How to Counteract Unbalanced Thrust

Posted by stephens on Mar 20, 2010 in Engineering Decisions, Equations, Progressive Dies, Transfer Dies | 0 comments

The debate on how to counteract unbalanced thrust lives on in the stamping industry.

I have seen it first hand in several markets. Automotive. Appliance. Medical device. The list goes on.

Here is a note sent to me by one of my DieGuy.com readers through the contact page of this site:

Hi Tim,

Really enjoy reading up on your blog & your die knowledge. I have a question for you, that may also lend itself to a blog topic.

Is there any hard and fast “rule” regarding thrust blocks or supplemental guidance in a die?

I understand the point is to counteract unbalanced thrust, etc. but I was thinking more specifically when to use a 6-post die set versus vee-style thrust blocks versus rectangular thrust blocks, etc.

What is the best style? How do we quantify this?

And if needed, the best places to locate thrust blocks and/or extra guidance members if you cannot get them right next to the thrust source.

This topic has always been a source of debate among many of us! It’d be great if you could clarify things! Thanks and keep up the great work!

A fan,
Greg Grigutis

Tool Design/CNC
Spalding & Day Tool & Die
Louisville, KY

This is a great question. So, let’s breakdown Greg’s excellent question into these five critical characteristics:

Rule

The rule on if and when to use some heeling device to counteract unbalanced thrust from cutting or forming is this:

If the lateral force exceeds the single shear load performance rating of the dowel pins that locate the cutting or forming device, then that device must be keyed, pocketed, or heeled to absorb the unbalanced thrust.

Type

There are two choices for heeling steels or components that are subjected to unbalanced thrust: Heel externally on the die set or heel internally near the source of lateral force.

Conventionally, most die engineers locate the heels externally. This is just wrong for one simple reason: these heels are too far away from the source of lateral force to be effective.

The die set guide pins will end up taking all the force. If the force is high enough, the guide pins will deflect and bend.

Style

What style of internal heels is best? The one that works for the unique requirements and constraints of your die design.

The generally accepted styles are machined pockets, solid keys, heel blocks, and wear plates.

That said, machined pockets and solid keys are the simplest method (no running clearance fitting) but are only effective if the cutting or flanging component has a base-to-height ratio of at least 1.5-to-1.

Some style of thrust block, like heels or wear plates, is best if the base-to-height ratio is less than 1.5-to-1. In this case, the engagement of the thrust block must be at the same elevation as the cutting or flanging work.

No matter which method you end up with, just make sure that the heeling device is close AND perpendicular to the lateral force vector.

This is why I am not big on vee-blocks. For vee-blocks to be effective, there must be two force vectors, each at 45-degrees from normal, such that each face of the vee is perpendicular to the force vector.

Assuming normal running clearances, using vee-blocks in a typical unbalanced thrust situation means that the face of the vee is 45-degrees to the lateral force vector. You effectively have more clearance before the heels take effect.

I prefer commercial wear plates if they fit or homemade aluminum bronze heel blocks if keys or machined pockets won’t do the trick.

Quantify

You already know how to quantify lateral force for cutting and flanging. The question now is how to quantify the internal heel so it is not too small.

This calculation needed here needs to find the adequate surface area of the heeling device, working within the compressive strength of that heeling device.

A simple equation for this is:

• Ah = (1 / CS) • Fut

where:

• Ah = Area of heeling device (mm²)
• CS = Compressive strength of heel material (kN / mm²)
• Fut = Force of unbalanced thrust (kN)

Note that most materials have compressive strength rated as Newtons per square millimeters. I convert this to kiloNewtons to work with the other die equations.

Location

Always use internal heeling devices as close to the source of lateral force as possible. Again, this is both in the plan view and in the elevation.

Don’t forget to keep the heel face (or machined pocket or solid key) as perpendicular to the lateral force vector as possible for maximum effectiveness.

Greg – I hope this clarifies things. My goal here was to be clear, concise, and precise. Let me know if we need to discuss any of this topic further.