuck and it has a spring-loaded pointed pin that centers the tap in the hole. The tap itself is held by a handle, which one turns very carefull, backing out when too much resistance is felt. How much to turn in and out depends on the material, the size, the condition of the tap, etc. Err on the side of caution... break a tap, and generally you scrap the part too.

    Bookmark   November 20, 2004 at 9:13PM
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"...break a tap, and generally you scrap the part too."

If the part is expensive enough you can use a broken tap extractor to remove the stuck portion. It is a holder with steel rods that are placed in the flutes of the brocken tap and used to turn it back out.
For really expensive items a broken tap can be EDM machined out of the hole with no damage (other than to your wallet).

    Bookmark   November 21, 2004 at 10:46AM
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When the tap breaks there is movement of the portion still attached to the handle. If the break occurs within the hole, then whne the upper portion turns without being able to go down into the hole, it will deform the threads above the broken portion. Additionally, backing out a broken tap means the freshly exposed sharp broken edges will be gouging new territory on their way out. Add to that any chips from the broken tap and the result may well be unsatisfactory.

Yes, EDM can be used, but that is a costly solution. Generally speaking, a broken tap is something to avoid if at all possible.

    Bookmark   November 21, 2004 at 7:55PM
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Back the tap out (yes, it normally creates an area of bad threads), remove the broken tip, drill and helicoil the hole and call it a day.

    Bookmark   November 26, 2004 at 3:04PM
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I keep the snap on ones or like brand taps and dies for starting virgin untapped holes, I bought a set up to half inch metric and standard taps and dies that i use to clean up existing threads for 3 bucks lucky? yeah Like kbeitz says snapon rules! like horseman says cheap ones from asia for cleanups....one more tip... If you are mad at the wife/husband dont try to tap holes they tend to be bent like you are just be patient and it will happen

    Bookmark   November 28, 2004 at 9:42PM
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Yeah, I guess if the workpiece has enough room for a helicoil. Do they make helicoils in 8-32? LOL.

    Bookmark   November 29, 2004 at 12:09AM
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