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Rotary Vacuum Adaptor

with Double Bearings


In the book Fixtures and Chucks (see home page), I describe in detail how to make a rotary adaptor for a vacuum system. That design, however, uses only one bearing to make the seal between the rotating and stationary parts. For the most part the adaptors work well, but with some it is obvious that considerable leakage occurs through the bearing.


This article shows a similar design that uses two bearings instead of just one. As expected, the performance is much improved with little if any leakage ever being observed.


Making this adaptor requires no metal working, and except for the bearings and perhaps a sheet of foam from a hobby store, there is nothing to buy. The parts are made of hardwood, MDF, or plywood – your choice. The adaptor described here is intended for a Powermatic but is easily modified to fit any other lathe.


Overview


The rotary adaptor makes the connection between the vacuum hose and the spindle of the lathe. It fits onto the hand wheel and is held there by suction – no screws or bolts required. The vacuum is communicated to the vacuum chuck through the bore of the spindle.

In addition to the bearings, the adaptor consists of the body (a disk), the bearing housing, a hose barb, and a centering tube that is inserted into the headstock spindle. A novel feature is that the hose barb and centering tube are turned from a hardwood spindle blank. Yes, a wooden hose barb works just as well as one made of brass.


The bearings are 6201-2RS with an OD of 32 mm, ID 12 mm, and thickness 10 mm. A search will turn up numerous sources online if you are unable to obtain them locally. They are not expensive – typically less than $5 each.



Make the hose barb.


It is convenient to make the hose barb first because it will provide a handle for the bearings when it comes time to fit them into the recess in the bearing housing.


Begin by drilling a 1/4” hole through a spindle blank 2 1/2” long that will true up to a diameter of about 5/8”. Any tight-grained hardwood will be good.


One way to do this is to mount a blank about 3 1/4” long in a scroll chuck, drill the hole, and then part off a section 2 1/2” long.


Once the hole is drilled, mount the blank between a mandrel and a cone center so that it is centered on the hole. Turn it to a diameter of 5/8”. Mark off a  20 mm section at one end. Turn it down to about 12 mm so the bearings will slide on easily.          


Turn the tapered section where the vacuum hose will slide onto the barb. You may wish to reverse the blank on the mandrel if you have a preference for turning in one direction over the other. Test the fit in the hose as you go.


I use a 3/8” ID thick-wall vinyl hose with my system. It is available at any big-box store. The thick wall is desirable so the hose doesn’t collapse and flatten out when the vacuum is applied.


Aim for a fit that requires a bit of force to slide the hose over the barb, but not a lot. If it’s too tight, the hose could be difficult to remove. You probably will not need a hose clamp.


Apply a coat of lacquer sanding sealer.



Install the bearings on the barb.


Wipe the metal surfaces of the bearings with alcohol, lacquer thinner, or acetone to remove any film of oil that may be present.


Apply a thin coat of epoxy where the bearings go and slide the bearings on. Be careful not to push up a ring of epoxy that might get squeezed over onto the seals of the bearings. Wipe off any excess epoxy with a paper towel moistened with alcohol.



Make the body.


Cut a disk roughly 3.25” in diameter from 3/4” plywood or MDF. Attach a waste block to one side of the disk. After the glue sets up, jam the assembly against a flat surface and form a tenon on the waste block.

Mount the blank in a scroll chuck using the newly-formed tenon.True up the edge of the disk. Form a recess 1 1/2” diameter and 5/16” deep. This will become the socket for the bearing housing. (A Forstner bit will make quick work of this if you have one available.)


Make the bearing housing.


Place a short fat spindle blank about 1 5/8” long between centers. Turn it to a diameter of about 2” and form a shallow tenon on one end. Mark off a length of 1 3/8” from the end with the tenon. Mount the blank in a scroll chuck.


Adjust the overall length of the blank to 1 3/8” by squaring it off at the line you drew in the preceeding step. Form the recess for the bearings.


The OD of the bearings is 32 mm, which is about 0.01” larger than 1 1/4”. So, we have the option of drilling out the recess with a 1 1/4” Forstner bit and then enlarging the diameter just slightly to allow the bearing assembly to slide in. If you don’t have the bit, you can do the hollowing with turning tools.


Aim for an easy sliding fit of the bearings into the recess. Make the bottom of the recess slightly concave to provide clearance for the inner race of the bearing.

Once the recess is complete, form a shallow tenon on the blank.


Reverse the blank in the chuck. Form the tenon that will fit into the recess on the body. The nominal diameter is 1 1/2” but test the fit when you get close to the target diameter. The width of the tenon should be 1/4”.

Finally, drill a 3/8” hole through the exposed face into the bearing cavity.


Attach the bearing housing to the body.


Return the body to the scroll chuck. Epoxy the bearing housing into the recess in the body. Use the tailstock to apply pressure and to be sure the housing is properly aligned.  





Remove the waste block and drill the hole for the centering tube.


Reverse the assembly in the chuck and remove the waste block.


Use a 5/8” Forstner bit to drill a hole for the centering tube. Drill the hole to a depth of 1/2”.


Switch the drill bit to 3/8” and continue the hole through to the bearing cavity.


Mark the location where the foam sealing ring will be applied.


Shape the profile.  


Reverse the assembly by jam chucking it against a flat surface. Shape the profile of the body and bearing housing to your liking. Apply a coat of lacquer sanding sealer to the entire assembly.




Cut out and attach the foam seal to the body.


An airtight seal between the adaptor and the handwheel is achieved by a narrow ring of closed-cell foam attached to the body. The foam can be purchased where craft supplies are sold.


The foam I’ve used in this application is made by Darice and called foamies. It is available in sheets that are 6 mm and 2 mm thick. The 2 mm thickness is better for this application. (2 mm is just slightly greater than 1/16”.)


One way to cut out the ring of foam is simply to use a pair of scissors. But if you want a really neat-looking ring, you can do it on your lathe as described in the article on this website, Cutting Rings and Templates.


Attach the ring to the body with the adhesive of your choice.  I’ve tested Titebond III, Elmer’s white glue, E6000, and model airplane glue and all produced a good bond.



Install the bearing assembly.


This must be done carefully in order to avoid pushing up a blob of epoxy in front of the bearings as they are inserted. Apply a very thin coat of epoxy to the side walls of the cavity, and also to the outer races of the bearings. If you use 5-minute epoxy, you must work quickly.


Push the bearing assembly straight into the cavity until the bearings are fully seated. Then pull them back out. Look inside the cavity to see that excess epoxy was not pushed to the bottom. Ensure that the outer races of the bearings are coated with epoxy, then re-insert the bearings and allow the epoxy to cure.



Turn the centering tube.


Drill a 3/8” diameter hole through a spindle blank that is about 2” long and a bit over 5/8” diameter. Mount the blank between a mandrel and a cone center as you did for the hose barb.


The tube should slide easily into the 5/8” bore of the headstock spindle (for a Powermatic). However, a 1/2” section at one end should be sized for a snug fit into the hole at the center of the body.



Install the centering tube.


Apply a thin coat of epoxy to the inside of the 5/8” hole in the body and to the end of the centering tube that was test-fitted to the hole. Push the tube into the hole, rotating it to ensure that the epoxy is distributed uniformly. Use a paper towel soaked in alcohol to wipe off any epoxy that squeezes out of the joint.


To make sure the tube is exactly square to the face of the body, insert the centering tube into the bore of the headstock spindle until the body presses against the flat of the handwheel. Leave it there until the epoxy sets up.


This completes the construction part of the project.

 

  

Strain Relief


Arrange mechanical support for the hose so that it doesn’t tend to pull the adaptor away from the handwheel when the system is not under vacuum. It will be held securely once vacuum is applied.


For my system I devised a clamping arrangement that is attached to the motor, but you may prefer a simpler method. (See the photo at the beginning of this article.)



Hose Clamps


To make a simple clamp, wrap two turns of soft steel wire around the hose and then twist the ends together. Tighten the twist just enough to cause the wire to dent the surface of the hose ever so slightly.


If you prefer to use a worm-gear clamp, be sure to get one that fits the hose almost perfectly when the clamp is about 25% closed. If you use one that is too large but make it work by tightening it almost to the limit, you will almost certainly create a leak because the clamp will pinch up a wrinkle at one point, and air will flow rather freely through the wrinkle.



Testing the Adaptor


Before connecting the hose to the adaptor, block the end of it with the shank of a 3/8” drill bit and check the vacuum. Most pumps used by woodturners will produce a vacuum of about 26 inches of mercury (in-Hg) in this configuration.


Connect the hose to the adaptor and install the adaptor onto the handwheel. Block the spindle by inserting a drive center into the Morse taper. Check the vacuum again. It will probably be a bit less than before, but anything above 20 in-Hg is quite satisfactory.


If the vacuum is low, make sure the adaptor is seated firmly against the handwheel. A thin layer of Vaseline applied to the foam ring will make for a better seal.


A possible but unlikely leakage path exists between the bore of the handwheel and the spindle shaft. Apply a fillet of Vaseline or grease to the inboard side of the handwheel to remove this possibility.


Run a fingertip around the seal of the exposed bearing while watching the vacuum gauge for any change. If you suspect the seal, coat it with Vaseline. With the single-bearing adaptors I sometimes note that a bearing will develop a good seal only after it has run a minute or so at high RPM.


Wiggle the hose where it fits onto the hose barb. Any fluctuation of the vacuum reading indicates leakage. Install a hose clamp or check to see that the one you have has not pinched the hose to create a leak.


To be sure no leakage is occurring around the drive center blocking the Morse taper, wrap the connection with a layer of masking tape, or better, plastic electrical tape.


You did put a coat or two of lacquer sanding sealer on the entire surface of the adaptor? Both solid wood and plywood can be very leaky, but the worst for leakage is MDF, surprisingly enough. Air seems to move quite freely along the laminations, parallel to the finished surfaces.




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