Shop Doc – Thread

Today’s Machining World Archive: February 2006, Vol.2, Issue 02

Dear Shop Doc,

We are a medium-sized job shop with single and multi-spindle machines, along with CNC lathes and mills. Our Brown & Sharpe machines are the most productive way to manufacture many of the parts that we make. The machines are rugged, precise and in good condition. However, we experience occasional threading and tapping problems, even with custom-made cams, which leave us wanting to move production off these machines. Any ideas?

By a Thread

Dear By a Thread,

Threading problems can indeed be very frustrating. Here are some tips on how to eliminate various sources of threading problems.

Tooling

The correct tooling can affect your thread quality. There are various styles of taps: forming or cutting, plug or bottoming. Matching the correct style to your part parameters can make huge differences in results. Likewise, the chamfer and grind of your chasers should vary with the material. The chaser fit in the die head is also a part of the equation.

Correct hole and blank preparation plays a major part in tool life and part quality. Various reference manuals are available. There is little margin for error when using a form tap; the hole size must be correct or the minor diameter will be too small, or your tool life will be very short.

Cams

The drive linkage on Brown & Sharpe machines is not perfectly linear. Cam makers that do not compensate for this can “build in” inaccuracy into the process. Many cam makers have special equipment or software to correct this condition. You may want to inquire whether your cam maker is making this correction.

Operators

A trained screw machinist has the skills to align tooling correctly, set the trip dogs properly and maintain the correct spindle clutch adjustment. These are skills that can be acquired with proper training. A spindle clutch set too light will slip when challenged by a coarse pitch thread, causing an incorrect feed rate. A spindle clutch set too tight will burn up the clutch or shift into neutral. A properly adjusted spindle clutch will provide millions of good threads.

Machine Issues

The turret and spindle must operate on the same axis. Adjustable tooling can correct alignment issues when the turret and spindle axes are parallel. As the thread gets longer, the alignment becomes more critical.

The 2nd speed ratio gears must be correct. The ratio gears are located directly behind the highspeed spindle change gears. These ratio gears can be changed, resulting in low-speeds (on 2-speed machines) that will not match the speed that the gear chart specifies. The ratio gears should not be changed.

Even less experienced operators can solve threading problems using proper problem-solving logic. I would recommend you keep records of the solutions to discover which are the most common. Next, determine what is the source (cams, machines, etc) and whether you can correct something that is common to many of the problems.

Servo-driven control upgrades are another possible solution to correct both cam and machine inconsistencies. With proper machine maintenance, tooling, cams and operators, you should be able to produce excellent quality threads on your Brown & Sharpe machines without moving the part to a slower or more expensive machine.

Greg Knight
Vice President, AMT Machine Systems. Columbus, OH

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