My versions of articulated hollowing jig *LINK*
Dave Paine
This post is longer than expected, but providing detail in case others want to try their hand at a similar jig.
In the March meeting of my local wood turning club, a member, Sonny talked to me about his future demonstration in Sept where he was going to show a simple hollowing jig with laser. He commented that he thought I would be able to suggest some improvements.
A wood version of a captured bar hollowing system.
After seeing the picture I offered to make an improved hollowing jig for Sonny to use over the summer and include in his demonstration in Sept.
I started work on the first version in mid-April. I did a few internet searches to find many of the various commercial designs, and some homemade designs.
I then decided on an articulated arm design. My aim was to make an inexpensive jig which was relatively simple to build with table saw and drill press. I ended up also using my metal lathe, but it should be possible to make some design changes to avoid the metal lathe at the expense of some flexibility such as accepting different tool diameters.
A couple of weeks later I saw this thread by John Lucas who also wanted to make an articulated design but was not sure of his skills on his Smithy lathe/mill.
See link below
I am including some of the details of how my design evolved. Most, perhaps all of my projects evolve during the design or build.
I wanted my design to use mount in the tailstock and use the Morse taper to hold the jig. This is to avoid the need to remove the tailstock for folks like me who do not have sufficient bed length to just slide the tailstock out of the way. I also wanted to avoid the cost of making a post and foot assembly to mount on the lathe bed. I feel the tailstock is more than capable of holding the jig.
A couple of commercial articulated arm jigs use the tailstock. One mounts around the quill, another mounts in the Morse taper, but does not use a drawbar.
Sonny has a Powermatic 3520B lathe with a hollow quill so can use a drawbar to hold the Morse taper in place.
To keep the material costs down and for a simple to construct design, I decided on a mix of wood, aluminium and some steel components.
Wood is used for the head, the base and an arm. Aluminium bar to connect between the wood components. Cut with a hack saw. Holes drilled on the drill press.
I thought about how to support the joints and allow smooth rotation. I wanted to avoid a design which required tight machining of a hole and pressing e.g., a roller skate bearing into the arm.
I decided to use thrust bearings in the rotating joints. The style I purchased from ENCO (now MSCDirect) consists of two ground washers and needle roller bearing. The design articulates easily - even under load.
This picture shows the thrust bearings in a test piece alongside the components - two precision ground washers and a needle bearing.
For the Morse taper, I used a purchased MT2 taper with 1/2in x 20 tpi screw which also was made with a tapped hole for 3/8in x 16 tpi drawbar.
I felt the 1/2in x 20 tpi thread was a bit smaller than I wanted to epoxy in the wood base so I made an aluminium bushing which was threaded for the 1/2in x 20 tpi screw. The screw thread was then reinforced by using epoxy to glue this into the inner thread of the bushing. The bushing was then epoxied into the wood base.
I drilled 7/8in dia shallow holes in the wood for the thrust bearing assemblies. Depth was just less than 1/8in. I did this to try and minimize the wood debris getting into the needle bearings.
I wanted everything to fit relatively tight to minimize "slop" in the jig. The bolts for example have a slight radius under the head. The diameter at the radius is a little bigger than the nominal shaft diameter.
I trimmed the underside of the 3/8in x 16 tpi bolt heads to be flat and to be the nominal diameter. In this picture you can see the trimmed bolts sit flat. The as-purchased bolts are a bit proud of the hole. A washer may have fixed this issue, but the washers I have are stamped and have their own issues of not being truly flat. Easier to fix the head of the bolts. I do not need the washers.
I prefer to try and use materials on-hand before dashing out to purchase things. I had a few feet of 1/4in x 1in aluminium bar which had been sitting in the shop for a few decades. A wanted to use thicker bar, but sometimes we need to make the design around what we have available.
This was my prototype test head. Drilled for 3/4in dia hole to accept bushings. A piece of 3/4in O1 drill rod for a bushing with set screw to hold the tool in place.
The hole in the lower right was for a bolt and barrel nut. This bolt applied pressure to a slit in the side intended to prevetn the bushing from rotating.
As with many prototypes, sometimes we learn what works, sometimes we learn what may not work as well as desired. This was the later.
I got the idea to make the bushings from 3/4in bolts. I drilled a 3/4in hole then marked out the hex pattern on the head and chiseled out a hex.
I then used the bolt and barrel nut to hold the bolt hex head into the jig head.
For this picture I screwed in the bolt by hand to hold the bushing, then slid the tool in the bushing. Once I mounted the tool in the bushing I found it was a pain to use a hex key to tighten the bolt.
I made a small bushing, epoxied to a 1/4in x 20 tpi bolt. Now this can be tightened by hand or with a wrench.
This was easier to use - at least when installing the bushing and tool. Sonny mentioned the hex bolt heads fit rather snug so are presently not as easy to remove as I planned.
In the above picture you can see I drilled and tapped the head for 3/8in x 16 tpi thread for a laser support. This wood felt to be sufficient density to hold the thread.
Sonny will be giving the jig back to me at this weeks meetings so I can fix the issues with the tight fit and improve the locking method.
This is main jig components laid out. The drawbar is a piece of 3/8in x 16 tpi all-thread. I made a bushing epoxied into the drawbar knob. I could have epoxied a nut into the knob.
I added a post in the middle of the two aluminium arms to increase rigidity of the arms.
In this design all the holes in the aluminium are through holes. Lock nuts are used on the end of the bolts.
The laser support components laid out. The cam clamp was intended to provide tool-free use. It was later replaced. It did not hold as well as desired so the laser slipped causing Sonny to blow through the first test piece. I replaced with a simple 1/4in nut tightened with a wrench and no longer slips.
The jig assembled for this picture on my NOVA lathe.
The jig being used on Sonnys lathe. I did not get the drawbar in this picture.
I have a local wood turner friend who has a Lyle Jamieson captured bar hollowing system. He found the set screws to hold the tools in the bushings contribute to vibration so he asked me to make additional bushings so he could epoxy each tool in its own bushing. He commented this may a big improvement to using his jig.
I suggested that Sonny epoxy his tools in the bushings, which he did.
Sonny has hollowed several pieces with this jig. His latest feedback was "The tool is still operating tremendously and is a joy to use."
The MT2 taper with the drawbar is holding the jig without any issues. Sonny really likes not having to remove his tailstock to use this jig.
I will save this post and add another post on version 2 which has some significant design improvements.
Since some of the items were already on-hand, I estimate the cost of the materials to be around $40 - $50.
John Lucas articulated hollowing tool thread
