Drilling and Tapping

p/n 33-D209 Rev A

Drilling & Tapping

It is often desirable or necessary to post machine a 3D printed part for many reasons including:

  • Correcting design mistakes without having to reprint the part.
  • Adding metal inserts for improved strength and robustness to repeated assembly and disassembly.
  • When the design calls for it for any practical purpose.

In these cases, it is common practice for 3D printed MJP materials to be post machined. The following diagnostic was created to demonstrate such capabilities.

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Drill/Tap, Press, & Screw Diagnostic (Black – Projet 5600, Clear Projet 2500 M2R-CL)

From left to right the following features are shown: 1) Drilled and taped hole with hardware, 2) Press fit precision dowel pins, 3) Screws added to 3D printed injection molded bosses, 4) Press-fit brass inserts, and 5) machined precision holes.

 

Drilling and Tapping

All the rigid & engineering materials for MJP printers can be machined, drilled, and tapped. These material are excellent for such applications. The VisiJet materials can be treated like traditional thermoplastics (acrylic, polypropylene, polycarbonate and/or ABS).

However, the 3D printed material is a rigid or semi-rigid thermoset and will not melt or easily gum-up due to heat  that is typically found with thermoplastic materials such as ABS. Regular high-speed twist drills can be used successfully. Slow to medium speeds

are typically preferred. When done correctly, chips can be formed with sharp and clean entrance and exit holes resulting is a wide range of extremely accurate holes. A standard tap can be used to create finely threaded holes for standard machine screws.

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Upper:  M2R-WT drilled with traditional bit and hand tapped threads

Lower:  M2R-CL slower speed drilling of small hole and higher speed drilling of larger hole

 

When drilling, jogging of the bit to remove chips and release heat is recommended.  The pictures above show dry drilling, but typically normal cutting fluid is recommended (water or water-soluble oil).    For drilling, be especially careful as the bit enters or exits the plastic.  As the bit enters the plastic it creates high compressive pressure and/or can grab the plastic causing fracture.  Also, the final cut through a part results in a very thin section of material that is under bending forces and is susceptible to chipping if too much pressure is applied.  Therefore, the slowest speeds are often required when entering and exiting the hole. 

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                                Clean and sharp entrance and exit holes drilled with standard screw bits

 

 

 

Standard practice of using smaller diameter drill bits to pilot drill the hole is also recommended.  A pilot hole will also reduce the tendency of the bit to wander and improve drilled hole accuracy.  A simple trick is to design a small hole into the part at the locations requiring post machining.  This is useful for alignment or locational purposes even if a larger hole will ultimately be drilled with a traditional machining process.  This has and equally important benefit of locating the holes without the need for complex measurements. 

Although standard high-speed drills may be sufficient for machining operations on 3D printed MJP plastics, it is often advantageous to use special plastic drill bits.  The relatively flat tip on a normal drill bit can aggravate entrance pressure and exit bending deflections of the final cut that grab and fracture the plastic.  A plastic cutting drill bit has a sharper point and less pitch than a conventional drill.  These plastic bits will reduce risks of damage and/or allow higher productivity speeds and feeds.  These bits can be found in most hardware stores or online companies like McMaster-Carr. 

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Plastic Drill Bits

 

Other specialty drill bits for hard plastics have a diamond dust bonded to the tip surface. These drills can offer superior performance. Also, regular high-speed twist drills can be modified to reduce the tool from grabbing by grinding small flats onto both cutting edges so the bit cuts with a scraping action.

Traditional bits can also be ground to the desired shape.

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Standard screw bits can be modified for plastic printing

Finally, if a bit is less than sharp, this can lead to increased heat and will require a slower feed rate.  Sharp bits can be drilled quicker, but more bit retraction may be needed to let the material cool and remove chips.  Once the hole is drilled one can use a counter sink bit to relieve the sharp 90deg corner from both sides of the hole where there is a possibility of stress cracks or cracking during taping or customer use.  It is common practice to adjust the speed and feed of the bit as well as the entrance and exit speeds and the cyclic chip removal interval to achieve the best and highest productivity process.  This is true for 3D printed MJP plastics just like traditional plastic materials.  In any case, it is always recommended to spend some time testing the process with a scrap part if available before drilling the actual part

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The first column of the diagnostic was created with 3D printed holes that were mechanically tapped.

The fifth column of the diagnostic is simply a machine cut hole using standard screw bits and shows no entrance or exit defects.