I find it odd that management spends numerous hours discussing organizational and scheduling issues but manufacturing personnel spend little time discussing technical issues. As a result, engineers and manufacturing professionals make the same mistakes again and again.

The following are suggestions for designers and engineers, who seldom receive feedback from shop personnel. The intention is to help designers and engineers improve the little things that, together, make manufacturing easier and more productive.

The function of most press fits is to locate and retain two items together. I've bored and reamed countless holes for press-fit applications and have concluded that a light press is all that is needed for most applications.

To avoid complication, I suggest a 0.0005" interference for a light press fit and a 0.001" interference for a standard press fit.

Heavy press fits have four disadvantages. First, there is a tendency for parts to gall during assembly and disassembly. Second, a heavy press can significantly change the diameters of parts. In the case of a bearing, a heavy press can cause bearing damage.

Third, a heavy press may not locate items as accurately as a light press, because there is a tendency for material to distort and, possibly, flow unevenly under the high stress needed to make a heavy press fit. Finally, the stress caused by a heavy press may promote cracking of the parent material.

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Multiple leader lines in one view can be difficult to sort out. It is often easier to see circular dimensions in a side view.

While some designers and draftsmen think that leaving hidden lines out of a drawing helps to reduce "clutter," it actually makes the drawing more confusing for the machinist, who uses hidden lines to check what he thinks he sees in another view.

If a hidden line he's expecting to see is not there, the machinist can't be sure he's interpreting the drawing correctly. If you are going to leave hidden lines out of a drawing, state it clearly on the drawing.

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While flex-type bar clamps work great when designed properly, some designers don’t allow for enough weakness in the flexing area. Without enough weakness, mechanics are forced to apply a lot of torque to the clamping screw, which eventually strips the threads.

This is not always possible, but it often can be done. In the shop where I work, plastic molded parts have tolerances within 0.001". It's a constant battle holding those tolerances, which guarantees a lot of downtime and creates stressed out inspectors, operators, molders and managers.

The fault is in the part design. One of the hallmarks of well-designed parts and assemblies is that they'll work with relatively loose tolerances.

This ProShop Machinist Article is adapted from information in the book "Machine Shop Trade Secrets: A Guide to Manufacturing Machine Shop Practices" published by ProShop Publishing. A special thanks to 'Cutting Tool Engineering Magazine' for teaming up with James A. Harvey to produce this article.