Counterboring Cast Iron

[Eddyde]

Hi All,
I need to counterbore 6 @ 9/16"diameter x 1/2" deep holes in a cast iron part. Question is, do I need a carbide/cobalt counterbore or can I get away with using HSS? I have drilled and tapped cast before but never counterbored it. I don't want to waste money on an expensive tool if I don't really need it nor do I want to trash a less expensive tool. any help would be greatly appreciated.

Thanks,

Eddy

 

[JimDawson]

HSS will work fine. Pretty slow speed and a reasonably heavy feed. Cutting oil is a good idea. If you are doing it on a mill, an end mill will work well.

 

[Arrak Thumrs]

C-I can easily be counterbored with a standard HSS tool. I agree with slow speed. 50SFM is a good starting point. Feed... yeah, use enough pressure to keep a solid cut as you go. How long you dwell at the bottom seems to affect depth a little. Just do it consistently on all six if depth matters. But I would say 'no' to using oil/coolant. Not normally needed; the graphite in C-I acts as a lubricant during machining. Also worse for clean-up of all those small, granular, dirty, abrasive particles when mixed with oil/coolant.

 

[T Bredehoft]

Cast iron was machined before HSS was developed, Cobalt and Carbides are for hard steel, only used for cast iron because they don't need sharpening as often. In the case of hobbyists, we don't work CI enough to justify using carbide on it.

 

[Eddyde]

HSS will work fine. Pretty slow speed and a reasonably heavy feed. Cutting oil is a good idea. If you are doing it on a mill, an end mill will work well.

Thanks Jim,

I would use an end mill but the piece to be counterbored is part of the Z axis drive on the mill. I suppose, If I can find a height that will work for all the tools needed in the operation, I could just lock the head in that position.

 

[Fabrickator]

I just have a comment to Jim's suggestion of using cutting oil. I'm not sure what today's norms are for machining cast iron, but in my day I was taught NOT to use lubes when machining CI. It's basically compressed dirt and oil builds it up instead of dropping it away like powder. So this is what I do to this day, machine it dry. Funny though, when you hone CI (cylinders, valve guides, close tolerances) you always use a light oil bath to protect the stones.

 

[chips&more]

Depending on the quality of the CI, hard spots could be a problem. I guess if the project in mind came from overseas, it would be prudent to take heed. If you use an end mill be careful of it walking. Having a pilot is a good thing. And an off the shelf end mill does not leave a flat bottom…Good Luck, Dave.

 

[Eddyde]

Thanks everyone for the feedback.
I never used lube when drilling or tapping cast iron but counterboring might be a little tougher on the tool so I'll try it both ways and let y'all know the results.

Depending on the quality of the CI, hard spots could be a problem. I guess if the project in mind came from overseas, it would be prudent to take heed. If you use an end mill be careful of it walking. Having a pilot is a good thing. And an off the shelf end mill does not leave a flat bottom…Good Luck, Dave.

The casting is from Taiwan but certified "Meehanite", supposedly a higher grade of cast iron.
Yeah, I am kind of backing away from the end mill idea as I don't have one in that size. also I don't mind buying the counterbore as I can use it for other projects in the future.