Hard spot in cast iron

[AndySomogyi]

Seem to have encountered some sort of a hard spot whilst scraping the dovetails, it’s about 5/16 diameter or so.

A sharpened carbide scraper just glides over it, and requires huge amount of pressure to get it to catch.

You guys ever seen this before?

Not really sure what to do, maybe hit it with a die grinder?

This dovetail is almost there, maybe needs 4-5 more passes and it’s done, but this hard spot is really messing with me.

 

[benmychree]

One might wonder how it was machined originally, but yes, some careful grinding would do the job.

 

[extropic]

I've heard of a spark plug ceramic being found in cast iron. Never seen it though.
I think the story was related to WW II production (time was of the essence).

Is that old American iron or ???

 

[AndySomogyi]

I've heard of a spark plug ceramic being found in cast iron. Never seen it though.
I think the story was related to WW II production (time was of the essence).

Is that old American iron or ???

It’s a Grizzly 8x30 knee mill.

 

[Ulma Doctor]

i had a couple hard spots on my VHM728 saddle
the cast iron was less than stellar in my machine, many voids can bee seen
on one spot there was a void about 4x6x2 mm that i filled with JB Weld, and scraped it down

 

[Braeden P]

on my 1890s Frankenstein lathe I got the bed re-machined on a planer giant hard spot covered 50% of the bed the tool glided over it and would not cut so we had to do three more passes then three more passes not changing the depth if it could be machined with high carbon steel then hss should work I will ad a photo

 

[NC Rick]

I haven’t seen it first hand but have heard of bearing balls, nuts and things found in castings from Asia. I do also think the casting process can cause hard spots if the final product.

 

[Braeden P]

Ive heard of drywall screws in straight edges dont know how.

 

[addertooth]

Yes, carbon content can be quite variable in cast iron, and it can create localized hard spots. We are spoiled by working with crucible steels today for a lot of our projects, which have better control and distribution of carbon and other elements. This tends to make us surprised when we learn the uneven distribution which can be found with Cast Iron. I run across this in antique Tamahagane steel Japanese swords quite frequently.

 

[Braeden P]

this shows only half of the bed the other side was worse (the dark spot is the hard spot)