Tapping Blind Hole into Cast Iron - Question

bcall2043

bcall2043

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Working on a project this afternoon and needed to tap four 1/4-20 blind holes into cast iron. I drilled the correct size hole and started the holes with a tapered hand tap. I used "Tap-Magic" cutting fluid out of habit on the first two holes. When the first tap bottomed and I went to clean the chips out and change to a bottom tap the chips were caked in the bottom of the hole and difficult to remove. I tapped the second two holes dry and it was a little easier to clean the chips out.

Now for the question. Is it normal to tap cast iron dry or is there a better cutting fluid/lubricant than "Tap Magic" to use? I did a search and found only one post that mentioned the subject:
http://www.hobby-machinist.com/show...by-Lew-Merrick-PE?highlight=tapping+cast+iron

Thanks, Benny
 
bcall2043 said:
Working on a project this afternoon and needed to tap four 1/4-20 blind holes into cast iron. I drilled the correct size hole and started the holes with a tapered hand tap. I used "Tap-Magic" cutting fluid out of habit on the first two holes. When the first tap bottomed and I went to clean the chips out and change to a bottom tap the chips were caked in the bottom of the hole and difficult to remove. I tapped the second two holes dry and it was a little easier to clean the chips out.

Now for the question. Is it normal to tap cast iron dry or is there a better cutting fluid/lubricant than "Tap Magic" to use? I did a search and found only one post that mentioned the subject:
http://www.hobby-machinist.com/show...by-Lew-Merrick-PE?highlight=tapping+cast+iron

Thanks, Benny
Click to expand...

Usual wisdom is dry with CI, but the only real downside to a lubricant is as you found, it makes the dust cake up.
I would back up the tap every quarter turn in CI as opposed to every half or full turn in mild steel.
If you do get the dust caked in the bottom of the hole, just use a pick to loosen it, then suck it out with the shop-vac; don't use compressed air to blast it out, it's very abrasive.

Others more experienced them me may have different advice, but I use Tap Magic and haven't yet broken a tap (famous last words!).


M
 
I have always tapped CI dry and keep in mind that the blind hole should be avoided if possible, a through hole is always a safer option. When a blind hole must be done ensure that the pilot hole is about 25% deeper than the length of the required threads.
 
I always tap with a cutting lubricant and use compressed air to clear the hole of chips and debris. I also use heavy grease on my taps if I want to control where the chips go like when tapping a spark plug hole or something that has orings or seals. That way I dont drop chips into a motor. Learned that from an old skool machinist. He had a jar od vaseline in his tool box that was just for doing that type of work... He was a magician woth metal so I paid a lot of attention to how he did a lot of different things to improve my skills as I learned. I miss the old guy, but he's making Angel wings for the new comers now.
Bob
 
Thanks for the replys with answers about my question. I checked my bedside reader last night to see if I had read and forgotten or had never read about tapping cast iron. I could not find anywhere where the subject was covered. in the 1940 copy of Machine Shop Training Course by F. D. Jones that I have. Think I will go with the majority for future tapped holes in cast iron and tap dry and blow out the chips from the blind holes.

Thanks, Benny
 
Well I'm a little late to this party but I tapped 4 wholes in 1/2 plate of cast iron last week and I dry tapped them.

Only difference was mine were through wholes.

Paul
 
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