CNC in the (Model Engineers’) Workshop
Part 9
Instalment 9 of CNC In The (Model Engineers’) Workshop was published in Model Engineers’ Workshop dated January 2014.
This instalment looked at circles and arcs. Along the way, it included some words about G2 and G3 commands; some arithmetic; and a rather cheesy example which didn’t quite reach its final conclusion in this instalment.
MEW Issue 211 : January 2014
Arithmetic
You may have thought the paragraph on arithmetic rather laboured an obvious point. I included the arithmetic because, for some of us at least, it’s a long time since early Secondary school and the rules we were taught about + and - signs. We will meet some more important uses of arithmetic in a future instalment not too far down the road, so the topic is relevant both now and shortly.
I gave my apologies for attempting to teach grandpa to suck eggs, but this really is important, and its one of those annoying skills which seems so obvious to those who remember and use it, but not so friendly for those who don’t.
Let me just give a few more examples:
2 + 3 = (if you don’t know the answer, just skip the rest of this series ...)
3 - 2 = ....... ho hum.....
2 - 3 = -1
-2 -3 = woah... let’s just back up and think about this one.
One way of thinking about this is to visualise a number line (or an axis), like the X axis, for example:
positive is to the right, and negative is to the left,
so wherever we are, adding a positive amount moves us to the right, and adding a negative number (or subtracting a number) takes us in the negative (left) direction, moving us to the left from where we are at the moment.
-2 -3 can be read as:
we are at -2, and we move 3 in the negative direction (i.e. 3 to the left) so we end up at -5
-2 -3 = -5
Another way of thinking about this (and this will be important in a forthcoming instalment) is to say that
(a) every number has its own sign, positive or negative. -2 and -3 both have negative signs.
(b) there is only one operation and it is addition. We are either adding a positive number or adding a negative number. If we add a positive number we move to the right. If we add a negative number we move to the left.
So -2 -3 is actually (-2) + (-3) and the answer is still -5
-2 +3 is actually (-2) + (+3). If we start at -2 and add on +3, we end up at +1, so -2 +3 = 1 (or +1 if you prefer).
More on this when we get to multiplication...
Typo
Page 9 of MEW211 contains a typo which I knew about but couldn’t correct in time. (You may insert your choice of adjective here.) My apologies. This is certainly my fault and not that of our editor or the printers.
In the left hand column, a few lines above fig 44, the line beginning:
“We can specify which mode we want”
should read:
“We can specify which mode we want by using the command G91.1 to set Incremental IJ mode, or by using G90.1 to set Absolute IJ mode.”
Note that the G92.1 printed in MEW is incorrect. We will meet G92 and G92.1 shortly, but it does something quite different. Meantime, I’;m off to clean my glasses, and take my memory pills.
Download the article here