Ever since the introduction of NC technology in 1950's in MIT, NC technology has seen a tremendous growth. The invention of the computer and its association with NC technology has added new features and sophistication enabling it to secure a strong foothold in today's world. The uses of CNC (computer numerical control) technology are enormous and it is fruitless to describe, as the reader might be already aware of them. CNC technology is something that we cannot live without, which might suggest that it is invincible. But nothing comes without flaws and this article is about some of the limitations of one of the most path breaking inventions of mankind.
"There is an exception to every rule"
The problem that I am going to outline in this post is related to the field of thin wall machining. For the uninitiated, it is the process of making thin walls whose thickness to height ratio is very low, making the walls very prone to deflections. This is a very crucial process for the aerospace industry as it helps to reduce cost by making parts out of single block of aluminum instead of assembling them from a number of smaller components. So, in thin wall machining automated CNC machining centers are used to remove material from a single block of metal. When the material is thick the bulk machining is error free but when it comes to finish machine the thin wall, the limitation of NC comes into light.
"Adversity brings out the best in man (and NC too!!)"
When the wall is thin it has very low stiffness and it deflects easily due to the cutting force. As a result, the material removal is less than what is expected. This problem is described in the figure 1 below:
Upon application of cutting force the wall deflects as shown in fig: points A and B moves to A' and B'. After the cutter has passed the wall "spring backs" to its original portion and we get dimensional inaccuracy as indicated by the yellow portion in the figure.
This error is due to the fact the cutter follows the exact path that is specified by the part program to the NC controller and doesn't account for the part deflection. But this is not the fault of NC technology, in fact, it is its most admired strength (ironical). But we have somehow got so much accustomed to "smart" systems that we expect the same from half-a-decade old technology. Anyways, there is a way out.
"For every knotty problem, there is an equally simple solution"
(I made this one, thanks for your appreciation)
The name is NC compensation (you may have heard of it :-)). Wait for part 2 to know how it works.
“Blessed is he who expects nothing, for he shall never be disappointed.”
-Alexander Pope
