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Eliminate Screw Failure
Eliminate Screw Failure By: Jim Szumera - MACOR Published on: 02/20/2016
We have all experienced screw and bolt failure in our careers to varying degrees. In some instances we were lucky. The damage was minimal. In other cases, not so lucky. The die crashed and resulted in broken details, strippers and punches. All things considered and assuming we have executed best practices in utilizing correct tap drill sizes, proper torgue and application, the screws still fail. Three vital, less known criteria to eliminate screw failure is as follows:

1. Only use quality name brand fasteners certified to meet or exceed specification, such as Unbrako, Holo-Krome and others. Do not purchase counterfeit screws and bolts. By counterfeit, I mean sub-standard material, heat treatment and machining. This problem proliferated from cost and resulted in poor quality imports from Asia, and Europe. It became widespread and was used in everything from bridge building and construction to high speed stamping dies. It became such a systemic problem that Congress passed the Fastener Quality Act of 1999.

2. In die applications the class of fit is critical. Many die builders go oversize on the tap drill size to facilitate CNC, tap life, and expedience in screw removal. While this may be good for die building economics, it spells disaster when trying to run these dies. Screws come loose and cause all kinds of havoc. Using a slightly smaller tap drill, or preferably fine thread usually works well. This is especially critical when fastening upper dynamic components, such as punches into hardened retainers and back up plates. Many die shops engineer punch removal in the press.

3. The last little known item is proper through hole preparation. Here we are referencing the inside clearance hole. It must have a chamfer to allow for the radius at the screw head. Often this step is overlooked in the die building process because it may not be called out on the prints or due to bad practice or negligence. This causes the screw to fail at the head. This may not develop into a failure mode if the counter bored hole is in soft steel. The screw will actually form the soft steel slightly when tightened. Check out the Parameters for Eliminating Screw Failure file in the Technical Data Tab.
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What Size Pilot?
What Size Pilot? By: Jim Szumera - MACOR Published on: 09/7/2015
What size pilot?
I have been involved with hundreds and hundreds of tools, dealing with every conceivable type of stamped part imaginable from .001 Kapton film to stamping 1/8 inch thick lawnmower decks, from .010 beryllium copper connectors to a 3/8 inch thick trailer hitch. In every case the pilots were .0002-.001 smaller than the pierce punch. Almost in every instance, the tools stamping the thicker materials had broken pilots, made miss hits and caused die damage. In a lot of cases the pilots had to be removed in order for the die to run. Not good. Many good die designers live in a false world of precision. They adhere to the notion that the pilots should be .0002- .001 smaller than the pierce punch and that is how to hold part tolerance. Not necessarily true. The pilots only purpose is to register the strip in its advanced position and help hold the strip straight. The station tooling should be so engineered to locate and form or blank. The exception would be a washer that has a center hole and must hole true position. The blank punch may have a center pilot installed in it. The pilot diameter is really a function of the metal thickness being stamped and its relationship to the pierce punch. Given strip growth stress and deformation, as a rule, if the stamped material is .025 or thinner I make the pilots .001 smaller that the pierce. I may use .0005 for material .015 or thinner. Material .025-.050 requires a pilot .001-.002 smaller. Material thickness .050-.090 require pilots .002-.004 smaller. Material above .090 thickness require pilots .004-.006 smaller. These sizes apply to all progressive dies. If the pilots are made too close in diameter to the pierce, progressive dies never run successfully. The dies will miss hit constantly, gall, break pilots and cause lower damage to the holes and bushings.
Having the right size pilot contributes to a smoother running tool without affecting tolerance. As a general guideline it is also recommended to spring load the pilots. This prevents further unnecessary die damage.
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Producing Quality Parts on Older Machines
Producing Quality Parts on Older Machines By: Jim Szumera - MACOR Published on: 05/20/2015
Tips and Tricks to Producing Quality Parts in Older Machines
Oftentimes I fall into the blame game. Is it the press? Is it the die? I have had new dies installed into presses only to make several hundred hits before the punches would shear. I have had draw dies set up resulting in cracked cups, wrinkling on one side only, galled and sheared. The initial tendency is to address the problem as a tooling issue; after all, it was the tool that failed. If the tool is machined and set up correctly, it can never be a tooling issue. The root cause often lies elsewhere. If a tool fails shortly after set up, one needs to look closely at the press variables. These can be alignment, wear, parallelism, speed, stroke, timing or lubrication. One of the most critical performance criteria is press alignment. Older presses, through neglect, age, off center loads, constant area loads and the floor foundation itself all contribute to miss alignment. Several simple checks should be made to determine the press accuracy and alignment. First, the surface of the bolster and ram need to be checked for flatness. After years of pounding, the ram and the bolster may have an area of impression. A good rule of thumb is .001 per linear foot. Any more than that is trouble. The bolster may have to be reground if it shows excessive depressions. The ram also may have areas of depression. Resurfacing the ram may be too costly and time consuming. To compensate for the ram I may elect to “soft mount” the upper shoe using a hard urethane sheet between the ram and the upper shoe. This acts as a “filler” and will compress slightly on the down stroke. Another method is to mount a pre hardened 4140 plate to the ram, provided you still have adequate shut height. Basically we are installing a new surface to the ram. All high volume heavier material stamping dies should be designed using a 4140 steel die shoe to begin with. The higher strength material will prolong the surface life of the ram and bolster. Using 4140 for the die shoe, in many cases, will allow the elimination of back up plates. That savings will more than offset the increased cost of the shoe. The lower shoe must be supported all the way through the bolster, wherever there are large cutout areas. I have actually encountered a lower die shoe so flexed that the shims behind the bushings slipped under the die block. In another case, slugs actually slipped under the bushings. The die shoes also need to be checked for excessive bending and corrected as necessary. If you are stamping thin materials, you may not experience these types of issues. Short of rebuilding the press, I will modify the tool to improve alignment and prevent shearing, galling or breakage. If I am dealing with older presses I may design my tooling with guided strippers and no press fits. On draw dies, I will allow the draw punches to float and find their own center. This will always make for a better looking even draw. In some applications, involving high speed dies, I will not bolt down the lower shoe. It is allowed to float. Any modifications to mounting methods should be done in a safe and protected manner.

We have listed several ways to help correct the press inaccuracies such as installing pre-heat treated plates to the upper ram, regrinding the bolster, supporting the lower shoe, and various tooling corrections. Other issues that can cause die failures are crank and pitman timing on double crank machines, galling of the press slides and gibs, short guide pins that exit the bushings on the upstroke. This usually results in deflection and “bounce” on the down stroke. The crank assemblies can be out of timing with respect to each other. Another critical component are the isolation units under the press feet. Are they worn or collapsed? The floor foundation is another factor. Does the entire press flex on the down stroke, given the weakness of the floor foundation. I have had instances where the floor was cracked between the press legs and actually flexed. In the idle state, the press checked out fine. Another issue that rears its head slowly is with open back incline presses or OBI type. These presses can "yawn" or open more in the front over time. It may be necessary to actually shim the front of the bolster to compensate. In any case, if you have evidence of press degradation you must set up a plan to address these issues. The plan may include increasing inventory, outsourcing, or running in optional available machines giving time to rework the press, and planning for new presses in the future. You cannot keep running presses in poor condition and expect quality parts every time. Post your ideas.
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