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.

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.

Making Screw Blanks

One of these 5 TPI Beech screws has been threaded while the other is still blank. The yellow blank is made of Osage Orange. It will turn dark brown with time.. The far blank will make two stub screws for an experimental press that should show up on the blog soon.

The method that I use to make threaded screws requires a precisely made blank.

First the blanks are made square; in this case 1-3/4″ x 1-3/4″. Then the corners are sawn off to make them octagonal.

After being sawn to length the blanks are rough turned on the wood lathe. The far end of the handle end is turned to 1-3/4″ (to fit to the master screw; more later) and the narrower diameter (which is to be threaded) is turned about 1/16″ oversize for its entire length (1-5/16″ in this case). A small groove is turned at the end. This is where the thread will start.

The “router lathe”. This device rotates the blank while slowly advancing the router from right to left. The bit of the router trims the blank to create a precise diameter on the part that will be threaded. In this case the finished diameter is just under 1-1/4″.

On the left is a blank roughed from the wood lathe. On the right is a blank that has been further reduced to a precise cylinder with the router lathe.

You can see the spiral pattern that the router leaves. The larger diameters near the handles will help them better fit holes bored through the press jaws. These are smaller blanks than the ones in the previous photos. These have 1″ shanks and 1-1/2″ handles.

The purpose of the starting groove is shown here. These were made a bit shallow to show how the thread starts.

The screw blank mounts to the end of the master screw with radiator hose and clamps.

Master Screws

This method of cutting wooden threads depends on master screws. These are used to guide the blanks through the die and establish the pitch of the thread being cut. Above are my three 1″ diameter master screws and two of the brass nuts that are essential for their use. The bottom screw is a ‘double start’ screw; meaning it has two threads intertwining (a double helix). Even though it looks like the finest of the three it is actually the coarsest at 3 threads per inch (3 TPI). Above that is a 4 TPI master screw and at the top a 5 TPI one.

The nuts and screws are easily switched out on the threading frame. The brass nut can be unscrewed and replaced to substitute a master screw of a different pitch. The black steel part on the right is a shaft coupling; in this case it connects the master screw with a bit brace which is used to drive the screw by hand. You can see this on the video, “Threading a Wooden Screw” (Earlier post)

At the opposite end of the master screw an identical shaft coupling is used to enable a connection with the wooden screw blanks. Here is the set up for my smallest (5 TPI) screw blanks which have 1-1/2″ diameter handles. The white plastic part is an adapter which makes the transition from the 1″ inside diameter of the shaft coupling to the 1-1/2″ outside diameter of the screw blank. For my standard (4 and 3 TPI) screws the adapter isn’t needed since the outside of the coupling is the same diameter as the screw handle (1-3/4″)

The Die Cutter

In my wooden screw making efforts a “Eureka!” moment was the realization that the thread cutting tool can be made of two rectangular blanks clamped together to form a “vee” cutter. The material is High Speed Steel M2 lathe cutter blanks. The two part cutter is easier to make and easier to sharpen.

Here you can see a cut being made.

The screw has been backed off a little to show the “vee” shaped shaving. Thinned raw linseed oil is applied to the screw while cutting the thread. This softens and lubricates the wood and helps the cutter stay sharp much longer.

Threading Die

The die that cuts the thread on wooden screws is the most complex part of the threading apparatus. Here the two main parts of the die are separated to show the ‘innards’. The main body of the die (in the back) fastens to the threading frame and serves two functions; it provides a channel to guide the wooden screw blank and a base for mounting the more complicated “shutter” (at the front) which is what I call the swinging part that contains the knife and the regulating mechanism.

Here the shutter is mounted in the body of the die but is open. The die is disconnected from the threading frame and is lying on its back.

The die is closed. The taller knob at the right is for tightening the shutter onto the screw blank. The smaller knob controls the depth of cut.

The channel through which the screw blank is guided uses HSS lathe bits as wear bars. The wear from making many, many screws is visible on the middle bar.

This shows how the channel closely fits the screw blank.

The two part cutter is mounted at the correct Pitch Angle in the shutter. This angle is calculated from the circumference of the screw and its pitch (in this case 3 threads per inch or .333″). The white ‘circles’ are the ends of acetal plastic cylinders. The two small ones are fixed ‘stops’ which are set to stop the cutting when the thread has reached full depth. After the thread is fully cut the stops will contact the surface of the screw and prevent the shutter (and cutter) from advancing further. The large cylinder is adjustable. When it is advanced fully the cutter cannot contact the wood. As it is retracted in stages the shutter will be allowed to swing inward and the cutter will be able to cut as the wooden blank rotates past it. The adjustable stop turns on a 1/4-20 threaded rod and is adjusted in 1/4 turn increments. Thus the average cut is .0125″; about 1/80″ or .33mm.

In this photo you can see the multiple cuts that were made to produce this 3 threads per inch hard maple screw.

The two parts of the cutter must be securely clamped together as the cutting action tends to pull them apart. Here you can see how six set screws are used to clamp the parts sideways and two more set screws clamp the halves against the mount. Also visible at the top are two adjusting screws which control the position of the cutter halves.

Here is a view into the channel of the die. The part on the right (the body) is fixed to the threading frame and does not move. The shutter (on the left) swings in a wide arc from a pivot at the top. If you look closely you can see that the adjusting stop (white) protrudes farther than the tip of the cutter so no cut is currently possible. If the stop is retracted the first cut can be made; a very slight spiral scoring of the blank. Between passes through the die the stop is retracted incrementally and the thread becomes gradually deeper until it is fully formed.

The cutter as seen through the slot through which the shavings are ejected. The screw has already been fully cut and backed up a bit to show how the parts relate.

The same screw as in the photo above. The shutter has been opened. You can see that the movable stop is completely retracted allowing the cutter to cut full depth. The two fixed stops are also visible.

A different screw size and a different die but this shows what a screw looks like after the first pass through the die.

The same screw after its third pass through the die…

…and fully cut several passes later.

Further thoughts on mould construction

Those of you who are interested in paper mould construction may wish to read comments posted by Serge Pirard of Belgium. Serge studied with the late Ron Macdonald and makes moulds in the same tradition. His comments appear at the bottom of my September 14th post about repairing a paper mould.