Tapping a hole


Old 10-03-04, 01:29 PM
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Tapping a hole

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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Old 10-03-04, 02:49 PM
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Hello: Chris

To drill and thread a hole in cast iron or metal of any kind, you will need the proper size drill bit and tap. Of which I am sure you already know. Used taps and dies can be purchased sight unseen, but only if you care to risk it or can be assured somehow the set is not wornout, damaged has lost pieces, etc.

Good priced tap and die sets are based upon individual needs and preferences. Cheap sets are not designed for extensive usages. Buy a quality brand if your intent is to use them often and want quality results.

Quality results and long service life is not often possible with cheap sets. That old saying, "get what you pay" for comes to mind. I have not had good luck using cheap sets personally. I prefer the quality of a good set to get good results and long service life.

Also adviseable to buy a quality drill bit suited for the task. The hole also has to be drilled perfectly straight to obtain quality results.

Since the nature of the question is based on drilling and tapping a hole in cast iron and not specific to the tools used, this topic is okay to have posted the question into.... Thanks. One less question to move...

Check back on your question several more times. Other members posting replies in this forum topic may offer you additional advice, their opinions and or suggestions.

Kindly use the reply button to add additional information or questions. Using this method moves the topic back up to the top of the list of questions automatically.

Regards and Good Luck. Sharp Advice.
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Old 10-03-04, 07:03 PM
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Sharp Advice pretty much summed up the issue of buying used and the importance of quality.

What I can add is that there are a couple of things to know about tapping cast iron.
One thing to remember is that because of the coarse grain structure of cast iron it isn't adviseable to use a tap finer than NC (national coarse).
Also, because of its brittleness, you have to be carefull to use plenty of lubricant and make sure to take small bites with the tap.
Old 10-04-04, 11:21 AM
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This link may also offer some assistance:

Old 10-14-04, 10:32 PM
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My job for the last three years before I retired was productionizing stamping dies for GM. I've drilled and tapped hundreds of thousands of holes in cast.

You can use a hand held drill, just keep it steady cause if you wobble it around you'll get shallower threads and they will strip easier.

Get a drill and tap chart, should be one online. Use the proper size drill, this will help prevent breaking the tap. A hand tap wrench would be advisable. Use plenty of lubricant.

Important: after tapping the hole put some grease on the bolt the first time. Cast can grab a bolt and you'll break it before you'll get it out.

If you tapping a blind hole (one that doesn't pass all the way thru the material) You'll have to constantly clean out the hole with a magnet or blow it out with air. If it requires threads close to the bottom of the hole you'll need a bottom tap to finish the hole.

For hand tapping you'll want to use a 4 fluted tap (has 4 cutting edges seperated with 4 flute for material removal). After the tap starts cutting back it off a half of turn then go forward the half you backed off plus one quarter of a turn then repeat.

To help you keep from breaking a tap I would completly remove the tap after every full turn of cutting and blow out the hole and relube the tap before starting again. Broken taps in a hole can make life unpleasant.

If the cast is a case for some kind of machinery you'll need to make sure not to get shavings into the mechanism.

The main cause for tap breakage is - One end stops turning (snap).

Take your time and don't force it. Cut a little, back off often and lube a lot.

Good luck.
Old 10-15-04, 06:09 AM
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For a tap and drill chart:

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