Boy Howdy, THAT was a chore.....

Linefinder

Got tasked with machining 430 parts that load into a fixture on the CNC. The fixture had to be within .0002" parallel of the Y-Axis travel of the machine over a distance of 19". Bet it took me almost 2 hours to get it right. You could have the fixture bolted down almost tight, but if you breathed on it it'd move more than that.

The parts themselves had a total of 34 seconds cycle time each.

After machining, the parts go to QA for inspection on an optical comparator. The fixture QA uses has to be parallel with the X-Axis of travel within .002" over a distance of 2".

I asked the engineer why did I have to hold within .0002" over 19" while the guy who decides if the parts are good or bad gets .002" over 2 inches?

"Because that's what the paper says", was his answer.

Sometimes I hate this job.

Mike

PFD

And it probably made sense to him.

zorba

OMG! Two tenths of a mil over NINETEEN INCHES?!?

Congratulations for pulling that off - and congratulations for NOT choking the engineer!!

Linefinder

zorba said:

OMG! Two tenths of a mil over NINETEEN INCHES?!?

Congratulations for pulling that off - and congratulations for NOT choking the engineer!!

LOL!!! It was a near thing, believe me.......

Mike

jbp-ohio

Now that you have accomplished it.... The engineer will tell everyone, "See... Told you it would be easy."

Linefinder

jbp-ohio said:

Now that you have accomplished it.... The engineer will tell everyone, "See... Told you it would be easy."

That is exactly how it goes. He'll be a genius and I'll be fighting 1000 of these a month for the rest of my working life......

Mike

bklysen

Mike, when you get an extra 2-3 hours, could you translate that all back into English?

Justsomedude

That was a job for a surface grinder.

Linefinder

Justsomedude said:

That was a job for a surface grinder.

Well, I'd agree, except you can't do much to a parabolic slot with a surface grinder.

Mike

Justsomedude

Linefinder said:

Justsomedude said:

That was a job for a surface grinder.

Well, I'd agree, except you can't do much to a parabolic slot with a surface grinder.

Mike

Ah, I was envisioning parallel milled parts. Thats my bad.

Linefinder

No, this was side milling a tiny, teensy bit out of an irregular curvature on one side only. If it had been flat or face milling.....walk in the park. I wasn't that lucky.

Mike

zorba

Can you post pix? Or not?

Linefinder

bklysen said:

Mike, when you get an extra 2-3 hours, could you translate that all back into English?

I'll try......

X-Axis is left to right. Y-Axis is front to back. That gets the directions straight I hope.

1 ten-thousandths of an inch equals an inch divided equally into ten thousand parts. .0002" equals 2 of those. That is pretty small.

Life being what it is, there ain't much in this world that's perfectly square, and the longer distance you have to traverse, the greater your initial error becomes. If I traverse 2" and I'm off by .001" (not uncommon on initial setup), I'll be .012" off in 24". So you tap the vice around, gently, with a piece of brass half the distance your test indicator is showing "out", and do it again, and again, and again.

Eventually you get "close", then you break out the test indicator that measures down to .0001".....and start over. It gets really interesting at that point.....You are now trying to correct to a precision 1/10th of what you were doing before......

You finally get it "right", tension your last hold down bolt, and wham........you're now .002" off, while the goal is 1/10th that. So you do it some more....and more....It's sort of like herding cats but not as fun.

Trust me.....    .0002" over 19" is a bitch to achieve. But I did it once. They're going to expect I'll do it more quickly next time.

At which point I'll hand them the .0001" indicator and say, "Okay, Karen".......

Mike

Linefinder

zorba said:

Can you post pix? Or not?

No. I even have to shred prints upon completion of a job. We're a contract manufacturer.

Mike

Justsomedude

Timing is everything. I had to use the rotary table up on end yesterday to make an extractor cut in a rebarrel job, so I obviously had my vice off. Swept the vice until perfect, tightened down the hold down nuts and of course the vice moved. Started over. Piss and grumble. On a happier note, here's my extractor cut.

zorba

Linefinder said:

zorba said:

Can you post pix? Or not?

No. I even have to shred prints upon completion of a job. We're a contract manufacturer.

Mike

I understand.

Linefinder

So today was fun. The parts I milled yesterday get 10% inspected by QA. Meaning out of 430 parts, QA would pull 43 to inspect and if a single one failed the entire lot of 430 gets scrapped.

The engineer in charge decided since this was a "Process Qualification" run, it'd be a good idea to check 100% of the parts before sending them to QA. (Sort of defeats the purpose of a PQ run, IMO, but....whatever). So, he had me check my own work....LOL!

I got to use an old fashioned optical comparator, complete with backlight, front light, side lights and a back lit projection screen. The fixture that held the parts is aluminum milled to an almost polished finish. The reflection from the front light, mildly stated, was intense.

After about 10 minutes of inspecting parts, any time I looked away from the comparator I'd see flashing blue spots wherever I looked. At one point I was truly afraid I was in the early stages of a stroke.

I finally figured out that while loading parts into the fixture to close my right eye and use my left. Then close my left eye and use my right to look at the screen. I went through 4 Tylenol during the process.

All the parts passed. I decided I'd had enough for one week and left. As soon as I stepped into the sunlight, I was blind in my left eye again. I drove home using my right. It's been over an hour now, and when I close my left eye I still see a blue spot, but it's a lot better than it was.

Next time.....I'm gonna grab a black Sharpie and tone down that aluminum fixture....

Mike

Linefinder

Justsomedude said:

Timing is everything. I had to use the rotary table up on end yesterday to make an extractor cut in a rebarrel job, so I obviously had my vice off. Swept the vice until perfect, tightened down the hold down nuts and of course the vice moved. Started over. Piss and grumble. On a happier note, here's my extractor cut.

Very nice!

FWIW, I've used a rotary table exactly twice in my life. I hate them with a passion. Along with boring bars. We have one completely manual Lagun mill in the shop, and it has a rotary table installed on it. But, the 2 Bridgeport E-Z Traks make relatively simple work of odd sized holes. If it has to be really close, the Haas CNC gets the job. If it has to be dead nuts on (molds, for example), you build an electrode with the Haas, then burn it in the EDM sinker.

It's not that rotary tables are bad tools, or can't produce fine results....It's just that I completely suck at using them. Kind of like manual lathes.....Were it not for CNC lathes I'd likely still be trying to produce a "to spec" part.

Mike

JerryBobCo

Can we change this discussion to something simple and easy to understand?  I'm thinking a discussion on differential equalions might be fun.

GunNut

JerryBobCo said:

Can we change this discussion to something simple and easy to understand?  I'm thinking a discussion on differential equalions might be fun.

Or Quantum mechanics...

zorba

The only thing I know about quantum mechanics/physics is this tidbit: "If you think you understand quantum mechanics/physics, you obviously do not!"

ilove22s

Linefinder said:

Got tasked with machining 430 parts that load into a fixture on the CNC. The fixture had to be within .0002" parallel of the Y-Axis travel of the machine over a distance of 19". Bet it took me almost 2 hours to get it right. You could have the fixture bolted down almost tight, but if you breathed on it it'd move more than that.

The parts themselves had a total of 34 seconds cycle time each.

After machining, the parts go to QA for inspection on an optical comparator. The fixture QA uses has to be parallel with the X-Axis of travel within .002" over a distance of 2".

I asked the engineer why did I have to hold within .0002" over 19" while the guy who decides if the parts are good or bad gets .002" over 2 inches?

"Because that's what the paper says", was his answer.

Sometimes I hate this job.

Mike

sometimes even the engineer doesnt have a say.

i wonder if the original designer/engineer figured out that the chances of getting 1 or a handful of parts was minimal.  or if there was a 0 missed out of the inspection?

PFD

I remember from High School shop (thank you Mr. Eugene Gazola) that if you have a 1" rod chucked up in the lathe and you need a 1/2" rod, you turned the tool in 1/4" . 😁

Linefinder

His math skills are certainly correct, but if you try to take 1/4" of anything in one pass, your tool is going to turn into shrapnel.

Truth is, in machining, you never "cut to the numbers", meaning if something needs to be exactly 1" long, and your stock is 1.050"  long, you don't immediately cut off .050". You cut off .025", which should give you a length of 1.025", but say it actually measures 1.030". 

So, next, I'll divide the amount I need to cut off in half, in this case .015" and cut that. Measure the part again, expecting 1.015, but it actually measures 1.010". Your part has heated from the previous cuts (grown in length so you actually cut off more than you intended), and  your tool has dulled slightly, which is another variable in consistency of cut.

So I now have to take off .010". I'll cut .005" and measure again. If it measures 1.004", I'll once again take off half, or .002" and measure again. From that point I'll "nibble" off the rest until I'm a few ten thousandths over length, than without making an adjustment, I'll make a "clean up" pass. That usually brings it to length, or close enough.

In theory. And that's on a block that's already flat, parallel, and square. On raw stock, it's a bit more complicated.

Imagine a mold with a few moving parts in it. I used to be amazed that a mold you could lift with 2 hands could cost upwards of $40K. Not anymore.

Mike

Linefinder

JerryBobCo said:

Can we change this discussion to something simple and easy to understand?  I'm thinking a discussion on differential equalions might be fun.

I don't understand the problem, Jerry.

Machining is simply taking something bigger and making it smaller.

Or....in words an arithmetic major can follow........"subtraction".  

Mike

Justsomedude

For most things I use the rotary table for, I just set it up next to my vise and don't mess with the vice at all but this time I had to set it upright and span the barrel across the x axis with a center point fixture in the chamber end, hence retramming my vice. Having to loosen the vice gives me a crap feeling in my gut because I know the pain that awaits me.

Linefinder

I know that feeling well. I hate removing my vise, but I have to do it quite often, and have gotten pretty quick at indicating it back in. What I hate is tramming the head after the rare, but inevitable, tool crash. Doesn't happen often, but, damn.....when it does........

If it happens late in the shift, I just cuss and clock out. If it happens early....well.....it's gonna be a long day.  

Mike

PFD

.

PFD

Linefinder said:

His math skills are certainly correct, but if you try to take 1/4" of anything in one pass, your tool is going to turn into shrapnel.

Truth is, in machining, you never "cut to the numbers", meaning if something needs to be exactly 1" long, and your stock is 1.050"  long, you don't immediately cut off .050". You cut off .025", which should give you a length of 1.025", but say it actually measures 1.030". 

So, next, I'll divide the amount I need to cut off in half, in this case .015" and cut that. Measure the part again, expecting 1.015, but it actually measures 1.010". Your part has heated from the previous cuts (grown in length so you actually cut off more than you intended), and  your tool has dulled slightly, which is another variable in consistency of cut.

So I now have to take off .010". I'll cut .005" and measure again. If it measures 1.004", I'll once again take off half, or .002" and measure again. From that point I'll "nibble" off the rest until I'm a few ten thousandths over length, than without making an adjustment, I'll make a "clean up" pass. That usually brings it to length, or close enough.

In theory. And that's on a block that's already flat, parallel, and square. On raw stock, it's a bit more complicated.

Imagine a mold with a few moving parts in it. I used to be amazed that a mold you could lift with 2 hands could cost upwards of $40K. Not anymore.

Mike

.I kinda knew about the. many passes thing, just using lazy math.

The rest was a fascinating and amazingly generous and concise sharing of the techniques employed to accomplish that degree of precision. 🙏

At that level of accuracy, does the ambient temperature affect the measurement at a given time?

Linefinder

Yes, it does, especially with a surface grinder, which as the name implies, doesn't cut....it grinds. Which imparts heat into the part which makes it grow at the same time your grinding wheel is wearing away.

So. dousing your part with water isn't a good idea,because it cools your part below ambient and it will measure smaller than it really is. So, when you're "close", you go outside and smoke a cig or two while the part attains "ambient". You go back in and do it some more........when you're close, you go back outside and smoke another cig or two while the part cools again.

Then your "supervisor" pokes her head out the door and asks "It's not breaktime...what are you doing out here?".

Then you say, "Okay, Karen".................and wink.

Mike

tennmike

PFD said:

.I kinda knew about the. many passes thing, just using lazy math.

The rest was a fascinating and amazingly generous and concise sharing of the techniques employed to accomplish that degree of precision. 🙏

At that level of accuracy, does the ambient temperature affect the measurement at a given time?

That would be a YES. If you go out in the steel yard and grab a round bar that has been sitting in the sun a while, then it will have 'grown' in both length and girth from temperature expansion. During cutting passes, most CNC machines I'm familiar with use either flood or spray cooling on the part being machined to keep the part from 'growing' during the machining process and causing machining errors after they've cooled off. And I don't know, but highly suspect, that Linefinder's work area is climate controlled and materials from which he machines parts are inside that area, if possible.

Fun fact. Railroad rails have a set gap between rails to allow for expansion of the rails without causing them to butt together and cause track distortion.

And high tension electrical transmission lines sag from expansion when under high current flow. Sometimes they sag considerably.