A Brief History of my screw making efforts

The following post gives an overview of the development of the wooden screw making methods I’ve developed. Along with the tools and methods and how they evolved I will provide a more personal story line for those who are interested.

Above are the first taps and dies that were made using the 1977 Fine Woodworking article. In this approach a tap is first made which is in turn used to make the die.

I have always been fascinated by mechanical things and was immediately drawn to this FWW article. I didn’t actually attempt making the tools until the early to mid 1980s when I was living in Madison, Wisconsin. My spouse, Pati Scobey, (www.patiscobey.com) was in grad school at UW-Madison studying under Warrington Colescott and Walter Hamady. Book Arts were a major part of her studies which encouraged me to pursue paper mould making and making bookbinding tools. We rented a small house next door to Ace Hardware on ‘Willy’ Street which was very convenient for an aspiring inventor.

The screws made with this tool were not very satisfactory. I was looking for a dependable way to make high quality wooden screws so that I could produce book presses for sale. At least half of the screws were unusable with badly chipped threads. I put my screw making ambitions on hold for a few years and concentrated on making paper moulds.

Around 1996 I was asked to teach a workshop at Paper and Book Intensive. By this time we were living and working in Michigan. (I had already taught a workshop in paper mould making for PBI in 1991.) As I recall I received a phone call from Pam Spitzmueller inviting me to teach and it was she who suggested the subject of making finishing presses for bookbinding. Soon after that I received a call from Tom Conroy (who I had never met) who generously offered a lot of very good advice about what makes a good bookbinding press. He also mailed me copies of his extensive notes and drawings and a copy of an article by Derek Beck which I found particularly helpful. (Derek Beck, New Bookbinder, Issue 1, 1981) What seems amazing in retrospect is that I agreed to teach this class without actually having any workable tools and methods to make wooden screws! I was 41 years old; evidently still young and foolish enough to give it a try. It worked out OK though.

This contraption was my next attempt at making good wooden screws built with the idea of using it for the upcoming class. It is a sort of hand powered machinist’s lathe complete with a lead screw. By changing the chain sprockets it was (theoretically) able to produce threads of several different pitches, including two, three and four threads per inch. Friction (a lot of it!) was a major problem as was excessive ‘play’ in the lead screw propelled tool post. However I was able to make 3 wooden master screws (in the foreground) which became essential parts in the next attempt. As it turned out these wooden master screws were used to make hundreds of screws before they also became obsolete.

Another view of the lathe and the master screws produced on it.

This is the first tool that could actually produce very nice wooden threads. This is one of two ‘threading frames’ that I took with me to Penland in North Carolina to teach the PBI class. As you can see the master screw is made of wood. I had discovered the method of making a two part cutter by this time (very exciting!) and the method used here is basically identical to what I still use. Since then it has been a process of refining things to make the same process easier and more reliable.

The 4TPI threading apparatus shown along with the original taps for 4TPI and 3TPI threads. There was also a larger tap for tapping a 1-3/8″ diameter hole for a small lying press. And there was another 3TPI threading frame that could make both 1-1/4″ 3TPI threads and 1/5/8″ 3 TPI screws. I still use all of these pitches and sizes. We used the same router lathe that is pictured in previous posts. The process was very successful; we made a lot of presses. But it was slow; the threading was driven by hand with the knobbed crossbar and the router lathe was hand cranked.

Now back at home in Michigan I knew that I could make good wooden threads but I wanted to speed things up a bit. The next idea was to drive the tap and die with a bit brace. With the die the bit brace was mounted in a wooden rack that traveled along on ball bearing drawer glides. The tap could be driven by hand with the cross handle or with the bit brace by means of the bolt head on top. This turned out to be a brief transitional phase as it soon led to the next step (below).

The next advance was making this ‘universal’ threading frame. It was designed so that dies and master screws could be switched out to produce any pitch wooden screw. As you can see the original parts of the first threading frame have been ‘cannibalized’ to make this tool which functioned quite a bit better than the one before.

I made a lot of wooden screws using this set-up. The original wooden dies were capable of making great threads but wear from use was a constant concern. As the parts of the die wore down the adjustment of the thread had to be closely monitored so the thread would be well formed.

For the wooden master threads to work well they had to be firmly fitted to their nuts without any play which would have caused a badly cut thread. This made for quite a bit of friction. I can’t remember when I thought of switching to steel master threads but it was a wonderful solution. The friction decreased a lot making the screw blank much easier to drive. And there was no significant play to worry about. They aren’t cheap but their use transformed the threading apparatus into a very efficient hand powered production tool.

I kept the wooden master threads lubricated with beeswax which built up at the margins. You can see the paper shim added to reduce ‘side-play’ which if excessive causes the thread be ruined.

The ‘half nut’ which was tightened against the master screw (loosened here).

Inside the ‘nut’. Wear was always a concern.

The last step was to re-think the design of the dies. The original dies controlled the thread depth by adjusting the position of the cutter in relation to the ‘hole’ (slot) n the die that the blank passed through. These were simpler to make but more complicated in use (than the later design). The screw needed to be held firmly against the back of the slot by the pressure bar on the left. The depth of cut was controlled by the ‘eye bolt’ at the bottom right. Each time the eye bolt was adjusted the other wing nut needed to be re-tightened. And for reasons too complicated to relate here the pressure bar needed to be constantly re-tightened too. (The pressure bar surface and die surfaces wore unevenly due to the multiple passes of the screw being formed as the thread became progressively sharper. I hadn’t started using very hard HSS wear bars.)

Another view of the die.

The pressure bar on the left (here slightly opened) would press on the surface of the screw blank to keep it against the back of the slot. The cutter mount (on the right) would be adjusted IN RELATION TO THE BODY OF THE DIE (compare with below).

The new style die (pictured in many of the posts) turned these concepts ‘on their heads’. In it the cutter is mounted in a shutter that is adjusted IN RELATION TO THE SURFACE OF THE SCREW BLANK. The adjustment disc (see other posts) has a dual function of both controlling the depth of cut while at the same time pressing against the screw blank to hold it in the slot.

The last step of all in ‘perfecting’ this method of wooden screw making was the master screw driven tap (not pictured here). It also makes for a much more efficient method than what came before in producing the internal threads. It is very dependable and low maintenance.

Summary

The process for making wooden screws that I’ve presented in the previous dozen or so posts is the culmination of many earlier efforts, including many mistakes and a few dead ends not covered here. Only this last method using a combination of ‘off the shelf’ steel master screws and fittings along with custom made dies really achieves a workable method of production for a small ‘cottage industry’ making wooden book presses or other similar tools. But now since I’m getting a little tired of making anything ‘in production’ I hope that someone else might build on what I’ve learned. It seems too good to waste!

Tap Cutter Geometry

This is my 14th post on the topic of making wooden screws. Most of the main points have been covered and the posts may even give enough information to create your own tooling. If any of the information is confusing or if you have questions please ask; you may provide topics for future posts.

The cutter for the tap is made from a piece of High Speed Steel drill rod, a little shorter than the diameter of the tap and approximately the same diameter of the thread pitch. For example, 1/4″ diameter would be good for a 4TPI thread. I make two at a time from one drill blank and then cut them to length. I can’t show the steps in making the cutter at this time but it the drawings and photos should give a good idea of the finished shape.

(above) View from the bottom showing the three relief angles at the cutting edges; one along each angled side and one to blunt the point. These are shaped last.

(above) A view from the end. The dashed (hidden) line shows the angled face that the (cup point) set screw will press against to hold the cutting face at the correct pitch angle in the tap. The flat cutting face is ground to half of the diameter of the cutter.

(above) A top view. The cutting face is shaped to the thread angle along its sharp edges. As mentioned previously my threads are angled at 80 degrees. The rectangular area is the recessed flat for the set screw to press against. (Side view below.)

A good way to start is to grind both ends to the thread angle.

Calculating the Pitch Angle

For a particular screw configuration both the tap cutter and the die cutter need to be set at specific angles to cut well. The tap cutter should be angled to match the pitch angle at the minor diameter of the nut. (see below) The die cutter (discussed in a previous post) should be set between the two pitch angles of the minor and major screw diameters so the cutter is ‘aimed in the right direction’ to follow the path of the thread being cut.

Below is an example of the method that I use to find these angles. A separate drawing must be made for each screw configuration. To get better accuracy the drawing is scaled up. I scaled this drawing up 2-1/2 times (1 : 2.5) because anything larger would not have fit my scanner.

The pitch for this screw (3 TPI or .333″) is represented by the vertical distance between the two parallel horizontal lines. Using Pi (3.14) and the major and minor diameters the 4 circumferences are calculated and laid out in vertical lines (A,B,C,D). Angled lines are drawn where the pitch and circumference intersect. These angles are measured with a protractor. This drawing shows that the recessed, angled face on the top of the tap cutter should be ground at 6 degrees to align the cutting face at that angle when the set screw is tightened. This orients the face of the die cutter at 90 degrees to the path of the thread at the inside (minor diameter) of the nut.

The two piece die cutter (see the previous post) needs to be set between the minor and major screw pitch angles; in this case between 6-1/2 degrees and 4-3/4 degrees. I would shoot for about 5-3/4 degrees. The main consideration is to leave space behind the cutting edges that is roughly equal on both sides.

Incidentally, this drawing is a good illustration of the fact that a screw is essentially a wedge wrapped around a cylinder. One of the most basic tools.

Some Interesting Facts about Wooden Screws

These observations may seem arcane, but I find them interesting.

Metal screws and wooden screws have some interesting differences. The geometry of screws is mind boggling (to me at least). As a practical matter the following observations are not very important. I spent a very productive decade or so making good, functional screws before I noticed that things were not quite as they seemed…

Metal screws are cut with a scraping action. The face of the cutter is set at the center line of the rotating screw blank. I have tried to illustrate this on the right in the illustration above. The radiating lines represent stages of the cut (every 10 degrees) to help visualize the shape of the face of the thread that is created. On the left I show the slicing/shearing action that forms a wooden screw thread. Again the stages of the cut are indicated by the radiating (but slanted) lines. When examining the finished screws, in either case you would be able to lay a straight edge along one of these (imaginary) lines showing no gap. But because the wooden threads were made with an angled, slicing cut they will be slightly hollow when viewed from the side. The drawing below is much exaggerated to show this effect.

If that isn’t enough…

Another way that the shape of the thread is distorted involves the V-shaped die cutter and its angle in the die. The sharp edges are ground to a forward slant on both ‘wings’ to create the shearing cut needed for cleanly cut wooden threads. But when the cutter is properly set the effect is to change the angles of the left and right cutting edges in respect to the axis of the screw. I have tried to show this above. The longer shearing cut of the left wing will create a greater concavity than the shorter shearing cut on the right. The two faces of the threads will be slightly different; more and less hollow and with slightly different angles. For large diameter screws with smaller threads this will be nearly undetectable. For small diameter screws with very large threads the difference will be more pronounced. (It may be possible to adjust the raking angles on the two halves of the cutter to minimize this. Perhaps I’ll try this sometime.)

You might be able to see the (very slight) difference between the left and right faces of the threads in this photo. This screw has the steepest pitch angle of my screws, with a large 3 TPI thread on a 1-1/4″ diameter screw and thus has the most hollow on the thread faces. Not much though!

Die Cutter Geometry

A die cutter is made from a pair of HSS lathe bit blanks. The blanks are square in cross section and around 2-1/2″ long. 1/4″ square blanks are big enough for my biggest thread, 3 TPI. A larger 2 TPI thread would require 5/16″ blanks. 3/16″ square blanks are good for my 4 TPI threads and I used 1/8″ blanks for my 5 TPI die cutter. In the future I wouldn’t use anything smaller than 3/16″ square; the 1/8″ blanks are too small to easily work with.

The most critical parts of the cutter are the two outside angled surfaces. These should be ground as smooth and flat as possible. They are equivalent to the back of a chisel or plane blade and will not be altered by honing. This jig grinds these surfaces to the proper angle for my 80 degree threads; 77-1/2 degrees. As you can see one edge of the jig is shimmed so that the magnetic chuck holds the jig at a slight slant to create a relief angle. The angled cutting edge of the 77-1/2 degree cutter makes a shearing cut. Since the surface of the screw is curved the 2-1/2 degree reduction compensates for the shear to create an 80 degree thread. I adapted this cutter geometry from a 1977 Fine Woodworking article by Richard Starr. His cutter used a 57-1/2 degree cutter with a 50 degree shear to make 60 degree threads.

This is a ‘mock up’ since I am not making a new cutter at present. I have clamped the halves of a completed cutter in the jig to show how it works. Normally this would be the first step in the process of shaping the blanks. The second step would be to grind the 50 degree bevel on the ends to establish the shear angle. Then the inside bevels would be ground away to produce the cutting edges.

The finished cutter must be set in the die at the pitch angle of the screw. I calculate both major and minor circumferences of the screw. Then I make a scaled-up drawing using these and the pitch to find the two angles. Set the cutter between these two angles to give clearance behind both cutting edges. The midpoints of the cutting edges at full depth (the parts actually engaged in the wood, not including any unused portion at the ‘wings’) should approximately bisect the center line of the screw being cut.

Thread Design for Wooden Screws

This post will attempt to explain a method of designing a thread of any size; a necessary step before starting to make the threading tools; tap and die.

(1) Choosing a Thread Angle

The first step is to decide on the THREAD ANGLE. Many threads in metal and in wood have 60 degree thread angles. I prefer a flatter thread of 80 degrees. This way the threads are less sharp and less vulnerable to chipping. They are also a little stronger since the forces created by the angled faces of the matching threads tend even more away from shear and towards compression as screws are tightened. All of the presses pictured in my website and posts use 80 degree threads. I will show how to design 80 degree threads using inch measurement. The same principles can be applied to other thread angles and to metric measurement.

(2) Establishing a ratio of Thread Height to Pitch

This triangle represents the geometry of a single thread in cross section. Measurements taken from this simple drawing can be used to calculate a wooden screw and matching nut of any size and pitch. There is likely a way to determine the height of an isosceles triangle with an 80 degree apex angle. Since I don’t know the math I use a protractor and rule to make an accurate drawing to measure from. Making the drawing large insures that the measurement will be more accurate. For this method all you need are a (somewhat arbitrary) thread PITCH (here represented by the 8.000″ base) and the THREAD ANGLE which is 80 degrees. Once I completed this drawing I took the measurement of 4.775″ from the drawing using a vernier caliper. This establishes a ratio which can be used to calculate the BASIC THREAD HEIGHT of ANY thread with an 80 degree angle.

(3) Using the ratio to calculate Thread Height.

Using 8″ as the pitch of this imaginary thread (which would make a truly huge screw) works very well for my collection of thread pitches. These include 4 TPI (1/4″ pitch), 3 TPI (1/3″ pitch), and 5 TPI (1/5″ pitch). All of these can be divided evenly into 8″ so determining the BASIC THREAD HEIGHT of any of these is simply a matter of dividing 4.775″ by the appropriate number:

8.000″/.250″ (1/4″ pitch) = 32. It follows that 4.775″/32 gives a basic thread height of .149″ for a 4 TPI thread.

8.000″/.333″ (1/3″ pitch) = 24. It follows that 4.775″/24 gives a basic thread height of .199″ for a 3 TPI thread.

8.000″/.200″ (1/5″ pitch) = 40. It follows that 4.775″/40 gives a basic thread height of .119″ for a 5 TPI thread. (I will use this thread as an example throughout this post.)

At some point I may attempt a 6 TPI extra small screw and a 2 TPI extra big screw. Both of these (or any pitch actually) can be calculated using the same 8 : 4.775 ratio.

(4) Truncated Thread Height

The next step is to calculate the TRUNCATED THREAD HEIGHT. It is best for the actual wooden threads to be truncated. Having sharp threads (internal or external) serves no useful purpose and makes the wooden screws vulnerable to damage. Subtracting 1/8 of the basic thread height from both the top and bottom of the basic thread height will give the truncated thread height. This leaves 3/4 so multiplying a basic thread height by .750 will yield the result that you need. As an example the truncated thread height for my 5 TPI thread would be .089″ (.119″ x .750 = .089″).

The drawing above shows how the original full-sized thread and the truncated thread fit within the context of a threaded hole.

(5) Establishing Major and Minor Nut Diameters

The best way to design a new thread is to start with a standard size TAP HOLE for which tooling will always be available. For wooden screws a Forstner bit used in a drill press is the best approach. Choosing the actual hole diameter and adding the truncated thread height (twice; once for each side) will yield the next two dimensions; the MINOR DIAMETER OF THE NUT and the MAJOR DIAMETER OF THE NUT. (“Nut” here refers to anything that includes a threaded hole even though it seems odd to refer to a press jaw as a “nut”). The minor nut diameter is fixed by the size of the hole which is bored to receive the threads. Using a tap to scrape away waste to create a thread to the depth of the truncated thread height yields the major nut diameter. Using the 5 TPI thread as an example the minor nut diameter is .875″ diameter (7/8″). Adding the truncated thread height (x2) to that yields a major nut diameter of 1.053″. (.875″ + .089″ +.089″ = 1.053″).

This drawing should be imagined as a screw passing through a threaded hole. The narrow spaces between the pairs of outlines represent the necessary clearance, ‘looseness’, or ‘play’ between the parts.

(6) Sizing the Screw for proper Clearance

The next step is to determine the dimensions of a screw to fit in the threaded hole. The screw needs to be sized to create the proper amount of CLEARANCE. Clearance is needed to insure that the screw and nut can work freely through all seasons of the year. As humidity increases in summer and decreases in winter dimensions change slightly. The pitch of the screw will remain constant (since wood doesn’t change dimension longitudinally) but the pitch of the nut will change (since wood changes significantly across the grain). If that isn’t enough the screw and the threaded hole will both become slightly oval during parts of the year. Creating enough clearance will prevent any of these fluctuations from becoming a problem.

To create clearance the nut diameters are reduced by same amount that was used to truncate the thread height earlier. To calculate the MAJOR DIAMETER OF THE SCREW subtract the same truncations (1/8 of the basic thread height) from the major nut diameter. Again I will use our example of the 5 TPI screw. First, divide the basic thread height of .119″ by 8 to get the result of .015″. This needs to be subtracted from both sides of the major nut diameter to create the MAJOR SCREW DIAMETER. In this case 1.053″ – .015″ – .015″ = 1.023″. This in fact is the size of the opening in my die which is used to make these 5 TPI screws.

Making the screw and nut to the above dimensions will still leave room for some adjustment. I prefer to tap holes deeply; leaving only a minimal truncation on the minor diameter. You can see this above. The internal threads are practically indestructible and only subject to wear, not breakage. Tapping the holes deeply leaves room to thread the screws with wider truncations on their major diameter to make them stronger and less liable to damage. You can see this below. I make the cutters (both tap and die) with sharper tips than those calculated above with the standard truncations; it is better to have the cutters more ‘pointy’ than less. How pointy is largely a matter of preference; I have seen screws cut to full depth with no flats left at the bottom between the threads. This would have no real effect on strength for either internal or external threads.

You may be wondering about the holes that need to be bored in the unthreaded side of a press. These need to be large enough for the wooden screw to pass through so than the two press halves can be pressed together. My solution is to choose the next bigger standard Forstner bit to drill these slip holes. A part of the shank is then turned oversize to fit these larger holes so that the fit isn’t too sloppy.

You can see the oversize diameters turned to fit the slip holes in the unthreaded half of a press. The thread can never be cut completely to the shoulder so a portion must remain at the smaller diameter as you can see on the left.

Making a Tap part 2

A reply to the comment on the last post. The perforated plywood piece is a sort of ‘adapter plate’ or ‘connecting plate’ used to attach things to the front. As you can see the plate bolts to the tap frame and the numerous holes can be used for screws to fasten things there. These screws would be facing pointed side out and would first fasten whatever cross pieces are needed to the plate which would then be bolted in place. Easier shown than described.

Scrap pieces have been screwed in place so that press blanks can be clamped using a rectangular piece of plywood and wing nuts. You can see those parts in the “Using the Tap” post.

Making a Tap.

I have posted previously about my 5 TPI bottoming tap which is pictured here, surrounded by tools and parts that will be used to make a new ‘through hole tap’ having the same thread. Both taps were made from the same 12″ rod of bearing bronze. The rough blank for the new tap is at the top.

“Making book binding equipment, like book binding itself, is a sequence of numerous separate steps. But any error or imprecision in an early step will haunt the outcome. I am fascinated at your wonderful grip of sequence and its control throughout the process. I hate to suggest a lapse into philosophy but we will also benefit from a larger understanding of command of sequence in manufacture of book binding equipment. We need a beginning (conception and design of jigs) and further reflection on the role of equipment maker in the crafts. Why make things to make things?”

Comment by Gary Frost on “Truing a Blank with the Router Lathe” 11/20/2020

A brief response to Gary’s comment

I liked this comment (and not just because of the flattering third sentence). It includes some interesting ideas: (1) Yes, I agree completely about the sequence of separate steps and the importance of precision from the very beginning. Attempting a high degree of control from the start helps to offset the gradual loss of that control that seems always to happen as one moves through the many steps needed to make something. Over-emphasis on precision (or a lack of emphasis) may be a bad thing in craft; possibly the trick is understanding the appropriate level of control. (2) I also like Gary’s call for understanding the steps in correct order. I never really considered trying to do this (it seemed overwhelming), deciding instead from the start to post about WHAT I have at hand, WHEN it is at hand. The thinking is that all the important steps will eventually get covered. It may be possible to index the posts later to make it possible to view things in sequence. One answer to Gary’s last question is to note that ‘making things to make things’ might just be very appealing to some people! (Though I think he was concerned more about the role of equipment makers than their motivation).

Making the new Tap

Here is the finished tap installed in the tap frame. The new part is made of bronze and is a fairly simple tool being basically a shaft which includes an adjustable cutter. It is rigidly connected to a master thread by which it is driven to create a spiral groove inside a hole.

This post won’t include making the cutter. Here for reference is a photo of the cutter for my 3 TPI tap. It is a little less than an inch long and made from a 3/8″ diameter HSS drill rod. You can see that the recessed area where the set screw presses is ground at a different angle than the scraping surface; this causes the cutting surface to be correctly angled in the tap.

This more clearly shows how a tap cutter is set on a slant to match the pitch of the thread being cut. This tap is made of an ‘off the shelf’ piece of ground steel shaft 1″ in diameter by 12″ long. It matches exactly the diameter of the master screws and shaft couplings and needed little modification. Making a smaller tap (7/8″ diameter) necessitated the switch to bronze in order to more easily machine it to fit standard shaft couplings and master screws which are not available in every diameter.

One end of the bronze blank is faced and then drilled to receive the lathe center.

This bronze rod is manufactured and sold oversized so that it can be turned to a 7/8″ finished diameter. This was done in several passes using the lathe’s lead screw to pull the table and tool post along. You can see the original ‘mill’ surface still remaining at the left on this first pass.

The final pass. This material is very easy to machine. This is the ‘working’ end of the tap; it is turned to .875″ (7/8″) diameter to tap a hole of the same size.

Next an inch and a half of the other end is turned to .750″ to exactly fit the shaft coupling that connects the tap to the master screw.

The working end is drilled with a 5/16″ hole. This hole will be tapped (with a metal tap; the kind you can buy in a hardware store) to receive a 3/8″-16 hex head machine screw. This will allow the new tap to be driven by hand with a bit brace and square drive socket.

Notice that the machine is unplugged. This is a hand operation. First the tap was chucked into the tailstock to start the thread by pushing on the (loose) tailstock while rotating the 3 jaw chuck by hand. This gets the tap aligned and started properly. After the thread has been started the tap is chucked into the tap handle (pictured) and driven the rest of the way by hand.

My 3/8″ tap may be getting dull. I had trouble with the bronze tap twisting in the lathe chuck. That is what caused the ‘stripes’.

This is a way to center a hole in a part. The plastic rod is turned in the lathe to the same size as the part to be drilled (.875″) and then drilled (while still in the lathe) to fit a drill rod (the shiny part in the photo). One end of this makeshift gauge is chucked into the drill press and the other clamped into the vise. This aligns the vise until it can be firmly clamped to the drill press table. Then when the new tap is clamped horizontally it can be drilled crosswise exactly through its center.

This ‘center drill’ gets the hole off to a good start so that the larger drill will not wander.

This drill bit is .201″ diameter and will drill the hole for the tap cutter which is made from .200″ diameter drill rod. The drilled hole passes right through the center of the tap and is at 90 degrees to its axis. The cutter is set at a very slight angle corresponding to the pitch of the thread. This is determined by the set screw pushing against the carefully angled ‘flat’ that is ground into the cutter shank.

The same cutter will be shared between the two taps. This also has the advantage of allowing the same gauge to be used on both. The gauge controls the depth of cut while a hole is tapped.

Here I have ‘laid-out’ the location of the set screw and punched the metal to help start the hole in the right place for the tapped set screw hole which will be drilled at 90 degrees to the hole for the cutter.

Here the set screw lies alongside a tap with matching thread (10-32).

Tapping this hole was very easy.

My ‘Smithy’ lathe/mill is being used here to mill a recessed area around the tap cutter to make room for the wood shavings that pile up in front of the cutter. Any ‘real’ machinist would most likely find this machine to be somewhat crude. But for my purposes it is excellent; I use it mainly for turning plastic and soft metals and find it incredibly useful.

The 3 jaw chuck had to be removed to make room for this operation.

This plywood bearing/disc must be changed for different diameter taps. Here the 1″ one has been removed.

A new disc has been installed. It has a 7/8″ hole to match the new tap. It is now a simple matter to put in the new tap.

The new tap has been connected with the 5 TPI master screw.

One more view. In order to use the tap a pre-drilled wooden press blank must be clamped at the front. The four holes visible here are for bolting on various attachments to make it possible to clamp the blanks. To see a tap being used view the video posted on “Using the Tap” 11/11/2020.

Truing a blank with the Router Lathe

The router lathe spins the screw blank between centers while a router slowly traverses from right to left, truing the blank. I made this around 1996 for a PBI class at Penland. It wasn’t motorized then so the students had to crank it by hand.

The blank is driven at the headstock by a nail that engages a plastic rod. At top right is a microswitch that shuts the machine off when a blank has been completed.


At the tailstock end the blank is held by another center. This one is pushed by a weighted lever, holding the blank securely between centers at both ends while it is shaped.

A view of the weighted lever that pushes the pointed center into the blank. The pressure can be released by using the plywood prop so that a finished blank can easily be removed and a new one inserted.

The tailstock center is made from a piece of 1/4″ drill rod with a 60 degree point. The hex bolt that can be seen is one of four. They enable very fine adjustments to insure the blank is turned to a true cylinder without taper. The diameter that is machined on the screw blank can be easily held to within a few thousandths of an inch.

A blank has just been finished. The white plastic part on the left is the thread follower. It engages the two small lead screws and controls the router’s forward motion. The router is tipped up to show the bit and how it is guided along the rails. The router is pulled along by a cord which is weighted at its other end. The bolt head visible at the left is a stop that can be adjusted to correctly trim the edge of what will become the screw handle.

The thread follower can be easily snapped on and off so the tool can be quickly re-set to make more screw blanks.

First the far side is snapped in place and then the front is snapped down on the other lead screw.

The thread follower is just visible beneath the front edge of the router base.

A 5 TPI Bottoming Tap

This is a way of tapping blind holes in order to fit legs to the Multiple Height Press that is described on the website.

A large Multiple Height Press clamped in the bottoming tap frame. Tipping the tap 45 degrees makes it easier to use and the shavings can still fall out of the hole being tapped.

In the original post (starting below) the tap was positioned vertically (90 degrees instead of 45). There is a little more information on the Making Wooden Screws page of the website.

The 5 TPI bottoming tap installed in the tapping frame. The wooden part is the frame. The metal parts are (top to bottom): the TAP (bronze), the SHAFT COUPLING which connects the tap to the MASTER THREAD which passes through the NUT (brass). The nut screws into a FLANGE which is bolted to the wooden base of the frame. The tap has been made from bronze rod. All of the other parts are ‘off the shelf’, though the master screw has been cut to length.

This gauge is used to align the hole in the press jaw with the tap.

The press jaw is clamped in place and the alignment gauge has been removed.

The tap is driven by hand. This is slower than using the bit brace but I haven’t figured out a better way. Orienting the tap this way allows the shavings to fall out of the hole as it is being threaded. This makes it more awkward to drive the tap. I may try a hand wheel at the bottom of the master screw to see if that works any better.

A view of the tap as it is aligned to enter the hole from the bottom.

The tapped hole. A thread cannot be cut all the way to the shoulder of a wooden screw. Thus it is sometimes necessary to have a larger diameter hole adjacent to the tapped hole to accommodate the unthreaded part of the wooden screw. I would normally not use this very ‘buggy’ wood but this is a press made as an experiment to test an idea.

Free Play of Wooden Screws

This post is in response to Gary’s comment on the November 12 post “Using the Tap”. Yes, an amount of “Free Play” or “Looseness” is important when making wooden screws.

One advantage of the thread making method covered in these posts is that the fit of the screw to the threaded hole is adjustable. A loose fit will prevent the problem of presses that become ‘stuck’ when the weather changes. Using large threads makes it possible for the threads to be quite loose. As seen above the screw can ‘wiggle’ in the hole. The hole is drilled at exactly 90 degrees; the screw can wiggle a degree or two in any direction.

The loose fit also allows the screws of the press to be adjusted independently (to a limited degree). This, I believe, is the issue that Gary was interested in.

This is a 4 TPI screw showing how it fits the internal threads. One might wonder if this loose fit compromises the strength. In my experience it does not…

I attach these two photos to illustrate this. As an experiment I tried to break a thread. I was curious to know what would fail first. In this case the handle split where the screwdriver was used as a lever. I tried again after putting on the hose clamps and was unable to break anything. The friction increased so much that the screw could not be tightened further, even with the use of the two plastic washers that I used in an attempt to reduce the friction. If this were done with two screws in a real press the amount of force generated by this extreme tightening would likely be ‘unhealthy’ for any book that was clamped there.

This shows how the handle split.

Using the Tap

Tapping a 1″ diameter hole to create a thread for a 1-1/4″ 3TPI wooden screw.

Two press jaws with holes bored with a Forstner bit. The one on the left has been chamfered with the router. They are ready to be tapped.

Changing the tap and master screw in the tap frame. The master screw must match the pitch of the tap; in this case 3TPI.

The shaft coupling securely joins the master screw and tap so they can work as a unit.

A press jaw loosely in place.

Here it is clamped so it won’t move while it is being tapped. As you can see in the video the tap is driven with a bit brace. I was able to find a bit brace incorporating a square socket drive to use with standard socket wrenches (below). I don’t know if these are widely available.

The tap cutter collects shavings as it scrapes in a spiral path to create the internal thread.

The gauge used to advance the cutter which must cut in small increments to work.

A view of the end of the tap after the thread is completely formed and the clamp removed. The head of the hardened machine screw is visible. A hardened screw is used to slow the wear from repeated contact with the socket wrench.

The completed internal thread.

A very narrow spiral of the original surface of the 1″ bored hole remains. The internal threads are very strong; they will not break. The wooden screws are more vulnerable to damage since the orientation of the grain of the wood makes it possible to chip off the threads, usually by some accident not related to the screws being used. A properly made screw thread will compress under strain (in the press) and this actually strengthens the thread making it unlikely to break. When a press is tightened with any normal amount of pressure friction will take over before damage can be done to either part of the threads.