Ignorant on the topic: Can I tap holes into cast iron?

[Kent A Bathurst]

I want to drill and tap blind [ie - not through] holes into cast iron. Specifically, into the main casting on my DC-380 planer.

Hole dia is not critical - 1/4" would be fine. Depth would probably need to be in the 5/8" to 3/4" range.

I have tapped aluminum bar stock before, but clearly this is not the same critter.

Is there anything I need to know besides lube often, align carefully, don't force it? Anything in particular about drilling the holes themselves?

Thanks

Kent

[Mark Ashmeade]

Kent,

I've drilled it before, no problem at all. No special lube required, it's apparently "self lubricating". However, I have not tapped, I can see why that would require lube though!

[Matt Meiser]

Should be almost as easy as tapping aluminum. Just be sure to back up every 1/2 turn or so to break the chips, use lube and you should be fine. If its a blind hole (meaning not all the way through) you'll want to get a bottoming tap as well as a regular tap, or just make the hole way deeper than you need.

[Steven Satur]

I have tapped cast many times. Oil when you drill, oil when you tap.
Means you are bottom tapping you need two different taps, a regular tap and a bottom tap (does not have a taper). Just know when to stop at the bottom. I have broken off a few taps in the hole. Then you will really have fun getting them out.

Steve

[Kent A Bathurst]

Thanks, guys. Good point about the bottom tap.....I can drill the hole deeper than I need to compensate. And - since they will be my neighbor's taps, I'd think not breaking one off should be a goal of mine. Fi for some reason I did break it off, I can assure you that sucker will stay there. The hole location will not require decimal precision.

Anything special about the drill bits?

[ian maybury]

No prob. The only real issue i've seen arise with cast iron is that it's very occasionally possible to run into a patch containing white iron which is a hard iron/carbon combination which can arise if the conditions during cooling are not quite right. It shouldn't be even a possibility with good castings...

ian

[Scott T Smith]

Kent, I've drilled and tapped quite a bit of CI, mainly back when I had my machine shop but also more recently in the wood shop.

There are two schools of thought about using a lubricant with CI - some say use it and others say don't. The "old school" was not to use a lubricant on CI, as it is considered to be "self lubricating. I've done it both ways, and usually gravitate to not using a lube.

It usually drills (and taps) like a dream compared with steel. One thing to consider is that is is much easier to break off a small diameter tap than a large one, so if "size doesn't matter".... then consider a 3/8" threaded hole instead of 1/4". For me CI drills faster and easier with a slow to moderate drill speed.

Usually you want to have at least 3 threads inside your material, so if you're drilling thin CI then consider a fine thread fastener (NF) instead of course thread (USS).

As Matt suggested, back the tap off about 1/2 turn every couple of turns to help keep from breaking it. Also chamfer the hole after tapping to make it easier to start the bolt.

No special bits required, just be sure that they are sharp. Don't hesitate to drill a small pilot hole and then enlarge it.

Best of success to you.

Scott

[Kent A Bathurst]

Kent, I've drilled and tapped quite a bit of CI, mainly back when I had my machine shop but also more recently in the wood shop.

There are two schools of thought about using a lubricant with CI - some say use it and others say don't. The "old school" was not to use a lubricant on CI, as it is considered to be "self lubricating. I've done it both ways, and usually gravitate to not using a lube.

The voice of experience. thnx.


It usually drills (and taps) like a dream compared with steel. One thing to consider is that is is much easier to break off a small diameter tap than a large one, so if "size doesn't matter".... then consider a 3/8" threaded hole instead of 1/4". For me CI drills faster and easier with a slow to moderate drill speed.

3/8" should be no problem. I was figgering on a slow-ish speed - thnx for the confirmation.

Usually you want to have at least 3 threads inside your material, so if you're drilling thin CI then consider a fine thread fastener (NF) instead of course thread (USS).

Nah - the main casting on my Delta DC-380 planer. Gotta be a bunch of arn there, given the weight of hte sucker.

As Matt suggested, back the tap off about 1/2 turn every couple of turns to help keep from breaking it. Also chamfer the hole after tapping to make it easier to start the bolt.

Yeah - that's what I've always done - good to go there.

No special bits required, just be sure that they are sharp. Don't hesitate to drill a small pilot hole and then enlarge it.

Good. You know - I meant to ask that very question.

Best of success to you.

Scott

Thanks, Scott..........and all the others as well.

These will be the only SAE fasteners on the entire machine..........There wouldn't be some allergic reaction to the imperial, would there? Maybe I should go metric just in case? :D.

[ray hampton]

Thanks, Scott..........and all the others as well.

These will be the only SAE fasteners on the entire machine..........There wouldn't be some allergic reaction to the imperial, would there? Maybe I should go metric just in case? :D.

Only if it is from a metric nation , if the cast iron are made in the U S A , then use SAE tools

[Matt Meiser]

Most anything made in the US is metric anymore.

[Jim Barkelew]

I agree tapping cast iron is relatively easy. Cast iron has graphite in it so it's self lubing. One thing I learned from a college machinist/instructor is that backing out a tap every half turn is actually shortening the tap life. Everytime the tap is backed out to break the chip it risks chipping the cutting edge. Its better to keep turning the tap until you can feel some resistance because of cloging, then back it all the way out to clear the chips. I have been hand tapping this way for decades without any issues. Try it, see how many turns you can do (in a scrap piece) before you feel resistance.

[Anthony Whitesell]

I agree tapping cast iron is relatively easy. Cast iron has graphite in it so it's self lubing. One thing I learned from a college machinist/instructor is that backing out a tap every half turn is actually shortening the tap life. Everytime the tap is backed out to break the chip it risks chipping the cutting edge. Its better to keep turning the tap until you can feel some resistance because of cloging, then back it all the way out to clear the chips. I have been hand tapping this way for decades without any issues. Try it, see how many turns you can do (in a scrap piece) before you feel resistance.

I just tapped a cast iron engine cylinder to helicoil the exhaust bolt holes. Using the tap for a 1/4-28 helicoil, I could go about a half turn before I felt too much resistance and feared breaking the tap. I don't back the tap out further than necessary to break the chip. I just back it out until I heard (feel) the snap of the chip breaking free. It doesn't usually take more than a quarter turn.

[Jim Barkelew]

That is true with cast iron. I was too general. Cast iron will turn to dust instead of making chips so it cloggs faster, especially with small taps. Larger taps can be turned many revs in cast iron and mild steel. Threading iron pipe can usually be done without backing up using my HF dies and an inch of threads.

[Rob Lindquist]

Can't stress enough to be sure the tap and drill bit is sharp! With that CI is easy.

[Mike Cruz]

Okay, since nobody else said it, I will... I don't know, Kent...can you? :D

[Bruce Page]

Okay, since nobody else said it, I will... I don't know, Kent...can you? :D

There's one in every crowd.. :rolleyes::D