Also, many computer-drawn parts are more difficult to understand, because often, only one surface is shown and it may be the inside or outside surface。 Computer drawings often show all

lines, including hidden features, as solid lines instead of dotted lines。 This leads to interpretation errors, which in turn leads to errors in the building of the die。

To better understand complex part shapes, it is helpful to build a "sight" model of the part using sheet wax, rubber skins or wood models。 Dimensional accuracy is not critical for these models, as they are used primarily to visualize the part。 Rubber skins and sheet wax also can be used to develop preform shapes and to develop the best positions for the part as it passes through each die operation in the progressive die。文献综述

Starting Material in the Die

Care must be taken to ensure that the strip is started correctly into the die。 Improper location of the lead end of the strip will do more damage to the die in the first 10 strokes of the press than the next 100,000 strokes。 "Lead-in" gauges must have large leads and a ledge to support the lead end of the coil strip when it is inserted into the die。 Large leads on the gauges are important so that the die setup person does not have to reach into the die, as well as for minimizing the time required to start a new strip into the die。 Also, one gauge should be adjustable to compensate for variation in strip width,。

The position of the lead edge of the strip is critical for the first press stroke, and must be determined for every die station to ensure that piercing punches do not cut partial holes in the lead edge。 This could cause punch deflection or result in a partial cut with trimming punches, which can result in an unbalanced side load as the strip passes through the die。 Any of these conditions can result in a shift of the punch-to-die relationship that may cause shearing of the punches。

Improper location of the lead edge of the strip also can result in an unbalanced forming or flanging condition that can shift the upper die in relation to the lower die。 Heels should be required to absorb this side load, particularly when forming thick materials。

A pitch notch and pitch stop can provide a physical point to locate and control the lead edge of the strip。 Brass tags or marker grooves also can provide a visual location, but these are not as accurate or as effective as a pitch notch stop。 The press can be prevented from operating with either a short feed or over feed by mounting the pitch stop on a pivot and monitoring it with a limit switch。

Part Lifters and Part Feeding

Progressive dies often require the strip to be lifted from the normal die work level to the feed level before strip feeding takes place。 This can vary from a small amount--to clear trim and punching burrs--to several inches to allow part shapes to clear the die。

Normally, all lifters should rise to the same height so that the strip is supported in a level plane during forward feed。 The strip must not sag between lifters; otherwise parts will be pulled out of their correct station location spacing。 Bar lifters provide good support and are better than spring pins or round lifters notched on one side of the strip。

Often, a good bar lifter system allows higher press speeds because feed problems are eliminated。 Although the initial cost is more than round lifters, performance is better and setup time is reduced。

As the strip is started into the lead-in gauges, the strip should be able to feed automatically through all the following die stations without requiring manual alignment in each set of gauges and lifters。 The strip also must be balanced on the lifters so that it does not fall to one side

during feed。 A retainer cap can be mounted on the top of the outside bar lifters。 This produces a groove that captures the strip during feed and prevents strip buckling。