Wednesday, December 13, 2017
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Basics of making custom robot brackets and skeletal parts

—Cutting Sheet Metal with a Sherline Mill

Check out how Matt Bauer cuts and shapes metal pieces to make robot brackets. Click here for an ERB.zip file that contains .nc g-code files provided by Matt for the Sherline mill and a “how-to” .pdf.

—the editors

I am the proud owner of several robots that range from simple toys to advanced humanoids, and one of them is seldom given the credit it deserves, given its abilities—my Sherline Products 2010 CNC mill. This precision machining tool is responsible for all of my latest custom and prototype work, and it’s the workhorse behind some of the lighter manufacturing we do at Bauer Independents Ltd. The mill dramatically cuts down on the expense of having small quantities of parts machined or laser-cut elsewhere. And it is much more convenient to be able to trek on out to the shop, mill the part there and test the finished product not long after first drawing it on the computer.

Items needed*Sherline Mill

12x6x5/8-in. slab of 6061 aluminum

12x6x1/4-in. sheet of acrylic

12 in. x 6 in. x 1/8 in. sheet of acrylic

No. 8 drill bit (alternative, 7/32 in.)

No. 21 drill bit

10-32 tap

4, 10-32×1/2-in. socket-head capscrews

8+ 10-32×1/2-in. truss-head screws*I already

had these items; you can use substitutes

to meet your own needs. In all honesty, 5/8-in.

extruded aluminum is overkill; you could easily

get away with using 3/8 in.

For many, the greatest benefit of owning a CNC mill lies in its ability to cut three-dimensional parts out of stock materials that range from semi-rigid plastics to titanium. The stock is usually relatively thick in comparison to the part being milled, and it’s held down with step-block clamps or placed in a vise. Rook’s Pawn III, our competition-level humanoid, has a number of parts made of solid blocks of Delrin stock. His aluminum brackets, though, were all cut out of 0.040-inch-thick sheet and then bent on a box-pan break to form 3D shapes.

It doesn’t take much effort to flex metal sheet that thin, and it can be a challenge to prevent the end mill from lifting a sheet that thin. This article provides hints on how to hold down sheet stock while you cut parts out..

I have received several emails inquiring about the cutting method that works best for me. A good solution was to make a tooling plate specific to my mill that could hold very thin sheet metal flat and secure at multiple points.

I recently discovered that Sherline Products does indeed list a mill tooling plate in its accessory catalog (item no. 3560). A tooling plate is typically used as an easy-to-modify work table for mounting job-specific tools or clamps. They are also an inexpensive alternative to exposing your mill’s tabletop to damage resulting from machining mishaps. If the thin sheet you’re cutting falls within a 4×10-inch footprint, this accessory is definitely worth buying, or you can make one from scratch.

Making a Custom Tooling Plate

I start by finding the mill’s maximum travel—its range limits—in both the X and Y axes. The 2000 series mills allocate 7 inches of Y and 9 inches of X travel. These dimensions may vary according to your model and the way in which your mill is set up. I had a slab of 5/8-inch-thick, 6-inch wide 6061 aluminum lying around—plenty big for most of the work I do.

In this example, the maximum size of part being cut from one single sheet will reside within an area of 5.5 inch. by 9.0 inch. I measured 2 inches down from the top of the aluminum plate and 1/2 inch over from the left. This marks the center position of one of the capscrews that will hold the tooling plate on the mill’s table. From there, I measured 1 1/2 inches down and marked the second position. Then, I repeated these measurements, this time 1/2 inch in from the right side (see photos with diagrams).

Tooling plate in extreme negative-Y position.

Tooling plate in extreme positive-Y direction.

Preparing the sheet on the drill press: drill out hold-down

points using a no. 8 bit.

Using a no. 8 bit, I drilled through the plate at each of the four locations previously marked. To prevent the capscrews from interfering with material placed on top of the plate, I made countersunk holes by pocketing out a 0.15-inch radius to a depth of 0.30 inch around each of the four holes.

Note: In order to achieve cut lengths closer to the 7″ mark on a 2000 series mill, you’d want your first mark on an 8″ slab to be something like ½” down instead of 2″. Excessive overhang, however, can create unbalanced forward stress on the mill table. Over time this could create a wear problem. So, I’d suggest using a table of that size only when the moment absolutely calls for it.

I secure my plate to the table using four 10-32×3/8-inch socket-head capscrews fed through the plate and into the T-nuts. Then, by inserting the nuts into the T-slots, I slide the plate easily into place. I mounted it so that its left edge is flush with the table’s left edge. This helps to take the guesswork out of wondering where the plate was last positioned between setups, and I can maintain a consistent X-Y zero point. After squaring everything up, I tighten the screws.

Customizing the Tooling Plate

From here, the plate can be tailored to meet any number of needs. Depending on your project’s tolerance requirements, it might be in your best interest to further slab-mill or face-mill your tooling plate’s surface. If you frequently use cutting lubricants, perhaps pocketing a trench system to capture the excess runoff might be important. Threading holes to allow an easier setup of additional milling tools and accessories is always a plus.

Preparing the sheet using a template: mark the

hold-down points using a center punch.

I chose to engrave in a 1/2-inch-grid pattern outlining the travel range of my table. After removing the plate from my mill, I plotted a hole pattern that would best suit the types of parts I planned to cut out. Using a no. 21 bit, I drilled through the plate and threaded each of the 58 holes with a 10-32 tap. If I ever need an additional hole or two, I simply remove the plate, make the changes and slap it right back on.

I usually draw the parts I make first using a CAD/CAM program and I convert it to g-code from there. For quick reference, the table and all of its screw points and screw-head diameters are displayed in the background of each drawing. After drawing a new part, I can reposition each piece on the screen so that I can check that the tool’s path will not collide with any of the screw heads when I cut.

Marking the Y and Y zero with permanent ink instead

of a punch ensures that you won’t mar the sheet.

To further protect the new tabletop, I made an additional barrier between it and the material being cut. I made it out of an inexpensive sheet of 1/4-inch-thick acrylic, and it has as identical hole pattern to the plate’s. Again, this is just a personal preference.

Sheet Preparation and Cutting

I recommend that you make a template whenever you will machine multiples of the same part. Using clear acrylic makes it easy to keep everything oriented. Knowing the outline of the part I wish to cut out, I drill a corresponding hole pattern that best reflects the means of securing the sheet to my plate without causing interference. After I’ve drilled the holes, I put the template is over the aluminum sheet, and I mark the holes I wish to use as hold-down points using a center punch. I also set up my templates to indicate the X and Y axes’ zero points. To avoid marring the sheet’s surface, I lay down the zero-point locations with a permanent marker instead of a punch. It’s then over to the drill press where I open the hold-down points using a no. 8 drill bit. A quick de-burring of the holes, and we’re set.

Countersink the socket-head capscrews so that they

won’t interfere with the material placed on the

tooling plate’s surface.

Let’s talk about screws real quick. I gain the greatest advantage using multiple truss-head screws around the perimeter of the part to be cut. They have a very low profile and a larger surface area than most of the other screws have. Where tool clearances would be an issue, button-head or pan-head screws may be a better choice in those tight spots. Moving in the other direction you could add flat washers under, or a sort of miniature leaf spring between the screws to compensate for larger gaps.

The aluminum sheet after the parts have been cut out. Note the number of screws around the perimeter of the negative space that keep the sheet held fast to the surface of the tooling plate.

Note the grid lines cut in the tooling plate (shown here in two extreme positions).

With the sheet on the tooling plate, I first start all the screws before I tighten them in place. If you notice any bowing or deformation in the sheet after you’ve tightened them, you probably have a discontinuity in your hole pattern. Simply remove the sheet from the plate and further enlarge the hold-down screw holes using a larger drill bit. Then set up the mill as you would for any other mill process. Set each axis to zero and let it rip.

The part after being cut out using the Sherline mill.

After a quick filing and buffing, the part nowlooks like this.

The part is positioned in my box-pan break.

and bent.

I verify that the angle is square before bending the

opposing end.

The finished product. The part can now be

anodized, powder-coated, painted, polished, or

left with its natural brushed finish.

After you’ve bent and smoothed the sharp edges, beautiful custom brackets can be part of your robot

creation.

Sherline Products mills are exceptionally useful for making robot parts. Now you know the secrets of fabricating metal bracket parts.

Click here for an ERB.zip file that contains .nc g-code files provided by Matt for the Sherline mill and a bracket fabrication “how-to” .pdf.

 

Links

Bauer Independents Ltd., www.bauerindependents.com

Sherline Products, www.sherline.com