Saturday, December 5, 2009

Planning the Steps Involved in Machining A Job

A CNC program is a series of operations performed for specific selections of the tool, for a specific orientation/fixation of the job and at a selected combination of settings of the machine.

The series of operations is decided by the design of the job. So lets start with a simple part such as that shown in the picture below.

Here is a picture of the stock available to you.

Based on the stock and the final object desired, we determine that we need to machine both sides of the stock. We also determine the specific procedures and tools needed for each side and make a list pertinent to each side. 

Based on the two lists, we can already know some of the tools we need to load into the machine. In addition, we also need to determine if some of the steps need to be repeated with a finer tool for better finish. This is usually determined by the machinist after test running a piece to see the quality of the output and checking that against the requirements of the customer. 

For the first side, we need the following procedures performed. Let us assume that we do not need to machine the two off center holes on the job drawing:

a) The top of the cylinder has to be faced in order to me make sure that the the surface is truly perpendicular to the height of the cylinder and to give it a smooth finish.
b) The central hole has to be drilled.
c) The chamfer on the outside rim has to be machined
d) The champfer on the inside rim has to be machined

This is what the job looked like after the first side was done.

For the second side, we determine that the following procedures need to be done.
a) The bottom of the cylinder has to be faced in order to me make sure that the the surface is truly perpendicular to the height of the cylinder and to give it a smooth finish.
b) The outer rim of the cylinder has to be machined for a smooth finish.
c) A large partial hole concentric with the central hole has to be machined.

This is what the job looked like after the second side was done.

Bob and I ran (alright, I admit I didn't write the program or pick the tools) the test run and determined that for the first side, step (c) needed a finishing step after the main step and the same repetition for step (d) for the first side and step (c) for the second side. Thus all in all, we actually ended up with 5 steps on the first side and 4 steps for the second side.

I find that CNC machines are very fascinating! Here is a shot of the machine in action.

Some useful tips to remember:
1. When you start a CNC machine at the beginning of the day, for the first job, you need to back off about a thousandth on your cutting tool calculations because everything is cold. This manifests as the cutting tool cutting a little less. 

2. Conversely, I also observed the opposite - after several runs (this was a 200 piece order), the tool bits do expand from heating and you might have to move in another half-a-thou. Otherwise the holes you drill may be the slightest bit loose.

3. Unloading and Loading tools into a CNC:  In order to change tools on a CNC, you can generally (although this is specific for Mazak) find a TOOL menu item which can be expanded to show several options. Choose the MDI (Manual Data Input) option. Then you will be prompted to enter the tool number you want to change (typically up to 10 tools) and hit INPUT. The machine will then move the tool block so as to present the request tool head to the user. Twist and pull out the tool. 

Putting in the tool must always be done in the same way, with a large dot on the tool always positioned either to the front or the back (depending on the machine involved), so be aware that there is danger for misalignment if this is not observed.

Above you can see some of the finished pieces stacked together.

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