Not the strippers you think, it’s the stripper you need.
Stripping when speaking of toolmaking means to remove the work from the punch. A stripper is the device for stripping.
Stripping in toolmaking can be grouped into two general groups. The two classes are if the stripper is motivated or moves or if there is a stationary stripper.
Let’s briefly describe the action of a stripper for the two different groups.
First, the die starts closed at the bottom of the press stroke. As the punch starts to move up or ascend, the stock material will have a tendency to cling to the punch. This is when the stock material will come in contact with a fixed stripper and the stock material is pushed off the stripper. This allows the stock material to move through the die.
Let’s now talk about pressure pad type strippers (spring). As the press is triggered the top half of the die descends toward the material. The pressure pad stripper contacts the material and applies load or holding pressure on the material. Next, the punch enters the material cutting the piece part. Now, the punch ascends back to the starting position through the pressure pad stripper. Shortly after the pressure pad also follows the punch releasing the material for the next cycle.
As a general rule, if a die has stationary punches the stripper must push the material from the punch, so a fixed stripper would not be appropriate. If the punch is stationary, the stripper must be travelling. A toolmaker has to look at the part to be made and consider the die life factor to decide which of the two types of strippers will best apply to the job. There are several variations for each of the two groups of strippers. For fixed strippers, a few examples are open gap strippers, hook pin strippers, or box strippers. For moving strippers, there can also be many variations.
Fixed strippers are desirable only because they are simple to design and build, as there are generally fewer parts, but they require more precision. If strong holding forces are required for a particular material or with dies that have a very high production, springs may require a lot less maintenance then a fixed stripper.
Hook Pin Stripper
The textbook that we are learning from makes a point to mention two kinds of strippers one being the hook pin stripper. It looks very much the way it sounds. Hook pins are made from cold drawn steel drill rod or cold drawn steel that is packed and hardened or tooling steel. If tooling steel is used, a Rockwell of 45 to 55 is generally desirable. When drill rod is used, it is described for the toolmaker to heat and forge the drill rod into the contour of the hook pin, then machine and file the part so that they will function. Hook pin strippers also serve as stock guides and stop pins depending on how the hook pins are arranged. Normally, because the location and direction of the hook pins may be critical, a toolmaker should put location flats on the hook pin facilitating ease of installation and fasten the hook pin strippers using set screws. Because of the nature of hook pin strippers, they have a tendency to distort the scrap skeleton. It feels as though the hook pin stripper would be practical in a low production die making blanks, this is a solid stripper.
Box stripper
Box strippers are almost exactly as they sound, because the stock guides are part of the stripper, and the material is boxed into a channel with the stripper above. This is normally done by channelling a slot in the die block. Before doing that, a toolmaker should analyze what is the best method of machining the slot. In a box stripper situation, it may be more efficient and adequate to machine clearance for the backing punch before the slot.
The back edge of the box stripper may act as a back gauge, keeping that in mind, a toolmaker must locate it properly. The width of the channel should be material width plus material cutting clearance. When using a box stripper, one must also make clearance holes in the stripper to allow the punch and pilots to travel through the stripper. This makes for ease of construction as there are normally clearance holes using the maximum formula of ½ times material thickness for the stripper clearance. Stripper opening clearance should not be less than cutting clearance when making a box stripper. The book warns that a toolmaker should be cautious using minimums and maximums for clearance and not to use them more than ½ material thickness for maximum clearance.
Separate Back Gage
A back gauge is normally not part of a stripper, but it may be combined in a box stripper setting. If the back gauge is being used with a box stripper, it may be necessary to have a double step in the stripper to allow adequate back gauging thickness and suitable tunnel height through the box stripper. The back gauge should be dowelled, as the location of a back gauge is normally critical. A toolmaker can also make a box stripper where the guides may extend past the stripper.
Material for Stripper Plates
Of course, like everything in toolmaking, the application and cost are paramount. In the majority of box strippers or simple low production dies, a cold-rolled steel stripper plate should be adequate. If the stripper plate warps or shifts because of machining, the toolmaker may anneal or stress relieve the metal. If the stripper plate is being used to support and guide the punch then another type of steel is necessary.
Here is a list of material in descending order of quality and durability for making stripper plates:
- Tool steel, hardened
- Pretreated, alloy steel
- Tool steel, left soft
- High strength, low alloy steel
- Hot rolled, low carbon steel
- Cold-drawn, low carbon steel bar stock
Material for Guide Rails
As we’ve said before, time and money are paramount in toolmaking and because of this toolmakers have come up with ingenious ways of making parts last as long as possible and as cheaply as possible. In low production applications, guide rails can be made from cold drawn steel. Low carbon steel can be hardened and used, but would require long pieces to be hardened; because of this, tool makers have come up with brazing or soldering hardened inserts on soft guide rails that have localized hardened spots. This saves time and allows quite a bit of money to be saved.
Guide Edges and Bulge Clearance
There are a few details that can get missed that may cause a toolmaker a lot of headaches later, if they are not aware. One of those conditions is that guiding edges should be sharp when using cold drawn steel. Guiding edges should be ground sharp and flat, using a small chamfer may be acceptable. Especially light gauge work will hang up and not progress if there are not smooth guides.
Another concept to be aware of, is bulging when blanking a thin material or, if the blanking takes place at the edge of the stock strip, it will expand or bulge. A toolmaker needs to account for the bulging along the stock guide and die section along the progressor or the stock strip gauge at each progression. With limited die build experience, I thought that this would be the exact details that I would miss. Most drawings that I have inspected have not included bulge clearance.
Pressure Pad Stripper
There are times, when cutting and forming some materials, it will be required to hold the material in place with pressure, not just locate the part in the die. When this is the case, a toolmaker can use a pressure pad stripper. A pressure pad stripper is defined (refer to text) as a die component which delivers holding pressure to the material being worked. Because the stripper is able to apply holding pressure, it can have a double use holding and stripping. Pressure pad strippers may be actuated by springs, hydraulics, rubber cushions, or pneumatically. According to the textbook, there are one or two application for pneumatic and hydraulic pressure pad strippers. When there is very high holding pressure required or if the holding pressure needs to be applied at a certain time during the die cycle, pressure pads are much easier to control
Normally, pressure pad strippers or pressure pads are necessary when there is a drawing process in the piece part or die cycle.
Stripper Springs
Springs can be used in a pressure pad situation but, in my experience, I have only used springs in a die construction. Spring strippers are normally the best option when holding force is necessary just to flatten the part rather than holding because of a draw process.
Stock Guides with Spring Strippers
The past examples have been examples of channel dies, which work well with hook stripping. There are other types of stock guides, one of them being spool type guides. Spool type guides are more cost-effective and normally allow for a smaller stripper plate which can be desirable when space is limited. Spool guides may require hardening to 50-60 Rockwell C. Some of the drawbacks to the spool type guides are that it may be harder to start a stock strip with spool type guides and they also generally don’t perform well as hook strippers, unless their placement is just perfect.
Spring Confinement
There are several ways to confine a spring for a stripper plate. I am going to try to describe the most conventional practices and any rules that I apply. One standard way of configuring a spring, is to pocket it into a hole drilled into the die section. Springs can be confined on just one end depending on how much travel is required. The spring can also be pocketed in one die section using a small pocket to confine it on both ends of the spring. Another way would be to use the pocket on only half of the spring and on the other side the spring can be confined using a spring locator plug and/or the head of a cap screw as a locator. Springs in tool making are normally standardized for their sizes. Keeping that in mind, the text has set some standard rules for spring pockets sizes. Springs ranging from ½ to 1 ¼ in add 1/16 to the outside diameter. For springs larger than 1 ¼ the outside diameter would be 1/8.
As tool makers, when we are building the die, I am always impressed with how designers have considered the whole of the die life; meaning, the die is designed to be serviceable and maintained. This is why compensator slugs are placed behind the spring. The reason for this slug is as the punches are resharpened or reground, which makes them smaller, which leads to the spring being further compressed. To correct this, if it is a very tight tolerance die, the toolmaker can grind the same amount that was taken off the punch off the compensator slug. That way the spring travel remains the same throughout the life of the die.
Another way of confining a spring, is to use a spring around a stripper bolt. There are some drawbacks and some advantages for this. Two of the drawbacks are that a die would require a lot of vertical space for this to work and a common mistake is that when constructing a die this way many conventional springs are too small. This is important to keep in mind. The two main advantages to continuing a spring around a stripper bolt are that the bolt maintains the stripper in the center of the spring and it also allows the spring to be pocketed in a large hole, yet maintained in the center of the spring.
The most common application of a stripper bolt is to use it in suspension, because of this, rarely would it be permitted for the head of a stripper bolt to protrude beyond the stripper. To prevent this from happening, the end of the stripper bolt should be at least 1/64 away from the bottom of the stripper plate. To ensure proper stripping the stripper should over travel slightly. For normal applications, this can be as little as 0.005 for accurate work. For heavy work an over travel of up to a 1/16 can be acceptable. Over time, as the punches on a die are sharpened, the over travel will increase. If this effect is not desirable, the excessive over travel can be corrected by using compensating washers under the head of the stripper bolt.
Conclusion
As you can see, there are many variations on strippers, in terms of how to activate them and how to use them. As I continue working through my notes, I will add more to this chapter. I am beginning to realize the devil is in the details, because each different application the stripper is used in, will have different nuances, tolerances and good practices. If you found this article helpful subscribe to our email list I’ll let know when I put out any new content or helpful offers.