Often with older steel string guitars the years of string tension pulls the neck up, and bends the bracing down at the sound hole. As the neck tilts upward the strings rise higher and higher off the fretboard. Ideally there should be around 4/64" to 5/64" space between bottom of the high E string and top of the the 12th fret. The bass E should be 1/64" or 2/64" higher.
Tip before buying a guitar: If the neck is straight, the action is high, and the saddle is already filed low ... don't buy it. The action on such a guitar can't be lowered without major work. When you sight down the straightened fretboard, by eye or with a straight edge, the tops of the frets should line up at or very close to the top of the bridge (the wooden part). Then the saddle (the white part) can do its work to hold the strings up to the correct height. Even on new guitars this can be off, even very expensive ones, so check before you buy.
This Washburn D43 is only 12 years old, but it spent a few years by the beach in Puerto Rico. The string height at the 12th fret was up to 10/64 inch and the saddle had already been filed as low as it could go. Fortunately the neck was held on by two bolts, and glue only under the fingerboard extension over the body. This makes re-setting the neck much easier, and cost effective for this guitar. To remove this neck I applied heat to the fingerboard to soften the glue, and with putty knives worked it loose. Remove the bolts and we're done:
Removing the neck from this 1956 Gibson LG1 is much more of a challenge. This is held on by a dove tail joint and glue. It wasn't until the 1970s that someone figured out the best way to remove these necks. First I removed the 15th fret and drilled a small hole through the fret slot. The drill bit has to find its way into the small space behind the joint. Then using automotive fuel line hose, a basketball needle, and a cappucino maker, I sent steam into the joint for a couple of minutes to weaken the glue. Some necks come out by hand, some need help from a clamp. This one needed both, as the heel had been cracked and came out in two pieces. First I scraped the old glue off while it was still wet, then I let the wood dry for a couple of days before gluing the heel back together and proceeding. The steam causes "blushing" in the lacquer, which clears off easily with a rag slightly dampened with denatured alcohol.
The frets on the Washburn pointed about 3/16" below the top of the bridge. The distance from the neck heel to the saddle is about 3 times the length of the heel itself; so if I want the neck to point 3/16" higher on the bridge I need to remove about 1/16" from the bottom of the neck heel. I use sanding sticks like a file until I get close:
Then I use sandpaper strips to fine tune the joint. I don't know who thought of this method but it's very clever. By pulling strips of sandpaper through while pressing the neck tight to the body you get an exact fit, a tapered cut that rocks the neck back perfectly. First I make several passes on each side of the heel with 100 grit sandpaper. Then I put the neck back on the guitar to check the progress. As I get closer I switch to 180 grit, or 220. There's only one chance, so I'd rather do a little bit at a time:
On the Gibson's dove tail joint I need to add a shim after the heel has been sanded. Using carbon paper to locate any high spots, I keep scraping, sanding, and checking until the joint is as tight as possible. The pitch has to be right, as well as the string alignment, and the fretboard has to be level side to side.
Now the dove tail joint is so tight I can string the guitar up without glue, and double check the final fit:
After glue up and set up they play like new.