Haas VF3 column bolts breaking?

HuFlungDung · Jun 11, 2018

I was hooking up one of my indexer cables this morning, and I noticed a hole was open in the column base of my 1996 VF3, and the bolt (a 1" Allen capscrew) was laying on the floor, a piece of it anyways. Sure enough, the bolt had broken off. There are 8 of these large bolts in total and I test torqued all the rest, and one more broke off.

They weren't too bad to extract. I thought the holes were tapped a bit on the shallow side, maybe getting 7/8" of thread engagement on a 3.5" long bolt, so I tapped a little deeper and put in longer bolts. I couldn't budge the other 6 bolts, so I left them alone. The broken off part of the bolt was about flush with the top of the tapped hole.

You other Haas owners might want to go around back and test torque the bolts on your machines.

rklopp · Jun 11, 2018

How much, if any, corrosion was on the threads? That kind of steel can be susceptible to stress-corrosion cracking. I had a CAT40 pull stud, which is made of similar steel, let go just when the drawbar pulled in, and the threads were corroded at the fracture location, which was strong evidence of stress-corrosion cracking failure. Fortunately, I caught the issue before the spindle turned on.

I doubt there was much stress on the bolt besides the steady tension from torquing, so fatigue is not likely the issue. Similarly, if hydrogen embrittlement were the issue, it would have occurred while the machine was practically brand new. Could the bolt have fractured in 1996 and sat there for 22 years?

HuFlungDung · Jun 12, 2018

Definitely some rust evident, although not enough to make removal extremely difficult. The one I broke off when re-torqueing also had evidence of corroded cracking, although a large portion of the surface was newly fractured.

DMF_TomB · Jun 12, 2018

HuFlungDung said:
I was hooking up one of my indexer cables this morning, and I noticed a hole was open in the column base of my 1996 VF3, and the bolt (a 1" Allen capscrew) was laying on the floor, a piece of it anyways. Sure enough, the bolt had broken off. There are 8 of these large bolts in total and I test torqued all the rest, and one more broke off.

They weren't too bad to extract. I thought the holes were tapped a bit on the shallow side, maybe getting 7/8" of thread engagement on a 3.5" long bolt, so I tapped a little deeper and put in longer bolts. I couldn't budge the other 6 bolts, so I left them alone. The broken off part of the bolt was about flush with the top of the tapped hole.

You other Haas owners might want to go around back and test torque the bolts on your machines.

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low strength bolts stretch quite a bit before breaking. grade 5 and grade 8 bolts once overloaded are damaged and when they break they typically just snap.
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i would replace ALL the bolts. i have seen slightly bent bolts that appeared ok and when tightened and or under load just snapped off with no warning.

YdnaD · Jun 13, 2018

That's certainly true (about the lack of elongation strength on the hardened screws) but I would think the screws could take a tremendous amount of force before that failure point. I would assume they're alloy steel screws, which are *supposed* to be good for like 150,000 PSI eh? (multiplied by their surface area).
I'm not a machine builder so I suppose there's a certain level of vibration that comes into the equation. But geez, two broken screws is certainly one hell of a coincidence...maybe replacing all of them is a good idea!

Milland · Jun 13, 2018

YdnaD said:
That's certainly true (about the lack of elongation strength on the hardened screws) but I would think the screws could take a tremendous amount of force before that failure point. I would assume they're alloy steel screws, which are *supposed* to be good for like 150,000 PSI eh? (multiplied by their surface area).
I'm not a machine builder so I suppose there's a certain level of vibration that comes into the equation. But geez, two broken screws is certainly one hell of a coincidence...maybe replacing all of them is a good idea!

Some US-spec Gr8 bolts and metric 12.9 bolts share a typical issue with low-alloy steels "overhardened" to meet strength specifications. This is less likely when the screws come from a quality manufacturer like SPS or Holo-Krome. I would have added Unbrako to the list, but apparently they're no longer part of SPS and have been sold off to an Indian business.

With the hardness comes brittleness, and when material is over ~140Ksi or 40HRC, greater risk of hydrogen embrittlement can be expected. Fastenal, while I don't appreciate their prices, has a very good article on fastener embrittlement here: https://www.fastenal.com/content/feds/pdf/Article - Embrittlement.pdf that's worth reading.

If you replace the bolts, I'd try to identify the old vendor for curiosity, but go with Holo-Krome for replacements. If the old bolts were a good brand, well, sometimes process control fails and you get a bad batch.

After you've replaced the bolts you're going to want to check the machine for tram and perpendicularity of the head and base, there's a chance that even removing and replacing the bolts one at a time could cause a shift of the two main castings and throw the axis off a bit. As to torque, maybe you can get the number from Haas, or use one of the generic specs available on the web.

HuFlungDung · Jun 13, 2018

I couldn't budge the other screws with a 4 foot pipe on the wrench. That's a lot of torque on just a 3/4" male hex socket. Couldn't really get at them using an impact wrench without having to use an extension. That was also a 3/4" drive. Just bounced on them, no movement at all, either direction.

The two bolts that broke were on the left side (I was facing the electrical cabinet), at either corner. The two center ones are still tight on that side.

Milland · Jun 14, 2018

HuFlungDung said:
I couldn't budge the other screws with a 4 foot pipe on the wrench. That's a lot of torque on just a 3/4" male hex socket. Couldn't really get at them using an impact wrench without having to use an extension. That was also a 3/4" drive. Just bounced on them, no movement at all, either direction.

The two bolts that broke were on the left side (I was facing the electrical cabinet), at either corner. The two center ones are still tight on that side.

Fastenal (again) has this chart for socket cap screws: https://www.fastenal.com/content/feds/pdf/Torque-Tension Chart for A574.pdf where the torque spec for 1"-8 bolts is between ~770 and 1K lbs/ft, depending on lubrication status. That's a lot of force, and when you add the "stiction" factor and dry threads I wouldn't be surprised if it took over 1200 lbs/ft to shift the screws.

So that's 300lbs on the end of your four foot tube. I know I'd have to go to a 6ft tube to have a chance at 200lbs unless I could brace fully and get my legs into it.

The bolts you replaced the broken ones with - any idea what your "real" torque on them was? And who was the manufacturer?

DMF_TomB · Jun 14, 2018

HuFlungDung said:
I couldn't budge the other screws with a 4 foot pipe on the wrench. That's a lot of torque on just a 3/4" male hex socket. Couldn't really get at them using an impact wrench without having to use an extension. That was also a 3/4" drive. Just bounced on them, no movement at all, either direction.

The two bolts that broke were on the left side (I was facing the electrical cabinet), at either corner. The two center ones are still tight on that side.

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maybe loctite on screws probably ok to leave in
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just saying i have seen high strength bolts slightly bent that failed suddenly with little warning when under load. even had some make fun of even mentioning it. aint no fun when part weighing a ton the hold down bolts break off during machining. you cannot tell a damaged (overloaded) bolt from a undamaged bolt most times. usually safer to replace it

machineit2 · Jun 14, 2018

If that bolt broke, I would assume it was defective. Socket Head Cap Screws are stronger than grade 8 or 12.9.

I am building a pressure chamber that uses 24 1" SHCS on each end. I had to buy a great big torque wrench that goes to 600 ft. lbs. to tighten them and I'll definitely have to use a assist bar on it. They better be right, as each end will end up with over 1 million pounds of pressure on it.

I would replace the bolt and move on.

Also, I too had a pull stud that broke in the threaded area and not on the slender shank area.

Good luck----Mike

machineit2 · Jun 14, 2018

Just for reference, grade 8 bolts have a minimum PSI strength of 130,000 psi and SHCS have a minimum of 180,000 psi.

And, something I learned in researching my chamber is that fine treads are stronger than course. Course threads were made to speed up assembly of things because they tighten more quickly with less turns. See you learn something each day!

HuFlungDung · Jun 14, 2018

I see 3/4" torque wrenches are usually rated at about 600 ft-lb at 48" handle length, so I'm going to assume I pulled nearly that much. Probably I should get a torque multiplier and use 1/2" input and 3/4" output, getting up to 1200 ft-lb.

The original bolts are marked BJLM. I replaced them with grade 8 capscrews for the time being. I'm leery of trying to pull 1200 pounds on a 3/4" male hex socket......sounds a bit optimistic. What say ye?

machineit2 · Jun 15, 2018

Yes, I do not look forward to have to tighten the bolts. The biggest issue may be holding the chamber still. It will have to be bolted to the stand before I can for sure.

HuFlungDung · Jun 15, 2018

Have you ever looked at cheaterwrench? Advertised as a lug nut remover, but has a ratio of 68:1. Should be able to snug up these mere 1" bolts with an inch-lb torque wrench

I ordered a knock-off version just to see if it really works in this situation (slightly cramped quarters).

RC Mech · Jun 15, 2018

machineit2 said:
And, something I learned in researching my chamber is that fine treads are stronger than course. Course threads were made to speed up assembly of things because they tighten more quickly with less turns. See you learn something each day!

A bolt w/a fine thread is stronger than an equivalent bolt with a coarse thread simply because the cross-sectional area is larger. A fine thread is able to take more stress and strain axially and applies more force for a given torque value owing to the lesser incline of the helix.

machineit2 · Jun 16, 2018

Are you disagreeing with something I said?

Milland · Jun 16, 2018

HuFlungDung said:
The original bolts are marked BJLM. I replaced them with grade 8 capscrews for the time being. I'm leery of trying to pull 1200 pounds on a 3/4" male hex socket......sounds a bit optimistic. What say ye?

Interesting, my older VF-5 and -2 use socket capscrews from the beginning. Yours had hex bolts? Did they have lines on them, and if so how many (I'd hope 6).

I'd think that a quality 3/4" hex would take 1K lbs, if that's what the screws will take. At worst, call the manufacturer and ask. And watch your knuckles... :angry:

HuFlungDung · Jun 17, 2018

Hex socket capscrews originally. I've ordered 8 replacements. The 2 hex capscrews I put in temporarily are grade 8 with 6 marks.

Milland · Jun 17, 2018

HuFlungDung said:
Hex socket capscrews originally. I've ordered 8 replacements. The 2 hex capscrews I put in temporarily are grade 8 with 6 marks.

Gotcha, explains my confusion over the hex bolts, as I didn't think Haas would use them in that location.

HuFlungDung · Jun 21, 2018

I finished the job today when my 'cheaterwrench' arrived. I marked the existing bolt heads for rotational position before removing them. I then loosened them off and then torqued them up to wrench tight (maybe 100 ft-lbs). I could see that this was still about 60 degrees of rotation out from where it was originally. So I installed the new hex socket capscrews (one at a time!) and torqued them up to about 100 ft-lbs and marked the position of the head about 60 degrees in advance of where it should line up. I borrowed a large torque wrench and a 4:1 multiplier and torqued them up to 1000 ft-lbs (best estimate using the 250 ft-lbs setting on the torque wrench and working through the multiplier). They didn't all turn another 60 degrees but it was +/-25%.

I was surprised that the 3/4" hex drive (UltraPro, NAPA) took all the un-torqueing and retorqueing with out any apparent damage. I did twist off a couple of my cheap 3/4" drive extensions (you know the one, the entire set of sockets and wrenches for $100

) I had about a 12 foot long pipe on the reaction bar of the 4:1 and managed to wedge it against other nearby machines while I operated the torque wrench.

None of the other 6 bolts failed during removal. Some had a lot of rust on.

Haas VF3 column bolts breaking?

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