How Closely do Jointed Edges Have to Fit?

[Steve H Graham]

I got my walnut guitar halves edge-jointed, but it looks like there is a 0.008"-0.010" gap at one end, and you can almost get a 0.002" feeler in the other end.

Before I try to figure out how to fix this, can someone tell me how tightly two edges have to fit before they can be joined with glue? I believe there is a miniscule hump on one board, and I am tempted to put the board on its side and hit it with the orbital sander.

These things are roughly 2" thick, 7" wide, and 18" long.173688http://www.sawmillcreek.org/images/misc/pencil.png

[Josiah Bartlett]

Don't sand it, you will round it off and then it won't fit at all. You need to take some meat out of the middle. How did you joint the blanks? It sounds like you got some jointer snipe at the ends.

[Steve H Graham]

Rounding it off is exactly what scares me.

Believe it or not, I jointed it by clamping it to my miter gauge and running it through the table saw. The blade has been adjusted to 90° with a Wixey magnetic protractor, and I tightened up the fit between the miter slot and miter gauge, but I think there is still enough wobble in one of the the miter gauge slots to cause a problem. One of the halves seems perfect.

I don't have a good reference straightedge. Closest things I have are cast iron tool surfaces, the bottom of an old Stanley plane, and a granite dining table.

I have an extremely flat steel DMT stone I could use as a plane. It would take about 3 days, but it would probably work.

I guess I could run it through the table saw again, shaving off as little as possible.

[Chris Fournier]

You are building a guitar and I think that your tolerances should be approaching perfection, not merely adequate or "I can get away with it".

Also you are at the very front of the build, or at the foundation portion of the build. If you accept less than perfection here, your project is really off to a poor start and will only get worse as you build on error after error, sloppy tolerance upon sloppy tolerance.

I would encourage you to engage in the process of guitar building rather than merely running through the steps in a desire to have an instrument in your hands. You will end up with a much finer instrument if you tackle the build with this mindset.

Truth be told the foundation of any successful project is knowledge and then careful planning. Not to be rude but it seems that you would really benefit from backing up a bit, putting down the wood and tools and doing some research and planning. John mentioned this and I totally agree with him.

A crappy instrument is forever!

Whether you have your guitar this spring or next is really of no consequence, whether it is beautiful and playable instrument is the only outcome that matters.

[Tony Bilello]

First off, lose the Wixey. The joint should be close enough that your eye can't see any spaces or gaps and you shouldnt have to use clamping pressure the force the joint closed. When you look at the joint when clamped under mild pressure, you should not see the joint at all. The only thing you should see to tip you off that there is a joint is the grain pattern change.
Unless I am not understanding this, why did you use the miter gauge to make the edge cut and not use the saw fence?

[Peter Quinn]

There should be no gaps at the ends. This will open up over time and fall apart. A slight gap at the center represents a sprung joint and is acceptable, by slight I mean a a few thousands of an inch. I am scratching my head why you would use a miter gauge to staight line rip a glue joint? If push comes to shove make your self a good straight line sled with hold down clamps, and set your blade at 90 with a square. The Wixey box is great for setting off square angles, but IME needs a 90 reference. A good machinist square is around $8' if you don't have one, get one. I'd just joint one edge and use the fence to rip the other parallel, which will facilitate glue up.

[Dave Zellers]

First off, lose the Wixey. The joint should be close enough that your eye can't see any spaces or gaps and you shouldnt have to use clamping pressure the force the joint closed. When you look at the joint when clamped under mild pressure, you should not see the joint at all. The only thing you should see to tip you off that there is a joint is the grain pattern change.
What he said.

[John Coloccia]

Ditto what everyone's said. When it comes to instrument building, there's no "good enough". It either fits or it doesn't. You can get away with a little slop for furniture building. If there's a tiny little bump or gap here and there, most people won't notice. I should show you some of the slop on an Ekornes couch I have. I'd be mortified if it left my shop like that, but for furniture it's acceptable I guess. If you have the tiniest little bump or gap on an instrument, however, someone's going to complain.

What you're finding is what most people find when they start instrument building. Building the instrument is easy in theory, but the tiny little details can be very difficult to get just right. Many just toss it together. Again, it's a good sign that you're perseverating over stuff like this :D

[Steve H Graham]

Okay, now I know what to shoot for. I was afraid I was being too meticulous, and that I was supposed to rely on the glue to fill in the last few thousandths.

I have some machinist's squares. I didn't know the Wixey was inadequate for this job. As it turns out, it worked perfectly, so I guess I got lucky. The cuts are perfectly perpendicular to the front and back of the slabs, as measured by a 6" machinist's square.

Just so people understand, I am not having problems with that angle. I am having problems getting a straight cut from one end of the guitar to the other.

As to why I used the miter gauge instead of the fence: the fence would give me a crooked edge, since it would rely on a crooked edge for reference. The guitar slab had an uneven edge, so presumably, the slab would have been able to rock against the fence as it went through. I figured the miter gauge was worth a shot, since it would only shave off a tiny amount of wood, leaving room for a different approach later. I think the wear in the miter slots makes them tighter at the end than the beginning, and that may make it impossible to get a really straight cut without using a two-slot sled, which would be more stable.

I found a way to sand the sides of the slabs without rounding them off. I clamped them one on top of the other, with the jointed edges facing up. I aligned them so they were flush at the ends. I was able to feel the place where one slab protruded past the other. I used the sander on the joint, and because I now had a 4"-wide surface to deal with, it was not easy for the sander to get so out of coplanarity that it would round anything. I got the gap down to around 0.003", but now that I know that's too much, I will either keep going or find a better method.

I intended to use the router table for this, but I realized it wouldn't work because I would need a bit with a cutting surface at least 2" long.

Would it be possible to do this by attaching a straight MDF piece to the side of the walnut and then using the router with a flush-cut bit, following the MDF edge and shaving a few thousandths off the walnut? I was reluctant to do it that way because I was afraid any excess movement would ruin the edge, and I would have to do the cut at least twice, changing the router height to get the full depth of the wood..

[Tony Bilello]

Why not take that same straight piece of MDF and run it through the table saw rather than the router. After all, the table saw was designed for ripping.

[Craig Ryder]

This is a nice site too if you don't already know about it.

http://www.luthiersforum.com/forum/viewforum.php?f=10101&sid=3605f7c1a155dac9e73767ec4ba91967

[bradley strong]

A straight edge and router can be sued to joint wood. I have used that method to make hundreds and hundreds of glue joints in solid surface counter tops. It's best to us ea bit that will make the cut in one pass. The straight edge must be perfect though. MDF is pretty flexible unless it is really wide. Make friends with someone in your area who owns a jointer. You mentioned having a hand plane. A few swipes of a well tuned hand plane should solve the problem.

[Steve H Graham]

It's really easy to fasten the walnut to MDF in a way that permits routing down the side of it. Two clamps, and I'm done. Far as I know, it's harder to do it in a way that will work on the table saw. Maybe it's the smartest way to go with no jointer, however.

Or I could spend a few days trying to learn how to use my hand plane.

[Pat Barry]

I'd go with the MDF straightedge idea and rip fence. Difficult to clamp though, so use double back tape to hold the guide in place. Make sure you do the bookmatch technique to rip both sides and take very light pass - just a shaving cut with a very consistent feed rate.

[Matt Benton]

Here's what you need to do: Lay the boards together like they will be when glued, then fold them like a book. Clamp them together, then flatten the edges on sandpaper on the granite table or CI extension. Mark the edges with pencil, and you will quickly see how straight they are (or aren't)...

[Steve H Graham]

I think I'm going to give up and make a sled that fits in both miter slots. It will be big and awkward, but it ought to get the job done. Either that, or I'll tape the slabs to MDF. That's a much better idea than anything I came up with. I keep forgetting double-sided tape exists.

The sandpaper idea sounds interesting, but my wild guess is that I would have to attach it with double-sided tape, and the tape would have so much give, it would mess up the whole effort. Am I wrong? That's how I picture it in my head.

Someone was telling me to get a drum sander. I'll bet this job would be perfect for that machine. Is it possible to do this kind of work on those cheap disk sanders that have belts and little tables?

I fiddled around with my plane on some scrap tonight, and I used it to take the major peaks off the back side of the walnut. My hat is off to anyone who can do quality work with one of those things.

I long for the day I can chuck stuff like this on the jointer.

[Harlan Coverdale]

A few dabs of hot melt glue can be more stable than double sided tape. Easy to knock the MDF off after you're done running it through the table saw, too.

[Steve H Graham]

I can't believe I don't have a hot glue gun. I better fix that.

[Russell Sansom]

Sandpaper will always leave an inadequate joint. The best way is by jointing it with a plane. First, though, you can get within less than .001" by ripping in on a table saw. Clamp a fence to the raw side as a guide along the TS fence. Assuming the faces are finished / parallel, TURN the 2nd board over and rip it. The angles will complement each other and the face will come out as one plane. To get them to align during glue up, you need two dowels or a spline. It doesn't have to be large, but the two pieces are guaranteed to slide around when you glue them.
Hand workers make this joint in seconds with a shooting board and a plane. You can make a temporary shooting board in 15 seconds: lay a 3/4" piece of lumber with parallel faces ( mdf ) on an MDF back board. If you search "shooting board" on the web, you'll quickly see how this works. Take a fine shaving. An almost imperceptible hollow along the length of the joint is what you want. You can generally clamp a joint like this with a single clamp in the middle. The clamp springs the tiny hollow shut. Most joints of this kind are done this way because the ends dry and shrink faster than the middle. This way, the joint won't pull apart in later years.
And I am of the other camp. My first musical instrument I built with an exacto knife. I used a couple heavy chairs for clamps. You will converge on making fine instruments more quickly if you give yourself some experience. There's a gap between where you are now and where you need to be to make fine instruments. Reading will help some, but there's no substitute for actual game time.

[Prashun Patel]

Another way to skin this cat is to glue the blanks together as they are now - edge to edge.

once it's pretty dry, rip the joint down the line on the tablesaw against yr rip fence. If you're 'pretty close' already, the glu line will be completely ripped, and you'll have perfectly complimentary pieces that should fit together even better.

I would, though use a blade that leaves as few scoring marks as possible unless you have a reliable way to remove them.

BTW, HD has their 6" jointers on sale in some places. Probably the wisest $250 I've spent in the last coupla yrs...

[Lee Schierer]

If I had .010 gap in the end of a board, I would joint it again and not force the joint together, even in a piece of furniture. Springing a joint is a recipe for failure down the road. Ten minutes to fix the problem now several hours to fix it later, it's your choice.

[doug faist]

Steve - I'm kind of surprised that no one here has mentioned the use of a hand plane for tuning up jointed edges. They're quick, not too difficult and surprisingly accurate with a little practice. For tweaking an edge by 0.001-0.002 like you're discussing, they're ideal.

Just another opinion.

Have fun with your project.

Doug

[Kent A Bathurst]

I think I'm going to give up and make a sled that fits in both miter slots..........Either that, or I'll tape the slabs to MDF............

1] double-faced tape will work fine. You can also screw the slabs to the MDF. There are areas in the raw slabs that will be gone when your project is done [ends, edges, maybe even routed out areas for pickups, controls, ???], and you can screw into there.
2] A sled that fits in both miter slots is more difficult that it needs to be. Getting one runner in place is not a problem, but getting two of them dead-nuts aligned to parallel with each other and the blade can be frustrating. Just take the time to get a precision width on the runner, so it doesn't wobble - use hardwood [and wax it] or UHMW.

Attach the runner, and run the jig through the TS to cut an edge. This edge becomes your cut reference line. Then, if you put some tee-track into the top surface [perpendicular to the runner/miter slot/blade], you can use destaco cam-clamps or the Rockler Deluxe Hold Down Clamps to secure the slab. You just use your fingers to "feel" the slab's overhang along the reference edge, clamp it, and go. This gives you a sled for ripping a straight edge that is adjustable for different widths. That's the way I handle it - I have a "sled" that is 8' x 18" with Destaco clamps in 4 positions. It's a one-trick pony, but it's a real nice pony to have with long, rough lumber.

[Steve H Graham]

I got myself a 2 x 4 sheet of 1/2" MDF and a glue gun. We'll see if I succeed!

[Steve H Graham]

Okay, I bought MDF and some tape and a glue gun. I decided to try the tape approach, largely because Home Depot was out of hot glue sticks.

I cut a 2'-long strip off one end of the MDF, labeled both sides "Factory Edge" in order to ward off the obvious senility-related mistake, and taped a piece of quasi-jointed scrap to the MDF. By "quasi-jointed, I mean I ran it through the table saw to smooth both sides a little. I ran the scrap through the table saw and put it on my granite dining room table, and it appeared to be stable.

I followed up by doing the walnut guitar slabs. I tried to check the perpendicularity of the blade with a 6" machinist's square, but it was impossible. I admit, I didn't try very long, but the teeth and the worn throat plate caused problems, and I decided to flip the second piece of walnut when I jointed it, so it would have an angular error complementing the error on the first piece.

I set the slabs up so something like 1/16" (probably less) came off the sides when I ran them through the machine. When I clamped them together, I was unable to get a 0.002" feeler between them anywhere, and the joint looked great. If you get your face right down on it and use good glasses, it seems like there may be a microscopic visible line, but I think I've gotten the best result possible with my negligible skill set and inappropriate tools.

Now I'll have to start a long annoying thread on gluing slabs together.

Thanks for all the help. Maybe I should cancel that jointer order! The money I spent on the jointer would buy a lot of MDF and tape.

[Wes Grass]

What Prashun said ...

I've done that, sort of, with slabs on sawhorses and a circular saw. Just keep sliding them against each other and adjusting the straight edge until they clean up completely.

My experience with trying to shave an edge on a table saw is that the blade will deflect slightly and generate a crowned edge. It'll be square at the start, and then lean over a bit. Cutting on 'both sides' of the blade fixes this ... hopefully.

As for sanding, over a thickness this big I wouldn't be concerned about loss of joint strength from a sanded surface. Self adhesive paper (the gold stuff from Stewart-McDonald, hint hint) on your table saw should work fine. A little tip, don't push-pull-push-pull on it. Go one way, probably push is more comfortable, and then lift, return, and repeat. I think the drag of the paper tends to wear the 'front' portion of the face more, resulting in a slight crown if you push-pull.

A bit hollow is better than crowned, as noted. But on a slab this thick, wide, and short, it'll be harder to pull together with clamps than a 1" thick table top 5 feet long. So go for as dead straight as you can. As opposed to the comments I made regarding gluing the faces, where you can pull the curve out with the clamps easily, this needs to be as perfect as you can get it.

Ideally, you'd book match the front and backs independently, flatten them, and then glue the stack. But I know sometimes that's not practical given the tools at our disposal. But I'm sure this will be just fine once it's together. It's an electric. They're 'easy'. And I think pretty hard to really screw up, actually. Look at the workmanship on some of the new name brand instruments for proof ;-)

Oh, one more thing. Regarding 'spring jointing', these are beams and the deflection you get from a given amount of clamping pressure goes up by the cube of the length. IOW, make them twice as long and the same clamp pressure will deflect them 8X as much. So if you're gluing up something half the length of your buddy, you've either got to be 8X flatter than he is, or use 8X the clamp force. The first option is more difficult. The second, probably not practical.

[Steve H Graham]

I just looked at Prashun's suggestion, and it occurs to me that for someone of my skills, there would be a good chance that the saw would end up outside of the original joint, by the time it exited the slab. You would have to have the original joint very parallel to the fence. The MDF method worked very easily, so thank God, I won't have to try the other ways.

Next time I want to make the middle of the guitar thicker and use 3/8" slabs on the front and back. I'll put the middle together first and then do the front and back, serially instead of together.

This has been a ton of fun, even with the frustration. I think this wood will become a guitar, in spite of all the errors, and I'm getting some weird ideas for embedding other contrasting woods in the next version. If I can rout out channels between the slabs and fill them with mahogany and maple and so on, I can get some neat built-in ornamentation once the guitar's outer shape is routed. I just hope the tone will be okay.