Fresh cast iron threads gulling and locking black oxide socket cap bolts

[tylersteez]

Hey guys

Sometimes after drilling and tapping threads in a cast iron part, when I go to thread the bolt in I'll notice it will randomly and immediately get harder to turn. if I was to thread the fastener in any further, the bolt would lock up into the cast iron part and will either destroy the threads on its way out or the head of the fastener will shear off. the bolts that I am able to get out always have destroyed threads.

I always use the correct drill bit for the tap I'm using and make sure to go back in with a bottoming tap if its a blind hole. it will happen after using a brand new tap as well. I always thoroughly blow out the threads of any chips and dust prior to threading the fastener in. I typically always use the bridgeport to drill the holes and I'm pretty careful about keeping the tap going in the hole straight while I'm taping by hand.

My method to prevent this is to drill, tap, and clean out the holes as stated above, then slowly start threading the fasteners in with anti seize. At the absolute slightest sign of resistance, I take the fastener out and chase the threads with a tap. Then I'll clean and re apply anti seize to the fastener and everything works out fine. I can't recall ever having this problem with zinc coated fasteners, only the black oxide socket cap fasteners that this part requires.

Any ideas why this occurs? Has had me stumped and on edge for years.

 

[crossthread]

Well it has to have something to do with the cast iron. Perhaps little crystals of the cast iron are breaking off from the threads and jamming up the works. Could be crappy cast iron with a lot of inclusions. Aside from using the wrong tap I can't think of any other reason.

 

[Forestgnome]

I can't say I've ever seen this problem in cast iron.

 

[tylersteez]

All of the parts come from the same foundry so it is possible something along the lines of that is going on. on other parts from the same provider I have ran into random hard spots before. it just seems odd since it drills and taps with ease.

 

[Mike1974]

Will your assemblies tolerate any lube? I would clean/wash, then put a sot of lube insided the threads. I would guess with the way cast iron cuts, you are (micro level) getting tears and chips in the threads.

 

[tylersteez]

I am able to use anti seize since these threads only mount a steel bar to a machine that periodically get swapped out for different bars depending on the diameter of material being ran thru the machine. typically all is well after a fastener is threaded in without issue which does seem like it has something to do with the initial thread finish after tapping.

 

[4GSR]

Sounds like you're not getting all of the crud out of the hole from tapping. Cast iron is bad about that. Blasting with air doesn't always get it all out. Don't say what size tapped hole it is. Take a can of Brake Clean or any other aerosol cleaner, mount the little red tube to the nozzle. stick it down in the bottom of the hole and blast it. Works better if you can get the hole in a horizontal position. Follow with a blast of air from air gun equipped with a straw that will fit down in the hole. Doing that, the bolt should thread into the hole without any problems.

Are you sure, you're not getting the fasteners mixed up with metric stuff? There's a couple sizes that can almost work but start to bind up a few threads down into a threaded hole. The one that I've encountered is 5/16-18 and M8x1. They darn near will interchange with each other when a 5/16-18 bolt is inserted into a M8 hole. Another is 10-32 and M3, I believe it is. Ken

 

[tylersteez]

I could see some left over crud causing this issue as well, I'd like to think I'm pretty thorough with the air gun but then again, cast iron makes fine sand compared to steel chips.

As for the threads, it is 3/8-16 and impossible for me to mix up with a close metric size since we don't have any metric fasteners in stock. (specialize in rebuilding an old american class of machines)

 

[kenton]

Sounds like you're not getting all of the crud out of the hole from tapping. Cast iron is bad about that. Blasting with air doesn't always get it all out. Don't say what size tapped hole it is. Take a can of Brake Clean or any other aerosol cleaner, mount the little red tube to the nozzle. stick it down in the bottom of the hole and blast it. Works better if you can get the hole in a horizontal position. Follow with a blast of air from air gun equipped with a straw that will fit down in the hole. Doing that, the bolt should thread into the hole without any problems.

Are you sure, you're not getting the fasteners mixed up with metric stuff? There's a couple sizes that can almost work but start to bind up a few threads down into a threaded hole. The one that I've encountered is 5/16-18 and M8x1. They darn near will interchange with each other when a 5/16-18 bolt is inserted into a M8 hole. Another is 10-32 and M3, I believe it is. Ken

M5 and 10-32. A 5/16-20 would probably thread right into a M8

 

[Dan from Oakland]

I'm thinking you still have chips in the thread as mentioned above. Rig up an air nozzle you can go into the hole with rather than just blowing at the hole. If there is any dust or chips remaining you will gall things up pretty quick. McMaster- Carr has a good assortment of small diameter wire brushes- stick one in a cordless drill and clean out your threads with that if they are thru holes.

 

[sealark37]

When you consult the drill/tap chart, what percentage of thread depth are you drilling for? A full 75% thread depth will often cause dissimilar metals to bind. Try a drill size that gives< what you are using now.

 

[Doozer]

Are you power tapping with a spiral flute tap
that happens to be a little dull?
I have had a situation happen just such as this
and what was happening was the high effort of tapping
was causing the helix of the tap to unwind just a
little bit. Instead of getting 16 threads per inch,
I was getting 16 and 1/4 threads per inch.
The bolts would go in part way and bind up.
Re-tapping with a straight flute hand tap fixed it all.

--Doozer

 

[boslab]

Bit radical but I’ve found it works on troublesome threads, helicoil the thing
Just a thought, you may have a casting that’s not homogeneous, solidification starts from the coldest point, the analysis varies from place to place, drives stuff like manganese sulphide and so on along till it hits a solidification front coming from a different direction, cast things hide a complex architecture internally
Mark

 

[kustomizer]

Something I like to do is actually check the hole size before and after threading then run a go-no go thread gauge

 

[MrSteve]

Are you sure, you're not getting the fasteners mixed up with metric stuff? There's a couple sizes that can almost work but start to bind up a few threads down into a threaded hole. The one that I've encountered is 5/16-18 and M8x1. They darn near will interchange with each other when a 5/16-18 bolt is inserted into a M8 hole. Another is 10-32 and M3, I believe it is. Ken

10-32 and M5:
.190” vs .197” basic major dia. and 32 vs. 31.75 TPI. Should get gradually harder vs. suddenly, but potential trouble all the same.

 

[pavt]

On cast iron, I've found it useful to spin a straight flute tap in by hand, just with my fingers a few times. Cleaning the heck out of everything in between passes. And a drop of oil on the bolt or stud threads.

 

[tylersteez]

This is along the lines with what I was figuring, I'll step up the drill bit size .002"-.005" and see what that does for me. These specific bolt holes always get stripped out by the machine's operators so I was skeptical of oversizing the hole and losing thread depth

 

[tylersteez]

I'm hand tapping these holes with straight flute taps but I could totally see that occurring. I'd rather have a spiral flute tap unwind a little vs snap in the hole lol

 

[Milland]

This is along the lines with what I was figuring, I'll step up the drill bit size .002"-.005" and see what that does for me. These specific bolt holes always get stripped out by the machine's operators so I was skeptical of oversizing the hole and losing thread depth

The additional grip strength offered by the minor peaks of the thread is minimal. If you use good quality fasteners, you'll get better holding power by lowering the "H Limit" of the tap to shrink the gap between screw and cut thread. It'll put more emphasis on thread quality and cleanliness, but more than make up for a small increase in drill size.

https://www.gwstoolgroup.com/tapping-tip-how-h-limits-relate-to-class-of-fit-part-2/

But if the threads are still getting pulled out in use, you might seriously consider going with helical inserts from the beginning. It'll do a lot more for thread robustness than just about anything else aside from changing from a cast iron to an alloy steel.

 

[Scruffy887]

On cast iron, I've found it useful to spin a straight flute tap in by hand, just with my fingers a few times. Cleaning the heck out of everything in between passes. And a drop of oil on the bolt or stud threads.

Very interesting. I toured the Kentucky Mazak plant about 15 years ago and I saw a worker start assembling on a large VTC base casting and he had a tap and a Dewalt battery drill, hand chasing all threads before bolting down the linear rails. Started tap by hand and in and out with the Dewalt.