Ronnie,
First off, you need to check the native units in mach to see that you are in inches.
If so, you need to make sure that you when you tell the machine to move one inch that the machine actually moves that distance.  Chances are your machine configuration is set wrong.  meaning your motor tuning needs to be configured to your machines hardware configuration. If your machine moves the correct distances, then you need to check which post processor you are using in artcam. 

then you need to check which post processor you are using in artcam. 

To ensure it's generating a G21 in the header for metric or a G20 for imperial?
As whatever you have Mach3 set to then a G21 or G20 command will override this?

Hi. All. Mach3 is showing G20 in the header. Native units set to inch. Motor tuning is steps per.2000 velocity.600 acceleration.4 step pulse and dir pulse.3. These settings are the same for all three axis. G.code inch is the only post processor I have found to work with this machine. It is a Stepmores. It is not listed in post processing options. Tried G.code mm. Machine travels excessive distances. Regards. Ronnie.

Those numbers are very wrong for a machine set up in inches, unless it's a very expensive, very high performance machine. Those are numbers you usually see for a machine set up in mm's. Since it's a chinese machine, that seems about right.
What are your steps/unit set to?

As for the post processor, you should be using a Mach3 post processor.

Hi. Gerry. Machine is v cutting a tree inlay at the moment, so will check when its finished. Where do I find that setting?

Right next to the other ones.

Assuming you mean in gen. Config. G20, G21 control lock DROs to setup units is not checked. And Distance mode is set to. Absolute and W mode is set to Inc.

No, the steps/unit is in the motor tuning.

Steps per.2000. Velocity 600. Accel. 4 G,s 0.0103607. Step pulse.3 Dir pulse 3 All three axis are the same. Spindle is different in step 1 and velocity 120

Ronnie, in artcam the model master post processors work well with mach3.  cannot remember if there is an inch one or not.
Have you checked to see if your machine is moving the correct distances?

Lay a ruler on the table.  Zero the X or Y axis over an inch mark.  Work cordinates to Zero. Use the MDI line. G0 x 1   press enter, did the X axis move from one inch line to the next?  i will bet money that it only moved under half the distance.