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7. Computer-Controlled Machining

This week I worked on how to design a table and cut by Shotbot.

Group Assignment

The design rules

  • Draw the table by Fusion. There’s something wrong with the legs. As leg lines were not straight or only single, and Fusion run slowly in my computer. So I started to use Rhino.

  • Enough space between lines that allow the end mills to go through, otherwise the lines could cover each other. We use Diameter 6.35 mill, the space could be above 6.5 mm.

  • As the end mill is cylinder, it’s hard to cut the 90 degree angle without breaking the production and losing materials.

  • The edge for X, Y axis is at least 30 mm.

  • Compared with Fusion 360, V Carve Pro is simple and easy to use and set the parameters.

  • We use the Veneer Plywood as the materials to cut. Its thickness is 18 mm, so the cutting depth can be 18.5 mm.

The end mills we use

  • For the ShopBot machine, we use end mills to cut materials instead of drills. As one drill( the left one in above picture) will move up or down during cutting the materials, and one end mill(the right one in above picture) will move left or right during cutting the materials.

  • The end mills have several shapes, including flat, ball and V-shape. Different shapes could used to cut different shapes and designs.

  • There are two kinds of flat end mills, they are upcut end mills and downcut end mills. The rotation direction of the two end mill is opposite. The chips produced by the upcut end mills come out from the top surface of materials, while the chips produced by the downcut end mills will fall down from the bottom of the materials. If I want to cut the veneer plywood, I use one downcut end mill for the top layers and one upcut end mill for the rest layers to keep the surface of the materials flat.

  • However, the flat end mill has its geometry limitation. As basic shape of the end mill is cyclinder, it’s hardly to cut the 90 degree cornor. So when I draw the design, each 90 degree is replaced by a curve. For example, a rectangle will change to a bone.

A cutting test

  • We cut one simple model with different feed directions and end mills with the same tool rotation speed (18000). During cutting ,we pause the machine, touch and check the chips produced to see whether the cutting process is good or not.

  • And we also use a micrometer to check the cutting depth of one model. If the depth is bigger than 6 mm, this is to say, the end mill loose and drop a little and the end mill need to fix again.

  • The final result shows that the conventional milling is much faster but rougher than climb milling.

  • We choose 3.6, 4.2 and 4.8 int/second for up cutting and find 4.2 int/second is good for up cutting. We choose 3.0, 3.6 and 4.2 int/second for down cutting, and find 4.2 intsecond is good for down cutting. The top layers of down-3.6-climb-cutting is pretty good. And the bottom layers of up- 4.2 climb-cutting is good too.

My design idea

I want to cut a table that can hold the computer and is small enough to put on the bed. I found a cute table online and draw the table in the Fusion 360. And my instructor suggested me to add support for the legs, and make the table more stable.

Then we found there’s any space for our legs to put through, so the legs are modified again.

Design Steps

VCarvePro—creating the toolpath files

  • Import the .dxf file in VCarvePro.
  • Input the values of Job Size and Material Thickness, the size of the whole plywood is 2440 mm * 1220 mm * 18 mm. And check the XY Datum Position is X 0，Y 0. And press OK.

  • Click”Toolpaths”, chose “2 D Profile Toolpath”, input the cut depth 6.0 for the downcut end mill. If you change to upcut end mill later, the Start Depth is 6 mm, and the Cut Depth could be 18.5~19 mm. Choose “Edit Tool”, open one dialogue, choose the right size of end mill, input the Spindle Speed 18000 r.p.m and Feed Rate 4.2 int/second, press OK. Then choose climb milling.

  • Choose “Add tabs to toolpath”, click “Edit Tabs ....”, right click the place that you want to put tabs and click “Close”.

  • Back to the original page and click “Calculation”, create the Toolpath and rename the files and save as .sbp files.

CNC machine - Shopbot

Steps

  • Fix the wood with nails, make sure the plywood is flat and very close to the plate.

  • Check the end mill, down cut mill for the first layer, and up cut mill for the rest layers. The way to switch the end mills is

  • Power on the ShopBot machine and the vacuum machine.

  • Zero X,Y by moving the mill to the starting point, and set X,Y zero.

  • Then set the Z 0 with auto-function.

  • Use VCarve Pro open the .dxf file and set the parameters for the down cutting, up cutting. Save the up cutting and down cutting toolpath separately as .sbp files.

  • Click cut part , choose the down & pocket cutting files to cut firstly.

  • After the ShopBot cut one shape once, click stop, and measure the depth of toolpath to make sure the end mill is fixed well. If the depth is bigger than 6 mm, this is to say, the end mill loose and drop a little and the end mill need to fix again. If everything is going well, clickresume.

  • After the downcut toolpath finish, change the downcut end mill to the upcut end mill, and continue to cut the rest layers. The final production：

My cute table

Problems & Errors

  • Choose the wrong files to cut and waste 140 mm * 1220 mm wood.
  • Choose the down cut for the 1st layer, but cut depth is 13.5 mm, which should be 6 mm. The productions I got is much more rough than it should be.

Files

CNC table

Last update: June 30, 2021