Securing piece to lathe for end grain hollowing

[Bill Bell]

I recently had occasion to cut some beautiful Bradford pear and have prepared a piece that I want to use for hollowing. I have roughed out the section of log to a tapered cylinder about 7 inches long and about 6 inches wide, with a tenon of 3 3/4 inches at one end (size of my largest chuck.). My plan is to bore a small hole in the larger end of the cylinder and then hollow out the interior. As the cylinder is basically just the turned piece of the original log, all this work will be done in end grain. My problem is that because the wood is green it is still quite heavy — probably around 7 or 8 pounds - and I question whether it be safely secured in a check for turning. I roughed out the exterior between centers but I obviously cannot use the tail stock when hollowing. Any suggestions on how to proceed?

[Steve Eure]

Whenever I have that problem, I turn to my steady rest. Mine is homemade, but works extremely well. It will support the weight of your hollow form very well.

[roger wiegand]

I routinely hollow pieces as large as about twice that size held in a chuck. No problems so far, at least using a chuck with dovetail jaws. You'll want to check that the chuck grip is tight from time to time while working. You also want to make sure that your tenon is made from sound wood. The good news is that when you're hollowing you are almost certainly standing at the end of the lathe, well out of the "line of fire". The outside can be shaped with the tailstock in place, eliminating some of the weight before you hollow.

[John K Jordan]

Bill,

What chuck and jaws are you using? Some will grip much better than others.

For hefty pieces I like to use the Powergrip jaws on a Nova Titan chuck. This jaw set is significantly deeper and more likely to hold better than jaws that are less deep. You probably know that any jaws grip much better when close to their closed diameter than when fully opened (when they only contact at 2 points per jaw segment.) A proper dovetail can help where appropriate.

A few points (I should say I almost always turn dry wood - I’d probably dry that blank first and turn it in the future):
- I shape the outside to the final shape with the end supported, may even sand and apply oil finish before working on the inside.

- A steady rest certainly would help (I don’t use one). Could rig a simple steady rest without too much effort. Since the outside is smooth before hollowing I can use my hand to steady the piece (and control vibration when the wall gets thin.)

- In addition to retightening the chuck occasionally, always tighten at each socket. I usually rotate about 3 times while tightening at each place.

- Might bore out the center with as big a Forstner bit as possible. An extension may be needed. (While drilling, I always direct a constant stream of compressed air into the hole behind the Forstner bit to clear chips and keep the bit cool.)

- Gentle cuts can prevent unnecessary side forces on the piece. No catches allowed. I watched a guy dislodge the same bowl three times with catches! I like to use Hunter tools for the inside - gentle cuts will take alit longer than aggressive cuts but unlikely to dislodge the piece. HSS hollowing bits might need frequent sharpening. For several reasons I like to turn at reasonably high speed.

- If turning green, there may be warping while working even with an end grain piece. I was taught to spritz with water to keep it wet while working. When taking a break or stopping for the day wrap with plastic or something to keep it wet.

Note that the warping in an end grain piece may be negligible or significant depending on the species and where the wood is cut from the tree. A price from a log/limb with the pith centered will warp less than one cut from one side between the pith and bark, especially with certain species. Once I roughed out a lidded box off center from Dogwood and it warped so much it couldn’t be finished turned. If you plan to turn to finish while green instead of roughing then finish turning when dry this might not be problem.

I love turning Bradford Pear - it’s hard, fine-grained, and cuts so cleanly with sharp tools. Gradually turns a golden brown with age.

JKJ

[Kevin Jenness]

Have you considered using a faceplate, with or without a glue block?

[Reed Gray]

If your chuck jaws close to 3 inches or so, that is plenty for a piece like this. Lyle Jamieson likes to lag bolt all of his hollow forms. I do prefer dove tail jaws for both bowls and hollow forms, though I don't do many hollow forms.

Are you going to once turn it, or twice turn it? Makes a difference in how big your tenon is, mostly making sure you have enough to true the tenon up for the second turning.

robo hippy

[John K Jordan]

Have you considered using a faceplate, with or without a glue block?

Faceplate screws might be iffy in end grain. Side grain glue block glued to wet end grain can also be a problem, depending. Perhaps some glues are better than others for this.

However, it looks to me like David Marks used a face plate for this piece. He did use a steady rest and it almost looks like he used epoxy glue to to reinforce the wood at the screws. I forgot what he said but I suspect he turned it green. He has/had a blog somewhere describing the process.

david_marks_vessel_3.jpg david_marks_vessel_2.jpg david_marks_vessel.jpg

JKJ

[Reed Gray]

When screwing or nailing into end grain, always 'toe nail' rather than drive straight in. If you go in at an angle, you get a much better grip on the wood fibers.

robo hippy

[John K Jordan]

When screwing or nailing into end grain, always 'toe nail' rather than drive straight in. If you go in at an angle, you get a much better grip on the wood fibers.

robo hippy

What type and gauge of screws do you use?

[roger wiegand]

I've used faceplates on endgrain when I've had a lot of wood to waste, but that's not generally the case and it makes the piece longer than it needs to be, exacerbating vibration. So it is rare. I use Spax construction screws (#10, as I recall); in end grain I use relatively long ones (like 2") and fill all the holes. I'll also pre-drill to avoid splitting the wood with so many screws in a tight area.

[John K Jordan]

I've used faceplates on endgrain when I've had a lot of wood to waste, but that's not generally the case and it makes the piece longer than it needs to be, exacerbating vibration. So it is rare. I use Spax construction screws (#10, as I recall); in end grain I use relatively long ones (like 2") and fill all the holes. I'll also pre-drill to avoid splitting the wood with so many screws in a tight area.

The late Lissi Oland, who sometimes turned bowls big enough to curl up in and take a nap, told me she used large sheet metal screws, one in each hole of the huge faceplate. I forgot the gauge and length. But I didn’t see any end grain pieces on her private gallery.

She cored with a chainsaw.
https://youtube.com/watch?v=1PMEJ7rirso&feature=share

JKJ

[Reed Gray]

I used the galvanized decking screws, #10 I think. Been a VERY LONG time since I used a face plate.

robo hippy

[David Walser]

Bill -- You've received a lot of good advice. Here's hoping I don't drag down the cumulative average:

First, I've hollowed many a blank as large or larger than the one you describe using nothing more than a scroll chuck. I believe a 3 3/4" diameter tenon should be more than adequate for a 6" diameter blank that is only 7" long. As others have suggested, a steady rest would be a big help. However, I really doubt one is necessary. Generally, I don't pull mine out unless the blank is over 12" in length. But, that's only my own personal rule of thumb. If a turner wants to 'hog out' the material during hollowing, a steady rest may be desirable with even shorter blanks.

Second, while a scroll chuck is adequate for your purposes, it's NOT the most secure method of holding such a blank. For that, I suggest the following:

• Attach a glue block to a faceplate using quality screws. Fill all the holes in the face plate with screws. The faceplate should be at least 3/4" thicker than the length of screws you'll be using and the screws should penetrate at least 3/4" into the glue block. Also, when turned round, the glue block should be about 2" greater in diameter than the tenon you've turned.

• Mount the faceplate onto the lathe and turn the glue block flat. Then, turn a mortice to fit the tenon you've turned on the blank. The tenon's sides and the walls of the mortice should be straight-sided. The fit of the mortice and tenon does not need to be perfect, but it should be very good.

• Glue the blank into the mortice using plenty of wood glue and allow the glue to cure fully.

This will give you a very secure hold. I've already said I don't believe it's necessary. A chuck should suffice. But then, you're the one who's actually hefted the blank and are the best one to judge your own comfort level.

HTH and good luck!

[John K Jordan]

...
• Mount the faceplate onto the lathe and turn the glue block flat. Then, turn a mortice to fit the tenon you've turned on the blank. The tenon's sides and the walls of the mortice should be straight-sided. The fit of the mortice and tenon does not need to be perfect, but it should be very good.

• Glue the blank into the mortice using plenty of wood glue and allow the glue to cure fully.
...

I really like that idea. Assuming the glue block is the typical side grain (face orientation) and the vessel is end grain (spindle orientation) this would give plenty of side grain glue surface. Do you have guidelines on the depth of the mortise relative to the diameter of the tenon and the size of the vessel? If the blank is green/wet and the glue block dry do you recommend a specific type of glue?

JKJ

[David Walser]

For length of tenon, my rule of thumb is 1/2" in length. Obviously, the length I would feel comfortable with would depend on a number of factors -- the quality of the wood (punky, spalted wooden blanks might benefit from longer tenons), the quality of the mortice/tenon in terms of size and fit (higher quality means shorter tenons should work), the size and weight of the blank, etc.

For glue, I'm generally going to use a PVA wood glue like Titebond Original or Titebond II. That would be true even when working with green wood. But, note that I'm in Arizona and gained most of my woodworking knowledge in a furniture mill in Utah -- both very dry climates. What I consider to be 'wet wood' may be different than the view of most other woodworkers. So, let me try to define my terms: At the furniture mill, we could edge glue boards if the moisture content was 25% or less. But, we wouldn't do anything else with the wood until the moisture content was at 12% or less. For example, if a truck full of lumber came into the mill, it could go through the gang-rip saw and be glued-up into table top blanks even if the wood was damp to the touch (moisture content up to 25%). Then, those table top blanks would be stickered and stacked until the moisture content fell to 12%, when they could be planed to final thickness, jointed and ripped to final width, cut to final length, etc.

Given that experience, I'd be comfortable using a PVA glue as long as the moisture content was 25% or less. Above that moisture content, I'd probably use a polyurethane glue, like Gorilla Glue. I hardly ever use a polyurethane glue. It's really hard to find (what I consider to be) wet wood around here.

HTH