Thursday, May 27, 2010

Fiberglass Cloth Cutting

Written By Mark Batson Baril

Can woven fiberglass cloth be cut into non-square shapes with a steel rule die?

Fiberglass cloth can be easily cut into shapes with a steel rule die as well as many other types of tooling. In order to make the cut well, with no hanging threads, a sacrificial belt system or an operator with great experience, or both, will have to be involved. The problem comes after the cut in the unraveling of the edges that have been freed during the cut. Like in a slapstick cartoon, a single thread can lead to the entire part falling apart, leaving you exposed to all sorts of problems. If you need just a basic, jagged cut, they can work well. If you need to retain the shape perfectly to pass on to the next operation (usually some type of gluing), you need to make a change to the material.

Add material to the fiberglass to give all the fibers a pre-bond. Either a permeable laminate or a specialty additive work well to give the parts some bond before you make the cut. Your fiberglass supplier should be able to do this for you. Besides adding something to the material to keep it together we have heard of no other way to cut this product and keep it from falling apart. Even a heat sealing type cut is hard to make work because fiberglass is heat resistant. Great question in a very specialty field.

Thanks to Mutual Industries and RP Associates for their technical advice and expertise on this one.

Thursday, May 6, 2010

Dies and Die Cutting Tolerances

Written By Mark Batson Baril

Several times over the past few years we have had to try to explain to our customers why their tooling costs skyrocket when they ask us to better the accuracy (tighten up the tolerances) on their finished parts. Let's try to layout the basic differences in tools and tolerances here between the steel rule die, the solid milled die and the male/female (matched metal) tool. For comparison sake our sample part is a 2.000" x 2.000" (51mm x 51mm) square with four .250" (6.35mm) radius corners cut from .010" (.254mm) polycarbonate. Keep in mind that this article is trying to compare just for tolerances. Each one of these types of tools has production advantages and disadvantages depending on your exact situation. Longevity, ability to be reworked, trouble free running, speed, delivery of parts, etc…, must also be considered along with accuracy.

The Steel Rule Die: Typical close tolerance tooling is guaranteed to be within ±.005" (.127mm) Typical final part tolerancing is ±.010" to ±.015" (.254mm to.381mm) for this sample type work. Sample Die Price $100.00 USD

Of the three types of dies, this type of tool is the cheapest and the least accurate. Typical steel rule die accuracy is limited by the base material, the method used to cut the base, the centricity (cut edge fidelity) and dish of the rule, the finishing operations done by hand, and the distortions that can happen during the impression. The number of separate parts and production operations involved in making a steel rule die make it inherently the least accurate of the three.

Laser cutting the steel rule dieboard base is typically accurate to ±.001" (.025mm) on every 12" (305mm) of cutting distance. This table movement accuracy, as well as perpendicularity of the kerf, effects the final tool size. Depending on the quality of the base material used and the storage of that die, the tool accuracy can be effected by warping, twisting and general contraction and expansion of the laminate layers. Run to run part sizes can change!

The rule itself has a ±.002" (.051mm) tolerance on where the center of the blade sits. If the blade is out by the full ±.002" (.051mm) allowed and two opposing rules are placed in the tool, the total accuracy of the tool can be effected by ±.004" (.102mm) because of this factor alone. Dishing on a typical rule is allowed up to ±.002" (.051mm) on 1.000" (25.4mm) of height. Who knows what this translates into as far as image size distortion, but it is one more factor.

Finishing operations include bending the rule, inserting the rule, grinding and fitting the rule as well as applying ejection rubber to the tool. All of these are typically done at least in part by hand and add more factors to the possible distortion of the final tool.
The final factor that makes this tool lose accuracy is the rule movement during the impression. The rule can bend, the ejection material can push the rules, and the base may give a little which can all lead to blade movement.

The Solid Milled Die or Custom Punch: These dies use the same crush cutting concept as the steel rule die except they are made from solid steel. This type of tooling has middle of the road pricing and accuracy. Typical tolerancing for this type of tooling is guaranteed to be within ±.002" (.051mm). Typical final part tolerancing is ±.005" to ±.010" (.127mm to.254mm) for this sample type work. Sample Die Price $300.00 USD

The accuracy of this type of tooling is limited by two things: The CNC milling or EDM cutting of the shape into the solid block of steel and the heat treating process done after the tool is cut to shape.

The machining accuracy can be held within ±.0005" (.0127mm) on most of today's CNC millers and can be held even closer with some types of EDM (electronic discharge machining) centers. Limits to the machining accuracy as well as the secondary operation of putting the cutting edge on the tool can help push the tolerance up the scale. Machining the cutting edge, takes this from 2-D to 3-D machining which can push out tolerances.

Most custom milled dies must be heat treated after they have been cut to the right shape in order to give them longevity in the press. The finished hardness of the part makes it impossible to machine after the heat treating is done. This heat treating can have a growth or shrinking effect on the tool and help add to the finished pull away from optimal. This type of tool is especially effected by the heat treating process when the size of the image is greater than 6.000" (152mm). The bigger the tool the more it may shrink/expand/twist/etc…. . This can be compensated for by building the tool in smaller segments, but it almost always remains a factor in at least the large tools. Once made and finished the tool will remain consistent from run to run and will experience no size changes. It is more rigid than the steel rule die and will result in less flex and distortion in general during the strike compared with a steel rule die.

The Male/Female (AKA - Matched Metal) Tooling: Built with a completely different concept, this type of tooling is made in two parts. One part passes through the other to make the cut. Matched metal tooling is the most accurate to cut with and is the most expensive. Typical tolerancing for this type of tooling is guaranteed to be within ±.0005" (.0127mm). Typical final part tolerancing is ±.001" to ±.005" (.025mm to.127mm) for this sample type work. Sample Die Price $2,000.00 USD includes a die-set and is made as a standard blanking style tool.

This type of tooling is typically cut from solid blocks of specialty steel via EDM Wire Cutting machinery after the steel has been treated to its' full hardness. Tolerancing for the machinery processing of both parts of the tool can be as good as ±.00016" (.004mm). Add some possible distortion during finishing and tolerances are still the most accurate of any die cutting tool made.

Because of the lack of hand operations in the milled dies and these matched metal dies, they are controllable and reproducible. This becomes a terrific advantage when a parts' finished size must be to very tight tolerances. Because the process is so controlled, fine adjustments can be made on subsequent tooling and revisions in order to perfect a process. On very precise projects that involve both known and unknown materials, you and your customer should expect large increases in tooling and engineering charges to get through this "dialing-in the size" process.

Other Options:
Often times a diecutter will look at these three types of tooling as either/or situations, when in fact some of the most successful jobs are won and run on tools that combine two or more of these types of tooling. Not only can you gain some production advantages, you and your customer can gain some pricing advantages in making the tool. Perhaps the outside perimeter of the part needs to finish at ±.015" (.381mm) but two interior cutouts need to measure within ±.001" (.0254mm). It's possible to make a matched metal tool to cut the interiors while incorporating a steel rule die or a milled die for the perimeter cut. By doing this you can expect that your tooling costs will be dramatically reduced.