It is highly recommended that all rough cord dies be made using the meeting point method to provide the correct bearing length to start the dies life.  This makes it easier to bump the die to the next size when it is worn and maintain its correct bearing length.  It is also strongly recommended that the die be ground and then polished to ensure the angle is made to a true conical shape and is concentric to the outside diameter of the die case.  Since rough cords have inherently inaccurate geometry, do to their manufacturing process, just polishing the approach angle can result in eccentric bearing lengths.

To cut a used die to a meeting point requires the bearing to be removed and taken to the desired meeting point size as shown in figure 2. 

This requires removing a large amount of material from the die but is the only way to produce an accurate bearing length.  For example to make a .150” (3.80mm) 12 degree 50% bearing length die, it has a meeting point of .136” (3.45mm) which means a jump in size of .014” (0.35mm).  The draw back is any sizes that could be used between the finished size you are looking for and the meeting point are skipped and the die will loose a number of potential recuts.  The practical problem with cutting a used die to a meeting point is estimating how much will it take to remove the bearing since grinding on the angle is not a 1:1 relationship.  This is due to the fact you are cutting into the angle not on the diameter as show in figure 3.

Bumping Method

The other method to make dies is to take them from one size to the next closest size needed which I refer to as bumping.  This can be done in rough cord dies that are purchased with a preformed bearing or more commonly with used dies.

Bumping advantage over the meeting point method is speed and number of reworks.  A die can be bumped in a matter of 2-4 minutes if you are only taking .002-.003” (0.05-0.08mm).  Unfortunately, as the bumping amount and bearing length grows the time increases quickly.  To bump a die .010” (0.25mm) it will take as long as using the meeting point method.

Rough Cord

To bump a rough cord it must have a preformed bearing to the correct length.  In the US only R7 and larger dies are supplied with a preformed bearing length which is typically 35%.  Purchasing R6 and smaller with a preformed bearing or a different bearing length other than 35% can be done but will increase the cost and lead-time of the rough cord significantly.  In Europe many rough cords are typically produced with a preformed bearing length of 35%.  This leaves other lengths to large quantity orders or an increased cost along with longer lead-times.

Bumping a die from rough cord or used size to a new size is very simple.  All you need to do is calculate the difference between the starting size and the desired size to determine the amount to remove.  The largest problem is if the bearing is increased in size without removing material from the approach angle the bearing length will grow from the starting length as shown in figure 4.

For this reason any time the die is increased in size it must be worked on the angle to prevent the bearing from getting too long.  The problem becomes how to determine the amount of material to remove from the angle to return the bearing to the correct length.  The other problem is there is no way to verify this on the die finishing equipment especially in a polishing cycle which means the die needs to be removed from the machine, cleaned and inspected.  If the bearing length is still too long it must be returned to the machine for additional material to be removed from the angle.  If the length is too short the die must be remade or more commonly it is sent into production which results in a shorter production life.  For example if you only bump a die .004” (0.10mm) there is not enough material to grind and polish the angle.  If you only use diamond compound and time the question is how much time do you set on the machine to remove .004” (0.10mm) from the angle and with what compound grade?  This still hasn’t determined if the .004” (0.10mm) is the correct amount to remove to keep the correct bearing length which would have to be done with a geometric calculation using the approach angle, increase in diameter, back angle and bearing length.

The other major problem with bumping dies is if you do not want a 35% bearing length as preformed into the rough cord die how do you change it?  To make a longer length you will need to start with a smaller diameter and cut the bearing to size first causing it to grow in length, then the length will need measured to determine how much to grind from the angle to get it to the desired length.  On the other hand if you want a shorter length you will need to calculate how much additionally to remove from the angle to cut the bearing to the shorter length.  None of this geometry is overly difficult to calculate but it is time consuming.

In most shops the die personnel do this visually instead of using geometry and determine what to do from their experience.  The problem is our eyes do not have a measuring capability, which typically leads to bearing lengths that are too long.  Physical tracing machines work very well to measure the bearing length but this requires the die to be removed from the machine, measured, calculated and returned to the machine for additional work.  This increases the working cycle and requires the investment into an expensive tracing unit.

Used Dies

Bumping worn dies has all of the same issues as discussed with rough cord dies in regards to bearing length along with a few additional concerns.  If you are only moving a die a couple thousands of an inch or hundredths of a millimeter to the next size it is critical that the wear ring/drawing marks are completely removed from the bearing and approach angle.  This must be done by removing an entire plane of material from both the bearing and approach angle as shown in figure5.

Typically, this requires more material to be removed than the amount to take it to the next size, which is why many die makers bump finished dies instead of, used.  In bumping a used die, die makers often do one of two things in this instance.  First, they polish the angle by hand to blend out the wear, which leaves very poor die geometry that will cause it to fail prematurely.  The other common method is to remove an entire plane of material until the wear is gone in the angle which shortens the bearing length to an unknown percentage which will shorten the dies production life by growing oversize too quickly.

Since dies typically wear more on one side it is important to visually inspect bumped dies to make sure the entire wear ring has been removed.  Also if the wire is pulled out of the die on an angle it will cause the die to be oval which might not be removed by bumping and can be difficult to see in a visual inspection resulting in out of round wire.

How to Choose

With all of these items to consider how do you choose the best method to use?  First, all new rough cord dies should be cut to a meeting point to ensure the correct bearing length so if the die is bumped a reasonable bearing length can be maintained.  If dies are purchased finished bearing lengths should have a tolerance and be inspected prior to use.  The main consideration is what material is being drawn and how much tolerance in the bearing length can be allowed.  Since the bearing length creates frictional heat shorter bearing lengths reduce heat generation but will increase the chance the die will wear oversize prematurely.  On the other hand too long of a bearing length increases the heat in the bearing typically causing lubrication failure that results in metal pickup.

To ensure correct bearing lengths the only method to use is the meting point.  It shortens the number of potential uses of the die but provides a consistent die that does not need to be inspected for its length.  Making dies to meeting points takes more time in the die shop than bumping.  For hard to draw materials and applications that require tough material specifications it is critical to make dies to the meeting point to control bearing lengths.

Bumping dies is a quick way to make dies but with out proper supervision or measuring equipment it normally leads to inconsistent bearing lengths.  If used properly it is an effective die making tool that increase the production of the die shop where bearing lengths do not need to be held to a tight tolerance.

In conclusion to produce consistent high quality wire it requires consistent die geometry.  An accurate approach angle, correct size and bearing length all effect the production life of the die along with the mills efficiency.  Using the meeting point eliminates one of these variables, which is why I strongly recommend their use.  It may take slightly longer to make dies this way but the payoff in consistent production results payback significantly.  Bumping can be used effectively but most cases it results in a varying bearing length in all the dies and lower production efficiency unless the dies are constantly inspected with the die ship personnel properly trained.