Quest for the Perfect Miter Joint

[Dave Anderson]

It's too late to help me with my current project but I'm hoping for some advice that will help me do better in the future. After all, isn't that the goal for all us hobbyist woodworkers, continuous improvement?

I made a wheel-around cart for LOML and wanted to practice my miter skills. So, I edge-banded the cart with oak and mitered the corners. Used biscuits and glue to join the banding to the cart. My CMS did a great job of giving me "perfect" 90* corners. The dry-fit was perfect - I could have shaved with those corners!

Then, disaster struck during glue-up. (Boy, those biscuits sure can swell in a hurry!) I was chasing the clock but got everything lined up and clamped in good order. Call it a day - let it sit in the clamps overnight and check the beauty of my work in the morning.

To my horror, the corners weren't perfect, as they had been in the dry-fit. Two of the corners were good but the other two had slipped a bit such that one 45* was sticking past the other and on top of that, there was a slight gap in the joint itself. A little creative sanding will hide the mistake from all but the most discerning eye.

Am I being too hard on myself? After all, I was "practicing" but LOML tells me I'm a perfectionist and adds: "it looks good to me, dear." What can I do next time to ensure that my perfect dry-fit is still perfect after glue-up? My quest is a perfect miter joint.

[David Rose]

I am going to follow Bob Lasley's suggestion and try splines on my next miter that will fit the chop saw. I suspect the biscuit swelling was th culprit.

Have you ever glued up a miter with nothing for alignment? Wow! I even got them "close" on a pine blanket chest. But "close" is not good enough for me either. I think we can do better.

For my next miters on a long joint, like the chest above, I'm going to get a miter lock bit for the router. With no table saw, it will be nice to be able to cut the miter and lock at the same time.

David

[John Lucas]

Dave, that is a great idea. I have been a fan of this bit for about 2 years. I hope you have read my how-to's on it. It makes setting up much easier and perfect corners. Here is one page: http://www.woodshopdemos.com/cmt-lm10.htm

http://www.woodshopdemos.com/cmt-lma10.jpg
even more exact method here: http://www.woodshopdemos.com/sss-4.htm

[David Rose]

I've got dozens if not hundreds of pages off your web site in a binder. ;-) I don't remember the UHMW trick in "your" folder. It is now. Thanks!

The pine chest that I built was mitered with a band saw, then hand planed to smooth it. I'm not sure that I am good enough with a plane on hardwood to keep the edge true without building a guide of some sort. And mitering a large panel on the jointer would be a trick for me. Therefore I was looking for a way to cut the miter accurately to spline the edge. When I started thinking about a miter bit, the lock miter came to mind as a way to save some steps. It sort of fell together naturally. Glad someone was ahead of me on the idea. ;-)

David

[J.R. Rutter]

Another factor to keep in mind for miters is wood movement due to the glue swelling the wood around the joint. This is most noticible when mitering wide, flat stock. If you start with a perfect joint:

Decreasing MC will cause the wide stock to contract, opening up the inside of the corner.

Increasing MC will cause the stock to swell, opening up the outside corner. Adding glue will obviously increase the MC and cause localized swelling. Tight clamping at the corners can help, as can cutting the joints so that they are *slightly* open on the inside corners.

-JR

[David Rose]

JR, are you speaking of a mitered frame like a picture frame or door facing? I can see how expansion/contraction effects this type of joint.

For a mitered corner like the edges of a box, I don't see how moisture would have that much effect. In this case the pieces would just move together wouldn't they?

Tell me if I'm missing something here.

David

[J.R. Rutter]

"This is most noticible when mitering wide, flat stock." Picture frames, casing, mitered cabinet doors are where you really need to consider it. I just like to add info to a thread that may end up in a search result - future reference...