woodworking, woodturning, woodcarving, wood, turning, AWGB, Association of Woodturners of Great Britain, woodwork, carving, carpentry, woodwares, treen, gifts, Robert Sorby, Henry Taylor, Ashley Isles, timber, forestry, bodging, gouge, skew, lathe, bowl-turning, spindle-turning, crafts, woodcrafts, trees, Willy Vanhoutte, wivamac, Peter Hemsley, Floyd, t/f/c, wood-turning, wood-carving, wood-working, chip-carving, gallery, calendar, events, seminar, veneer, yew, maple, oak, ash, sycamore, burr, burl, plane, chisel, holz, schnitzen, schnitzer, schnitzarbeit, schnitzwerk, drechsler, drechslerarbeit, holzarbeit, holzwerk, zimmermann, holzdrechslen, drechslen
Screwcutting

Practical Screwcutting with Bill Jones
courtesy of Crown Hand Tools Limited, Sheffield, UK

Practical Screwcutting with Bill Jones

Every turner sooner or later needs screw threads. Taps and dies can be used for a few things but there is no more convenient way of making screw threads than by hand chasers. But using them oneself presents a hurdle one must jump as in riding a bicycle or learning to swim. And once you've got it you're there!

At least four factors have to be considered:

  1. Lathe speed
  2. Position of tee rest
  3. The chaser itself  condition, length of handle, etc. and
  4. Method of striking the thread.

All may be arranged differently with equal chance of success, the only inviolable rule being that, regardless of lathe speed, for every complete revolution the screw tool or chaser requires to be traversed to the left a distance equal to the interval between two of its teeth. This is purely academic as it is really 'getting the knack of the thing' that counts .... and you mustn't take no for an answer!

New chasers need a little preparation. They must be free to slide easily over the tee rest or armrest, so the rubbing surfaces must have the sharp corners or any rough places removed on the grinding wheel and oilstone. Sharpening is done on the top face which I hollow grind. The fine grinder leaves the chaser perfect for use but sometimes a little 'fierce' so I often soften it by rubbing it flat on the India oilstone. If the first, or leading tooth is dull  and this can be seen if you hold it to the light  the chaser will not cut because the leading tooth does the essential work. The top surface of the tee must be perfectly smooth and rubbed with an oilstone. Additionally a rub with a candle helps the free movement greatly, not only for chasers but for turning tools as well.

Chasers should be fitted into comfortable handles not less than 5" (125mm) long. The overall length of tool in handle should be between 9" and 12" (225300mm), the longer handles being necessary for coarser threads.

For the purpose of practice any odd pieces of hardwood can be used  anything between 10 and 50mm diameter will be suitable. Fix a piece in the lathe, turn it cylindrical and face  you may round the leading edge for that is the first place where the thread begins. A medium chaser 20 t.p.i. will be easier than anything coarser.

Fix the tee rest parallel with, and about 20mm away from the work and a little below centre height. Hold the chaser at the end of the handle with the right forefinger running along it and place the tool upon the tee. Now, whereas the woodturner will have most of his left hand above the tool, hardwood turners do not, because it's a different way of turning. The fingers of the left hand are all below the tee with the thumb on top, or at the left side of the tool. Further, whereas in normal woodturning the cutting tool is brought from above downwards into cut here, the tool is brought from below into a scraping mode with no bevel rubbing.

Figure 1

The strike is made upon the rounded corner, Fig. 1, but the chaser is sloping downward as in Fig. 2. Everything is done very lightly to allow the tool to cut its own thread. This is the vital message  you never force a chaser to try and cut your own notion of the correct traverse, but you move it gently to the left keeping it always moving to the left in one second movements, instantly removing it each time. If you stop for just an instant the spiral will be broken and a series of rings will result.

Before starting the lathe, try making the movements to the left on the corner and sloping as in the illustrations. Hold the tool very lightly to allow it to cut its own thread. When you feel comfortable with the movements, start the lathe. 200  400 r.p.m. is about right and see how you get on. If you have no success here are some reasons:

Figure 2
  1. The wrong traverse: this is arrived at by trial and error, so vary it until you get a thread.
  2. You are holding the tool too tightly, so loosen up and merely move it along, allowing it perfect freedom to cut it's own thread.
  3. The tee is wrongly positioned  the top should be at centre height and at least 12mm from the work to give plenty of freedom: experiment until you find it works for you.  When success is yours, turn all threads off the work and strike more until you gain confidence. Any enjoy it!

Using an inside chaser is no harder than the outside if you work on a bottomless hole such as a hardwood ring. Fix a disc in the chuck 1/4" 6mm thick and open a hole out 25mm diameter.

Fig. 3 shows the position of the tee rest with the left end close to the ring and the crook of the armrest projected over it.

figure 3
Figure 4

The handle of the armrest is held into the side by the left arm so that it allows the inside chaser to make contact with the work at about 8 o'clock as shown in Fig. 4. The left thumb holds the chaser down, the fingers under the tee. The chaser handle is held lightly in the right hand as a gentle traverse is made into the hole allowing it to strike a thread. Each traverse must move into the cut and be instantly removed and withdrawn. It is never allowed to remain still like a turning tool because it will break the spiral and make rings.

To get a true thread the speed of the traverse has to be the precise one, commensurate with the lathe speed but in hand chasing this is simply a knack acquired by practice. We are practising on a ring so we'll try one traverse per second with a lathe speed of  say 400 r.p.m. If the traverse is too fast, we might not get a thread. If too slow it might be a double start thread. If it is otherwise incorrect it might be a kinky thread! But indubitably you can't possibly try for long before you'll find you've cut an excellent thread. Enjoy it for a while, feeling your chaser entering evenly and truly and experiencing "the smooth crisp truth o' the screw". Then, with an inside tool, remove all trace of the thread and strike a new one exactly the same. That wasn't hard  was it?

Sixteen t.p.i. is a good size for hardwood boxes. 18 and 20 t.p.i. are also permissible and a little easier to cut. The more coarse the thread, the more material it removes, which requires a little more skill. If harder materials are used, such as plastics, ivory, bone etc., much finer threads  which are very easy  may be used. Two common sizes are 26 and 32. In fact, I'd say the following sizes are most commonly found: 16, 18, 20, 24, 26 and 32.

To be a good screw cutter you have to do a lot of it. Set yourself a specific job of making 50 screwed boxes. Even if half are useless you'll find by the end that you CAN chase thread and that puts you head and shoulders over the common or garden woodturners. And the best of success to you.

Bill Jones is the last of a family who have been ivory and hardwood turners for 150 years.  Bill has over sixty years of experience as a professional and now spends his time passing on his unique skills to a new generationi, in the hopes that some will survive.

©1997-2004 P. Hemsley.  The information on this website is the copyright property of Peter Hemsley.  Whilst reasonable efforts are made to ensure the accuracy of information presented, no liability can be accepted for errors in this information nor for contingencies arising therefrom.  If you are inexperienced in any aspect of woodworking, we would strongly counsel that you take a course of formal instruction before commencing to practice

The Floyd Consultancy

woodworking, woodturning, woodcarving, wood, turning, AWGB, Association of Woodturners of Great Britain, woodwork, carving, carpentry, woodwares, treen, gifts, Robert Sorby, Henry Taylor, Ashley Isles, timber, forestry, bodging, gouge, skew,

lathe, bowl-turning, spindle-turning, crafts, woodcrafts, trees, Willy Vanhoutte, wivamac, Peter Hemsley, Floyd, t/f/c, wood-turning, wood-carving, wood-working, chip-carving, gallery, calendar, events, seminar, veneer, yew, maple, oak, ash, sycamore, burr, burl, plane, chisel, holz, schnitzen, schnitzer, schnitzarbeit, schnitzwerk, drechsler, drechslerarbeit, holzarbeit, holzwerk, zimmermann, holzdrechslen, drechslen, bois, ééniste, charpente, travails du bois, sculpture, sculpture sur bois, ébénisterie, tournage, tournage sur bois, hout, houtdraaien