machining help needed
machining help needed
(OP)
Everything that I have been taught and had proven to me shows that using the offsets in a machine is bad. Here's the way I have come to look at it. If it has been proven that the process is capable than there should be no reason to edit with the offset for production purposes. If parts are coming out of the machine out of spec than there must be something else wrong (ie, tool wear, chip build up on fixture, bad parts going in, etc.) What are others opinions about this? My current position is that if parts are not coming out to spec we need to find the root cause as to why this is, not to just adjust the offset in the machine to bring the dimension back. I am currently in disagreement with a production sup. with about 20 years with the company. Thank You for your opinion and letting me vent.
Michael
Michael





RE: machining help needed
RE: machining help needed
Michael
RE: machining help needed
If you are getting large variations between measurements during a production run, then obviously something is wrong. You will get extremely minor variations, not every piece is ideally the EXACT replica of the former! On a mirco scale, machining depends on both the physical process and chemistry of the piece being cut, which are random in nature statistically speaking. The machinist must be diligent is checking a few pieces during the production run!
I use a random number table and have the machinist check those pieces for a few dimensions, again at random. For example, you need 20 pieces of a particular item. Then check the first two for ALL dimensions, then 5 for diameter and threads, 9 for length and thread pitch, check 11 for thread pitch with a master gauge, 16 for all dimensions. Then statistically speaking you have 6 of 20 pieces that have been proven out of the batch. You can work your magic to find the average, standard deviation of dimension and accuracy of the machining.
Clearly this is part of the ISO standard. I just call it smart production. Your machinist must show more due diligence and attention to his work! Check your float, it sounds like this is the problem in this case, in addition to not random sampling a batch run!
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: machining help needed
RE: machining help needed
As is often the case, if you can't find the problem, it's time to start eliminating assumptions.
RE: machining help needed
Michael
RE: machining help needed
One reason for wanting to be able to use offsets is so you can use reground cutters.
you might also want to use the offset command to compensate for normal tool wear as the job progresses.
Depending of course on what your tolerances are.
I know you said in your original post " If parts are coming out of the machine out of spec than there must be something else wrong (ie, tool wear, chip build up on fixture, bad parts going in, etc.)". However compensating for tool wear is a normal part of machining ( move it or change it). The idea is to give the machinist the ability to give you good parts.
RE: machining help needed
If you have parts that have liberal tolerances, you have a machine that is capable of fabricating the parts, and you have parts that are built out of tolerance; And you have a machine shop supervisor with 20 years experience that has a fix for you. Why are you fighting him?
RE: machining help needed
Michael
RE: machining help needed
RE: machining help needed
RE: machining help needed
That's exactly where I was trying to get to. That's how I feel a shop should be run but some of the "old schoolers" are fighting it. This afternoon I need to go see a customer that we just made many bad parts for. An ispector, that has checked this part several times in the past and always good, for some reason checked a part with a different methode. Needless to say the dimension didn't come out right so he had the operator change the program and run the rest of the parts (of course they were all bad then). This is getting very frustrating.
Michael
RE: machining help needed
Do you have an SPC data collection system in place in the shop?
My comments about offsets are to get the parts right.
Once the part is being produced to the tolerances, then the machine should be run without adjustment, and the results plotted on a data collection chart. One of the things this should show you is the machine noise ( Inherent errors and slop in the machine.) from this point on the only adjustments should be to get back to the center of the bell curve. You have stated that you know the machine is more than capable of holding things in tolerance. Have you run a backlash test pattern or are you relying on the supervisors word?
RE: machining help needed
On one hand, you have a proven NC program that a machinist "feels" in need of updating. Then you have personality issues between "old school" verses "new school", employees, a resistance to change in operations.
What you really need is a strong manager with a heavy hand. I guess floats and tolerancing due to machining the piece aren't the problem afterall.
Good luck with it.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: machining help needed
My apologies, I did not realise your guys had been tweaking the actual program, Changing a dimension in the program is an entirely different thing to messing with the offset's. As far as I am concerned that is a totally different issue, and is, or should be a big no no.
RE: machining help needed
unfortunately this will probably never happen so I add offsets (specific and well defined by man-readable code) where ever possible. I feel this is much preferrable than trying to figure out how the midnight shift hacked up the original program to get the (hopefully) good parts comming off the machine when I return the next day. Or a week/month/year later if I don't happen to catch it sooner on my own.
I have 14 years in CNC, offsets are not the enemy.