Custom three post progressive metal stamping die design
There are special requests toward the progressive stamping die that must be included in a tool and die design that ensures trouble free long service as well as following guidelines set by the client.
Stamped part must be flat
High volume production
High speed punch press is available for the tool
Coil will be used. No room for the exiting coil. Coil must be cut to small pieces
Progression is controlled by a feeder. Manual operation may be required
Stamping die must be easily accessible for in-tool adjustment, maintenance
Stamped part must be flat
When the stamped parts must stay flat, pressure plates must be used both from the top and from the bottom as well. Without the use of pressure plates, the stamped parts will be slightly bowed. As the punch forces the part to cut out from the stock it applies a lot of force and stress to the steel. The die plate supports the cut part on its outer perimeter with no support within.
As a result, the stamped part will try to bulge downward from the blanking pressure before the stock or sheet metal yields and separates from the rest of the strip. This minor deformation remains in the part. Sometimes the flatness of the part is important, and this minor distortion is not acceptable. To overcome this problem and ensure that the part remains flat, the blanked part must be supported well while separated or blanked from the stock. A lower pressure plate with the same profile must be under the part, preventing it from bulging. The top face is supported by the cutting face of the punch, while an upper pressure plate holds the strip down, preventing from bulging upward around the cut edges. The force behind the pressure plate must be greater than the force that could result deformation. Standard die springs are generally not powerful enough to produce so much pressure, especially when the blanked parts are small. For smaller parts smaller sprigs are used due to space limitations.
Since the part is trapped between the lower pressure plate and the punch, it can not exit the tool with the same stroke. The cut part is pushed back into the strip by the lower pressure plate (usually partially). It then carried forward with the progression and then knocked down in an empty station, separating it from the strip.
The use of nitrogen filled gas springs can do the job well, while lasting much longer than conventional springs. E.g. a 1 inch diameter 2.5 inch long extra heavy duty spring (strongest available) can produce around 340 lbs at 15% compression or 0.375 compressed distance. At 0.200 compression the force is less than 120 lbs.
A nitrogen filled gas spring, also 1 inch in diameter can produce between 955 – 1,700 lbs.
No pre-load is necessary, so a lot of force can go against the blanked part, preventing it from deformation. As an upside, it saves the thread of the shoulder bolts and its retainer. Since no pre-load required, the shoulder bolts could have e.g. 0.005 slack of free movement with no stress on them whatsoever.
High volume metal stamping part production
When the amount of parts to be produced are high, you want the tool last long, without down time, without sharpening. Generally used tool steel for punches and dies are suitable for most applications. When the stamped part is harder, and the amount is high, you want to use a suitable tool steel for the cutting components. At this design CPM10V tool steel was used.
Coil will be used
When coil is used to stamp the parts from you need to consider what to do with the coil as it exits the die. Either it should be wind back again and dispose the used or stamped coil when run out, or cut the stock at each stroke, so only small bits are exiting the tool as scrap, that is easy to collect and dispose. For this reason, a scrap cutter is designed to separate the exiting portion from the coil.
High speed punch press
There are different punch presses with different speed. The stroke per minute (SPM) ranges from 30 and up. Average high-speed presses run around 200 spm. Some can get much higher.
This speed means more friction and more heat. Not only the tool steel should be chosen accordingly, but the die set that will slide up and down, while precision guiding the components. The die set should not overheat and jam. Most presses run at 30 – 80 spm can use standard guided die sets that require regular lubrication, grease while in production. If the same die set with standard guide pins and bushing would be used running at 200 - 400 spm, it would not take long before the sliding components would swell from heat expansion and would seize.
There is a solution for that. Ball bearing guided units are available. There is a called ball cage, a bushing and a die pin. The ball cage retains the hundreds of ball bearing balls. The balls are tightly roll between the bushing and the pin when fully engaged. This guide system is designed for high speed production. For ball bearing guide components grease is not recommended. They work better dry. This special three post design allowed to use a smaller footprint die set while giving plenty of room for the stock pusher, and more room for in tool adjustment or maintenance
Progression of metal stamping die controlled feeder
Manual operation may be required
When coil is used to produce the parts from, it is necessary to use a feeder that can be set for the proper progression. The feeder advances the stock forward by the amount of the progression it was set to. To control the progression as precisely as possible, internal, mechanical components are used in conjunction with a feeder. These are e.g. French stop, also called side trim or side notch. Pilot punches are also used for precision positioning. The pilots are about 0.002 smaller than the hole diameter that they go into and were pierced usually at the 1st or 2nd progression. Progressive metal stamping dies can have easily over 10 progressions before the finished part is complete. From this point a complete part is made at every stroke. As part of the of the proper progression and its effectiveness, stock pushers should be used to keep the coil or strip coaxial with the feed direction and at a relative position, e.g. datum face, in this case the rear edge of the coil.
French stops are the first line in progression control. Its distance is set by the design and manufactured accuracy. IT does not change from time to time. The feeder could slightly over feed while allow slipping to prevent strip buckling. If used properly the progression amount always equals the dimension of the French stop which has the dimension for the progression. Many feeders have options for pilot release. This is to temporarily release the grip the holds the coil to be advanced. At that moment, the pilots can freely position the strip for the optimum position before the next hit. Feeders without a pilot release can fight the pilots, easily under or over feed, that can cause error and crashes. Another advantage of the use of French stops is that the tool or rather the strip advance can be done manually in case the feeder is out of commission.
The French stop or side trim cuts a portion off the edge of the strip with the length of the progression. You advance the strip until it can not go any further. The back end of the notch reaches a stationary stop block. The block is made thicker than the stock thickness to prevent the strip going over the stop. It is not possible to over feed or make the progression more than the cut notch length. However, it can be under fed, when the back end of the notch does not reach the stationary stop.
To prevent such events and when the feeder is in use, a misfeed sensor can be used. In this design instead of using a French stop based pivoting electro- mechanical misfeed sensor, a different one was designed. It eliminates moving components as well as gives more free space at the front. The sensor was placed above the strip. When the coil is passing through it makes the sensor give out signals, either when it detects metal under or does not detect anything because there is a hole. When the proper progression is reached, the sensor can give signal to the PLC and then to the feeder and to the press to go ahead and make the next stroke. When the strip is e.g. jammed for any reason and could not fully reach the set progression, the sensor will not give out the signal to proceed, therefore both the feeder and press is stopped until corrections were made and cleared the jam.
Such event should not happen ever but if it does, it means tool crash with considerable down time, repair costs and possible injuries.