| Re: Engine swap on the C1 |
Subject: Re: Engine swap on the C1 by BeachBum on 2008/11/20 18:06:44 Quote:
The only thing I will add to this dilema is that the above statement is very far from the truth.... there are so many variables with a CNC machine that its incredible..... Operator error is by far and away the most common mistake made on a CNC machine. In addition, most CNC machines have a considerably amount of positioning error in them as they sit. In the metalworking industry, there is a certain amount of scrap that is tolerated and actually built into the quoting they do. The scrap is generated by a wide variety of means, but all have to do with the CNC machine not doing what they were hoping it was going to do. In your situation, its sounds like the program itself is okay considering it has run other same brand cylinder heads successful, but keep in the G-code program they utilize for the path is simply just that, a programmed tool path. But, then a lot of good things have to happen for the CNC machine to actually follow that path. The most common operator error is the loading of the offsets, you have multiple offsets for most CNC programs, in the case of a cylinder head, it can literally be a dozen or more. Starting with the correct tool offset, the operator has to touch the tool off and then load that length offset into the tool table either manually or automatically. Then they have to remember to enter in the tool radius. If the tool was previously set-up, has anybody changed that tool in the tool changer? and if yes, did they accidently put in the wrong tool in either length or diameter.... for example, a very, very common mistake is for the operator to change a worn out tool in the ATC, and puts in the correct type of mill such as an end-mill, but doesn't notice that he put in a 3/8" diameter tool rather than a 1/4"..... which means he has to manually change that diameter in the tool table after he touches off for the tool length, if he forgets, then the entire cnc program will be off by that amount, Ie meaning, the cut is too deep or too shallow. Same goes with the work offsets, the corner of that cylinder head may be one offset, then the actually corner entry into the port may be another... they have to call the correct work zero offsets and hope they were correctly entered in the first time. Unless they have the perfect part fixtures, these will change from cylinder head to cylinder head..... and regardless, they should be reset for each cylinder head anyway, which requires the operator to re-touch off or probe the corners. To give you an example of what happens when a work offset is off, the path will be shifted over the value of the offset error, meaning, typically you will cut too deep on one side of the port, and you'll cut air on the other side. What makes this that much more difficult, on a cylinder head, the cnc program is typically not one long part program based upon one offset, but instead, they'll do a port, step over, call the new offset, and then repeat the program, which is typically a subroutine they are calling. When using this method, an indication that an individual offset for a port was off is that one port is clearly damaged, but the others appear to be generally okay. (The good news about a work offset, is that if the machine has a good home reference, each offset is un-related to the other, thus you do not have a domino effect.) There are many, many other operator errors that can and do damage parts everyday, plus there are many CNC machine set-up parameters & compensation that must be set-up correctly or the machine will not be acurate. In fact, most CNC machines utilize a rotary encoder for positional feedback, this alone is a big problem considering 90%+ of these are directly mounted on the Servo Motor..... thus, it does not take backlash, screw error or cross compensation into effect.... it instead reports the position of the axes as per the motor vs the actual table position. The more expensive option of Linear Encoders will vastly improve this situation, but due to the added expense, most CNC machining centers do not have them. The above errors account for literally Billions of dollars in losses that are pretty much deemed acceptable every year to Manufacturers of all types of widgets..... just the nature of the game in the machining business. CNC manufacturers are constantly in competition with each other on how to make a CNC easier and more fool proof.... but alas, its always an uphill battle. Having said that, with your particular situation, I have no idea what happened.... I'm just saying, keep an open mind and also do not for a second put your total faith in a CNC machine..... I think without a doubt the most common technical phone call that a CNC machine manufacturer receives from their customers is " Help !... my CNC machine will not repeat !" Good luck with a decent resolution. |
