Multiple start threads combine the close fit of a fine thread with the quick axial motion of a coarse thread. Any time a design engineer needs a fine fit combined with fast motion, it is likely that he or she will use a multiple start thread.
Every so often, I like to present some of the Web sites I’ve come across. Many have been submitted to me for placement on the CNC links page of my company’s Web site—www.
The most common workholding device for turning centers is the three-jaw chuck. Setup people remove and replace top tooling (jaws) during every setup, and this task can be relatively simple if quick-change chucks are used.
When programs are too long to fit within a CNC control’s memory, one alternative is to use a direct numerical control system. This kind of system is called a DNC system, but must not be confused with a distributive numerical control system (also called a DNC system) that simply transfers programs to and from the CNC machine.
Almost all current model machining center controls use fixture offsets for program zero assignment. With most, the distance from the machine's reference position (commonly called the zero return position) to the program zero point must be determined for each axis.
All current CNC controls have a special program verification display screen page that shows a few upcoming commands in the CNC program, the current absolute position, the distance-to-go and currently instated G and M codes. Many control manufacturers refer to this display screen page as the program check page.
Entering data at the machine is usually tedious. Although control manufacturers provide full access to any data that must be entered or modified by the setup person or operator, many do not make it convenient to do so.
In my last two columns, we covered steps one and two of verifying CNC programs. Now, let's move on to step three: verifying the correctness of the first workpiece.
All CNC programs must be verified. While new programs present more challenges than proven programs, operators must be careful and alert during every step of a program's verification.
Companies vary when it comes to how they use CNC people. In some companies, such as contract shops, one person is responsible for the entire CNC job, including programming the job, setting it up and running production.
Every dimension specified on a workpiece drawing has a tolerance—either explicitly specified with a tolerance band or implied in some manner. Explicit tolerances are specified in different ways (plus or minus a value, plus one value or minus another, or high limit and low limit).