I mean like a thread for somebody like me that is not a machinist!

I design electronics for a living - but am currently tasked with getting a combo mill/lathe (Shoptask) machine up and running to run prototype quantities of mechanical parts for our products (LED lighting).

I need to get this done by next week - so no pressure

What I need to do here is use this machine as a sort of automated drill press to drill holes in a sheet metal "pan" that is ~18" square.  These holes will be for mounting 4X LED PCB to the pan - the finished product being a 24" square light fixture.

I would imagine that my required accuracy would be on the order of +/- 10 mil across a dimension of ~18" in the Y axis and ~8" in the X axis.  I am basing this requirement on the size of the fastener (screw) and the size of the hole in the PCB.  Due to the 10" limitation of movement about the X-axis - the pan will need to be rotated 180 degree during the process.

This machine has three stepper motors to drive three axis - X,Y, and Z.  Currently I just need to use for drilling sheet metal which I would imagine would be a maximum of 100 mil thickness (probably less) and the material would be steel.

I have managed to perform a cursory calibration of the machine as far as movement goes with the Mach3 software.  I do not have a set of gauge blocks so first "roughed in" the calibration using a steel ruler for 4" moves - and then fine tuned it using a dial indicator for 1" moves.  I currently have the Y-axis +/- 1 mil, X-axis +/- 3 mil (this can be improved), and Z-axis +/- 6 mil (the Z-axis on this machine has a problem).

Is this calibration technique adequate for my needs?

Will the "pan" need to be pre-punched to guide the drill?

Should I use normal drill bits or pilot drill bits?

Is there some "convention" as to what is "zero" for each axis?  I currently have dead center of the X and Y axis as zero - and Z-axis zero is fully retracted - i.e. Z only moves in a negative direction.  Is this "normal"?

What about feed rate?  I know from using a manual drill press that this matters - but I am not sure I understand the concept using Mach3.  Is this simply a function of the acceleration profile setup during the motor tuning calibration - or are there other ways to control this?

...Due to the 10" limitation of movement about the X-axis - the pan will need to be rotated 180 degree during the process.... Is this calibration technique adequate for my needs?... Will the "pan" need to be pre-punched to guide the drill?... Should I use normal drill bits or pilot drill bits?... I currently have dead center of the X and Y axis as zero - and Z-axis zero is fully retracted - i.e. Z only moves in a negative direction... What about feed rate?... ...Is this simply a function of the acceleration profile setup during the motor tuning calibration - or are there other ways to control this?

To take points in turn. If you need to rotate, make sure your fixture is repeatable. Without this, any accuracy in the machine is meaningless. Calibration looks good for what you are doing (Z doesn't need to be super accurate for drilling). Pan won't need to be punched. Use the correct drill bit and it will be OK. Use the shortest, stubbiest drill bit you can get and chuck it up as short as possible. Rigidity is your friend. A four facet point will help with centreing. Z is more conventionally set as zero at material surface; X and Y to suit yourself. Acceleration profile is quasi-independent of feedrate. Acceleration is how quickly the machine ramps up its speed from zero to the desired feedrate. I say 'quasi-independent' because if your acceleration is low, the feedrate is high and the move is short, the acceleration will govern the actual speed the machine reaches. You control it with the F-word (the G-code one!). E.g G01 Z-0.25 F60 would send the Z-axis 0.25 (inches) into the material at 60 inches per minute (i.e. 1 inch per second, or roughly 1/4 of a second for the manouvre). LittleMachineShop has an online speeds and feeds calculator, where you tell it material, tool type and diameter and it gives you a guideline feedrate.

To take points in turn. If you need to rotate, make sure your fixture is repeatable. Without this, any accuracy in the machine is meaningless. Calibration looks good for what you are doing (Z doesn't need to be super accurate for drilling). Pan won't need to be punched. Use the correct drill bit and it will be OK. Use the shortest, stubbiest drill bit you can get and chuck it up as short as possible. Rigidity is your friend. A four facet point will help with centreing. Z is more conventionally set as zero at material surface; X and Y to suit yourself. Acceleration profile is quasi-independent of feedrate. Acceleration is how quickly the machine ramps up its speed from zero to the desired feedrate. I say 'quasi-independent' because if your acceleration is low, the feedrate is high and the move is short, the acceleration will govern the actual speed the machine reaches. You control it with the F-word (the G-code one!). E.g G01 Z-0.25 F60 would send the Z-axis 0.25 (inches) into the material at 60 inches per minute (i.e. 1 inch per second, or roughly 1/4 of a second for the manouvre). LittleMachineShop has an online speeds and feeds calculator, where you tell it material, tool type and diameter and it gives you a guideline feedrate.

Thanks for your response, djc.

I thought the fixture would be important.  The pans will come with two holes on each flange, for later mounting of the assembly to the finish frame, or bezel.  I plan to have a holding fixture with tapered steel dowel pins that fit those holes and locate the thing properly on the machine, with a couple of quick release type clamps to hold it from lifting or twisting.

Ultimately we will order the pans drilled from the fab shop - but will run a couple of hundred to finalize the design.

I'll try some of your suggestions today as I going to get some scrap material and start working on the program.

Sounds like an ideal application for Mach3. I'd vote for little stubby bits as well, they are ideal for sheel metal.

If you need very precise holes drilling then you can use an endmill and a G13 command to make a pocket - This give you a lovely "adjustable drillbit" efect that lets you specify the hole size exactly.