How to Use Wood Taps and Screw Boxes

One of the less-known hand tools, yet one most home craftsmen often need, is the screw box, a device for cutting threads on a wooden spindle. The screw box is matched with a corresponding tap for internal threading, the two permitting all of the varied applications of the screw thread. Tap sizes run from ¼ to 1-1/2 in. in diameter, the corresponding screw box cutting a somewhat smaller spindle diameter. Practically all of the tools are handmade, with the tap matched to the screw box and sold as a set. Other than tap diameter, specifications vary considerably so that the table in Fig. 6 is only an approximation.

General Construction: The working parts of a screw box and tap are given in Fig. 6. The metal thread cutter fits in a recess in the wooden body and is non-adjustable except that it can be driven forward to compensate for wear. The thread shape of the tap is approximately 68 deg., 8 deg. wider than the standard 60 deg. used for threads in metal. Screw boxes are available for cutting thread shapes from 65 to 110 deg. Taps present no difficulty in conditioning or using. The cutting action can be improved slightly by giving the cutting edges more curve with a tiny grinding wheel as in Fig. 5.

Using the Screw Box: The spindle to be threaded should be turned to a snug fit inside the lead hole of the screw box, and the end of the work should be tapered a little to allow the thread to start easily. The screw box is turned like a tap wrench, Fig. 4, using a little pressure at first to get the cut started. If the cutter is sharp and properly set, the job is simple and fast. Evidence of a good cut is a continuous peeling chip, as shown in Fig. 4.

It is helpful in all cases to oil the work. Use linseed oil, rubbing oil, or castor oil – anything to bind the surface of the wood. However, don’t expect oil to work wonders. If you are getting a fair thread with a slight amount of tearing, oil will make it better but it will not overcome a dull cutter or improper setting of the cutter.

Using Taps: The tap will tear the wood slightly on entering and emerging. Where appearance is important, the best way to eliminate this is to use thick stock and then slice off 1/16 to 1/8 in. to remove the torn portion. Fair results can also be obtained by clamping the work between two wood blocks. It is impractical to tap into end grain. When possible, the tap should enter from the flat grain side instead of the edge grain. Getting a straight start is the only difficult part of tapping. Chucking of small taps in the drill press, Fig. 2, is recommended. Tapping should be done before the work is cut to size, Fig.1, as strain on delicate edges, especially near ends may cause splitting.

Cutter Mechanics: The main secret of a clean thread is minimum clearance behind the cutter. As a refresher, it is worthwhile to take a wood chisel and do some cross-grain paring, Fig.7. You will find that a low angle of clearance – practically touching the wood – is essential for clean cutting. Observe also that if the chisel is held at a slant across the direction of cut a better slicing action will be obtained.

These two features – minimum clearance and skew angle – apply to the screw box cutter, Fig 8. Each side of the “Vee” cutter can be considered as a separate chisel. The clearance behind the cutting edge must be minimum and the cutting edge should attack at a skew angle. If a new box does not cut, a systematic check should be made. First, see if the cutter is set too deep (too far forward). This is a common fault in a new box and can be corrected by regrinding the cutter, carefully maintaining the same angles. Next, check the cutter position in relation to the guide thread in the cutter body. The cutter point must align with the thread, as can be seen in Fig. 6.

Now turn the spindle into the box and check the slant of the cutter – it should align with the pitch of the thread as shown in detail A of Fig. 11. Any departure from this position will increase the clearance angle on one side of the cutter and decrease it on the other side as shown in detail D. Carefully examine the cut thread on a spindle at the point where the cutter stopped. If the cut is shallow and appears polished, it means the cutter lacks clearance on one or both sides. If you want to check this all-important point exactly, paint the underside of the cutter with bluing and then test to see if the bluing rubs off. Actually, the bluing may rub off with a perfectly set cutter but it should not rub off heavily. Rubbing can be corrected by regrinding or pushing the cutter back in its slot. However, in no case should the chisel wedge be reduced to an angle of less than 20 deg. If the bluing does not rub off, the cutter evidently has clearance, but it may have too much clearance, and this is the condition that causes tearing. See Figs. 9 and 10. Excessive clearance can be reduced by regrinding or by pushing the cutter forward. It should be noted that the inner of the two chisels forming the Vee knife requires less clearance than the outer chisel because of the slant of the thread. Slight changes in the clearance angles can also be obtained by wedging the cutter up or down in the slot, but any radical change obtained by removing wood from the cutter slot should be avoided until everything else has been checked thoroughly.

Thread Shape: In addition to giving the cutter a slicing action, the skew bevel controls the shape of the thread. A skew angle of 50 deg. will give a thread shape of about 80 deg., as shown in Detail C, Fig. 11. Threads up to 90 deg., Fig. 15, are satisfactory for most work. If the skew angle is decreased, either by grinding or by tilting the cutter, the thread will be sharper, as shown in Detail E. The small skew angle also gives more clearance. It should be noted that the skew angle is measured on a diagram of the work and is not the angle of the cutter itself. Attack and tangent angles, Detail C, are not fixed values. The cutter will work at any point around the circle and can be set from exact tangency to about 40 deg. above the tangent from the contact point.

Most cutters will cut a more or less buttress thread, although not the extreme shown in Fig. 14. If the cutter is working well, it is not worthwhile bothering about this condition. However, if you are using inserted threads, Fig. 13, it is obviously good practice to reverse the thread as needed to resist the load. A thumbscrew should be reversed from the Fig. 14 position, and the same applies to a hand screw, Fig. 16. A definite buttress thread can be corrected by cocking the cutter slightly in its slot or correcting the cocking which has caused the off-balance thread.

Shoulder Threads: With the cover plate in place, the screw box will thread to within about ½ in. of a shoulder, Fig.13. With the cover plate removed, and the cutter clamped in place, Fig. 12, threads can be cut to within 1/8 in. of a shoulder. If a complete thread is required, the remaining 1/8 in. can be filed by hand or an inserted thread can be used.

Grinding the cutter: Cutters are carbon steel and the thin chisels (20 to 30 deg.) must be ground carefully, wetting the cutter to avoid burning. The best grinding position is shown in Figs. 17 and 18. The bevel must be perfectly flat. Grinding will raise a small projection at the extreme tip of the cutter as indicated in Fig. 20. This is controlled by the round at the base of the Vee groove – the more round, the more projection. Normally, the slight round obtained with a triangular file is sufficient. The cutter should not be ground unless chipped. Once ground, it should be honed to a perfect edge, and will stay sharp by repeated honings. If grinding is required for any reason, the cutter will retain pretty much its original Vee, but will have less height. Surplus metal at the bottom of the cutter should be ground back squarely, Fig. 19.

Woods to Use: Threading a wooden spindle is a delicate job of cross-grain cutting and only a few woods will hold the cut without crumbling or tearing. White birch and hard maple are commonly used. Other good woods are apple, pearwood, cherry, and walnut. Hard maple is by far the best and, since it is readily obtainable, there is no point in using anything else. Fig. 3 gives typical examples of how different woods thread. The tap will tear on the cross-grain portion of almost any king of wood, but at the same time it will make a workable thread in almost any kind of wood. As this part of the work is concealed it does not demand the perfection of a spindle thread.

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