Lathe: Feed-rate issue at corners

Hi,

I'm turning a small profile and I noticed when I run:
1 G1 F100
2 G1 Z17
3 G1 X0 F3

it will cut a circle at the corner of line 2/3; while the circle runs at a feedrate of 100 which pushes the tool too quick into the material.

Setting G64 P0.01 fixes the issue ... but G1 X0 F3 should never run at a feedrate of 100 to my understanding.

As soon as it reaches line 3 it should switch the feedrate to F3...

I suppose it depends on whether th blend move belongs to the Z move or the X move.

It is trying to maintain 100 until the Z move is complete, which is only true at the end of the blend.

If you can, increase the acceleration limits. Low acceleration is the fundamental cause of wide cornering.

I wonder do you know what is the reason for blend moves at all?
Does it have any practical background, or is it just because that's the way linuxcnc was designed?

Do other CNC controllers also do blending at the corners?

Yes. Look up constant velocity contouring.. If you want the corners to be perfect - run exact stop mode. G61. (Or do what you did with g64 and a really low tolerance)

MRx wrote:

I wonder do you know what is the reason for blend moves at all?

"Ye cannae change the laws of physics"

If the machine has limited acceleration (and all real machines have) then it can't do infinitely sharp corners. 
In just the same way as a car with limited tyre grip can't go round infinitely tight corners.

You tell the controller what the max acceleration of each axis is in the INI file, and it plots a trajectory within those limits. 

If you want absolutely sharp corners then you either need to stop at the corner then start again, or (for external corners) do a 270 degree loop (wearing rather than tacking, in nautical terms) 

 

MRx post=247794 userid=28403I wonder do you know what is the reason for blend moves at all?
Does it have any practical background, or is it just because that's the way linuxcnc was designed?

Do other CNC controllers also do blending at the corners?

Yes in constant velocity mode, all controls either say they are blending and allow you make adjustments to the compromises taken when blending occurs, or they lie about it, and don't really do constant velocity or don't tell you about the blending they are doing or a combination of both.