save money & boxwood!
There is really no need to assume that expensive boxwood or even osage is required for chasing good threads. Although an even grained hardwood will yield excellent threads, I recommend that you address the problems with the chasing tool first, and practice on other materials before you dive into exotics. I regularly get clean perfect threads (20 threads per inch) with no chipping on hard maple, but there are a few important things that make this possible.
First, the original grind on most chasers is an angle of less than 90 degrees, which is much too aggressive for anything but the finest grained woods. I think you will find it will cut much finer with the addition of a honed angle on the top of the teeth tips-producing a negative rake that slows down the scraping action.
I have had great success with using a cheap, simple lubricant on the area I intend to thread, which prevents woods like maple from chipping. My secret formula is a 50/50 mix of dishwashing liquid and glycerin, sometimes with a dash of denatured alcohol to help it penetrate. The resulting cutting action produces a very fine cut and a sort of ‘slurry’ rather than distinct shavings. I don’t recommend C.A. glue before cutting the threads, though it may strengthen existing threads somewhat. Personally, I don’t think it adds much, and tends to make most finishes put over it look splotchy. Plus it stinks.
The first strike of any thread is always the critical motion, as the cut can only be deepened. Hesitating with the first cut produces an irregular ‘drunken’ thread that it almost impossible to correct. Critical factors of a good first strike are RPM, tool rest height, tool presentation angle, and proportion of contact area to the number of teeth engaged in the cut.
One way to learn hand chasing is to start off with practicing on a material that is cheap and very even in consistency (no grain)- namely short sections of white PVC pipe (1†to 2 ½†diameter) that you can buy from any hardware store or home center in long lengths with no knots or cracks. (Don’t buy the endangered old-growth variety
). Start with a short (less than 4†long) section on your lathe – you can even do this between centers at first- just turn a cone shape for the headstock and use one of the live center cones. You may need to true it up a little. (OK, this also stinks a little)
Start by engaging the threading chaser into the surface running between 300 to 600 RPM. Once you get a feel for cutting an even thread, try stopping it at a particular point- first a drawn line on the PVC, then a shallow step cut with a parting tool.
Cutting interior threads can be a little tricky, but isn’t that hard once you get a feel for it. I won’t go into a long explanation here- others have written complete descriptions that should get you started.
With properly sharpened chasers, the right RPM, and plenty of practice, I think you should get good results even on hard maple. (if you have trouble, write back to me or come take one of my classes). Hope this helps.
-Michael
