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.