I've always wanted to adapt my vacuum press so I could use it for vacuum chucking small bowls on my lathe. This way, I can finish up the bottoms without showing any machine markings. One nice thing about this add-on is that it allows you to easily monitor the vacuum level on the work piece. When the needle on the gauge is above 15" of Hg, you know that the project is ready to be turned. And with the vacuum valve and bleeder, it also allows you to pre-fit the work piece with light vacuum so you can center the project on the jig easily without having to have an extra set of hands to control the vacuum source. When the project is perfectly centered, simply flip the valve handle and full vacuum is applied. You'll even see the project get pulled deeper onto the jig!
Before You Begin...
The most important thing to do before you build the vacuum chuck is check the headstock on the lathe. It needs to be hollow from the outboard side to the inboard side of the headstock spindle. Some lathes have indexing holes drilled in the headstock spindle. This is fine as long as the holes are not drilled fully through. These "through" holes will prevent vacuum from forming inside the system. My lathe is a piece of junk from Grizzly. Amazingly, it did have a hollow headstock spindle. If your lathe doesn't meet the requirements above, you might want to look at the VacuuMaster jig from Woodworker's Supply.
If your lathe meets the above requirement, you'll also need an outboard sanding disk or faceplate in addtion to the parts below. And don't forget, you can get a parts kit for this article at VeneerSupplies.com
Be sure to read this article entirely and verify your lathe will accommodate these instructions before you begin building a vacuum chuck. It's likely that some modification to this article may be necessary unless you have a lathe that is exactly like mine.
Vacuum Pumps and Systems
This kit is designed to be used with each of the vacuum presses found here at JoeWoodworker.com and also at VeneerSupplies.com.
Venturi Based Vacuum Presses (Project: V4 and V4 Premium)
Please note that this jig will cause the unit to cycle on and off frequently. This will not harm the electronics or pneumatics in the system. No further modification is needed to these systems for use with the vacuum chucking jig.
Auto-Cycling Electric Pump Vacuum Presses
The rapid on and off cycles that will be caused by this add-on are not good for the electronic components on the vacuum pump. Additionally, the pump may not cycle often enough to maintain a minimum of 15" of vacuum inside the vacuum chuck. The good news is that there is a very easy fix for this. You can simply wire in a jumper switch to convert the system to a continuous-run system. This switch will bypass the vacuum controller and allow the pump to run non-stop.
Click here for instructions to add this switch to your system.
Continuous Run Electric Pump Vacuum Presses
No modification to your press is required.
There is no safe suggestion as to what size vacuum pump is required for vacuum chucking. You would need to know three things...
- What is the surface area where the chuck attaches to the project?
- Is the project material porous? (some species of wood are very porous)
- How much weight will be held on the chuck at turning speed?
With these bits of information, you can calculate the clamping force and that will help you determine if the pump is safe to use for chucking.
Clamping force is based on a calculation using the maximum sustainable vacuum level and the surface area where the project is attached to the chuck. A smaller attachment area means less clamping/chucking force. A large industrial pump capable of pulling a very high vacuum may not be suitable for chucking if the vacuum surface area of the project is small. You have to calculate the vacuum surface area on your project first. Then determine what amount of vacuum you are capable of applying to the project. With this information, you can determine the total clamping force on the project and consider whether or not it is enough to safely hold your project while it is being turned. Here is the method for calculating the clamping force.
- Determine the surface area of the vacuum attachment point. In most cases, the vacuum chuck will be circular so use Pi (roughly 3.14) to determine this number. The formula starts by taking the diameter and dividing by two. This gives you the radius. Then multiply the radius by 3.14 and then multiply it again by the radius. Let's use a chuck diameter of 6" and determine the surface area. Divide 6 by 2. That makes 3 so then multiply 3 times 3.14 (Pi), then multiply that again by 3.
6 ÷ 2 = 3
3 x 3.14 = 9.42
3 x 9.42 = 28.26
So here we have 28.26 square inches in a 6" diameter chucking surface.
- Determine the maximum sustainable vacuum level that you can reach with your project piece attached to the vacuum chuck simply by placing it on the chuck and turning on your vacuum pump. A non-porous material that is chucked to your lathe will typically achieve the same level of maximum vacuum that your pump is capable of achieving. In other words, the project material itself allows no loss of vacuum therefor the pump will achieve its best available vacuum.
In the following example let's assume that the project material is non-porous and that the pump can generate 25" of Hg. For every inch of Hg vacuum, you get .49 lbs per square inch of pressure. Multiply .49 by the vacuum level that the pump can create.
.49 x 25 = 12.25 (this is how much pressure in pounds that you are getting per square inch)
- Multiply 28.26 x 12.25 to get 346.19 lbs of force holding your project to the lathe.
- Now only you can decide if that is enough chucking force to safely hold your project.
What if the chucking project is porous?
Unfortunately, there is no way to determine this without actually placing the project on a vacuum chuck. You would need to determine the actual maximum sustainable vacuum and this will vary across each project you try out depending on the size and porosity of the material.
I can only suggest one thing... the bigger, the better. Several of our customers are using 3 CFM vacuum sources for chucking but most are opting for 5 CFM` vacuum sources.
The Three Parts of the Vacuum Chuck
Manifold – Controls the vacuum level and shows the vacuum level in the jig
Outboard Spinner – Allows the manifold to free-spin without losing vacuum
Vacuum Drum – Holds the work piece to the lathe with vacuum
Part 1a: Build the Manifold
The manifold is just a bunch of brass fittings from the hardware store. It looks like there are more parts than necessary. I admit that it could be built without the gauge, valve and breather fitting but with these parts, the jig is safer and easier to use. It's a bit more costly to go this route, but I think it's worth the extra money since it's something that will be in your shop for a long, long time.
Warning: Brass products may contain chemicals known to the state of California to cause cancer or reproductive toxicity.
Vacuum gauge (1/4" NP, lower mount)
Brass pipe union (x2)
Brass street elbow (x2)
Brass tee (x2), (1/4" NPT)
Brass close nipple (1/4" NPT)
Vacuum valve (1/4" NPT, male to female)
Brass tank adaptor
1-3/8" Double-sealed ball bearing (7/16" wide)
Using thread sealing tape, assemble the manifold as shown in the picture. Warning: Brass products may contain chemicals known to the state of California to cause cancer or reproductive toxicity.
- Attach a brass street elbow to the long side of a brass tee as shown in the picture.
- Attach a brass street elbow to the short side of the assembly from step 1. Note the orientation of the fittings. The fitting must be facing the proper direction. Use the image shown for help.
- Attach a brass close nipple the remaining opening on the brass tee that was used in the previous steps.
- Attach the long side of the second brass tee to the close nipple in step 3.
- Attach the brass tank fitting to the final long side of the second brass tee.
- Attach the vacuum valve to the short side of the second brass tee.
- Attach the breather fitting to the open side of the vacuum valve.
- Attach the vacuum gauge to the street elbow that was attached in step 2.
Part 1b: Make the Bearing Adaptor
This part is easy if you take your time and work slowly. I can make this statement without hesitation because I made a dozen of these adaptors before I took the time to do it right.
The bearing adapter consists on only two parts. I call it a bearing adapter because its purpose is to adapt the ¼" NPT pipe threading on the manifold to the inside diameter of the bearing. You'll notice that a ¼" NPT pipe union fitting is marginally larger in diameter than the inside diameter of the bearing. Luckily, brass is highly machinable with ordinary lathe tools.
Lightly chuck one end of the pipe union fitting onto the lathe. For this step, a 3-jaw chuck works very nicely. Using a parting tool, shave off a very small portion of the thread from the end of the fitting and test fit the bearing. Continue shaving the pipe union fitting until the bearing has a snug fit. If you cut down too much and the fitting is loose inside the bearing, you'll need to start over with a fresh piece of brass so cut slowly and carefully to get the fit right. You might find that a cutting a slight taper on the brass fitting is an easy way to get a good fit.
To permanently attach the brass fitting to the bearing you'll need a package of JB Weld or a stick of metal epoxy. Mix up a very small amount and apply it to what remains of the threads on the brass fitting. It's a good idea to apply some to the inside of the bearing as well but be certain that it doesn't get on the rubber seal on the front or back. Assemble the two pieces and let the epoxy cure overnight.
After the epoxy is cured, you can attach the pipe union fitting to the manifold. Be sure to use thread sealing tape.
Part 2: The Outboard Spinner
The outboard spinner allows the manifold to free spin when the lathe is running. To make the vacuum chucking jig, you'll need the items listed below. My lathe came with an outboard sanding disk that is about 9" in diameter. You'll need one of these or an outboard faceplate to make the outboard spinner.
Outboard Spinner Parts
Fender washers (x3)
Wood screws #10 by ¾" (x3)
HDPE (3/4" thick), Baltic birch plywood, or a non-porous hardwood
Outboard faceplate or sanding disk (you can order these from the lathe manufacturer)
Build the Spinner
First you'll need to rough cut a piece of ¾" thick HDPE slightly larger than the diameter of the outboard faceplate. Then center the HDPE on the faceplate and marked where the screw holes are. Drill the pilot holes in the HDPE with a 9/64" drill bit. Do not drill all the way through the HDPE. The entry area of the pilot hole may feel slightly raised. If so, use a chisel to slice off the protrusion.
Temporarily attach the HDPE to the faceplate with four #10 x ¾" wood screws. The HDPE accepts the wood screws like softwood so be careful that you don't over-tighten the screws. Trim the HDPE so it's flush with the faceplate edge. I used a flush cutting bit on my router table to do this. You could also do this on the lathe if you have an outboard tool rest. When you are done, remove the HDPE from the faceplate.
To house the bearing, there are 3 flat-bottom holes that need to be drilled into the HDPE. Forstner bits are required for the next steps. You need to carefully drill the absolute center of the HDPE. This is critical. If this part is done wrong, the whole jig will wobble fiercely when in use. There are at least two ways to get this done right. I've used both of the methods below and had success.
- Mark the center of the HDPE while the lathe is running. I used the point of a nail to scrape the HDPE. I then removed the HDPE from the faceplate and used a drill press to drill the holes mentioned below.
- I also found that it was quite easy to drill the holes by attaching the HDPE to the extra large "jumbo jaws" on my four jaw chuck. Click here to see the picture.
Hole #1: This hole is 1-3/8" diameter and drilled 7/16" deep. Be careful that you drill this exactly right. This hole houses the ball-bearing.
Hole #2: The second hole is 7/8" diameter (you can also use a 1" bit) and 1/16" deeper that the first hole. The purpose of this hole is to allow the inside rim of the bearing to spin freely.
Hole #3: The last hole is ½" diameter. This time drill the hole completely through the HDPE. A forstner bit is not required for this hole.
The structure of holes should look like the picture shown.
You can now re-attach the HDPE to the faceplate jig. Apply a small bead of caulk to the back side of the HDPE. It is important that the ring of caulk is located just inside of the radius of the screw holes. Re-attach the HDPE to the faceplate with the four screws.
Attach the Manifold to the Spinner
It's time to put the outboard spinner and manifold together. I tried several ways of attaching the bearing and manifold to the HDPE and found that the easiest way was to use 3 fender washers and #10 wood screws. Place the o-ring inside the HDPE jig and attach the manifold assembly.
You'll note that the bearing no longer sits flush with the face of the HDPE (see image). Now, lay out the three fender washers so they're over the bearing's edge as shown. Mark the center of the holes in the washers so you'll know where to drill the pilot holes for the screws that hold the washers in place. Use a 9/64" drill bit to create the pilot holes in the HDPE (and again, be certain that you don't drill all of the way through).
Now you can mount the manifold to the faceplate jig using the three #10 x ¾" wood screws. Tighten the screws enough so that the fender washers put pressure on the edge of the bearing. This will seal the back edge of the bearing against the o-ring and the HDPE.
The Vacuum Drum
The next part of the vacuum chucking jig is often called the vacuum drum. It's the part on the inboard side of the lathe that holds the work piece in place with vacuum. This part is pretty easy to build.
Vacuum Drum Parts
5" x 5" x ¾" cherry or other non-porous hardwood (4 pcs)
Vacuum tape (1/16" thick by 3/4" wide)
Using yellow glue, laminate 4 pieces of 5" x 5" x ¾" cherry or other non-porous hardwood together (alternating the grain direction). Then rough cut the cherry block on the bandsaw to save on turning time. Place a thin bead of caulk on the faceplate and attach the cherry blank with four screws. It took me a while to figure out that the caulk is rather important in making a chuck that doesn't leak. Trust me… the caulk part is worth doing.
Turn the cherry until it is perfectly round and then create a ¼" deep recess in the face of the vacuum drum leaving a 3/8" wide rim. Then round over the rim as shown in the picture. Now, use a 3/8" drill bit attached to the tailstock on the lathe to drill a full-through hole in the vacuum drum.
The last part is to attach a vacuum gasket to the rim on the drum. After I (unsuccessfully) tried several types of gasket material, I found one that did exactly what I wanted. There are three things to consider when selecting a vacuum tape for a specific job. In this project, you need a tape that is thin so the work piece will not shift with pressure from the lathe tool. I found that 1/16" thick material worked best to accomplish this. It doesn't allow any perceivable shifting of the work piece which would make a good finish nearly impossible. Remember, it's best to use this jig when working on the foot of the work piece where pressure is against the headstock.
The other factors in selecting a gasket tape are resilience and durability. I know from experience that All-Star Adhesives has some of the most durable gasket tape available. I called my rep and got exactly what I needed. Well sort of. I really didn't want 250 rolls of it but I thought other woodworkers would want some too.
To attach the gasket tape evenly, start at the top of the rim on the drum and carefully shape it around the entire perimeter. Then trim the gasket to be a snug fit where the ends butt together. Then use gentle pressure to form the gasket tape over the edges of the rim.
Using the Jig
Here's the fun part. After you've got a work piece turned and ready for the vacuum chuck, securely attach the manifold to the lathe. Get your vacuum press plugged in and pressurized. Attach the lock-on connector to the brass fitting on the bottom of the manifold. Make sure the vacuum valve on the manifold is open.
Open the valve on the vacuum press. There will now be enough pressure at the vacuum drum to loosely position the work piece. Try to center the work piece perfectly. Test it by spinning the lathe with your hand. If its not right, carefully pop it off the vacuum drum and try again. It usually takes a few tries to get it right. Your vacuum press will cycle on and off a few times while you are doing this.
Once the work piece is centered on the vacuum drum, you can close the vacuum valve on the manifold. This will apply the full vacuum to the work piece. If you suspect that your work piece is extremely thin or fragile, the best thing to do is close the vacuum valve slowly and listen for any cracking. If you hear a crack forming, the vacuum chuck should not be used to finish the project.
You'll find that most projects will have no trouble with the pressure from the vacuum. It is generally accepted that 15" of Hg is the minimum safe vacuum for most small to medium sized projects. As always, exercise caution and good judgment. Wear appropriate protection and don't crank the lathe up to 8 million RPM. And for goodness sake, make sure the project is centered on the jig before you get the lathe up to speed.
When the turned project is complete, turn off the lathe and allow it to stop spinning (duh!). Then shut off the vacuum press and open the vacuum valve on the vacuum chucking manifold. Get ready to grab the finished project because when the vacuum drops, the project will pop off.
You may find that the completed system appears to leak. The leak is usually on the threads of the inboard and outboard spindles. To remedy this, simply wrap the threads on the spindles with a couple layers of thread sealing tape.
Remember, the vacuum chuck will not work on turning which are made from porous woods like oak!
Don't forget, you can get a parts kit for this article at VeneerSupplies.com
Questions or suggestions?
If you've got a question, suggestion for improvement, feel free to contact me.