Tapping & thread strength in cast iron vs. semi-steel vs. steel

P.Brewster

P.Brewster

Joined
Jul 25, 2007
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Does "semi-steel" have any advantages over cast iron in terms of machinability and thread strength (of an M8 tapped hole)?

I know steel would probably be better, but the cost would be 3x that of cast.

thanks
 
According to Wikipedia:

Semi-steel casting is a lower cost method to produce a casting that is not quite as strong as a steel casting but less expensive to manufacture. It was used more commonly as a marketing term.

The carbon and silicon percentages are reduced to the amount approximately consistent with those in steel. This is done using pig iron or gray iron casting scrap and reducing the amount of carbon through the addition of relatively pure steel or wrought iron scrap in a well heated cupola. The percentage of carbon is typically between foundry cast iron and wrought iron.

No idea about the machinability of it though... :(
 
and this is related to blades how?

not being flip, just want to know what's going on...
 
This is for some tooling that I'm building... I'm hoping some machinists might chime in.

thanks
 
I've never worked much with semi steel. I've modified a few things made of it, but never made anything from scrap.

I don't like working with cast iron because it forms a tiny dusty abrasive chip that makes a mess, gets past strainers into the sump and stick together into big clinkering chunks in your strainer pans. But it is unsurpassed for building certain kinds of tooling because it does not gall to steel and it damps vibration. So it is used for thinks like lathe chuck back plates and vice bodies.

What are you building?
 
Top Secret... the part is a 3 x 3 x 3" angle plate with modifications. I fabbed the first one from 1018 - I made it oversized and had it machined to final spec (Perpendicular to ±0.03° or ±0.005 over 10", with 5 holes in each face, and modified gusset), but a $10 cast-iron angle plate or a $35 semi-steel angle plate is looking pretty appealing, because it is already flat and square.

I won't be machining this myself, so I am more concerned about long-term thread strength than I am about machining difficulty.

The M8 bolts will be tightened/loosened on a daily basis, hence my concern for longevity. I realize that a hardened threaded insert may be the best bet, but I'd like to avoid that if possible (to constrain costs).

thanks!
 
The steel will probably tap a little cleaner than the cast iron, but the cast iron will probably wear a little better than the steel. If you have a machine shop do it, you can request thread milling instead of tapping, which will give cleaner threads than tapping, and you can get a 100% full thread form, which gives full thread engagement which will wear better.

Regardless of the material or process, I'd be surprised if the threads wear out unless they're really torqued down tight.
 
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