Tuesday, January 30, 2018

Plywood and Fiberboard Sheet Goods

This post discusses many different types of plywood and fiberboard sheet goods.  These are engineered wood products made out of wood plies or wood fibers. Types of fiberboard include particle board, medium-density fiberboard (MDF), and hardboard.

The great majority of the material featured on this page are available from Fingerle Lumber in Ann Arbor, Michigan. Others can be purchased from All American Plywood in Detroit, Michigan.

Note: For information on hardwoods and veneer please see Wood Basics.

Plywood

Plywood is a composite material made up of thin layers or "plies" of wood veneer. The plies are glued together with the wood grain of adjacent layers rotated relative to one another. There are several important benefits to the cross-grain orientation of the layers:
  • It reduces expansion and contraction of the panels due to moisture changes in the atmosphere. This provides much improved dimensional stability over hardwood.
  • The strength of the panel is consistent across all directions. 
  • The wood is less likely to split when nailed or screwed near the edges of the parts. 
Below are some videos describing the process of making plywood:

Plywood Grading

Plywood grades are established by the APA – The Engineered Wood Association (the initials APA come from the associations previous name of the American Plywood Association). In the United States, there are four basic plywood veneer grades: A, B, C and D.

Each plywood sheet will have two grades, for example AC. The first letter is the grade of the face veneer, and the second is for the back veneer grade. Some plywood sheets have a third letter, X, that designates them for exterior use.

A Grade: This material is the highest quality. It is sanded smooth, and is paintable. Some neatly made manufacturer repairs are acceptable, however you can generally expect the material to be free of repairs and knots.

B Grade: This material is a solid surface with some repairs, usually football-shaped patches and/or wood filler. It may contain tight knots up to 1 inch in diameter, however no chunks of wood are missing. There may be some minor splits.

C Grade: Contains tight knots to 1-1/2 inches in diameter with knotholes to 1 inches in diameter. There may be some splits and discoloration.

D Grade: This is the lowest grade and the material may contains knots and knotholes up to 2-1/2 inches in diameter. Some splits are likely present. Generally no repairs have been made to the material.

Baltic Birch Plywood

This material is commonly used in CNC routed projects. The wood species is Birch and it is sourced from the regions surrounding the Baltic Sea. In the case of the material we get in the Taubman Fab Lab it comes from Russia.

It's a veneer core material meaning all the plies in the sheet are free of voids. Well in theory... as you can see in the examples below there are sometimes small defects which you can see in the cut edges. In general however you won't find any large voids as you would in lesser grade stock.

There is often a superior face side and a lesser side. The non-face side may contain what are known as footballs. You can see one in the lower right of the piece below. It is a patch placed into a defect area of the veneer. It looks better than seeing a knot, but it does break the flow of the grain across the sheet. Keep this in mind as you are designing - if both faces are visible in your final product you may want to position your parts on the sheet for cutting as to avoid the area with footballs.

Paint Grade Birch Plywood

This is a low-cost product which accepts paint well. Generally the face veneers are not as clean in terms of the grain but they are free of defects so the paint application will be smooth.

CDX Plywood

CDX refers to "C-D Exposure 1". The faces are graded C and D and the glue used in the plywood is exterior glue. While the material is moisture resistant it cannot be exposed to outdoor conditions for an extended period of time. This is a low-cost material commonly used in construction. 

Radiata AC Plywood

This is a 7 ply material with thick exterior plies. The A grade side is high quality and has a marked grain pattern to it, similar in appearance to Pine.

Face Veneer Plywood

Below are a few examples of plywood with pre-finished veneers or lower quality material in the internal plies with high quality hardwood face veneers.

Pre-Finished Baltic Birch Plywood

This is a very high quality material composed of 13 thin plies. Both faces are pre-finished so it is ready for use immediately after cutting. The edges are free of any significant defects (voids).

Red Oak Veneer Plywood

This material has 5 plies and a Red Oak veneer on both sides.

Birch Veneer Particle Board

This material has 5 plies and a Birch veneer on both sides.

MDO (Medium Density Overlay) Fir Plywood

MDO is an exterior grade plywood with smooth, brown medium-density plasticized overlay. It's good for accepting paint and resisting exposure to moisture. The painted surface won't show any grain.
This material is available with the overlay on one side or two. 

Other Sheet Materials


Below are some videos describing the process of making Medium Density Fiberboard (MDF) and Oriented Strand Board (OSB):

Medium Density Fiberboard (MDF)

MDF is an engineered wood product made by breaking down hardwood or softwood into wood fibres, combining it with resin binder and a small amount of wax, and forming panels by applying high temperature and pressure. MDF is generally denser than other sheet goods such as plywood and particle board. This material is heavily used in the furniture industry.

Ultralight Medium Density Fiberboard (UDF)

This material is a light-weight version of MDF. It is much more porous than MDF and so is often used as spoilboard material in CNC routing applications. UDF allows the vacuum pressure from the CNC machine's vacuum table to be transferred right through itself to hold down the material resting on it. It can be planed flat over and over to restore a smooth surface. 

Melamine

Melamine has a compressed wood core, similar to particle board, which is covered with a resin and paper finish. Melamine is often used for cabinetry when a durable, smooth, wear resistant finish is needed.  The top coat is smooth and resists moisture. It's not as durable as a layer of plastic laminate (the melamine surface is much thinner) but is often used as a low-cost substitute. Sharp cutters are needed as edge chipping is sometimes an issue.

Particle Board

This is an engineered wood product manufactured from sawdust, small shavings or wood chips combined with a binder, which is then pressed and extruded. Particle board is cheaper, denser and more uniform than plywood and is used when cost is more important than strength and appearance. Particleboard can be painting or covered with wood veneers to improve the appearance. Though it is denser than conventional wood, it is the lightest and weakest type of fiberboard.

Birch Veneer Particle Board

A singly layer of Birch veneer is applied to a particle board core.

Maple Veneer Particle Board

This material has 5 plies and a Maple veneer on both sides. 

Oriented Strand Board (OSB)

Oriented strand board (OSB) is an engineered lumber formed by compressing layers of wood strands in combination with an adhesive (usually a resin that has been coated onto the flakes). OSB has desirable mechanical properties that make it suitable for load-bearing applications in construction. It is also sometimes used in furniture construction where a strong, low cost material is needed.

Cedar Flake Board

Flake board is another name for oriented strand board. This material is composed of strands from Cedar trees. It's strong and has the pleasant smell of cedar. It is used in closet lining or chest lining applications.

Sound Deadening Board

This fiberboard product is lightweight and porous. It is used as wall and ceiling material in noisy environments to absorb reflected sound waves to decrease the overall volume level in a space. 

PVC Azek

This material is different from all the other materials in this post because it does not contain wood fibers which can promote mold growth. It is strong and lightweight. It can be cleaned with soap and water. The material is color-fast and stain resistant and is extremely scratch resistant.

Bending Material 

Some types of plywood or fiberboard are designed to be used in applications where the material needs to follow curves or be bent into shapes. The material is classified into barrel type (4' edges bend) or column type (8' edges bend).

Bending Plywood

1/4'' Neatform Bendy MDF


Sunday, January 21, 2018

Tolerance Issues in Joint Fit

This post is a discussion of tolerance in getting parts to fit together well. It particularly deals with laser cut joints, water jet cut joints, and CNC routed joints.

Material Thickness

In fitting joints together the material thickness is a key parameter. You must not assume the thickness as advertised is correct. You have to measure it for yourself. It's a good practice to measure the material in several places because the thickness can vary across the piece. For a good fit you need to measure to 3 decimal places of accuracy using a digital caliper.

Fit Offset

Parts cut to exactly the same size will not fit together. Why? Because the cuts are not perfectly smooth, nor perfectly accurate. There needs to be a little extra material removed to allow them to slide past one another into place. To give this quantity a name we'll call it the Fit Offset. It varies based on the machine and the material.

Kerf

The amount of material removed by the cutting process is called the Kerf. For example if you are cutting a sheet of plywood on a table saw, the saw blade removes material. Often, a table saw blade is 1/8" thick... so that's the amount of material it removes. If you cut a 4'x8' sheet of plywood in half to get two 4'x4' sheets, either one or both will be less than 4'x4' because a kerf width of material is removed.

Laser cutters and water jet cutters also have a kerf although it is much smaller than a table saw blade.

To visualize the kerf we'll use this simple box joint as an example. On the left the parts join edge to edge. On the right they join perpendicular to each other.

In the drawing below the black lines show the part edges which are drawn at exactly the same size. The kerf is exaggerated and drawn in red. The laser itself is drawn as a red dot.

You can see how cutting right along the centerline results in two parts that don't fit.

You need to compensate for the kerf in the design of the joint to get a proper fit.

You can use a feeler gauge to measure the kerf in laser cut joints. Simply run a test cut of a line and measure the width of the kerf. In the sample below the kerf is 0.015".

Make sure when you cut you use the same machine and same power settings. Otherwise the kerf is sure to be different.

Laser Cutter / Water Jet Cutter - Kerf Compensation

The laser cutter and water jet cutter have a relatively small kerf. The amount is very dependent on the specific machine and how well it's adjusted. A typical laser kerf is between 0.005" and 0.020".

Water jet cut kerfs in our lab at Taubman are usually 0.030" to 0.040".

There are two possibilities for compensating for the kerf:
  • Cut centered on the line. In this case the one-half of the kerf width is removed from the part. So the drawn part needs to account for this difference. It needs to be wider by half the kerf. Then when cut the part winds up the correct size. 
  • Cut offset from the line by the half the kerf. In this case the part is modeled to its desired size and the compensation is done in the CAM software rather than in the CAD model itself. 


CNC Router - Kerf Compensation

CNC routers typically are set to cut with the tool to one side of the cut line rather than centered on it. They automatically offset half the width of the cutter so the edge cuts exactly on the line. Therefore the kerf is not a factor in the size of the parts. However, the Fit Offset still needs to be used to get proper fitting joints. For CNC cut joints I usually use 0.005" offset per side. So for a mortise and tenon joint the tenon is 0.01" smaller than the mortise (0.005" is taken from each side for a total difference of 0.01").

The other issue with routers is that the cylindrical end mill cutters are not able to get into tight corners. Here's an example - the white circles represent the bit doing the cutting. It cannot reach into the inside corners.

The resulting cut material will look like this - the bit couldn't reach into the corners and thus they remain rounded. This joint obviously would not fit together.

In order to make the joint fit you need to route or drill out the corners. Here a 1/4" drill removes just enough material to clean out the corner for the mating part to fit.

After drilling the parts look like this:

Here's how the joint appears as the parts meet at a corner:

If you cut the parts with a smaller router bit, say 1/8" diameter, you can drill with a smaller bit as well.

This leaves a smaller gap where the parts meet:

Here's a comparison between the 1/4" end mill/drill on the left and the 1/8" tools on the right. The holes are much less noticeable on the right:

The CNC Corner Fix Utility I wrote for Rhino is useful for creating the geometry necessary to cut the corners out.


Monday, January 15, 2018

Autodesk Fusion 360 - CNC Cut Plant Stand

I've been interested in parametric solid modelers for years. A decade ago I started into Autodesk Inventor but got turned off pretty fast. At that time I could see the obvious potential, but found the software in use to be quite annoying!

In the last few years I've been using Rhino for modeling and Mastercam for the CNC toolpath programming (CAM software generates the code to move the machines to cut the parts). When I heard about Autodesk Fusion 360 I was very curious. It's a constraint based, parametric solid modeler. Constraint based means you can define relationships between the parts. And as changes are made the constraints keep the parts related to one another just as you'd like. Parametric simply means you can define variable values in the model and the model will update automatically when the variables (called parameters) are changed. CAM is built into Fusion. So when you change the model you simply have to regenerate the toolpaths and they are ready to go again. This is as it should be!

So I devoted the last few months to doing tutorials and practicing with it. I can summarize by simply stating I AM IMPRESSED. This is great software, very robust, with a slick, easy to use user-interface. And a pleasure to work with. I'm hooked - for future furniture design projects it's Fusion for me.

This post documents a small CNC cut project I made with it - a parametric plant stand. Here's a screen grab of the design:

The following parameters can all be changed and the model will rebuild itself:

A key parameter in the model is the material thickness. When you change it the model fully adjusts, including all the joinery and the stock size used to set the set relative toolpath depths. As the table is made from different thicknesses of plywood it can be measured with a digital caliper and that variable entered as the parameter value: 

Modeling

This section lists the sequence of commands used to create the parametric 3D model:

Because I wanted to use CAM I made each part its own Component. Components have their own work offset which means you can position them where you like.

So the first step was to right-click on the main component at the top of the Browser and choose New Component. This becomes the first leg.


I wanted to be able to angle the legs and control how far apart they were using dimensions. So I created a sketch on the ground plane and drew a line. Then I dimensioned the distance of the line away from the origin and the angle from the X axis.

Next, to get the leg profile sketch I used the Construct > Plane at Angle command and used the line drawn above to define it.

Next, I created a sketch on that plane. Onto it I drew the profile of the leg. I started with a Rectangle and drew a Spline for the curved face.

Note how the control points of the spline are dimensioned horizontally. This constrains them if I alter the overall width of the leg. I can still drag them vertically to change the proportions (or adjust the dimensions).

The height of the leg is a parameter as are the distance from the origin and the width of the leg:

The leg sketch is extruded. The Direction is Symmetric, this means it goes to both sides of the sketch plane. The Distance uses the thickness of the material (MtlThickness). This gives the leg the necessary heft (twice the material thickness).

Next up is to create a tenon on the end of the leg. This could be done in the sketch above. But I choose to do it as a separate sketch as viewed from the top of the leg. I used the Sketch > Project / Include > Project command to bring in the edges of the leg top. Then I offset these inward to create cut outs.

The sketch is extruded. Note how the Distance is set to 2/3 of the material thickness. So the tenons will leave 1/3 of the material in the top. The Operation is set to Join so the tenons attach to the leg body.

Once the leg is created they are mirrored about the origin twice to get all four legs in position.


Next the stretchers are created. This starts, as usual, with creating a New Component for them. Then an Offset Plane is used to specify the height from the ground for them. My adjusting the parameter for this plane offset I can move them up and down the leg easily.

The centerline curve for the stretchers are parametrically linked to the leg edges. They are constrained to always hit the midpoint of the leg edge, and are held perpendicular to it. To do this the leg edges are brought into the sketch using the Sketch > Project / Include > Project command. Then a construction line is drawn perpendicular to the leg edge. The centerline curve is drawn as an 3-Point Arc where the endpoints are constrained to the midpoint of the leg edge and tangent to the construction curve:

In this way, as the legs rotate and move in or out, the curve remains perpendicular to it.

Next the cross section of the stretcher is drawn. The plane chosen is the inside face of the leg. This is drawn as a Center Rectangle. The width is set at 1" and the height is set to the material thickness parameter:

The result is a curved stretcher which spans between the legs and meets them perpendicular to them:

I wanted through tenons on the stretchers. So a new sketch is created on the end of the stretcher. The tenon is offset on only 3 sides (not 4). This allows it to be CNC cut but without having to flip the material over and route from the other side.

As before, Sketch > Project / Include > Project is used to bring in the stretcher edges. Then the Sketch > Offset command is used to offset the 3 sides.


The Extrude command is used to create the through tenon. The Operation is set to New Body so the tenon can be mirrored to the other end of the stretcher.

The edges of the tenon are chamfered (Modify > Chamfer). Then tenon is then mirrored. Then the 3 bodies making up the entire stretcher are mirrored across to create the stretcher on the other side:

I wanted to repeat this feature on the top of the design. So the Move/Copy command was used to duplicate them higher up the legs:

The next step is to create the top. I created a new component, created a sketch on the top of the leg (not the top of the tenon) and started with a Center Rectangle. Then Sketch > Arc > 3-Point Arc was used to create the curved sides:


The top was extruded by the material thickness. Then Modify > Combine was used to Boolean out the tenons at the top of the legs from the underside of the top. The edges of the top are chamfered.

There is also a lower shelf. Very similar idea to the top. Offset a plane from the top of the stretchers. Sketch on that plane, extrude and chamfer the edges.

The stretchers are cut from the lower shelf to form curved dados for the stretchers to attach. In plywood this is fine. In hardwood there's a cross-grain glue-up issue here. Not a problem - I'm making it from plywood:

Next step was to Boolean out the tenons from the leg. This is a simple Modify > Combine operation. The operation is Cut and Keep Tools is checked so the tenon doesn't disappear:

The leg is extruded twice the thickness of the plywood. By splitting it in half it can become two separate components. These can then be laid out and routed without any clearance holes. This is done with Modify > Split Body:

There are a number of ways the above few steps could have been done. For instance the Boolean Cuts could have been repeated to every leg. I chose to simply turn off the original legs and mirror the finished leg components again - easier.

A stock block is created - this is the sheet good material the parts will be cut from. Simply a rectangle extruded by the material thickness. In my case the material was Baltic Birch plywood. The parts are laid out for cutting using the Assemble > Joint command, once for each part.

The CAM side of Fusion will be covered in a separate post.

Fabrication

I cut with 3 tools - 1/4" brad point drill, 1/4" downshear endmill and 1/4" upshear endmill. In this image the downshear was the first pass on the contours and pockets to eliminate any tearout when the cuts are complete with the upshear. You can see the pockets for the through tenons.

Here are the parts after some quick sanding.


Here's the dry-fit. It came out great. Not too loose, not too tight. It's being held with rubber bands on the bottom of the legs.

There are a few things I chose to fix up manually (that is not using the CNC). I wanted to chamfer the legs, stretchers, top and shelf edges. This is easily done on the router table.

I also didn't want the look of drilled out corners where they are visible. So instead of drilling these out at the CNC I used the bandsaw to remove that corner material. The only place where this is the case is on the through tenons on the end of the stretchers. This is such a small amount of material that it was removed in a second long touch to the band saw blade.

Here it is glued up, after some sanding, with a coat of gray stain applied:

Overall I'm happy with how this turned out. I'm going to make a different version of the table with changes to the parameters to play with the proportions. I'll document the layout and CAM side of Fusion in that post.

I've wanted to be able to quickly tweak the proportions of my designs for years. It's so satisfying to easily change everything and respond to it visually.