Micromilling Techniques I: Securing Thin Plastic Workpieces for Precise Milling

10 Jun 2015

David J. Guckenberger¹, Theodorus E. de Groot¹, Alwin M.D. Wan², David J. Beebe¹ and Edmond W. K. Young^2,*

¹Department of Biomedical Engineering, Wisconsin Institutes for Medical Research, University of Wisconsin-Madison, Madison, WI, USA

²Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, MC313, Toronto, ON, Canada.

* Corresponding Author, E-mail: eyoung@mie.toronto.ca; Tel: +1(416) 978 1521

Why is this useful?

Micromilling is a highly efficient method for fabricating microfluidic devices directly in polymeric materials like thermoplastics. A recent review article highlights the use and relevance of micromilling in the field of microfluidics.¹While milling is most popular among machinists, the technique is becoming cheaper and more accessible to many others. Thus, there is a need for disseminating technical know-how for achieving quality micromilled parts. One common technical issue is ensuring flat work surfaces, and eliminating warping and bending of thin plastics during micromilling, which can lead to uneven milling and large errors in feature dimensions. Here we present a simple technique to achieve flat work surfaces – leveled to within 40 μin / in (.04 μm / mm)) – and properly secure thin plastic workpieces to minimize warping. Please refer to the review article by Guckenberger and co-workers for a primer on micromilling

What do I need?

Tools:

CNC mill (PCNC 770, Tormach)
3/8” set screw holder (#31820, Tormach)
Granite block (6” x 12” x 2” Grade AA, Standridge)
T-slot clamps (#32580, Tormach)
Drop test indicator (#24-315-4, SPI)
Wrench (choose appropriate size for clamps)

Materials:

Plastic workpiece (e.g., polystyrene, cyclo-olefin copolymer (COC), PMMA)
Surface protection tape (#6092A24, McMaster-Carr)
Two-sided or transfer tape (#9472LE, 3M)
Silicone rubber, 1/16” thick (#8632K41 – Durometer Hardness: 40A, McMaster-Carr)

Fig. 1 Tooling and Materials. (A) Hardware necessary for setting up the granite block. Assembly of the T-slot clamps is shown in Fig. 2. (B) (Top) Components needed for the drop test indicator: (i) collet/tool holder, (ii) aluminum adapter, (iii) drop test indicator, (iv) screw to attach indicator to the adapter. (Bottom) Side view of the assembled components. (C) Materials needed for adhering workpiece to the block: (i) surface protection tape, (ii) silicone rubber, (iii) transfer tape, (iv) polystyrene sheet.

What do I do?

Preparation: The dial indicator must be affixed to the head of the mill. To do so, mounting brackets can be purchased or custom-made, as we have done using a block of aluminum. This aluminum serves as a simple adapter between the dial indicator and the collet.

Step 1: Place granite block on the milling table. Note that a stiff yet compliant material is necessary between the block and the milling table to allow for minor adjustments. The particle-board feet affixed to the granite block by the manufacturer (Standridge) are sufficient.

Step 2: Assemble the clamps (Fig. 2A), and position the clamps in either a 3-clamp (Fig. 2B) or 4-clamp (Fig. 2C) configuration. The front of the clamp (i.e., the portion that contacts the granite block) should be the same height as or lower than the rear of the clamp.

Step 3: Hand-tighten each clamp, then tighten an addition 1/8^th turn using a wrench.

Step 4: Level the granite block by using the drop test indicator to measure the height across the entire block. The following steps are recommended:

a)      Lower the indicator onto the center of the block.
b)      Move the indicator along x-axis to the two ends of the block, and note the height of both ends.
c)      Position the indicator on the high side of the block and tighten the clamps until the height matches the low side.
d)     Repeat steps (b) and (c) for the y-axis.
e)      Repeat steps (b) to (d) until the block is level to desired tolerance.
f)       Tip: When using the 4-clamp configuration, clamps should be equally tightened in pairs to maintain levelness in the other direction.

Fig. 2 (A) T-slot clamp setup configuration. (B) Granite block configuration using three clamps. (C) Granite block configuration using four clamps.

Step 5: (Optional) Apply a protective adhesive film to the workpiece. This will simplify removal of the transfer tape from the workpiece (see Video).

a)      Peel a piece of protective adhesive film from the roll of film.
b)      Place a piece of silicone rubber on the roll of tape or other cylindrical roll.
c)      Place the protective adhesive film on top of the silicone rubber with the adhesive side face up, assuring the tape is flat.
d)     Roll the workpiece onto the piece of tape. The silicone rubber will provide compliance below the tape to prevent bubbles in the tape.
e)      Press the tape down to make sure it is fully adhered to the workpiece.