Musings from the Combat Pits

Computer-controlled foam cutter, Part II
See Part I | See Part III | See Part IV

By Buzz Wilson

This is the second of three articles on my quest to build a computer controlled foam cutter. See Part I. See Part III

I ordered the electronics kit complete with stepper motors. My initial plans were to assemble the mechanical piece of the project and save the electronics for the winter months. With the shop cleaned up and time on my hands waiting for props to cure, it is time to start building the beast. But wait a minute, I cannot build this per the plans; obviously I can make improvements.

Mechanical assembly

The design I chose called for mounting the stepper motors to hollow wall tool holders. Well, a custom product would work better. I used aluminum “L.”

The mechanical piece uses fence post and drawer slides. Home Depot did not have the fence post without the base. The first thing that had to be done was to put an abrasive cutoff wheel on my chop saw and cut the ends off four fence posts. Next step was to take two of these and assemble them as one unit.

Once these were together, then the linear motor was mounted to the bracket and temporarily attached to one end of the assembly.

Since I did not have the lead screws, I used a piece of 1/4-inch music wire and a piece of silicon tubing to hold it to the motor. Carefully measuring I set the dimensions for the tail bracket and drilled the hole.

Dimensionally everything lined up. This was repeated for the second side and then the two vertical towers. The assemblies were removed and set aside. Next came the attaching of the drawer slides to the assemblies. Careful marking of the post and drilling established the holes for the rivets. The drawer slide needed to have additional holes drilled in them. The problem is drilling and not getting metal chips in the drawer slides. I managed to do this, but had a problem on setting some of the rivets. I had to drill them out, and in doing so got chips in the drawer sliders. I tried to take the sliders apart and clean them, but to no avail. The brackets for the vertical towers have to be drilled and the backlash bracket and nut mounted.

The assembly is then mounted to the drawer slides.

As an engineer who worked on production improvements, I often calculated rates where I had to account for scrap and rework. This can get costly when it happens on your own projects. I managed to scrap two drawer sliders and three brackets. That is the bad news. The good news is the towers are together.

From the pictures of other projects, the foam cutters were set on a bench. I decided to build a dedicated table using T-Slot. I ordered this and assembled the table. Much to my dismay, there was considerable movement in one direction. No matter how tightly I torqued the bolts. The only solution was to weld the joints - defeats the purpose of the T-Slot but makes for a strong table.

I ordered the lead screws and couplings. I needed to make bushings for the end pieces. Nylon standoffs were cut down and pressed into the L bracket. I next took quarter-inch brass nuts, drilled them out to fit over the lead screw, and tapped them with a 4-40 tap. Using a 4-40 SHCS, I made a set screw to hold them to the lead screw. After using the machine, I placed a metal washer between the bushing and the brass nut. This was done on both sides of the bushing.

With the subassemblies complete, it is time to assemble them to the frame.

Electrical assembly

I ordered the electronics kit complete with stepper motors from Hobby CNC. This is owned by Dave Riggotti. Dave's son flies indoor and his brother flies stunt.

The kit recommends a PacTec case; off to the web to find a case. Now, when I build a kit of any kind I do not like to be given choices because I invariably make the wrong one. The case requires you to make decisions before you are ready to cut.

With electronics kit and case in hand, it is time to prep the case. The kit provides templates for preparing the case - front, back, and top. I ran copies of these, attached them with paper glue, and drilled them out. There are two large holes that require a little extra work. The first is the access hole for the printer cable connection and the second is the hole for the fan that mounts on top of the case. I drilled a pilot hole for these and used my Dremel jigsaw. This takes a little work because heat from the friction of the blade on the plastic wants to fuse the cut. I cut to the inside of the line and cleaned up the cut with a little sanding.

With the holes cut, I temporally mounted the switches and fan. The bottom of the case needs drilling to mount the circuit board. The case has been set aside and is awaiting the completion of the circuit board.

The circuit board kit comes with a bag of parts and step-by-step instructions. The board begins with installing resistors. I sorted all the resistors by color code. Once sorted by color code, I checked the resistance against the code. These were then placed into envelopes with the Step numbered on the outside. The workbench that I use for small projects has a glass top. I next sorted the parts by step and used a Sharpie to write the step number on the bench top. A magnifying glass and good task lighting are a must in this step. Although the instructions make reference to names that I was not familiar with, visits to Google provided information and a level of comfort with what I was doing. Before I started soldering the board, I pulled out one of the practice kits and practiced soldering. From reading post on the user group site, it appeared that the majority of the problems people are experiencing are from poor soldering.

To put the correct bend in the leads I used a small pair of pliers and began to assemble the board. The board is well laid out and the instructions are good. Things were going well until Step 4 which said a fan and heat sink is not required at 1.25 A or less Steppers. Well my Steppers are 2A. I looked through the instructions but did not see any specifics on an additional fan or specific instructions on heat sinks. There is a piece of aluminum and 4-40 screws and nut along with a note to use with fan when using 1.25 amp or more. Well not to be deterred on to the next steps. Step 25 gives you a choice to mount a LED either on the case faceplate or on the board. Well I had already drilled the face plate for the LED but the instructions did not say what gauge wire to use nor did they say how to mount the LED to the face plate. I did not see any problem with bypassing this step so on to the next ones. Onward toward completing the basic construction.

Upon completing Step 36, it is time to run some tests. The test called for applying at least 9 V DC. I was not clear whether a 9 V DC battery would be adequate; however, Dave confirmed.

At this point, it has taken me 8 hours to assemble the board. I now needed some additional information from Hobby CNC. I wrote an e-mail with my questions to the user group and got a quick response. The first had to do with the heat sink and fan. The heat sink is used with the driver chips. These are inside an antistatic protective bag and I had not looked at these. The instructions on the heat sink call for 1/8 inch holed located from 18 pin drive chips. Each chip has two mounting holes. The template inside the bag with the heat sink is not full size, but there is nothing saying that. There is a full size template on the sheet with the schematic diagram. Initially the statement about the fan was confusing, since a fan is installed at Step 4 in the General Construction Notes and the driver board is built under Step 3 of the General Construction Notes using its on set of instructions. Dave's response was that an additional fan was not required. Since I have been following a linear process, it was clear after looking at the chips and Dave's response that the fan being referred to is the one already included.

The wire size to connect the LED to the board is 28 ga. Off to find 28-ga wire - no luck at Radio Shack or the local electrical supply houses. Just when I thought I would have to buy a 1000-foot roll to get 6 inches of wire, I thought I would ask the local hobby shop. To my surprise, they had an entire rack of small gauge wire. As far as mounting the LED to the case, I will use Silicone sealant.

Here is the completed board.

See Part I of computer-controlled foam cutter article
See Part III of computer-controlled foam cutter article
See Part IV of the Computerized Foam Cutter project

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This page was upated Oct. 29, 2013