Roughness Plate Testing

My roughness test set up using a Mitutoyo roughness tester.

At Desktop Metal, I investigated how to get different roughness for various machining processes. The company’s production series printer uses binder jet technology to print metal parts. Therefore, it is important that the base, which has parts printed on top, is consistent and stable. However, we didn’t know whether the base plate’s roughness affected the print quality. So, my goal was to understand how to control the average roughness (Ra) of a piece of stainless steel and whether it affected print quality.

Generating Roughness Patterns. Please Wait . . .

A close up of the piece I machined for testing machining processes and surface roughness.

The planned machined surfaces on top of the printer’s baseplate.

First, I took a piece of 17-4 stainless steel and machined four surfaces, two on each side. I would place the piece in a milling machine and have a fly cutter with one-bit cut knurled surfaces. I would have the cutter move at a fixed speed but changed the feed rates. The slower the feed, the more separated the knurling cuts were. Since the cuts were circular the edges of the surface had a lower Ra than the centerline. In addition to knurling, I applied various grits of sand paper using an orbital sander onto the surfaces.

What’s The Roughness?

Average roughness of a knurled surface versus how far away the sample length was from the centerline.

A close up of the print bed with a machined surface. The other half was left blank as a control surface.

After machining, I used a roughness tester from Mitutoyo to understand how roughness changed depending on how close the needle of the tester was to the centerline of the knurled pattern. For 12 tests I took a minimum of 30 measurements. From the tests, I found the average roughness was directly proportional to the feed rate of the fly cutter during machining. Furthermore, machining surfaces yielded Ra values above 1.5 microns, while any form of sanding yielded Ra values less than 1 micron. After I finished collecting data, I worked with Desktop Metal’s machinist to machine surfaces onto the printer’s bed. After it was installed into a prototype printer, technicians printed several metal parts. They found no significant change in each part’s density and strength when printed over different surfaces.

Previous
Previous

Lift Cart - Desktop Metal

Next
Next

Other Projects - Desktop Metal