Biggest Machining Mistakes 1

[lewtam]

One of the self evident risks in manufacturing overall is as you assume more resposibility, you have the opportunity to make much bigger mistakes.

I'd like to hear about any mistakes/mishaps that members are willing to share. We'll have a laugh of course, but hopefully we'll learn something as well.

I'll start the ball rolling:

Worst Mistake: Sending air hardening steel shafts (15" dia) away to be heat treated with the old heat treatment specs which called for a water quench. I could fit my hand in the quench crack. Learning experience - a change can be a good thing, as long as you follow through properly.

Mishap: Bench grinder wheel exploded while I was using it. Fist sized piece of aluminum oxide ricocheted off the back of my hand and hit me in an area that is usually meant to be treated with the greatest of tenderness. Learning experience - welding gloves and apron may save your ability to have children.

LewTam Inc.
Petrophysicist, Head Stockman, Gun Welder, Gun Shearer, Ski Instructor, Drama Coach.

 

[Colega]

I work for a pump manufacturer. Once we made 500 pump casings the wrong way because the part drawing called for a thread, but didnt specify if it was left or right handed, the machinist did not ask any questions and made them right handed.

Learning experience - Assumption is the mother of all...mistakes.

Colega

http://www.corporacioneg.com

 

[lewtam]

Were they salvageable or did you have to melt them down?

 

[rmw]

I'll relate one that happened to my best friend when we were young college students. We were told never to use a standard universal adapter on an impact wrench. But, like a lot of instruction that young guys get, we weren't told why.

My young peer, not given to taking instructions that he did not know the reasoning behind, began one day to use an impact wrench to chase the threads of a larger bolt in and out of a threaded piece to lubricate and free up the threads.

As he turned the piece counterclockwise, backing the threaded shaft out, he pulled the impact wrench away from the head of the bolt, and the socket slipped off.

I heard a resounding 'thwack' from his area of the shop, the operation he was doing being a very common one, and no one was paying much attention to his activity otherwise, to see him stagger away from his work, free hand to his forehead, with blood streaming from between his fingers.

The socket, with all the possible directions that it could have gone in, came off the universal as it turned 90° and struck him in the forehead. He has a scar to this day. It took several stitches to close it. I vicariously learned a valuable lesson from his mistake.

rmw

 

[aamoroso]

I could write a book on the things I have seen.

First one that comes to mind, I saw a guy take a Case Harderned 48"x 1.5" x 2" bar (cut on one side like a flight of stairs in .125 steps w/ 7Degree angle to them)... anyway he took this hardened bar and was getting setup to finish grind it. He checked the flatness on a surface plate and it was bowed in the middle. So he put it in a manual press and tried to flatten it. Needless to say it popped shortly after he put some pressure on it, snapped the part like a toothpick. I kept a 4" piece of it, it now serves as an example of case hardening for me. You can see the shell all aournd the outside of the profile and the center looks powdery.

Maybe I'll come back with some more later.



Fill what's empty. Empty what's full. And scratch where it itches.

 

[BillPSU]

We were making hydraulic cylinder barrels 4" OD x 24" length internally threaded on a through hole CNC lathe. Young machinist with limited experience with special threads. Comes up to me an complains he changes the offset .001" and the threads go oversize. He tirelessly gages each thread. I go out and take one look. The print says 10 degree buttress but the parts are 60 degree V.

60 pieces of scrap. Lesson learned the hard way.

 

[Colega]

Stright back to the foundry they went. Nobody got fired for this (at least thats what I was told).

Colega

http://www.corporacioneg.com

 

[Eltron]

This probably isn't the worst thing that has happened to me, but it has left me aware of my mere mortal status. I had a job to make a tiny titanium knuckle for a joint, and it was pretty complex. I had to turn it, put on a radius, drill holes, tap threads, and polish it all by hand. It took me about a day and a half of staring at the thing to get it perfect. The finished part was beautiful and couldn't have been bigger than 3/8". I was so proud. On my way to clean it off, I grabbed an air line to blow off some of the coolant. Did you know that coolant makes things slippery? Well it does...The poor little thing blew out of my hands and down a drain in the floor it went. I nearly sprained my tongue cussing myself out.

 

[teb1]

What is a gun welder- gun shearer?

 

[lewtam]

Aussie Slang 101

gun
adjective:- to be gun at something is to be very good or the best at something , e.g.., a gun shearer is the best shearer.

May be taken as a slight a slight exageration on my part.

LewTam Inc.
Petrophysicist, Head Stockman, Gun Welder, Gun Shearer, Ski Instructor, Drama Coach.

 

[rorschach]

My favorite one was a mistake made by a co-worker. He was a drafter that was quite obstinate and refused to follow shop practice when it came to: drawing to scale, drawing in 2D instead of 3D Autocad, and most importantly, using associative dimensioning. He drew an expensive downhole (oilfield) hydraulic tool with a number of gun drilled holes at odd angles that had to intersect at specific places in order for the hydraulic valves to fit in and to work as designed. the angles and depth dimensions shown on the print would not result in a functional part, even though the picture showed it would. The checker, expecting the dimensions to be driven by actual geometry did not lay them out to verify that they would.

The result? A whole bunch of expensive high performance staninless steel alloy parts scrapped, and worse, a very tight shipping schedule shot all to hell.

 

[psychomill]

Ahhhm, gundrills. One comes to mind...

Some winters ago, working at a shop in Denver, I was moving a part from one machine to another on a HMC cell center. The tool matrix were all virtually the same with a few exceptions. Well, the program called for a 1/2 em which spun at 14k, the magazine had a 7/16 gundrill (11.0" gage length) in it. Imagine my surprise when that gundrill fired up to 14k. I slammed the door shut, and before I could hit the E-stop, the drill snapped and shot through the roof of the machine. Found the shank on the floor next to me but never did find the carbide end.

That spring as the snow began to melt, we noticed the roof leaking over the machine. A maintenance guy went up there to fix it, and you guessed it, he found (on the rooftop) the carbide end of the gundrill that broke a few months prior.

 

[unclesyd]

I have enough of them for a small book.

One that I was involved in was a PV head with a 60" bolt circle. We had put an new flange on the vessel and a very obstinate design engineer had designed a new head and supervised its fabrication and machining of site. When the head arrived on site I ask if he would like our group to verify the dimensions, in short answer NO!. We had insulted him. The head was brought to vessel for installation and while hanging on the monorail I noticed the bolts that held the protective cover on were in a odd position, but didn't say anything. The cover was taken off and swung into position and as a mechanic was set to install the top bolting he turn and told the designer there is no hole on the vessel flange. He had one holed the head while body flanged was two holed.

We manufactured in house 8" x 1l" 1" thick SS extrusion dies for extruding synthetic fiber. A particular type die was supposed to have 1280 holes equally spaced in two sets. Even though the holes are manually drilled the position is set with an xy table. For some reason they number of positions were only 638, this included both the back and front side. Some one counted the holes in a set and revealed the problem. The operations group rejected the lot dies (50) due to the missing holes. The cost of dies was in the neighborhood of $35,000 at the time. The manufacturing group work 24/7 through the holidays making new dies.
I put a rejected die on a comparator and compared the pattern with one that had been in service for several years, all the same. The rejected dies were slipped into the process one at a time over several years.

The biggest one was an expander wheel made from an A286 forging. The block was forged, skinned machined, and tested. After this it was sent to be rough machined and tested again. At time it was sent to rough out the blades. It was then shipped to the final machining. About half way through the final machining one of our engineers on vacation stopped by to see the machining process. To his astonishment they were machining a wheel with the wrong hand. A the time it was a $150,000-$200,000 mistake. I never heard the final resolution of problem. The wheel went to scrap in 2000.

 

[lewtam]

Unclesyd - your obstinate design engineer in the first example reminds me of a similar man I've encountered. After a particularly humbling experience, he said to me 'I'm never so dumb as when I know I'm right'.

It's something I really keep in mind today, and I've been able to climb down off my smart horse before I make a fool of myself a few times.

LewTam Inc.
Petrophysicist, Head Stockman, Gun Welder, Gun Shearer, Ski Instructor, Drama Coach.

 

[kenvlach]

At the University of Wisconsin-Madison, a graduate student once rolled about half of of his index finger into a small piece of steel. He evidently forgot or hadn't paid attention during machine shop safety training, but luckily his head hit the emergency stop crash bar.

The piece of steel, with a sizable indentation and clear fingerprint pattern, is passed around during safety training. It gains the students' attention.

 

[pmmch63]

I've got one from my days in a small job shop many moons ago. There was only one part that was scrapped and it wasn'
t a high dollar loss, but what was significant was that there was a potential for a serious injury ( to me!!!) which luckily didn't happen. We had an order for a shaft (I forget the mat'l grade, 1144 range maybe) from a wire manufacturer which included shrinking a hardened D2 toolsteel sleeve on a center portion of the shaft. The sleeve dimensions were approximately 8" ID x 10" OD x 15" long. The only part of this job I was given was to cut the sleeve OD to within .005 of finish size to prepare for grinding and polishing. (after it had been installed on the shaft) I forget how much mat'l. had to be removed, but it wasn't much, maybe .020. Well, I made the cut and was within 1/8" from the edge on the 15" long sleeve when without warning that nice pleasant spring day was shattered along with that D2 sleeve! That sucker absolutely exploded apart and sent shrapnel everywhere! The customer had sent the prints including dimensions and amount of interference between sleeve and shaft, which I would say were right on the money. The problem was the shop had a qc guy that inspected parts before going out the door, but failed to inspect these parts before assembly, which was a mistake. We later found the error must have been in the sleeve....the shaft size was correct. The lesson I learned....always be very inquisitive, especially when following up on someone else's work.

 

[wmalan]

Most common mistake in aerospace:

"Supposed to make the opposite dash number - not the as shown dash number"... if I had a dollar for everytime I've seen that one.
I've seen this in parts valued over $100k. Hopefully the vendor eventually buys the other dash.

--
Bill

 

[lewtam]

Wmalan - I don't like seeing these words in the same sentence:

'common'
'mistake'
'aerospace'

LewTam Inc.
Petrophysicist, Head Stockman, Gun Welder, Gun Shearer, Ski Instructor, Drama Coach.

 

[BillPSU]

lewtam
I agree with you but I don't see how a plane could fly with (2) left hand wings or anything else.

Go to a fab shop and ask the pressbrake operator how many times he has bent the part the wrong way and made the wrong hand part.

 

[DDRSS10]

A little story about the importance of good cutting oil. I worked in a small shop that frequently made welljet heads that reqired a 1-1/2 npt to be tapped into one end. Nothing fancy but a big cut without a reamer. We had recently switched cutting oils. Long story short about 6 turns into the power tapping the tap exploded like a hand grenade. I ended up with a broken set of saftey glasses and a shard of hss about an inch long imbedded in my chest through 4 layers of clothing. I had done about three hundred with the other oil, didn't make it through one with the new. I good learning experience.

Nick

"Speed costs money boys, how fast do you want to go?"