June 15th 2015 was World Elder Abuse Awareness Day, a day set aside to help raise awareness of elder abuse, elder care, and ways to identify, treat, and prevent abuse of elders in our communities. June 15th also fell during a week proclaimed by the President of the United States as a National Week of Making.
Victor Cardenas is a member of the Alamance Makers Guild who also works in Adult Protective Services at Alamance County Department of Social Services (DSS). He became aware of plans by DSS to purchase a park bench that they planned to paint purple (the color for World Elder Abuse Awareness) and donate to the Kernodle Senior Center here in Burlington to raise awareness and as the climax of a half-day long seminar on Elder Abuse. Victor suggested that the Alamance Makers Guild might be able to design and build a unique bench using CNC Router technology that would be a better value for the money than a mass produced bench purchased at a big-box store.
Victor came to us with the idea during one of our busiest times of year and we knew that most of the work to design, build, and paint the bench would fall during a week when we were also going to be traveling to Washington DC to attend the kickoff for the National Week of Making and also exhibiting for two days at the first ever National Maker Faire. During the week of Making we were also producing an event at Alamance Community College (a talk about the Maker Movement) and producing the first ever "Burlington's Maker Takeover" an event that was as large and complicated as our first Burlington Mini Maker Faire. In spite of all of these obstacles we felt this was a worthy project and that we were up to the challenge!
Learn more about the project, design, and solutions to problems we faced in this video:
Step 1: Designing the Bench
The criteria given to us by DSS for the bench were:
- Buddy Bench that could comfortably and safely seat two adults (therefore 4 feet long)
- The bench had to have a back (not just a flat bench)
- Color had to be Purple (for World Elder Abuse Awareness Day)
- The bench should survive outside in the elements and not require a lot of maintenance
- DSS would pay for materials (budget of around $250 or less, the less money spent the better)
- Alamance Makers Guild would design, build, and paint the bench
- The bench had to be ready by early morning June 15th 2015 (for their Seminar)
Other limitations that we faced:
- Time was our most limited resource due to all of the other projects and commitments we had going on around this date
- Time for design and approval
- Availability of AMG members to help out with the project especially with painting
- All the people most involved in the project would be in Washington DC from Thursday June 11th till Saturday June 13th (driving home late that night)
- The Weather... It was a fairly rainy and humid two weeks prior to the deadline of June 15th. All of our painting had to be done outside so we were at the mercy of the weather compounded with the rest of our schedule.
- 24" x 18" working area for the ShopBot Desktop (the CNC Router we have access to)... no part that we made for the bench could be larger than this area (and benches with backs average 28 to 30 inches tall)
- Use of PVC Lumber as the slats to make up the seat and back portion of the bench to avoid rotting and to reduce material costs. PVC Lumber would also reduce labor costs because it comes pre-finished with smooth edges and therefore no need to spend time to sand it. It also has a rough surface resembling wood grain pressed into its top surface that would give a nice slip-proof surface to the seating area. This lumber comes in 8-foot long pieces (simple to cut in half for two 4-foot pieces). The question, would it be strong enough?
The Design Process
- Research the dimensions and ergonomics of Bench design. We did this through a little online research and by taking measurements from existing benches.
We found that a typical bench has the following dimensions:Height at front of seat: 16" on average
Height at rear of seat: 14" to 16" on average (but typically slanting backwards)
Width of seat: 12" on average
Height of back (above back of seat): 12" on average
Width of 2-person bench: 4' on average
Base: Typically wider than seat with extra width at the rear of the bench (to prevent tipping over)
- Test the PVC Lumber for its ability to safely support the weight of two adults:We were planning a three leg design for the bench so that each slat making up the seat and back would only have two-feet between supports.We tested a 2-foot long scrap of PVC lumber that had the same width and depth as the pieces we intended to use for the design by placing it between two cinder blocks and standing on it with one foot (testing to approximately 190 pounds) and found that although the lumber had some minor deflection that it did not deform or crack.
The requirement of a 12" wide seating area plus the fact that the PVC lumber comes in 3.5" and 1.5" widths meant we could use two 3.5" slats (in the center) and two 1.5" slats (one on the outer edge and one in the rear) to make up the seat. If one 3.5" piece could support 190 pounds then 2.5+ times this width should be able to offer support for up to 475 pounds (more than adequate for an average adult) We felt comfortable with the use of this extruded PVC Lumber as the material for seat and back slats.
- Translate our initial thumbnail sketch ideas for the bench + design criteria into actual geometry for an initial design + estimated material costs + physical model:I set up Vcarve pro CAD/CAM software for a blank material size of 24" x 18" x 3/4" and started to play around with our design criteria to see how to best utilize the material to cut out base/leg segments and how to support the back of the bench. The limit of 24" cutting width meant that the legs and back support could not be machined out of one solid piece. I decided to go with a split design where the seat slats would go through mortise joints on the base/legs and through L-shaped supports for the back support.Glued-up pine boards are available in lengths of 6-feet in widths of 18". This is important because we need to work with the strength of the grain of the wood in the design. We knew we could cut these pieces into three nearly 24" segments times two boards to make six panels. This meant to 3/4" wide leg supports for each end and the middle of the bench.In laying out the slats I kept a 1/4" gap between pieces (at their maximum gap) and made sure that the height of the edges at each gap was equal (so that there would be no raised edge to catch on clothing). I then varied the angle of each piece to make a sculpted curve that would hopefully be more comfortable to sit on and lean back against than a flat surface. I also tried to make sure that none of the pieces would be parallel to the ground (at zero degrees) so that water would be more likely to run off and not pool on slats when it rains.
Once I'd worked out the basic geometry and layout of the pieces I printed out the 2D design onto paper, taped those sheets to a piece of cardboard, and cut out pieces with a hobby knife to make a scale cardboard model for the approval of DSS. This step would have been easier with access to a laser cutter by printing the file to a PDF and then cutting out of a material, saving me labor.
Based upon our design we knew we would need the following bill of materials:
2 - 18" x 6' x 3/4" Pine Boards (each cut in thirds making a total of six leg/base supports)
3 - 3.5" x 8' x 3/4" PVC Lumber Boards (each cut in half to make five 4-foot long slats)
2 - 1/5" x 8' x 3/4" PVC Lumber Boards (each cut in half to make 3 Slats and 1 Base Stretcher)
75 - Brass #10 Wood Screws (ranging from 1.5" to 2.5")
White outdoor primer paint
Purple outdoor finish paint
Total cost would be less than the $250 maximum budget. Our concept, model, and budget were approved by DSS.
- Refine the approved design into a final CAD design plus CAM (Computer Aided Manufacturing) to create toolpaths that the ShopBot Desktop could use to cut out the pieces.
I added a few details to the CAD design for the bench pieces including:
An oval cutout in the center of each base/leg piece to help reduce the mass of the bench and make it a little more visually interesting
Added tenons to the bottom of the legs so that PVC feet could be added via a mortise joint. These feet will support the bottom of each "sandwich" of two base/leg pieces and will provide a rot-resistant surface to place in contact with the wet ground. Adding these feet also allows for a few inches of increase to the width of the base of the bench to help prevent tip-over risk.
Added 1" holes to the bottom of the outer pieces for each base/leg that can be used as a place to run a hold down chain to prevent unauthorized movement of the bench.
Added a through-mortise slot in the base/legs for a 1.5" x 3/4" stretcher that will go across the bottom of the bench and help to hold the bench square.
Added hold-down holes for machining purposes. These holes are placed at strategic points around the design so that they will be clear of any cutting action by the CNC machine. These screws will keep the board in place as the CNC machine removes material.
Added a hold-up piece to some of the waste area of the boards that would later be used to hold pieces off the ground while painting the bench. This material would go to waste otherwise so why not make use of it for the painting and assembly of the bench?
All of the pieces of the bench are identical except the two outermost base/leg pieces. These pieces have stop-mortises to fully enclose the seat slats instead of through-mortises like all of the other pieces. This required the creation of an additional cutting file for the left-end piece and the creation of a completely different file for the right-end piece. The right-end piece was mirrored left-to-right so that it could be cut out with the correctly facing mortise pockets. I set up the cutting paths as modular jobs so that I wouldn't have to make a cutting file for each of the six boards, but instead could use different operations in order to make the pieces I needed. I printed out a check sheet that listed the six boards and exactly which of these tool paths would need to be cut (in order) for each of them.
Step 2: Cutting out the Pieces with the ShopBot Desktop
Cutting out the Base/Legs and Upright Back Supports
- Install the Spoiler Board and Plane it Flat:Since we would be cutting through material it was important to place a spoiler board onto the ShopBot Desktop's bed in order to protect the ShopBot's bed and the cutting bit from damage. I used a 24" x 24" x 1/2" thick piece of MDF Particle Board as a spoiler. Even though the ShopBot Desktop has a maximum cutting width of 18" there is more than enough room to mount a wider board on the machine. I had used this particular spoiler board previously for another project so I used a 1/2" planing bit to plane the surface flat (taking off a few thousandths of an inch) prior to machining any real parts from the board. We weren't doing any engraving or height sensitive work in this project but its still a good idea to start off from a known flat surface compared to the travel of the cutter tip. This process takes a few minutes.To plane the spoiler board first zero the X and Y axis of the machine (using the built-in limit switches on the ShopBot and the built-in routine in the ShopBot control software). Next move the cutting head to the rough center of the spoiler board and use the Z-zero plate to establish the top surface of the board. There is a board planing routine in the ShopBot control software that will generate the G-code (Open ShopBot Code) file necessary for planning your spoiler board flat. This routine simply asks the width and length of the board, and the desired planing depth.
- Mount the blank material to the spoiler board establishing safe hold-down:For this project we are cutting pieces out of a larger blank of material and there is a good 1/4" margin around the cutouts around pieces so down to 1/1000" tolerance on material placement isn't critical. I placed the blank board on the spoiler board at the origin position and roughly parallel to the X-axis of the machine.The next step is to place the cutting bit into the spindle of the ShopBot. For this project I used a 1/4" diameter 4-fluted flat end mill with a 1" cutting depth. A single fluted cutter would have produced larger chips and less sawdust but I didn't have one that was capable of cutting through 3/4" handy. Bit RPM and machine cutting rate (Speeds and Feeds) are important to figure out for a project, but I got good results with the standard settings associated with my cutting bit.There is no need to re-zero the X and Y axis since we've already established the 0,0 origin in the last step and haven't powered off the ShopBot desktop between steps.
You do have to re-zero the Z-axis since we've removed material from the spoiler board in the last step AND since the machine/control-software have no way of knowing how thick your blank material is.
Once the machine is re-zeroed for Z I ran the cutting file for the hold down screws. This cutting file peck-drills the surface of the board to 1/4" at the location of each hold down screw. A very fast operation. There is no need for serious hold down methods for this operation because there is no lateral force on the blank material AND the mass of the board itself is enough to hold it in place along the Z-axis for these simple peck-drilling operations.
Once the ShopBot had finished its operation I used a hand-drill to drill pilot holes through the blank and into the spoiler board. I then attached the blank material to the board using 1.5" long drywall screws.
- Run the cutting files (in order) to cut out the pieces.Again there is no need to re-zero the X and Y axis because we've already done that in the previous step and all cutting paths will work relative to the origin we've established.There is also no need to re-zero the Z axis because we've already established the top surface of the blank material in the last step.We've added tabs to all profile cutting paths where parts will be cut totally free from the blank material. These tabs are not cut by the machine and instead stay in place to keep the part you are cutting out from being able to move once all of the rest of the material holding it to the blank is machined away. Without tabs the final cut-through on a part might catch the edge of the part (especially small and light ones) and cause the damage to the part or to your cutting bit.
Once the machine finished cutting out each piece the hold-down screws are removed and the blank (with parts still in place thanks to tabs) is removed from the spoiler board so that the tabs can be cut out by hand liberating each part from the blank.
The process of hold-down-screws and then running the cutting files was performed six times (once on each of the six blanks to make the six base/legs and the three upright back supports)
Cutting out the "Special" Pieces
There were two sets of "Special" pieces for this project. Special because of how they had to be cut out. The first special cut was a pocket for one of the back slats that would later receive a plaque engraved with information about the bench and a link and QR-code back to this very article. The challenge... the back slat is 4-feet long and the ShopBot Desktop CNC Machine can only machine pieces that are a maximum of 24" long, the pocket has to be placed in a precise location along the width of the board leaving an equal margin on top and bottom. The solution... something similar to the idea of "plating"
- Create a pocket cut-out on the spoiler board that can be used to hold the back slat cantilevered out from the front edge of the machine. This tool path will cut a pocket from near the back of the cutting depth of the machine (18" away from 0 on the Y axis) and 2/3 of the depth of the spoiler board. Since the PVC lumber is very dimensionally stable its possible to measure its width to an accuracy of thousandths of an inch and then cut the width of the hold down channel with a very small clearance so that the board can be held in place by friction.
- Create the geometry for the cut-out for the pocket on top of the same coordinates as the hold-down channel and create tool-paths in Vcarve pro.
- Zero the Z-axis to the top of the spoiler board and cut out the hold-down channel pocket.
- Place the 4-foot long back slat into the hold-down channel pocket (a tight friction fit) and make sure the length of the board sticking out of the front of the ShopBot is supported. This step might require some sanding along the outer wall (furthest from the Y axis) if you haven't left enough clearance or due to tolerances in the machine.
- Re-zero the Z-axis to the top of the back slat board.
- Cut out the pocket tool path for the plaque.
The second set of "special" pieces were the feet made out of PVC Lumber. The design of the bench left behind a 4-foot long section of 3.5" x 3/4" slat that I planned to use as material to make feet for the bench. The feet would each have a two mortise joints (cut out as pocket tool paths) to house the tenons on the bottom of each base/leg piece. Additionally the mortises had to be 3/4" apart and parallel to make up each "sandwich" panel for the legs (which are made up of two boards with the upright/back support sandwiched between them). I also wanted to cut a radius to relieve the corners on each foot piece and a curved back for the rear feet.
Cutting ON a part (at a precise location) is different than cutting OUT a part from a blank, i.e. the exact location of the blank matters when cutting ON and isn't as critical when cutting OUT. To solve this problem for the feet I used a technique similar to the channel hold-down for the back slat.
- Create a pocket in the spoiler board for hold-down of the front and back feet blanks. (the front foot is narrower than the back foot). The pocket is set to 2/3 of the depth of the spoiler board and machined to the precise width of the PVC lumber blank. The length of each pocket is determined by the desired length of the part which is then cut manually on a chop-saw to this same length.
- Create the geometry and toolpaths for the rounded corners and the mortises in Vcarve pro at the precise locations of the pocket hold-down geometry.
- Add the blanks to the pockets (sanding or adjusting the pockets as necessary for a snug fit) and then reset the Z-zero for the ShopBot Desktop at the top surface of the blanks.
- Remove the blank from the spoiler board.
- Run the tool path for the profile cuts (cutting away the corners and adding the oval-cutout to the back feet) without the blanks on the board. This is necessary to remove additional material from the spoiler board that accounts for the kerf of the cutter while making these cuts. This avoids problems of encountering different densities while cutting out the PVC later and avoids pushing any MDF sawdust into the PVC material.
- Place the PVC Blanks into the spoiler board and run the cutting file (first the mortise joints and then cutting away the corners)
Step 3: Finishing and Assembling the Bench
- Use a carving knife (or wood chisel) to cut off the tabs left behind from cutting out the bases/legs and back-supports.
- Sand the wooden pieces down (getting rid of the "fuzzies" left behind by the router bit we chose to use.)
- Test fit the pieces by assembling the bench.
- Drill pilot holes for the brass screws that are used to hold the bench together. Each 3.5" slat (back or seat) gets two screws and each 1.5" slat (back or seat) gets one screw. Use a counter sink bit to make a counter sink for the screws head so that the screws will sit flush to the boards. Pilot holes are necessary to avoid splitting any of the lumber. Since this operation is done at right angles to the pieces they must be done by hand, as a result each screw hole is uniquely placed. Make sure to keep track of which seat/back slat goes in which location.
- Paint the pieces of the bench with a primer coat. We painted all the wooden pieces with a white base coat (on all surfaces including inside mortise joints) to seal the wood against moisture and the elements. We painted the PVC seat and back slats using the finish purple paint because we determined that the PVC's natural white finish and non porous surface wouldn't require the extra sealing white primer coat.
- Reassemble the bench. For this step we ran into a slight problem. I hadn't allowed enough clearance in the mortise joints to account for the addition of primer paint to both the joint and the seat/back slats. I wanted a tight joint but the end result was a little too tight. We were able to avoid sanding out the paint by using a rubber mallet to gently tap the seat and back slats into place. To avoid this in the future I'd add a larger tolerance to the mortise joints expanding them by several thousandths of an inch.
- Finish Paint the Bench with Purple Paint. We painted all surfaces except the bottom of the PVC feet (did not want to leave paint to scuff off on the pavement depending upon the final position of the bench). We made sure to get into all corners and areas to provide a seal against the elements. We ended up putting two coats of paint on the bench.
There are many more annotated photos describing each step of the process of designing and building the DSS Elder Abuse Awareness Bench at the Alamance Makers Guild Facebook Page in This Photo Album. (should be open and view-able to everyone, even folks without a Facebook account, just click-through any sign-up notices)
Bonus Video: Bench Dedication and Elder Abuse Awareness Day Seminar
In the video below Ben Harris (founder and director of the Alamance Makers Guild and primary designer of this Bench Project) talks briefly about the process of building this bench and the importance of Community Maker Projects during the Bench Dedication at the Kernodle Senior Center in Burlington on June 15th 2015 (Elder Abuse Awareness Day)
While at the Kernodle Senior Center on June 15th Ben was able to record some of the Elder Abuse Awareness Seminar presented by Alamance County Department of Social Services for local law enforcement officers, healthcare professionals, government officials, and other interested members of our community. The information in the video should be interesting to anyone who wants to learn more about Elder Abuse and ways to identify, treat, and prevent it.