Top 10 Considerations When Buying a CNC Machine for Composites

A considerable amount of time and investment goes into finding the right CNC solution to efficiently and effectively machine composites. The last thing manufacturers want is to invest in the wrong machine and be paying for that mistake for years to come. With little room for error, it is important to understand how different materials, machinery characteristics, and machinery options affect machining speed, precision, service, and overall production.

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1.  Spindle HP & RPM - Consider high RPM, low torque

Material density plays a key role in determining the best spindle RPM, HP, and torque for an application. To aggressively machine hard materials, such as steel and Inconel® , some traditional machining centers typically are equipped with high torque, low RPM spindles that operate at a maximum of 12,000 RPM. When it comes to composites (such as foam, tooling board, or carbon fiber), a high torque, low RPM spindle is too slow to reach optimal chip load and thus results in inefficient (slower) composite machining. Instead, high RPM (18,000 - 24,000), low torque spindles are more efficient in reducing cycle time, lengthening tool life, and improving overall spindle reliability. For the best of both worlds (composite and nonferrous metal machining), consider a spindle capable of running at 20,000 - 24,000 RPM and higher feed rates for lighter-duty materials but also can operate in the 10,000 - 12,000 range for harder materials that need more torque. 

2.  Dust Containment / Collection - Improve employee safety and machine longevity

Machining composites often creates a large amount of dust and debris that can cause health problems to workers and damage to the machine. Some composite dusts can lead to lung damage if inhaled and some are electrically conductive, so they can damage machine circuits and cause spindle or machine wear at an increased rate. Thus, choosing a CNC solution with sufficient dust collection and properly sealed and covered components is imperative when working with composites. Some additional considerations for handling abrasive material machining include air knife systems, downdraft tables, and/or full enclosures.

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3.  3-Axis vs. 5-Axis - Depends on the part’s geometry

When machining complex 3D composite components, a 5-axis machine is more efficient than a 3-axis machine and provides a greater return on investment over time. With a 5-axis machine, multiple sides of a part can be machined without having to manually reposition material or swap out tooling for angled heads. A 3-axis machine can perform multi-face machining as well but at a much slower rate as it requires an operator to stop the machine and reposition the part after each operation. To achieve 5-sided milling, drilling, tapping, and/or sawing operations without a 5-axis machine or without having to manipulate parts, consider adding a 4th axis and utilize angled heads, known as aggregates.

4.  Rigidity -  For speed, tool life, and machine longevity

Due to the abrasive and unique structure of composites, it is worth investing in a CNC machine with fully reinforced structural integrity and rigidity. A sturdy design reduces vibrations and tool deflection to provide top acceleration/deceleration speeds, long tool life, and low maintenance and repair costs over the lifetime of the machine compared to less rigid, light-duty machinery. From an upfront cost perspective, a machine can be made cheaper by reducing the structure and quality of its components; however, it will fail to have the longevity of a better built machine and ultimately will result in a higher total cost of ownership.

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5.  Work Envelope - Make sure it fits, and consider the benefits of larger tables

In addition to the type of composite material being machined, factor in its size, and select a work envelope accordingly. At a minimum, the work envelope needs to be larger than the largest part being machined. However, depending on production goals, buying a machine with twice the work envelope that supports pendulum processing, where materials are safely loaded/unloaded while the machine is still performing cutting operations on another part, will significantly speed up overall production time. Other advantages of large tables include batch processing which allows you to fixture multiple parts at one time and machine continuously without having to unload and reload.

Note - When searching for 5-axis machining solutions, be sure to consider the work envelope while the machine’s spindle is at 90 degrees.

6.  A Second Spindle (option) - Doubles the throughput

A second spindle is valuable when machining small or long composite parts at high volume. Adding a second spindle doubles the machine’s throughput without increasing its footprint. Depending on the machine manufacturer, take it a step further by adding up to 8+ spindles for maximum part production.

7.  Two Tables (option) - Continuous operation

Similar to a pallet changer for a traditional CNC machine center, a second table allows an operator to load composite material onto, or unload finished parts from, one of the tables while still machining on the other table, so production never has to stop. Additionally, on some machines, two independent tables can be electronically “locked” together to process extra-large parts.

Dual Process -  Combine a multi-spindle option with a multi-table option to unlock the ability to perform what is known as Dual Process machining. With this technology, one spindle performs an operation on one of the tables while the second spindle performs a completely separate operation on the other table, essentially turning one machine into two in the footprint of one machine.

8.  Return on Investment - Planning for the long haul

The cost of the CNC machine matters, but more importantly is the return on investment. A cheaper or lesser machine that fails to meet the unique challenges of machining composites and that has to be replaced after only a few years can cost a company more in the long run, not just in direct costs but in lack of part quality, production downtime, program delays, and added frustration. As John Ruskin says, “There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man’s lawful prey.” It is worth the time to work with experts in the field to ensure the machine being purchased is the very best for the specific composites manufacturing application.

9.  Support and Service - Questions to ask before buying

Asking the right questions provides an understanding of the service and support offered by an original equipment manufacturer (OEM) or dealer. Before making a purchase, some key questions to ask include:

• How many dedicated CNC technicians are there (not for other machinery)?
• Is there 24/7 support for emergencies?
• Is there remote login support?
• What is the lead time on replacement parts?
• What is the lead time on replacement spindles specifically (in the event of a crash)?
• At what point do machine parts become obsolete and are no longer made or carried?
• Where is the service department located?
• Where are spare machine parts manufactured and/or stored?

10.  American Made vs. Imports - Some food for thought

Buying an American-made CNC machine means:

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that really depends on the type of work you are doing. The 4th is just another tool like a circular saw. I could tell you a bunch of cool ways to use your circular saw while framing a house but that wouldn't help much if you use yours in a cabinet shop breaking down sheet goods. You dig? No one can help you if you don't even vaguely define the problem. A circular saw?? I think of a 4th as the other half of a 3 axis mill. As said it depends on what parts you make. I like to put a bunch of parts on a fixture/vise held between the 4th and a tailstock so I can hit all 6 sides of a cube in 2 ops and swap fixtures with 20 seconds of spindle down time between cycles, but that's for the parts I have always made. I work in a job shop so I make be making a round part or I might be making a square part. Not to worried about setting up round parts I feel like I am decent at that. The square/rectangular parts is where I struggle with. I made a fourth axis trunnion and I have a faceplate I can bolt my vise to but I struggle at getting it either one of them to work right. There's nothing really special about running a 4th axis. It just opens some really handy doors that you wouldn't have otherwise. It can reduce the number of ops required by giving access to multiple sides in one setup. It can effectively make your table larger by using a tombstone which has more surface area than your table. And it allows more flexibility when 3D contouring with a ball nose. How best to use these capabilities is up to your particular machine and parts.

There's nothing really special about running a 4th axis. It just opens some really handy doors that you wouldn't have otherwise. It can reduce the number of ops required by giving access to multiple sides in one setup. It can effectively make your table larger by using a tombstone which has more surface area than your table. And it allows more flexibility when 3D contouring with a ball nose. How best to use these capabilities is up to your particular machine and parts.

I agree, you can keep it as simple as indexing for simple 3+1 operations, or have a much better contouring program for parts that require high levels of detail on several sides. You may get more help from the forum if you choose a part you need to run and ask for input on how to process the part. In a basic sense it gives you some of the capabilities of a millturn with a lot less rigidity which is something to keep in mind.

I work in a job shop so I make be making a round part or I might be making a square part. Not to worried about setting up round parts I feel like I am decent at that. The square/rectangular parts is where I struggle with. I made a fourth axis trunnion and I have a faceplate I can bolt my vise to but I struggle at getting it either one of them to work right.

You say you struggle twice but don’t describe the trouble. Without knowing what aspect of 4th axis usage you’re struggling with, it’s hard to give advice.

You say you struggle twice but don’t describe the trouble. Without knowing what aspect of 4th axis usage you’re struggling with, it’s hard to give advice.

When I was using the Face plate and i would try to drill it would push my part out of the vise. With the trunnion I try programming it where I do not need multiple Offsets and anytime I would rotate my part up to put drilled holes in the end it would be off.

When I was using the Face plate and i would try to drill it would push my part out of the vise. With the trunnion I try programming it where I do not need multiple Offsets and anytime I would rotate my part up to put drilled holes in the end it would be off.

The first example is a workholding issue, not really a 4th axis issue. As noted above, dovetailing is a good technique.

Are you programming to center of rotation? Dynamic fixture offset? Work coordinate tracking macro? Using a CAM software and which? So many unanswered questions. For 3-jaw round parts, it's straightforward as you pointed out.

With prismatic parts, it's not difficult, but it can be finicky and tedious. If you're struggling to get the workholding dialed in perfectly on center, then your next best bet is to set different work offsets for every index position.

My general opinion is that using a 4th axis rotary in some cases is a huge waste of time. Employers would be money ahead biting the bullet and buying a trunnion like the TRT210 with the WIPS/DWO option. It'll save you hours on every job.

The first example is a workholding issue, not really a 4th axis issue. As noted above, dovetailing is a good technique.

Are you programming to center of rotation? Dynamic fixture offset? Work coordinate tracking macro? Using a CAM software and which? So many unanswered questions.

I program to the center of rotation and I dont use a dynamic fixture offset or Macros. I use Bobcad V27 I do not have the multi axis capability unlocked so all i can really do is 3+1 stuff

I program to the center of rotation and I dont use a dynamic fixture offset or Macros. I use Bobcad V27 I do not have the multi axis capability unlocked so all i can really do is 3+1 stuff

If you are programming to center of rotation and have positional errors at indexing then it's typically one or both of these errors....
Your center of rotation is not accurately set.
Your tool length offsets are not accurately set.

What is the most accurate way to find the center of rotation?

Vancbiker had a good suggestion for this but if you are working from a 3 jaw chuck you can perform the same with a piece of round bar and cutting a few flats.​

You could also just true up a piece of ground stock or even the shank of a busted endmill and indicate to find center.​

Once you find the center leave that as your Y zero in g55 or g54 and never change it. I have to remove my 4th for big work from time to time so I write it on the back of the control with a sharpie to at least give me a very close starting point for reindicating or to recover it if I accidently reteach that Y. Once set use a different G5x for anything not 4th axis related. For us G54 is always the 4th and G55 is anything mounted to the table. My vertical machining center came with a 4th axis that was almost unused with untouched paint, Now I understand why. it's too large, a 10 inch rotary table is painful to use when your trying to hold small parts. The biggest issues are clearances and long workholding and toolholders required. it weighs 280 pounds and requires the crane to load, and most of the 40 inch table to needs be emptied of vises except a single 6 inch at the end. so it's inconvenient to do anything else with the machine when the rotary table is loaded, otherwise you would have a collision when trying to reach the table like normal. They also can take a while to tram and align properly when you are doing long parts that would need a tailstock.

I might use a 4th axis a lot more often if I had a small 6 inch one that could be picked up by hand and held inside a Kurt vise, or a 5C lever collet closer indexer that was Fanuc servo controlled. But because I am wanting to sell this mill next year I am hesitant to purchase another smaller 4th axis for it.
Instead I just use a manual Hardinge 5C indexer for most work because it's faster to setup for small quantity jobs.

After asking other shops their advice for 4th axis setups they gave a simple answer, get another VMC and leave it permanently setup only for 4th axis work.