Tapping cast iron

[blackrose]

Is it any different to tapping mild steel? I guess cast is a bit more brittle?

[Tracy D'arth]

Drill with very sharp drill.    Always with paraffin lube.    High speed tap with lube. Keep it cutting else it work hardens.

[mark99]

Used to hand drill and tap (taper) cast iron pipes up to 48" diameter up to  2" holes BSP. Wall thickness was circa up to 1/2 inch. Took a long time. Iirc we used tallow as lubricant.  The ratchet bar lever was about 1 foot long.

 

In the old days they used to form a hole in CI with half round and diamond shaped chisels with a hammer.

Edited August 31, 2020 by mark99

[BEngo]

Cast iron is filthy stuff to machine, cos it is full of raw graphitic carbon.

 

For small (up to an inch, ish) sizes, cut it dry using HSS or carbon steel straight flute hand taps. The free carbon will do the lubrication. Use the normal process of backing off every few turns to free up the swarf, which is powdery.  Don t let the tap flutes get full of dwarf or you will get crap threads and broken or jammed taps.

 

Avoid  fine pitch threads if possible.  Metric coarse OK but  Whitworth or UNC

better any day.

 

N

[IanD]

How do the dwarves get into the hole? ?

[ditchcrawler]

31 minutes ago, mark99 said:

  we used tallow as lubricant.   

Russian Tallow, I liked the smell of that

[blackrose]

It's just a M10 thread that I need to tap. The hole is already drilled. I didn't have any particular issues drilling it, that was fairly easy. Is it really going to work harden if I'm just tapping the hole by hand?

Edited August 31, 2020 by blackrose

[WotEver]

4 minutes ago, blackrose said:

Is it really going to work harden if I'm just tapping the hole by hand?

No. 

[blackrose]

Ok thanks

[mark99]

9 hours ago, ditchcrawler said:

Russian Tallow, I liked the smell of that

 

I recall a dog grabbing/running away with a block of tallow I had on the roadside.

[Tracy D'arth]

10 hours ago, blackrose said:

It's just a M10 thread that I need to tap. The hole is already drilled. I didn't have any particular issues drilling it, that was fairly easy. Is it really going to work harden if I'm just tapping the hole by hand?

Yes if you don't keep it cutting, sharp new tap.

[hovrin]

Just tap as normal.

Start with a taper tap then use a plug tap.

Take your time and turn 270 deg forward and then 180 deg back to clear the swarf.

[WotEver]

13 minutes ago, hovrin said:

Just tap as normal.

Start with a taper tap then use a plug tap.

Take your time and turn 270 deg forward and then 180 deg back to clear the swarf.

And note that the swarf will be more powder-like than you’d expect with mild steel. 

[blackrose]

Just did the job this morning. As some said it was really no different to tapping steel. It was very easy with no issues and no work hardening of the cast iron.