Broken pull stead

Broken pull stud

My shop had a pull stud break today. We were fortunate, no major damage, no injuries.

Stats:
.75 2 flute endmill, 2.0 LOC
10 IPM @ 1500 RPM
Fanuc Robodrill, BT-30 adapter

The tool was clearing out material from the floor of a feature 2.0" deep, no contact with the walls. The operator noticed the tool pulling into the stock, and heard the noise, and stopped the machine. when he retracted the z the tool stayed down. I'm not sure how they got the pull stud out, I imagine they executed tool change to an open spot in the carousel and the rest dropped out.

This pull stud is fairly new, less than 2 years old.

Scary.

Had that happen on a Brother, wasn't so lucky..... spindle taper looked like you'd laid a few beads in it with a welder. Course, it was running 10k and the tool was a Ø2" x 3.75" LOC inserted end mill (36 PCD tipped inserts). It is automated, so nobody around until it started making lots of noise. Way too late by then. We put in a PM to change studs on any heavy cut tools every 6 months. Pull studs are CHEAP compared to a spindle, downtime and 36 PCD tipped inserts.

Any thoughts as to the condition of the spindle taper? If it's bell-mouthed, the toolholder could be rocking as it cuts, putting a bending load into the stud right where it broke. If you have a new, quality toolholder you could blue the taper and check mating surfaces.

Tony, that's a painful loss - I wouldn't have the guts to push a tool that size in a 30 taper machine...

We have a 6" face mill going into a 40 taper mill.
I bought a J&M stud for it.

(Just scratching the surface with it)


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Think Snow Eh!
Ox

barbter said:

Tony - I'm asuming it was fatigue that broke your pullstud, hence changing them every 6 months?

Click to expand...

Correct. They had a coolant through pull stud on it when they shouldn't have as the tool wasn't coolant through. Lots of chevrons emanating from a little place on the ID, under the coating. Looked like a drill swirl (ditch) where it started, and it was before it was black oxide coated. You could see how the fatigue crack spread until it failed in tensile overload. That stud had somewhere around 300,000 cycles on it. At the time, that 6 months was half that figure.

Just thinking about it since you asked the question, that wouldn't be frequent enough now on that tool, but I'm not using that tool anymore. Running about 35k a month on that machine now. I need to revisit a couple other pull stud PM's on that machine also. Thanks for the reminder.

Take a look a the german deckel pull about four times the metal mass on 40 tapper . tkanks Mike

lucky13dave said:

My shop had a pull stud break today. We were fortunate, no major damage, no injuries.

Stats:
.75 2 flute endmill, 2.0 LOC
10 IPM @ 1500 RPM
Fanuc Robodrill, BT-30 adapter

The tool was clearing out material from the floor of a feature 2.0" deep, no contact with the walls. The operator noticed the tool pulling into the stock, and heard the noise, and stopped the machine. when he retracted the z the tool stayed down. I'm not sure how they got the pull stud out, I imagine they executed tool change to an open spot in the carousel and the rest dropped out.

This pull stud is fairly new, less than 2 years old.

Scary.

Click to expand...

I would guess that those studs were not heat treated properly.

Pull studs are sized such that the tensile load doesn't come anywhere near the drawbar pressure, so it was likely crack propagation that destroyed your pull stud.

Tony's example shows two extremes - his pull stud went through 300,000 cycles and there was a visible flaw when the coolant hole was drilled. The fact that the stud lasted 300,000 cycles suggests that the stud was heat treated properly, but the flawed coolant hole should never have passed inspection.

In the OP's case, I'm guessing there was a tiny step between the fillet and the ground body (where the break occurred). That's the stress riser. It's hard to say what the material is and the type of heat treatment that was performed, but it likely resulted in low fracture toughness.

Making pull studs is not a trivial task. There's a lot more to it than getting the proper hardness and yield. I've heard of companies testing pull studs by whacking them with a hammer. That's a bogus test that reveals absolutely nothing, since pull studs don't get stressed anywhere near their yield or ultimate tensile strength limits in normal use. A proper test would be an air piston that loads and unloads the stud 1 million times.

Who made your pull studs?

Just to clarify, it wasn't really very visible to the naked eye. I looked at it under a Sony video microscope/measuring machine. Looked like a rapid retract ditch from the drill tip. Maybe 0.001-0.002" deep..wasn't much but that is where the crack started. I found it odd that it started on the ID but it did. We use studs from both Retention Knob Supply and from Nikken...... I don't remember which brand it was.

Are You aware that there are 2 different (or maybe more) studs available for BT30 taper ? One has equal angled chamfers the other not. One has tru-coolant, other not. Actually, its quite possible to mistake. So, double and triple check ! I've had this problem once.

Looking at your picture, it seems that this is the case.

Madis Reivik said:

Are You aware that there are 2 different (or maybe more) studs available for BT30 taper ? One has equal angled chamfers the other not. One has tru-coolant, other not. Actually, its quite possible to mistake. So, double and triple check ! I've had this problem once.

Looking at your picture, it seems that this is the case.

Click to expand...

The stud appears to be of the correct geometry.

The upper chamfer is not critical. The lower chamfer for a Robodrill is 45-degrees, and that appears to be the case here. A Brother machine would require a pull stud with a 60-degree lower chamfer.

Tonytn36 said:

Just to clarify, it wasn't really very visible to the naked eye. I looked at it under a Sony video microscope/measuring machine. Looked like a rapid retract ditch from the drill tip. Maybe 0.001-0.002" deep..wasn't much but that is where the crack started. I found it odd that it started on the ID but it did. We use studs from both Retention Knob Supply and from Nikken...... I don't remember which brand it was.

Click to expand...

Interesting - 0.001-0.002" certainly isn't a large enough flaw to warrant a part rejection (undetectable by most methods).

300,000 cycles could be a lot or not that many - depends entirely on the fracture toughness of the material.

Orange Vise said:

Interesting - 0.001-0.002" certainly isn't a large enough flaw to warrant a part rejection (undetectable by most methods).

300,000 cycles could be a lot or not that many - depends entirely on the fracture toughness of the material.

Click to expand...

I should have taken it over to the Materials Lab in the foundry and put it in the SEM to see if there might have been a material flaw there, but didn't.
If it ever happens again I will.

Whatever safety factor might have been calculated for pull studs, it was surely done before thru-spindle coolant appeared on the scene. So that hole is basically an afterthought.

Any hole thru a stressed part like a stud almost doubles the area available for a stress raiser, while increasing the stress on the remaining cross section. For example, gundrilled racing axles used to be honed to remove the internal tool marks (I haven't owned any for 30 years so I don't really know whether that's still the case, but it's a logical step).

I quit using thru-spindle coolant because of the inevitable rust problems, not because of breaking a stud, but it was obvious that they were a potential weak point.