How Flat Do Glue Block Surfaces Need To Be?

[Dom Garafalo]

This is probably a dumb newbie question but I'm curious to know the answer.

I've starting using glue blocks along with Titebond II glue for practice turning some small bowls and plates and wonder how flat do the surfaces of the glue block and the bowl/plate blank need to be before gluing?

Wood turning is a new hobby for me and I have been trying to square both pieces on the lathe then checking the flatness of each with a straight edge held up against a light source to see if there are any gaps. I also check with automotive feeler gauges to insure any slight visible gap or deviation is not greater than .005.

Am I being too precise or not precise enough?

Thank you

[Brice Rogers]

When I use a glue block, I make sure that the outer edge is very flat and check it with just a straight edge. I just use my eye (and sometimes a flashlight) to check for a gap and haven't felt the need to use a feeler gauge.

Some other comments: use a recess in either the glue block or your practice piece. The major holding power occurs on the edges and if you are parting the piece off, it makes the removal easier.

I screwed a piece of MDF to a faceplate and attached a self adhesive sanding disk to it. It is mounted on a lathe I seldom use. So I will sometimes will use the sanding plate to flatten either my glue block or (more likely) the piece I'm going to turn. I use light pressure and regular manual rotation to keep the piece from spinning in synch with the sanding plate. Another way to do the same thing would be to glue some 80 grit to a very flat piece of wood or something that you have verified is flat and just press it against the surface you're trying to make flat.

When I attach a glue block I will either clamp it or weight it to help get a tight joint.

[Dennis Ford]

I don't use feeler gauges but do use a straight edge on both surfaces. Turning a recess like Brice suggested not only makes removal easier, it also makes it easier to create a good glue surface. A gap of 0.005" should be OK but if both surfaces are slightly concave (or worse, convex), that tolerance on each would result in a failure.

[Dom Garafalo]

I screwed a piece of MDF to a faceplate and attached a self adhesive sanding disk to it. It is mounted on a lathe I seldom use. So I will sometimes will use the sanding plate to flatten either my glue block or (more likely) the piece I'm going to turn. I use light pressure and regular manual rotation to keep the piece from spinning in synch with the sanding plate

This is a good idea and one that I'll adopt.

[Geoff Whaling]

use a recess in either the glue block or your practice piece. The major holding power occurs on the edges and if you are parting the piece off, it makes the removal easier.

Dom, Are you mounting the glue blocks to a face plate or into a chuck? Is the blank & glue block dry or green? There are a few important factors to consider when selecting glue blocks and they vary slightly depending upon how the glue block is attached to the lathe and the materials being used.

If attaching to a faceplate, the size of the glue block and the grain of the wood used, plus how you install the screws into the waste block must not allow the grain / wood to split when mounted or when parts of the glue block are turned away.

When using glue blocks with chucks, the same principles of making secure tenons or recesses apply to glue blocks as to making them on or into a turning blank.

The cross sectional area of a tenon, annular ring of the recess, or a glued joint must be appropriate for the materials shear properties, i.e. some woods split more easily than others, so a tennon or a glue joint may fail more easily under "normal" turning loads / forces.

The area of the actual glued portion of the glue joint must be strong enough to hold the material and to resist the shear forces while turning, and not allow the material to move during turning. Creating a recess in either of the mating surfaces of the gle joint reduces that available area or weakens the glue joint because the thickness of the glue does not allow a strong joint to form in that portion of the joint.

The "major holding power" comes from the total area of the glued joint - and it is true that a significant area of glue joint can be achieved with only an outer ring of glue but it still must be strong enough to withstand the turning forces imposed.

Much of this is not documented, so it is more a case of learning from and relying upon the wisdom of a mentor and other more experienced turners. As a newbie it is better to be conservative, nice large flat well made glue surfaces in association with a quality well stored glue (you guys can actually freeze glues - in the tropics thats not an issue) will the reduce hazards of a bowl coming off the lathe from a catch etc. When you gain confidence, better tool skills and more knowledge then start playing with modified glue joints.

[Ben Pierce]

I like to use Gorilla Glue for green blanks. Wood glue doesn't do well w/ green wood- it just can't cure properly. If the blanks are dry, doesn't matter.

[Dom Garafalo]

Are you mounting the glue blocks to a face plate or into a chuck? Is the blank & glue block dry or green? There are a few important factors to consider when selecting glue blocks and they vary slightly depending upon how the glue block is attached to the lathe and the materials being used

I've been experimenting mounting glue blocks to both the chuck and face plates. The blanks I use are all dry and the glue blocks are mostly maple, cherry or poplar.

One thing I found when mounting the glue block with a tenon to a chuck is that a catch will unseat the glue block in the chuck enough to change its rotation so you have to reset the block in the chuck. This is making me think that screwing a glue block to a face plate may make more sense for me at this point.

[robert baccus]

Screwing the GB to a faceplate or single screw chuck is advisable. Be aware that gorilla glue is 50% weaker than any other glues in s recent fine woodworker test. Thick CA loves wet wood and will hold as well as any glue tested. Also it does not need clamp pressure to develope strength. Same for epoxy but it cannot tolerate wet wood. Also i would not even consider poplar as a GB--maple, dogwood, beech and elm are great. Being thick thick CA will provide gap filling in imperfect joints--same foe epoxy. Regular wood glues and gorilla do not.