Tuesday, February 25, 2020

A Rare Book Display Stand.

I was asked by our college librarian if I could design and build an original stand she could use to display large rare books on. Once she provided overall dimensions for the stand I set about sketching out a few alternative ideas.
Initially I wanted to use Northern Hard Ash, but after finding a couple rough Ash boards in the college wood storeroom and planing them flat and clean I discovered they were riddled with bug tracks. My local source for hardwoods would also not sell any Ash for the same reason.
While I don't usually care for white oak thanks to its busy face grain and color, I bought 26 board feet of it for this project. I ripped it into 1" wide strips and turned them 90 degrees to glue them into quarter grain panels for most of the stand parts. After the glue dried I scraped one side flat before running them through the planer. The resulting planks look beautiful showing medullary rays and far more interesting color variation than the original face grain ever did. The Probotix.com CNCs I oversee in the college shop were used to cut the edge and joinery details needed to create the parts and assemble the stand.
The top of this stand is 25" deep by 45" wide. Most parts are made from layered quarter sawn white oak. It is held up by two 3" x 2" posts connected with 4 bowtie tenons through-cut in their top end grain. The tenons are slim waisted which increases the contact area between mortise and tenon, and also resists racking of the joint which can lead to loosening.

As this is a stand for books I added a couple of famous quotes about books/reading on the visible faces of the two stretchers.The librarian also requested that the stand would have a shelf to keep a box of cotton gloves on.

Rear view. Shelf to hold boxes of cotton gloves.

I could find no better words than these about the value of reading.

You can see the varied grain patterns in the top.

The design is a result of my pursuit for simplicity in structure.

The 2 columns of 4 bowtie tenons are endgrain of the posts peeking through the top.

Photos were taken in the finish room of the fabrication lab where I teach. They were taken right before I delivered the stand to the college library.


Wednesday, June 13, 2018

A Simple Box, with Four Different Corner Joints.

This is an assignment sample I made for our beginning Workshop One class.  I designed it to show the students different ways to connect wood at a right angle.  All the joints on this example were cut using my CNC.  Simpler versions of each corner joint can be done using more conventional tools in a woodworking shop.  To show them what a CNC can do if they think out-of-the-box each corner has a feature that would be very difficult to do with conventional tools.

Miter.  The conventional way to reinforce a miter joint would be to use a spline or biscuits to cross between the two halves.  Using my CNC I've made a hidden alternating mortise and tenon row that disappears inside when the joint is closed. Much easier to glue up as this joint won't slide in any direction when clamped.

Tongue and Groove.  Normally a simple tongue in a simple groove is enough to make a good corner joint.   Using the CNC I've added a row of 5 small through-tenons that center the joint and add some aesthetic detail to this corner.

Box Joint. The CNC enhancement for this row of fingers was to make them each a different progressively larger width.  Tough to do using any other jig or tool, but fairly easy to draw up for the CNC to cut them.

Shallow Rabbet.  Conventionally we would reinforced this with screws covered by wood plugs.  Using the CNC I simply cut 3 round tenons from the end grain of the short side to replace the screws and plugs.   The end look is the same. The tenons align the joint.

The bottom is 1/4" (6mm) Baltic Birch plywood.  Rather than install it near one edge of the box, I'11 put it 1/3 of the way up from one end.  This will divide the box into two compartments with one twice as deep as the other.   Flip the box to access each.  The lids won't open unless they are on top. Twisting latches slide into the lid to keep it in place when it is on the bottom. A shallow perimeter rabbet on each lid aligns it to the box. When on top just twist the latches then the top can be lifted out.


Monday, July 3, 2017

Dowel Splice Joint

I've got a closet in my spare bedroom (storage room) that I wanted to add another clothes hanger rod to.  Among my scrap pieces I had two 1.25" diameter closet rod sections that together would be long enough, but neither was long enough on their own.  The distance to span was roughly 22". 

This is a CNC-cut splice joint I came up with to solve the problem.   My test cut done on smaller scraps is shown in the photos.  I'm calling it my radial finger joint.

The finished spliced rod is now loaded with clothes and is handling the weight with no complaint.

To hold the dowels on my CNC I used my adjustable angle clamping jig set to vertical with a vertical scrap of plywood clamped to it.  The dowels were trapped/clamped against the jig and edge.


Saturday, December 31, 2016

Low Profile CNC Table Vise

I often need a way to clamp small parts on my CNC bed without using clamps that hold them down from the top.  In the past I've been able to use a manufactured vise borrowed from my drill press table, but it is rather tall and my CNC doesn't have a lot of Z axis clearance.

This prompted me to come up with this low profile vise:

All the wood parts were cut out on my CNC from 1/2" (12mm) and 3/4" (18mm) baltic birch plywood. The bottom plate that holds the moving jaw down is from a scrap of old 3/16" thick plywood.   Follow-up procedures included drilling pilot holes for screws that hold the main parts together, and tapping threads in a drilled out 5/16" hole for the 3/8" x 16tpi threaded rod I used.

I've added some 100 grit sandpaper to the jaw clamp faces for more friction than the plywood edge itself has.  In use it now holds anything tight enough that when being cut by the CNC it doesn't move. The four bolt holes were spaced to align with the t-track slots in the bed of my CNC.  At the moment I use a box end wrench to turn the end where two nuts are jammed together.  I embedded a vertical steel post in the moving jaw for the threaded rod to push against.  It also helps keep the plywood from splitting.   Brass screws were used to hold all the parts together, as well as to keep the individual plywood parts from splitting.  The CNC will just mill through the brass screws should it accidentally encounter them.

I've posted a file that includes all the vectors I used in .CRV (Vectric's VCarve) format on their forum site:  http://forum.vectric.com/viewtopic.php?f=2&t=24361#p189439.  Direct download link: http://forum.vectric.com/download/file.php?id=53303

If you are interested in making your own feel free to use my file as is or modify it to fit your CNC's bed mounting/clamping system.   I know I'll get considerable use out of mine.

The software I used to cut the parts out is Aspire from Vectric.com.  Aspire is their most expensive and full featured product, but they do have lower priced options that would have been fine for drawing up and cutting out this project.

Speaking about Aspire, it was recently upgraded to version 9.  Version 9 has enhanced drafting features, and a much improved rendering engine to show how your parts should look after being cut.  below are two images of the base plate of the vise, all done in one file and once rendering.  The new and most appreciated feature is being able to show what 2-sided cuts would look like together.
Bottom side.

Top side.

2/18/2018 - Update for the bed vise:   I've taken it apart and re-assembled it so there is no more conflict with the hold-down bolts.   I also CNC'ed a slot for a 3/8"-16tpi square nut and the nut slid in there tight.   I drilled out the wood threads and now am confident the vise won't strip out in use. Steel threaded rod through a steel nut now.   I found that square 3/8"-16 tpi nut in a coffee can full of assorted hardware living in my garage shop.  Now it has a home.

Saturday, November 19, 2016

Original CNC-Cut 3-way Interlocking Mitered Corner Joint.

This joinery sample was inspired by several 3-way miter joint solutions posted on YouTube.   A common one you can find is a solution with three unique parts that slide together using normally hand cut tenons into machine or hand cut mortises  Side one tenoning into the second, with the third part using two tenons to lock into the previous two. All the clever cutting hides inside the mitered facades. Optionally a tenon end or two can peek through the outside faces.

My goal was to come up with one unique part I could make three times using the same file on a CNC router.  They have a single tenon protruding inside their miter-cut shell. The core of the joint is a floating cube of wood with three mortises through it. Each of the mitered parts slide into one face of the cube.   The mortises in the cube are cut using the same single tool path file, with the cube rotated to a different face for each of the three cuts.

When assembled no end grain is showing. There is no external clue of what goes on inside this joint. When clamped together all parts bottom out against the cube making it easy to pull together tightly with no slipping of the mitered edges.

The tool path files for this joint can be scaled to any size within range of the router bits available.    This single corner example was done in 6/4 oak with a 1/4" diameter end mill to pocket around tenons and a 1/4" diameter ball nose bit  for the mitered edges.   I've made a 1/4 scale model of an 18" parsons table using this joint.  A 5/16" long 1/16" diameter end mill was all I needed for all the tiny tool paths required.


Friday, May 20, 2016

Simple Desk Design. Tenon Array Joint.

My local Menards store had 1.5" thick butcher block birch counter/bench tops on sale recently.  I needed a new desk to replace a plastic topped desk that I use as a computer desk.  The plastic top rests on a metal tube base, but has started to sag and crack under the weight and heat from my PC.

I previously came up with a tenon array joint that makes a great connection between legs ends and table tops. The 1.5" thick hardwood is plenty thick for a good joint. All I would need would be something with a little cross sectional area to use for legs to support it. No stretchers required.

I've used cedar 4x4 posts in the past as legs for shop benches and my CNC cart.  With a little picking through of what was available I found some straight and clear 8' cedar 4x4s to use.

I wanted my desk to be 29" to the top when done.   I cut 4 leg sections roughly 28.5" long to allow for 1 inch tenons to penetrate the top.
   My CNC just barely had the capacity to hold the 48" x 25" top.  I used it as shown to cut the four mortise arrays each centered 1/5 of the length and width from the edges of the top.
The legs were first cut at a compound angle to tilt 4 degrees in the long direction and 2 degrees in the short direction.  Then I clamped them into my compound angle clamping rig and cut the tenon arrays with the CNC.  I made the tenons .004" smaller than the mortises for a slip fit and some room for glue.

I left the top clamped to the CNC while I tested the fit of the joints. I made a mistake when cutting the end on one of the legs, so I had to recut it a little shorter.   The tenons ended up .75" long and the mortises I cut .8" deep.  The legs slipped in with just a little wiggling back and forth. They come out using the same technique.

 If the legs would not have fit easily I could revise the CNC file for the mortises and recut them a little larger.   I didn't need to in this case.  All legs fit fine.

I haven't glued the legs in yet.  They need some sanding and finish, which will be easier with them detached from the top.  The outward flare of the legs give the desk a more solid stance, with less tendency to rack or twist than if they were vertical and all parallel.
The 3.5" x 3.5" mass of the legs has room for and begs a more interesting profile.   In use though I'll never see these legs. The birch top will look great with some clear finish on it.   I may paint the legs white or black to make a less distracting setting for the top.

I used my CNC to chamfer the bottom of the legs. With the bottom plane at a compound angle each corner bevel is slightly different.  A little hard to see in the photo below as most of the 1/2" chamfer sinks into the carpet.  The chamfer should keep the leg bottoms from getting snagged/worn as the table is moved/scooted around.


Monday, May 16, 2016

Improved Light Bracket for my Overhead Camera Rig

My first light bracket design hasn't failed. That I can perceive how they would fail has moved me to come up with a new design that eliminates the weaknesses of the first. This is not an uncommon happening as I work through the design of a thing.  Making the first prototype is more about being able to discover where an idea might be weak than being done with the design.

This version was cut from the same scrap of red oak as the first were. I removed much of the initial potential mass in the first design. This version removes only the outer bevel where no forces would pass through. The beveled edge was cut with a 3/16" end mill using the fluting toolpath that is available in Vectric's Aspire software.  The side hole was drilled out with a #7 drill bit then tapped for 1/4-20 threads.

The center cone spreads force from the screw head across the bottom area against the rig post.  The side bolt simply threads into the initial mass for strength.

The lights now do a great job of illuminating whatever I want to make a close-up photograph or  video of.  Next for this camera rig I'm working on an improved camera hanger bracket.   So far I'm not satisfied with my first or second ideas. The second idea is shown in the last photo above. It worked, but not as well as I hoped it would.   That tells me there is a better solution out there.  Now just to find it.