Because something is going to have to hold this die together
There are generally only so many fasteners that you are going to encounter in the trade and, from what I am understanding, a good toolmaker can normally look at a block of steel and know how many bolts and dowels it should take to hold it securely, just from experience. That experience comes from applying some fundamental rules of fasteners and Dowling in tooling.
There 7 fasteners that are used in tool making:
1. Socket head cap screws
2. Socket button-head screws
3. Stripper bolts
4. Allen nuts
5. Dowels
6. Socket flat-head screws
7. Socket set screws
Threaded Fasteners
There 7 fasteners that are used in tool making: The ideal situations are where a tap size drill is drilled through the components and then tapped from the mounting surface to a suitable depth. There are advantages to using a through-hole whenever possible, some of those advantages are that the through-hole allows more quenching medium to flow through the holes for more even hardening, through-holes also allow easy and quick tapping because there is clearance for the chips to clear away. Drill through holes also make it quicker to mount the new die components because the die maker can clamp the threaded components to its mating counterparts and can use the tap size drill to spot the mating counterpart making it faster and more efficient.
Tapping Depth
As an apprentice, I have only come across two rules when it comes to thread depth. In most applications, the depth of the thread should be equal to or more than one and a half times the diameter of the screw. The second rule applies to heavy workforces when it is recommended that two times the diameter of the hole should be threaded, for safety reasons.
Blind Tapped Holes
When through-holes aren’t possible, normally because the block you are working on is far too thick to make it reasonable to drill right through, you will have to make some blind tapped holes. The hole must have the required thread engagement ( 1 ½ – 2 times the diameter of the hole), but it should also have enough clearance in the bottom to allow the tap to thread deep enough, while still having chip clearance to avoid crowding the tap and breaking it. There is a shorthand to figuring this out, in our textbook, it explains to first scale measure the chamfered part of your tap and add that to the depth of two threads plus the depth of usable threads and that will help you efficiently decide how deep a threaded blind hole should be. Normally, threaded blind holes are transferred to the mating component by use of transfer screws.
Tap Drill Sizes
For die construction, there are rules for determining the appropriate tap drill size and the factors that influence that decision are:
1. thread strength
2. tapping effort
3. standard drill sizes for practicality
For most common practices in toolmaking, depending on the material, generally 75% thread depth is sufficient. There can be an exception to this rule, if working with certain tooling steel, where the tapping depth is more than twice the diameter of the tap, the toolmaker can reduce the thread depth to 60% by selecting a large tap size drill. The rules are critical when tapping size is ¼ inch and smaller.
Countersinking Tapped Holes
A general rule with all tapped holes in tool making and machining is, when you tap a hole, the tap has a tendency of pulling the material up on the first thread, causing it to be thicker and making a bump on the surface of the joining parts. This can cause play or throw tolerances out. The best way to avoid the bump caused by the first thread, is by countersinking all tapped holes. This also has a third benefit, which is that, now that the first thread is recessed, it is protected if it gets smashed by another component.
Clearing Holes After Heat Treat
It is a very good practice to check all your threaded holes for thread depth and engagement after heat treating. Heat treating can cause the metal to move or a slag to form from the quenching medium, which can fill the tapped holes, not allowing the fastener to join properly. The textbook way to check threaded holes post heat treatment, is to get a bolt of the right size and put a longitudinal groove through the thread of the bolt, use some tapping oil, and then run the bolt through the tapped holes. The groove allows the build-up of debris to fill and clear the thread.
The Function of Mounting Screws and Dowels
In tool making, it is very normal for any die component to have a combination of several bolts and dowels to locate and fasten components securely and accurately. Dowels are used to provide accurate and repeatable locating and screws are used to retain components in the doweled position.
Cons to Dowels
Dowels aren’t readily used in small dies because incorporating dowels would weaken the die, due to having to use thinner walls. The tool’s one other disadvantage, that wasn’t obvious in the beginning as an apprentice, is that dowels do not tolerate lateral forces.
Pros of Dowels
There are quite a few advantages to using dowels in die making and, of course, they always have to do with efficiency, accuracy, and cost. Using dowling for locating is really the simplest way to very accurately locate a component when installing it on a die. Because of the ease of installing dowels accurately, and how readily available they are, dowels are finished to 0.0002in. larger than nominal diameter with the tolerance of +/- 0.0001.
When a die is regularly serviced and dowel holes become damaged, oversized dowels are available allowing the toolmaker to open up the hole to the next available size quickly and easily.
Requirements For Dowel Quality Holes
How accurate a dowel hole is depends on many things and how a die should be doweled also has many factors that help decide how a die is to be doweled. For example, a low production die that doesn’t require tight tolerances of the piece part doesn’t require the same dowel hole treatment as a high production, precise die would. Factors that affect doweling procedures include fits and clearance required for the die, accuracy of the piece part, production requirements (die life) of the tool, and maintenance requirements. Due to these factors, some dowel holes will require grinding and lapping after heat treatment. A ground dowel hole will give an accuracy of location, produce a round hole and a perfectly perpendicular hole. For the majority of work forms, my understanding is that lapping is all most dowel holes actually need. One way of quickly lapping a dowel hole post-heat treatment, is to use a rod with a slit put in one end and sliding a piece of emery paper in the slit and running the rod clockwise in a hand drill to clear the hole of any debris from hardening.
Conclusion
As you can see, fastening will have some general rules to follow, but a lot of it will be applying common knowledge. Once a toolmaker understands the forces involved in the tooling he or she is working on, fastening things for safety accuracy and efficient die life will almost become second nature. I work with toolmakers that will often just walk by a section of tooling and tell me, “Hey, you should put one more socket head cap screw here or another dowel there it will stabilize the component” or they will say, “It will make it NICE if we have one more SHCS here.” In my fourth year now, I am already seeing these things before they are mentioned which is nice because I know the principles are setting in. This article, like most of the Die Build articles, is a summary of what I think is most useful from the text that I am using in preparation for my C of Q test. If you found this useful information and would like more, please subscribe to our email.