CNC machining is usually the most cost-effective way of putting mounting surfaces, holes, screw threads and precision bores into parts that are either cast or machined from solid. However, there are situations where the disadvantages of precision CNC machining outweigh the advantages.

Tolerances and Repeatability

Precision machining refers to maintaining tight tolerances. What constitutes “tight” depends on the size of the part and feature being machined, the type of machining process and the machine tool used.

For small parts, precision machining usually means maintaining tolerances of:

• Milling – +/-0.0004”
• Turning – +/-0.0004”
• Grinding – +/-0.00004”

A skilled machinist with a well-maintained machine tool shouldn’t find it hard to achieve this level of precision on one part. Maintaining the performance over a large batch of parts is a bigger challenge.

Computer-Controlled Motion

In Computer Numerical Control, (CNC) each axis of the machine tool is driven by a servo motor. Position feedback tells the computer when it reaches a predetermined position.

A series of movements are compiled into a sequence of steps the axes follow to machine one or a number of features. This sequence is part of the program. Programs are sometimes written directly at the machine, but it’s more common to program offline.

In manual programming the programmer reviews the part drawing, decides the sequence of steps, and enters the move instructions into a software program. Alternatively, this is done by a Computer-Aided Manufacturing (CAM) program. Most machine shops use a combination, where CAM produces a program that is reviewed and adjusted manually to optimize speed, finish and tool life.

Advantages of Precision CNC Machining

Letting a computer control cutting tool movement and speed delivers benefits in three main areas:

• Combine Operations
• Lower Labor Costs
• Higher Quality

Combining Operations

On a manual machine tool, particularly a milling machine or grinder, the machinist sets up to perform a single operation. After running the batch of parts through once, either the machine is reset for a second operation or the batch moves to another machine.

A CNC machine can combine machining steps by repositioning the cutting tool to remove metal in different places. In addition, if equipped with a tool changer, the machine can switch between various styles of cutting tools.

Combining operations like this move a batch of parts through a machine shop in less time and saves handling.

Lower Labor Costs

While it’s cutting metal, the machine needs no operator input. This means, that on jobs with longer cycle times, one operator can supervise several machines.

Higher Quality

CNC maintains the same speeds and feeds on every cycle. Plus, position feedback ensures the tool moves to the same point on the workpiece every time. This results in high accuracy, and excellent part-to-part repeatability. Further quality gains result from reduced part handling.

Limitations of Precision CNC Machining

There are only a few scenarios where manual machining has advantages over CNC precision machining. The limitations of CNC are:

• Part Programming
• Tool Presetting
• Visibility and Access

Programming imposes a delay before machining can start. Plus, to avoid the risk of collisions, it must be proven-out before mounting a workpiece in the machine.

Tool presetting is needed to ensure the cutting tip is where the CNC system expects it to be. (CNC drives the axes to the required positions but doesn’t know where the tool tip is.) Pre-setting involves mounting each tool in its holder in a way that ensures it will be in a known position. Alternatively, the tool tip position is measured in the holder and this value is entered into the program as an offset.

CNC machine tools are enclosed to protect operators from flying chips and coolant and keep them away from moving machine components. On manual machines these hazards are smaller because the axes are under machinist control. That’s why manual machines don’t need big guards that restrict access to and visibility of the cutting area.

When Manual Machining May be Preferable

Manual can be better than CNC for one-off jobs and machining prototypes. As there’s no programming, it can be quicker to get a single piece machined, and the superior visibility allows fine adjustments on-the-fly. Manual machines are often used for large workpieces that won’t fit in a standard CNC machine and for repairing parts.

CNC Machining Specialists

CNC machining is capable of precision, speed and repeatability. It’s cost-effective in most applications and has largely supplanted manual machine tools. However, there are situations where the latter have advantages. If you need CNC precision machining, Impro can probably help. Contact us to schedule a discussion.