How G28 Works. Why two pushes of CYCLE START?

How G28 Works. Why two pushes of CYCLE START?

How G28 Works

Every wondered why G28 takes two presses of the CYCLE START Button when you are in single block.

How G28 Works

Be amazed you are about to find out.

One of my pet hates, and at my age you have thousands, is when people say

“Oh I don’t know we’ve always done it”

I just think, well you are stood at this machine for eight hours a day why not find out what’s going on.

How G28 Works

How G28 Works

G28 tells the machine to return to its home position. This is usually a convenient point for a tool change.

The command on a machining centre looks a bit strange.

G28 G91 X0 Y0 Z0

Why G91?

Well G28 means return to your home position via an intermediate point.

So if you programmed G28 X0 Y0 Z0 then your machine would rapid down to the workpiece (probably crash) and then go to it’s home position.

What G28 G91 X0 Y0 Z0 tells the machine is this……

  • The reference point is incrementally zero from where you are.
  • So the machine does not move.
  • Then it goes to it’s home position.
  • Hence the two presses of cycle start.

On a CNC Lathe we use U and W for incremental. Read this post if you don’t know.

So if you programmed G28 U0 W0 your X and Z axis would return to their home position. This being because the reference point is incremental.

If you programmed G28 X0 Z0 you would probably get a collision like below.

Well at least it went home.

First move is to X0 Z0 which is the front of the part. (This is your reference point.)

The machine would then move back to zero return.

G28 is fine on a CNC Lathe but on a CNC Machining Centre you must remember to change back to G90 (Absolute).

My preference is to use G53 to get back to your home position.

Read this article if you never heard of G53.

Please note some older machines don’t have G53. Oh and it is classed as an option!

If you are affected by any of the issues raised in this post or need CNC Training then contact us.

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