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Tapping cast iron, 1.5x bolt dia. good enough for thread engagement?

marrt

Aluminum
Joined
Dec 19, 2008
Location
Virginia
For an ungraded steel bolt in a non-critical application, is 1.5x bolt diameter sufficient for thread engagement in cast iron? I realize "it depends." But I'm looking for a rule of thumb to avoid drilling through the material (I'll add depth for tap clearance of course).
 
1.5X bolt diameter into an engine block will break any standard engine fastener. 2.5X will break any available fastener. I did some testing on custom engine studs we make here and was quite surprised at how little thread is really needed.

Ed.
 
I used to deal with engine engineers a lot and was always told 1D was the minimum they would allow.

This was on a 3/8" exhaust stud hole btw.
 
Go with a finer thread???

My rule of thumb is higher strength/ductility of materials allows finer threads, lower strength/ductility calls for courser for tapped holes.

So with a cast iron of unknown grade, OP is better off sticking with course threads. Especially since at 1.5 diameters depth, he's got plenty of holding power with a similarly low-grade bolt.
 
A finer thread is not weaker. It's actually slightly stronger because the larger minor diameter means a larger area in stress, so lower pressure per square inch.
 
A finer thread is not weaker. It's actually slightly stronger because the larger minor diameter means a larger area in stress, so lower pressure per square inch.

That is very true for the bolt, not for the threaded hole. The larger minor diameter of a threaded hold is in the air.

CarlBoyd
 
That is very true for the bolt, not for the threaded hole. The larger minor diameter of a threaded hold is in the air.

CarlBoyd


It certainly is weaker. The fastener will be stronger because the minor diameter is larger. The threads are smaller and weaker. If your idea was correct we'd have fasteners with 100 threads per inch instead of grade 8 hardware.
 
I have tested hardened roll threaded studs both for clamping force and breaking strength. Fine thread gives about 10% more clamping force for a given torque and has about 10% higher breaking strength. This is with 9/16 fasteners testing both fine and course threads.

Ed.
 
I'm sorry I spoke imprecisely. I was trying not to write an entire dissertation.

In both the male and female threads the diameter at which threads usually fail is larger and there is a correspondingly greater area to resist failure. This is an established principle: Fine Thread vs Coarse Thread Bolts, Which One to Choose? - Nord-Lock Group

We don't use 100tpi threads instead of grade 8 for many reasons, the one that's applicable here is that the increase in strength diminishes as the thread count increases.

The article I cited has a nice list of reasons that we don't use fine threads most of the time.
 
I'm sorry I spoke imprecisely. I was trying not to write an entire dissertation.

In both the male and female threads the diameter at which threads usually fail is larger and there is a correspondingly greater area to resist failure. This is an established principle: Fine Thread vs Coarse Thread Bolts, Which One to Choose? - Nord-Lock Group

We don't use 100tpi threads instead of grade 8 for many reasons, the one that's applicable here is that the increase in strength diminishes as the thread count increases.

The article I cited has a nice list of reasons that we don't use fine threads most of the time.


I have occasion to use Nord locks products and that BS site really surprises me. They seem to be confusing thread strength and fastener strength. And they are so full of shit in places, such as saying that a fine thread is stronger in shear. The threaded area should never be in shear. A shear situation should not be threaded. . For a good example , look a back hoe/ excavator. All of those hydraulic cylinder ends are in serious shear. NONE of them will be threaded. NONE of them will be in tension. When a fastener is torqued to be in tension it stretches and thus the diameter is reduced. It"s then weaker, and that is why a pin in shear is never threaded and tensioned. The confusion lies in not knowing what shear and tension are and do. I didn't know it when I was much younger and questioned an engineer from GWB (Home Page | GWB Drive Shaft) as to how 16 20mm bolts con transmit 6000 hp with out breaking. His reply: They don't. They pull the flanges together so that the friction between them transmits the torque. They are only in tension if properly tensioned.If they are not properly tensioned or the flange faces are not flat they will break with much less than even a normal load.
Again, with a fine thread the minor diameter is greater so the fastener itself is stronger. With a coarse thread the fastener may be weaker but the threads are stronger and will not strip as easily
 
Here's a classic example of the shear out of cast iron in a product I designed many years ago. I don't remember the specifics at what it sheared at. The 1:1 times of thread engagement was not enough from the original design for cast iron vs steel. This was a 1.125-7 UNC thread. It sheared out at about 60,000 pounds of force. Design strength was at 55,000 lbs. too close for comfort. The sample stud shown in the picture was not used for the test, one similar with no undercut was used.
When designing a thread engagement, you have to take in account the chamfer at each end of a thread as being one less full thread of engagement, regardless. Next, you have to determine which is the weaker material, in this case, the OP says the cast iron is what the steel fastener is going into. That says cast iron is the weaker member, the steel screw is the stronger member. That means the thread will fail in shear in the cast iron long before the steel thread will fail. Also the failure will happen at the shear line of the cast iron, at the actual OD of the external screw thread.
Now, how many threads per inch should you use in cast iron? Cast iron does not like fine threads, period! What's considered fine threads? Anything finer than 14 threads per inch or it's metric equivalent. i still design in the Imperial Inch System. Sorry. The problem with cutting fine threads in cast iron is getting a good thread that is not torn, jagged, etc. Getting back to the subject. Most threads I design in cast iron are bigger than 2" in diameter, many up in the 8-16" in diameter.
The number threads or what I call "wraps" a thread makes in an engagement is what determines the holding strength of a thread. It don't manner the T.P.I. it's the number of wraps of thread engagement you have. Each wrap, contains an section area of material. This area is generally defined as the PD, if the materials are similar, times Pi, times the number of wraps, divided by two. Oh, don't forget to reduce the number of wraps by two for lead in chamfers and such. The result is the area of engagement. Take this area times the shear strength of the material involved. This will give you a "ball park" strength of what the thread will handle load wise. There's many other formulas out there that will get much finer results, I find this to work pretty close for the work I do. I work with lots of special threads, too. Ken
 

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Heh, lots of "stress" ;) when we get into the details of fastener strengths and pullout loads.

I'd say we're over-thinking this a little bit. OP has unknown grade cast iron and commodity low-strength steel hardware, and since generally it's considered that the first couple threads take almost all the loads when tightened, a 1.5 times engagement will be more than enough.

Yes, if you have a really brittle material you're tapping into then additional testing may be warranted, but again, the OP said non-critical application. He should have no issue as long as he's not tapping old cheese...
 








 
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