| Re: Best Castings in the Market? |
Subject: Re: Best Castings in the Market? by BeachBum on 2008/12/23 16:23:28 Oh well.... I'll re-do the short version. Jsup, there are ton of advancements from a CNC from 5-10 years ago to now, its night and day, and this includes accuracy and how you get to that position in space and what happens when you get their. Starts with "true" 5 axes tool compensation (RTCP) in which the CNC automatically modifies the tool path to keep the tool perpendicular to the part at all times. This creates a much better surface finish and accuracy as well.... it also helps eliminate those tool marks & gouges frequently seen on parts. (not talking about the tool path levels you see when you get close, but instead the marks you see on part contours, in between the levels & at places in the part you have directional changes) This is because the tool is no longer cutting the part on angles or sides. Actual positional accuracy has GREATLY improved over the last several years as well..... this is for a variety of reasons, one of which the CNC's feedback port frequency has increased in speed 5 and even 10 fold over the last several years, thus allowing for finer Linear Encoders which allow faster machine speeds with higher resolution encoders. In addition, the old positional feedback method was utlizing a quadrature TTL AB pulse train with a linear encoder which gives you an incremental feedback solution.... meaning, when you go forward the CNC reads the pulse train for position, when going backwards it subtracts based upon the pulse train. But, with this method, if you're off due to the progressive error.... if you have positional error at Point A, you'll still have it at Point B, the CNC has no way of correcting itself short of stopping the cnc program and doing a homing sequence. Nowadays, "Absolute" linear encoders are utilized, thus eliminating any progressive error at all..... most modern day VMC's are equipped as standard with absolute technology. Maybe the biggest advancement is in the CNC - Servo Drive communicaiton, which used to be done via a +/- 10vdc signal that would vary based upon a DA converter and the amount of following error. But, nowadays utilizing Sercos communication protocol and Digital Drives utilizing Fiber Optics, the CNC communications via a high speed layered digital command to give the servos their positional commands. Keeping in mind, these commands are updated multiple times per millisecond based upon the actual CNC command via the part-program and the position reported by the feedback device. The value of this, is that the machine is much more accurate, smoother and faster...... To understand more about what is happening, you have to understand that a typical 5 axes VMC utilizes your XYZ linear axes and then via a trunion tilting 2 axes rotary table (A & B axes), you have 5 axes the CNC must control in an Interpolated simultenous method.... what this means is that all 5 axes "have" to be at a constantly changing position at an "exact" specific time. To just position an independant axes is as easy as typing G01 X5.0001 F50, but to get all 5 running a high speed program simultenously on an advanced 5 axes path is a completely different animal. There is much, much more, but I don't feel like typing all morning, but you do have to understand there is a huge difference between VMC's of just 2 years ago and the VMC's of today.... just like the desktop/laptop PC market, the technology is changing and advancing so fast its alarming. Which is a big problem with the machine they have, it is a Fadal VMC with a Fadal CNC.... the problem with this is that Fadal really isn't a CNC developer, they are a machine builder who originally developed a PC based CNC on a DOS platform, but they just haven't really been able to keep pace with the dedicated CNC manufacturers. Hence, why as of just a few years ago, now offer Fanuc and/or Siemens CNC controls on their machines..... Fanuc & Siemens are the world leaders in sales and probably technology...... thus the Fadal has improved dramatically with this new change. But, that Brodix has an old Fadal CNC on it..... Don't get me wrong, I am not insulting the Brodix 5 axes VMC.... at least they have one and it apparently works, which is more than many can say. I also know the manufactuer well, have been to their manufacturing plant in socal many times as well as their primary competitors (Haas) and watched the battle between those two that has been dubbed the "Alley War" considering what seperated the two HUGE manufactures of CNC machines was only an alley for many years. But thats another story. |
