So if you got into the habit of using G94 Facing Cycle CNC Turning.
When you want to add more cuts it’s easy, piss easy.
So if this blank had 3mm on the face it’s really easy to add some more cuts.
G94 Facing Cycle CNC Turning
Oh just one other thing
This cycle is modal. Notice how all we did was add some Z figures and it just did the facing.
Well just like your wife when you got drunk 15 years ago and urinated in the bed. Well……
It remembers.
So that means we need to stop it. Just by adding a rapid command G0 or G00. It stops. It forgets.
Oh and don’t try n space the code out here because if you leave an extra block in it will face the part again.
I think I need to be honest with you here and confess that until recently I have never seen the point of this G94 code. But now I have seen the error of my ways and I think this demonstrates just how useful it can be.
Now I know some smart prick will say “Oh well I just use G71” . Yes that’s fine but don’t you think it is rather complicated just to face the front off a part that’s too fuckin long?
G76 threading start point, when you use a G76 threading cycle on a CNC Lathe the position that you start the tool at is really important.
This is the position you rapid the tool to just before you call the G76 threading cycle.
For example how does the control know if you are producing an internal thread or an external thread.
Actually I learnt this the hard way. It was on an old machine.
Maybe not that old. Anyway I couldn’t get it to perform a G76 threading cycle. It was cutting an external thread but the tool was working it’s way outwards.
What to do Next?
Everything rushed through my head, senile dementia, flashbacks from a misspent youth, did I feed the dog?
Anyway I suddenly realised I didn’t have a dog and that the start position of the tool was wrong.
G76 threading start point how was it wrong?
Well that’s how the machine knows whether you want an internal or external thread. Your start point needs to be inside or outside the thread. What a fuckin dipstick I was.
I had the tool just below the external diameter of the thread. If you do this the machine thinks the thread is internal and therefore cuts outwards.
Correct
So providing the tool is initially placed outside of the maximum diameter it will do exactly what you want.
Incorrect
Put it here and you are in deep shit. The tool will work from this position backwards and you will be totally confused.
So be careful because you might not notice that tiny amount.
Of course if you have a Siemens control Sinumerik 828 840D then this does not apply because you have to specify if the thread is internal or external in the cycle.
Front Clearance (G76 threading start point)
Now let’s think about how far in front of the part you need to be with the threading tool.
Maybe you never thought about it?
Well now’s the time.
Spare a thought for people like me who lie awake at night ruminating about these things. If you don’t know what ruminating means then maybe you shouldn’t be reading this article. Or maybe you just like the pictures.
This is my puppy his name is Donald he is learning to programme a G71 woofing cycle.
Now I need to feed at 6000 mm per minute so this pitch problem will be twice as bad.
Can you see where this is going?
Eat more fish.
No not really, well it is good for you but it’s not what I meant.
The faster you go and the courser your thread pitch the quicker the machine will have to feed.
If you have a coarse pitch thread in aluminium you probably will end up with a super high feed-rate your machine can’t even achieve.
This will all depend on the age of your machine but you must always make this calculation just in case you are trying to cut a thread at the speed of light.
G76 threading start point in Z Axis
This start point distance is really important because it will allow the machine to get up to speed.
I can’t give you exact figures for this, not because I can’t be arsed but because it will vary depending on your machine.
The machine will have a maximum feedrate and it will be in the manual.
You know that old book that you use to prop the door open.
The bigger you make this Z figure the more time your machine will have to get up to speed.
So don’t forget if you are starting at Z1. then the poor old machine doesn’t have much time to get up to speed.
Thanks for watching and reading
If you have been affected by any of the issues in this post or need CNC Counselling then contact me.
On a Fanuc or Haas control G02 G03 are the two G codes we use to move around clockwise and counterclockwise circles. You don’t need the leading zeros so from now on I will call them G2 and G3.
And by the way never use the letter O it’s number zero
GO2 GO3 looks almost exactly the same as G02 G03. But your control will blow it’s mind and get real mad with you.
It will also do some really weird shit like trying to start a new program. Anyway just don’t do it.
Oh and please don’t call it interpolation as it annoys the shit out of me.
Linear interpolation is movement in a straight line.
Circular interpolation in moving in a circle.
Interpolation is movement.
Now that’s the telling off out of the way let’s move on. It’s ok I don’t hold a grudge.
Which one of you bastards said interpolation?
Let’ Talk About Milling A Shape
When you are milling a shape and you want to move in a circular motion you use G2 or G3
For a clockwise arc use G2 and for a counter clockwise arc use G3.
They both work the same way. On a modern control you programme the endpoint (an XY figure on a milling machine) and the radius you require.
Then your CNC controller magically creates your circle. You can programme any arc like this with one exception.
Do you know what it is?
Ok I will tell you later, it’s OK to not know. (Just means you are little thick).
Lets go back to school.
Remember construction? Of course you do.
We are going to mill the top section of this part
Let’s see what the control has to do to work out where the centre of the circle is.
It knows its current position and in your G3 line you tell it the end point and the radius you want.
Well it gets out its little compasses and draws two circles at the radius you told it.
It positions the compass first on the start point and draws an arc. Then on the end point and draws an arc.
Where these two arcs cross it places the point of the compass. Now it can strike an arc that touches your start point and your end point.
Get yourself a pair of compasses and try it. It’s great fun.
That’s how the maths works.
Tell me did you really believe that inside your CNC machine there is a little man with a pair of compasses?
Fuckin hell I give up.
Sorry I lied it’s all done with simple trigonometry. But at least I tricked you into understanding the principle.
So you should now begin to work out why you get an alarm when the radius is too small. Your two arcs just don’t cross. In other words you are trying to fit an arc between two point that simply miss one another.
G2 G3 No Pot of Gold
Your control will give you an alarm “End Point Not Found” which makes sense because it can’t find one. It’s like the pot of gold at the end of the rainbow.
Oh Before you Go More on G02 G03
What about the question. When can you not use endpoint and radius with G02 and G03 ?
Answer: When you want to machine a full circle using G02 and G03.
You can do it but you would have to break it into two halves. I’m not going to explain anymore because this method is for wimps and peole who can’t be arsed to read my articles.
You know who you are, ordering Chicken Korma in an Indian Restaurant.
Well…… what follows is a full, blow your bollocks off, shit your pants the next day, vindaloo.
And Now Ladies and Gentlemen the Infamous I and J
At the CNC Training Centre we don’t get too excited about I and J when it comes to programming circles. That’s because you don’t need to worry your pretty head about it anymore. You almost never need it.
Did he say almost?
Well yes almost. You could programme forever and never bother about using I and J with G02 G03. So just go off and watch some porn for awhile and I will explain to the real programmers when you can use it.
In the old days I and J was the only way to programme an arc. You would tell the control where the centre of the circle is and the endpoint. This is cumbersome and a bit tricky. That’s why I’m not telling you about it.
Anyway one really good simple way to use I and J is for a full circle. Oh and it’s bloody easy. That is once I’ve explained it to you.
When I was a boy hundreds of years ago I had a toy called a Spiro Graph.
Spirograph those were the days
It consisted of a series of plastic gears. You stuck your pen in a hole in the gear and rotated it around another gear. With a bit of practice you could make some really nice pictures. Kept us happy for hours. The crime rate dropped dramatically in my area when these things came out.
What’s this to do with CNC programming you ask? Well if you tried to use end point and radius for a full circle this is what you would get:
If your start point was X0 Y0 you would program G3 X0 Y0 R100. assuming radius was 100mm.
So in a full circle your endpoint is the same as your start point. There are millions of circles the computer could pick. It would be just like the shapes above. If you do the compass trick you will see what I mean. That’s why it’s impossible.
In Comes I and J
So some clever bloke thought fucks this I’ll invent I and J.
If we use I and J there are four options as above.
You would simply program as below for a full circle (20mm Radius)
G3 I-20.
G3 I20.
G3 J20.
G3 J-20.
And that’s it.
So these are the four options
So there you have a fantastic way to programme a full circle without having to break it down into two halves or be clever like a newsreader or some twat off mastermind.
Why do you only need the I and not X and Y Dave?
For fucks sake don’t call me Dave my name is David.
I had this really nice guy phone me from Africa the other day. He explained to me that he needed to get his inheritance of 10 million pounds out of England and if I helped him he would give me £20,000. All I had to do was give him all my bank and credit card details. Oh and a small one off payment of £1000.
Of course I agreed, what a bargain. Just as I was about to do this the cheeky bastard called me Dave.
I immediately told him to fuck off. I just hate being called Dave.
I gave him my 96 year old mothers phone number because I thought at least she can benefit from it all. Her name is Mary so he couldn’t shorten that. She only has £1200 in savings so it would be doing her a massive favour.
Explanation
You don’t need to re-state the X and Y because you are already at the endpoint
