Jam Chuck – Wood Lathe Magic Trick

The jam chuck is our “unseen” partner in the wood bowl making process that helps us, bowl woodturners, look like magicians.

When someone picks up a bowl looking for an obvious lathe connection point and says something like, “how was this made?” We have the jam chuck to thank for this wonderful trick.

The jam chuck is perhaps the cheapest, yet most useful and versatile accessory for your wood lathe.

Why you ask?

Well, because you can create a jam chuck from almost any piece of scrap wood laying around and custom design it to fit your exact needs. Just like the headline magicians that build their own tricks.

In this article, we’ll go over what a jam chuck does, how to make one (or several), and how to use the jam chuck.

The simplicity of the jam chuck is magical and, when done right, invisible.

Jam Chuck Defined

In basic terms, a jam chuck can be any device designed to hold or support a wood turned item on the lathe.

As the name implies, the jam chuck is jammed up against a reversed wood bowl interior surface to make a firm contact point.

For our purposes, we are discussing how jam chucks work in the wood bowl turning process, but jam chucks also work in all sorts of woodturning.

When a bowl has been completely turned and is ready to have the tenon removed, by the way here’s an article that discusses removing the tenon from wood bowls, a jam chuck is used to hold the bowl in position for tenon removal.

The jam chuck is usually attached to the headstock either directly or with a four jaw chuck and drives the rotation of the reversed bowl during the tenon removal process.

Think of a jam chuck as one end of a reversed wood bowl sandwich, with the other end being anchored by the tailstock.

A jam chuck provides a firm even contact with the wood bowl’s interior surface and applies even pressure to the piece as the foot is shaped and finalized.

While the jam chuck provides no grasping connection to the reversed wood bowl, the lateral surface pressure applied by the tailstock is enough to hold the bowl securely in place during turning.

Five Jam Chuck Advantages

There are several advantages that jam chucks provide during the reversed wood bowl, tenon removing process.

1. The first advantage jam chucks provide is customization. Because we’re making jam chucks from scrap wood, we can shape, size and customize them as needed.
2. Jam chucks can be reused and repurposed. A jam chuck that once worked perfectly for reversing a particular wood bowl can be quickly modified to accommodate a different bowl shape.
3. Unlike most other accessories used on the lathe, the jam chuck is virtually free. Besides the little bit of time it takes to make one and a scrap of wood, there are no other expenses to a jam chuck.
4. When used properly, a jam chuck leaves zero evidence of its contact to the wood bowl. Because of this, there is no worry that the tenon removal process may mar the finished interior of wood bowls.
5. Jam chucks are quick and easy to use. I have a box with all my jam chucks ready to go. When I’m ready to reverse a bowl, I pull out the best-suited jam chuck and keep working with no wasted time.

Jam Chuck Designs

The design of a jam chuck is straightforward. Start with a cylinder and add a curved shape to the end to ease the contact or pressure points on the wood bowl interior.

Depending on what types of bowls and what sized bowls you turn, dictates what kind of jam chucks you’ll need.

I turn a variety of different bowls and, as you might imagine, I have many different sized and shaped jam chucks.

For medium to large bowls, I have a “goto” jam chuck that is about six inches in diameter and has a gently convex curved face. This jam chuck works well to make a firm connection with almost any bowl from just over six to 10-12 inches in diameter.

Smaller bowls require smaller jam chuck designs. Very large bowls require more contact surface and wider jam chucks. If needed, a thick plywood disk can be fixed to the end of a turned cylinder to add the needed surface area for a very large bowl.

The interior curved shape of a bowl can also affect the jam chuck performance or surface contact connection. Deep bowls may require more of a elongated or cone-shaped extension to the face of the chuck.

Also, the length of the jam chuck plays a valuable role in reversing deeper or larger bowls. A more extended jam chuck allows a deeper wood bowl to be reversed and not come into contact with the headstock area of the lathe.

Convex Vs. Concave

I recently made a discovery that now seems so obvious when I look closer.

Most of my jam chucks have a convex face to them. This works great if the convex jam chuck curve matches the profile of the reversed wood bowl interior.

However, when the convex curve of the chuck does not match the bowl bottom shape, contact can be limited to just the center of the chuck face. This limits the chuck support and creates issues when centering the bowl.

A simple modification of the chuck face from a convex to a concave profile provides more contact support for the wooden bowl and works more universally with many different bowl bottom shapes.

Loose Vs. Tight

I usually use loose fitting jam chucks to match the bowl bottom interior surface with a piece of padding for cushion in between.

For precise tight-fitting jam chucks, a rim receiving edge is turned on the jam chuck to perfectly receive the exact size and shape of the bowl rim.

The uses for this type of jam chuck are limited pretty much to one bowl at a time. So that bowl must have a rim that is close or exactly vertical or ninety degrees to a table surface.

Inward or outward turned rims or natural edge rims don’t work well with a tight-fitting jam chuck.

The advantage of a tight-fitting jam chuck is that, if the fit is tight, tailstock removal is possible, and the full bowl base is accessible, without any additional special equipment.

Even if a perfect snug fit doesn’t occur, tape can be applied around the rim-to-chuck connection and make an excellent hold, also without the tailstock.

The downside is you need to make a new jam chuck or modify an existing chuck for every piece. Now, if you’re making that one-of-a-kind masterpiece, this might be a great option.

Cushioned Contact

Because of the surface-to-surface contact of the jam chuck to the wood bowl interior, it is wise to cushion this contact point.

Basically, anything that is uniform, soft, and won’t scratch the bowl surface works well for cushioning the chuck surface.

I usually use a small disk of thick, dense packing foam or a scrap of carpet padding.

Permanently adhering, with contact cement, a padded surface like rubberized shelf liner or other material to the face of a jam chuck can be a nice touch. However, the chuck won’t be easy to modify once padding is glued in place.

Be sure any loose cushion is just big enough to cover the face of the chuck with no excess material hanging free.

Especially do not allow any cushion material to extend beyond the edge of the bowl, as it can potentially get wound in the headstock. That would not be a desired special effect!

IMPORTANT – when adding loose cushion material between the chuck and the bowl surface, be sure the cushion is centered, even and not folded over. The slightest variation can make the bowl very difficult to center on the tailstock.

How To Make A Jam Chuck

Like I’ve mentioned, jam chucks are customizable and easy to make. Thanks to that ease, it’s great to make jam chucks as you need them and before you know it, you’ll have one for almost any situation ready to go.

Start with a piece of scrap wood large enough for the desired sized chuck needed. The wood should be a relatively sturdy species and not soaking wet, as we don’t want the chuck to shift or alter during reverse chucking or later as it drys.

I usually turn my chucks between centers and keep the same side grain orientation as I do with side grain bowl blanks. So times I’ll make end-to-end jam chucks as well.

Grain orientation is critical to note because when turning side grain end-to-end on the lathe do not use a spindle roughing tool.

The end grain of the wood, on a side-grain turning, can be too difficult for a spindle roughing gouge and cause issues. Turn the cylinder shape using a regular bowl gouge.

1 – With an end-to-end connect between spur chuck and tailstock, turn an even cylinder.

2 – On the tailstock side of the turned cylinder, make an ideal tenon with a clean flat shoulder and a dovetail angled edge.

3 – Remove the chuck from the end-to-end connection and attach the newly created tenon to the four-jaw chuck now in the headstock.

4 – If you’d like, you can take your time and use a profile gauge to shape the face of the jam chuck to match precisely the interior of the bowl profile.

5 – Round over the outer edge of the jam chuck and hollow out the center a bit to form an outer ring with an easy smooth curved surface.

Matching the interior wood bowl surface exactly is not necessary and eyeballing a relative chuck face shape works fine. The cushioning material also aides in making up the difference in the two profiles.

It’s that easy to shape and create a jam chuck.

Special Vacuum Jam Chuck Design

Another option when making a jam chuck is to make the chuck center hollow to allow vacuum chucking.

An additional vacuum system will be necessary for this process. I use the Hold Fast Vacuum Chuck System which attaches to any basic air compessor. Making custom vacuum chucks adds excellent flexibility with this system.

The primary advantage of a vacuum jam chuck over a regular jam chuck is the tailstock can be pulled away at the end to allow full access to the wood bowl bottom. This is great for making fine details on the base of the bowl.

Even though vacuum is holding the jam chuck, for safety, I still keep the tailstock in place until the very last moment with only a couple small finishing passes remaining.

Making A Custom Vacuum Chuck

The same steps apply for making the chuck as described above, with a couple of modifications.

First, start with a wood that is dense, dry and non-porous. Most hardwoods work fine for making a custom vacuum chuck.

Hollowing the center requires a Jacobs chuck, drill bit, and wood thread tap.

1 – First, round the cylinder, then add a tenon to the tailstock end.

2 – Flip the chuck over and mount the tenon in the four-jaw chuck and attach the Jacobs chuck to the tailstock. (At this point, I shortened the jam chuck cylinder in the photo series below.)

3 – Turn the face of the cylinder smooth and flat.

4 – Depending on your headstock size and thread count, you need to match the drill bit and tap used. For a 1” x 8 TPI headstock, drill a 7/8” initial hole and tap with a 1” x 8 TPI wood tap. For a 1-1/4” x 8 TPI headstock, drill a 1-1/8” initial hole and tap with a 1-1/4” x 8 TPI wood tap. Drill a hole deeper than the length of your headstock threads but do not let it contact the four-jaw chuck.

5 – Then, with the lathe off, slowly tap the hole with the threaded tap to form the threaded headstock connection. The tap I used has a small indentation on the end so I pulled up the tailstock to initially keep the tap centered in the hole.

6 – With the threads tapped, remove the cylinder from the four-jaw chuck and the chuck from the lathe. Thread the new thread connection directly to the headstock. Be sure the wood seats firmly on the shoulders of the headstock.

7 – Form the opposite end of the vacuum chuck and turn out the center area until the drilled opening is exposed.

8 – A vacuum jam chuck will need a permanent cushioned edge that is both soft and airtight. Use contact cement to carefully attach rubber shelf liner to the entire rim of the custom vacuum chuck. (not pictured)

Like a regular jam chuck, make variously sized vacuum chucks to fit your needs.

Also, if you want to save the time of using a four jaw chuck to hold the jam chuck, you can make self-threaded chucks with or without full vacuum openings for speed and easy lathe mounting.

How To Center On A Jam Chuck Effectively

Using a jam chuck effectively is all about centering.

The reversed bowl needs to be as close to centered as possible to make the tenon removal process work well. Here’s an article all about removing the tenon.

If you’re using a loose fitting jam chuck, mount the chuck and have the tailstock close by and ready.

Hold the bowl in position and place the cushioned padding between the chuck and the bowl interior. If you’ve made a small tick mark in the center of the tenon base, align the tailstock live center with that point.

Apply enough pressure with the tailstock to hold the bowl in place, but not too much yet.

Position the tool rest near the outside edge of the bowl and rotate the bowl manually, with the lathe off. Place your thumb stationary on the tool rest and let your thumb tip rub the bowl surface.

Ideally, the bowl is centered when the bowl rubs evenly on your thumb tip all the way around the bowl.

If the bowl is wet green wood, this may not be possible as the wood may be actively shifting.

Usually, when you rotate the bowl, there are areas that rub and other areas that miss your thumb tip. Rotate a few times and find the high spots or the areas that touch your thumb the most.

Move that high spot to the top of the bowl rotation, gently loosen the tailstock just a bit and lower that high spot ever so slightly. It doesn’t take much.

Continue doing this until the bowl rubs your thumb tip as evenly as possible.

Move the tool rest down to the tenon, snug the tailstock just a bit more and turn the lathe on slow. Using a pencil on the tool rest, slowly move the pencil lead forward until you contact the tenon.

If a small area of the tenon has pencil marks, that is the high tenon spot. Stop the lathe, loosen the tailstock, and move that high spot just a touch inward towards the center and tighten the tailstock again.

It may take several adjustments to center the bowl on the jam chuck. Take your time and with some experience, this process, like everything, becomes much more natural.

To learn more about removing a tenon from a wood bowl, see this article. And while you’re at it, be sure to see this article all about making the perfect tenon.

Wood bowl turning depends on a secure connection, and a correct tenon is a keystone to turning smoothly and without vibrations.

Vibrations are a whole issue that needs serious attention. If you’re experiencing vibration on your lathe, read this article for assistance.

Jam Chuck Conclusion

Jam Chucks are simple and easy to make. Make several as you need and keep them handy.

As simple as they are, jam chucks play a critical role in the whole bowl woodturning process.

The funny thing is we appreciate them most based on what we don’t see. Reversing the wood bowl using a jam chuck makes that big ugly tenon and lathe connection disappear, just like magic.

Think about that the next time someone looks closely at one of your bowls and asks, “how did you make this?” 😉

---

You’re probably going to want to read these articles next:
• WOOD BOWL TENON REMOVAL – 3 EASY WAYS
• WOOD BOWL MORTISE OR TENON – WHICH IS BEST?
• 5 WORST TENON SHAPE WOOD BOWL (FOOT, SPIGOT, ATTACH)

---

Happy Turning,
Kent