Pseudo Tensegrity End Table

Held stable with only tension straps.

Most tensegrity projects have a rigidly connected post up from the base and another that projects down from the top.  They are connected by a simple tension cable. An unstable connection.  To stabilize this usually 4 more cables run from top to bottom at the outer corners. The end result is barely stable.  An unexpected side push will usually collapse such a table. 

My project starts with a rigid post that is not attached structurally to either the top or the bottom.  An unstable assembly. Both ends are rounded over and sit in a slippery socket at the top and bottom centers. The center post is threaded.  Right hand threads on one end and left hand threads on the other.   

Bottom and top of the center post rounded over.

Screw holes patched with walnut plugs.

Holes repaired

The ball ends of the post rest in sockets cut in HDPE that inset into the bottom of the table top and top of the base. 

Socket test.

A good fit.  Post spins easily in the slippery socket.

Hub nuts screw onto the post. They have slots around their edge. Webbing straps will slip into the slots and be pinned in place with a 5mm pin though a grommet in their end. 

With both RH and LH threaded hub nuts, a twist of the post will tighten or loosen the straps. I cut the threads using the threading toolpath available in Vectric.com's Aspire software and a side cutting V bit. I hook the straps to the underside of the top and the hubs in a slot with a pin dropped in a hole or slot.  The pin passes through a grommet in the ends of the straps.  The straps end in a slot perpendicular to the pin. 5mm x 5/8" steel pins.

Strap connection to hub nut.

Shelf pins pin the strap end in place.  I made a friction fit hole for the shelf pins so they fit snuggly and will not drop out easily.  It took some test holes to find the best size of drill bit to use.  A #9 drill bit provided the closest fit. 

The top has 4 perimeter slots for straps, and an inset HDPE section with a ball socket cut in the middle.  Socket test seen above.   

Underside of the table top.

The key to this project is how the straps are connected to make the project work. 

The non-threaded section of the post is where you grasp the post to twist it.  It might be easier to grasp and twist if it's shape was a hexagon or octagon cross section.  

For detail continuity the base is a scaled down version of the top. The contoured surface is on its top rather than its bottom. 

Base

I made both top and bottom inserts from some 1" thick HDPE I have, re-sawn to just under 3/4" thick, then milled down to 5/8" thick on my CNC. 

Socket inserts for the top and base.

A  test assembly.  Straps were installed with the nut lower on the post.  Once attached the post was twisted to lift the hub nut higher to tension the straps. 

Repeated for the top.

Repeat the straps and hub on the other end to hold the table top securely horizontal.

Making straps for the design I've realize matching strap sets is a hit or miss problem. It is easy to make two or more the same length.  As those in the long axis aren't the same length as those in the short direction, and all have to be a precise length to tighten up at the same time. Finding the length of a set to go with the other set is the challenge.  Making and installing the set for the long direction would be easier if they could be adjusted in length after installing.  Change them from being a fixed length with a grommet in both ends to an adjustable length with a grommet in one end. The other end wrapping around something to come back and attach to itself.  The attachment point should be adjustable to change the total length of the strap.  Something I'll look into should I use straps in future projects. 

The wood parts of this table need some final sanding and a finish applied.  I like tung oil on walnut.  This table does provide an interesting glance down at the base while in use.  More interesting than any typical table. It was an enjoyable exercise to design and build. 

Straps in tension make a stable assembly.

4D

 

Making a Push Button Depth Stop for my Wen Drill Press

I've made push button releases for a few past projects. In every case they made adjusting or attaching/releasing much easier to do.  This post is about how I make one to serve as a quick setting depth stop for my benchtop Wen drill press.  This is an initial prototype. Part of what it is for is to show me if it works well.  Any flaws that show up will be noted and fixed in the following prototype.  Test, update, and test again.  

Depth Stop

My Wen benchtop drill press currently uses a knurled nut you have to spin up or down on a M12 threaded post to set a stop. Tedious. For the push button upgrade I'll need an M12 fine tap.  I ordered this one from Amazon.com:

M12 fine tap

The depth stop works by pushing a threaded section against the threaded post. Push the button in and the threads are pushed back so the post can slip through the depth stop body. Release the button and the spring pushes the thread back against the post to stop it from sliding up or down. 

To make the push button return when released I'll need a small conical compression spring. I found this one that should work at McMaster-Carr.com:

Conical Springs

A 12mm drill bit is needed to make a hole so the button can slide easily up and down the 12mm post. I ordered this one from Amazon.com: 

12mm drill bit

To drill the start hole for the tapped threads I'll need a 10.8mm drill bit. I couldn't find a 10.8mm bit, but 27/64" is very close to 10.8mm.  I already have a 27/64" bit so I'll use it instead. 

27/64ths

For the prototype depth stop parts I'll use some HDPE. I have some on hand. I've chosen HDPE for this project because it is easy to machine, and somewhat self-lubricating. 

I started with a 1" thick scrap of HDPE. Milled an area down to .75", then cut the 12mm shaft hole and outer shape. 

Depth Stop Body

I removed the bar from the CNC to tap the hole. I trimmed off the bridges using a flush trim bit in a hand held router clamped in my bench vise.

The front edge of the depth stop now needs a 5/8" wide elliptical hole, 0.7937" deep for the button.

HDPE Body

In the bottom of the 5/8" hole another smaller hole .0.42"d and 0.1288" deep is needed to center the conical hinge. For precision I clamped the depth stop body vertically to pocket those holes using my CNC. 

The button itself is the key part to make this work. I cut the elliptical button from a 1" thick HDPE block. The steps to make it included:

1.  Start by drilling the 27/64" (10.8mm) hole through the side.  My low profile vise held the HDPE blank while this hole was cut.

2. Tap the 27/64" hole with the M12-1.25tpi fine tap. 

3. Next to, and overlapping 1/2 of the tapped hole drill or pocket out a 12mm hole though the blank. There is 0.121" between the centers of the tapped hole and the 12mm hole. 

4. Flip the block up 90 degrees, then use the CNC to shape the outer end of the button and cut the ellipse perimeter from the blank. 

5. Cut the button to final length to release it from the block.  I used my compound sliding miter saw to cut it. 

I made the button and hole it slides into an elliptical shape rather than a simple cylinder. This keeps it aligned with the post. The long axis of the ellipse is horizontal. There is no width to sacrifice given the 12mm hole is cut through it. 

Once assembled this prototype works as expected. It keeps the depth stop from moving up or down when the button is not pressed in. When the button is pressed in the stop can be easily moved up or down the post. 

On my Wen drill press

The one flaw is due to the post having a flat face rather than being threaded all around.  If the post was completely threaded then the depth stop could be fine adjusted by spinning it around the post. With a flat front the threads inside the depth stop disengage when spun to the front. Keeping the depth stop button at the front provides approximately 0.05" per step adjustment. Slightly less than 1/16" (0.0625")

For Aesthetic reasons only I'll likely put a small chamfer around the top and bottom edges of this depth stop. For the next prototype I'll see if I can make one from aluminum. 

For a more useful stop a shaft with 16 threads per inch would allow 1/16" per step adjustment.  A half turn of the button would move it 1/32".  A quarter turn would allow 1/64" step precision. 

Comments and question always encouraged and appreciated.  

4D