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Milling Composites, Part 2: Tooling, Workholding, and Machine Requirements

Mar 7, 2016   //   by Laura Warfield   //   Blog, FeedsSpeeds, Manufacturing, Techniques  //  3 Comments

This is Part 2 of our Composite Milling series.  In Part 1, we talked about some of the general properties of composites.

Tooling for Composite Machining

Diamond Tooling

PCD

PCD tooling is preferred over carbide to resist the abrasion of composite machining.  Image courtesy of Fullerton Tool.

One of the first factors that comes up in any consideration of tooling for composite machining is the material’s abrasive nature.  Particularly with carbon fiber composites, there’s no chip making.  Instead, the cutting edge shatters the material and the carbon fibers.  It is this process that makes cutting these composites so abrasive to the tool.  With such materials, carbide can be used, but it will wear very rapidly.  The preferred tool material for composites is polycrystalline diamond (PCD).  Given how hard diamond is, these tools can stand up to the abrasive nature of the composite machining process much better than plain carbide tooling.  A good PCD tool can run 3x faster in composites and last as much as 25x longer than carbide.

PCD is a synthetic diamond that is actually tougher than natural diamond, though the hardness is the same.  The reason they’re tougher is that the diamond layer consists of diamond particles that are randomly oriented and do not have any cleavage planes or soft wear directions. This layer is applied to a carbide core to make a PCD Cutter.  The PCD cutting edges are a lot like the brazed on carbide edges seen in some lathe tooling.

Different sizes of particles are available too, typically fine grain for finish operations, medium grain, and coarse grain for extremely abrasive roughing applications.  PCD inserts are also available.  Another useful property of PCD is that diamond has the highest thermal conductivity of any cutting material–this means it can move heat away from the edge quickly.  Recall that it is hard to get the heat out of composites and it soon becomes apparent why the thermal charactertistics of PCD are also important in this application.

While we’re on the topic, PCD tooling can be used for materials besides composites, but it does have a strong affinity for iron and so it should not be used in ferrous materials.  Worse, it has a fairly low temperature threshold which also argues against its use in ferrous materials. Typically, PCBN (Polycrystalline Boron Nitride) is preferred over PCD in ferrous applications.

PCD tooling cost will at first glance seem extremely expensive.  It’s important to compare on the basis of cost per linear foot machined so as to get an apples to apples cost comparison with carbide tooling.

Tool Geometry

GuhdoMill

This image, courtesy of Guhdo, makes the brazed on PCD cutting edges more visible on an endmill…

As with any material, there are geometry considerations for tooling intended for Composites.  For example, 90 degree lead angles are preferred with indexable tooling because a 90 degree lead generates mostly radial forces.  Axial forces are bad for composites because they cause tearing out and fraying of the fibers.

The shattering rather than chip cutting nature of composites means it is hard to carry the heat out in the chip.  Very high positive rake is also favored in the geometry of tools intended for composites because it encourages a sharp quick break in the material.  Another important geometry consideration is to leave plenty of clearance angle so the tool doesn’t rub and build up heat that way.

Special Purpose Composite Cutters

Downcut endmills change the direction of cutting forces so as not to chip and rip composite material upward…

Given the nature of many composites–they’re some sort of tough fiber embedded in a resin, one of the primary issues for machining composites is to maintain surface quality so that edges, walls, holes, and surfaces near the cutting are not chipped and there are no frayed fibers exposed.  Such considerations are not unlike what’s seen when machining wood and similar materials on CNC Routers.

Because of that, we see many of the same special purpose cutters as are found in CNC Routing applications:

  • Downcut Endmills:  Downcut endmills change the direction of cutting forces so as not to chip and rip composite material upward.
  • Compression Cutters:  Compression cutters can actual draw the material to the center of the cut wall, avoiding chipping on both top and bottom surfaces.
  • Diamond Toothed or “Rasp” Endmills:  These specialized cutters can be even more gentle on the material where finish quality is particularly important or difficult to achieve.

We’ve covered these in detail in our special article on Cutters for CNC Routers, so I will refer you there rather than going into great detail here for more information about these cutters.

Another special tool geometry would be staged drills, also called “tapered drill reamers”.  Such drills have tapered diameters that drill a pilot hole and then gradually ream it to the required diameter so as to minimize fraying, splintering, and delamination of the composite material.

Still other geometries and specialized tools result from a need to work on composite stacks where there are layers of composite and metals such as aluminum or titanium combined in a stack.  The two types of materials have pretty radically different requirements so specialized tools can really help.

Workholding for Composite Machining

The signature requirements for workholding when machining composites are the ability to hold relatively thin components that have a large surface area and that are frequently curved.  The roughest approximation of shape comes from the molding process and machining is used to refine the shape and add holes.

This is not your typical workholding situation.  The most common solution is vacuum based workholding, but engineering a system to reliably hold complex curved parts can be the most difficult and expensive part of a composite machining job.

Machine Requirements for Composite Machining

Given appropriate tooling and workholding, what else is required for successful composite machining?  In particular, what are the machine requirements?  It turns out the machine requirements are not hugely different, but there are a few things to consider:

Coolant and Dust Control

As mentioned, dust is a problem with composites and so is heat–the composites do not carry heat out with the chips very well.  Too much heat will ruin the composite in a hurry by damaging the resin.

Coolant can help or hinder.  The choice to machine wet or dry is made based on the machining operation.  If that operation will generate a lot of heat, coolant must be used.  Coolant can also help keep cutting edge temperatures cooler, which is critical for diamond tooling since the diamond degrades at much lower temperatures than traditional carbide tooling edges will.  The use of coolant can also aid in dust control.

Vacuum is another approach to dust control.  The choice of coolant or vacuum for dust control is often based on how large an area is generating dust.  For many single-point turning operations, dust is concentrated in a small area and vacuum works well.  When milling large panels, it may be harder to apply vacuum, so coolant is preferred for dust control.

For most composite machining, pure water is the preferred coolant.  If rust control is important, a small amount of rust inhibitor (1 to 2 percent) may be added.  It is important to use a water-soluble coolant, because oils can adversely affect paint bonding.  Given the porous surface of composites and a tendency for the fibers to wick oils and solvents into the interior of the part, cleanliness is important as it is nearly impossible to clean undesirable contaminants such as oils after the fact.  Any coolant should be tested and verified that it won’t interfere with the paint or adhesives planned for use with the composite material.

The last issue to consider for coolants is that when using coolant to contain dust, the result isa coolant/dust mixture that is a slurry.  Disposing of such slurries often requires post-treatments such as removal of the excess water, before it can go to a landfill.  This additional cost is one reason to prefer vacuum over coolant dust control when the cooling properties of a liquid are not needed.

Tool Life Management

Any machine features that can help with Tool Life Management are a blessing when machining composites.  It is important to track machining time on tools since tool wear is much greater due to the abrasive nature of composites.  This creates two issues for managing tool life.  First, it makes it harder to maintain tolerances when the tool changes size so quickly.  This adds a greater burden on the operator to manage that change in order to maintain tolerances.  Second, as the tool dulls, it will have a greater tendency to snag and pull out the fibers underlying the composite.  This produces unacceptable finish quality and can scrap a part in a hurry.

Diligent Tool Life Management enables wear adjustments and changing tools before they become dull enough to damage the parts.  Given how expensive it is to scrap most carbon fiber parts, it’s much more critical to change the tool before it has dulled rather than just waiting to tell if the tool is dull from it’s effect on the work.

5-Axis and Sturz Milling

SturzMilling

The organic curves of composites often require 3D profiling or create access and rigidity problems due to excess tool stickout.  5-axis machining may be required for optimal results.  A technique called “Sturz Milling” takes a conventional tool with a tip radius (not necessarily a full ballnose) and turns it on its side 3D profiling applications.  The results can be dramatically faster and with longer tool life and better surface finish than using a conventional 3-axis machine with ballnose endmills.  This can lead to competitive advantages for shops that have invested in the 5-axis machinery and necessary CAM software to employ these techniques.

How does Sturz Milling help?

Consider a ballnose sitting vertically.  The range of surface speeds and therefore cutting forces vary dramatically from tip to the largest radius (full radius of the cutter).  The very tip is hardly moving relative to the diameter.  This creates unequal loads that can fray delicate composite fibers.  But it also forces the tool’s motion to fit the lowest common denominator.  Now tilt that same tool so we’re largely using a faster moving part of the tool and we get more uniform forces, less material fraying, and the potential for faster feeds.  All good stuff!

Size Matters

Many composite machining applications involve relatively large parts.  Having machines with sufficient capacity can determine which jobs shops can and can’t bid on.

Next

In our third installment, we discuss using G-Wizard Calculator to figure Feeds and Speeds for PCD endmills.  Make sure you don’t miss out on the future installments by signing up for our Weekly Digest of Blog Posts below.

 

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Milling Composites, Part 2: Tooling, Workholding, and Machine Requirements
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