(Un)Plugging Cooling Water Passages

One of the moulds for my Monotype Composition Caster, specifically the type U Lanston display mould fitted for 36 point body size, had completely blocked cooling water passages. This would seriously limit how fast the caster could run, even more so than the already slow speeds recommended for such large type.

There is a tool which can be attached to the underside of a mould, which fits two small cylinders to the cooling water ports, allowing one to install a small piston and, by striking this with a hammer, force liquid through the cooling passages under high pressure. This is often useful for minor blockages, but seemed to have no effect on this particular mould.

In order to clear out the passages, I disassembled the mould well beyond the recommended amount, even removing the intermediate base from the main base. This is not a recommended procedure because it loses the factory-set alignments of the parts. It is, however, the only way to reach all the water passages, or at least to understand how they are connected. Now that I know their layout I may find it possible to clean them without this extreme disassembly.

In the process I’ve mapped out the water and oil passages, something to be documented in a future post perhaps.

The cooling passages are a series of drilled holes that intersect within the parts of the mould, with brass screws plugging most of the drill entry holes. Together these holes join to form a continuous single path for water flow through the mould.

Clearing the passages involves drilling into them as was done at the factory, but this time the drill removes the blockage (probably mostly consisting of iron oxides and old oil) rather than steel. Unfortunately, most of the brass screws were either filed flush to the surrounding surface and so had no screwdriver slot any more, or were sufficiently seized in their holes, that they had to be drilled out too.

This leaves the problem of replacing the screws, the main point of this post.

This mould is almost identical in structure to the 3E mould shown on pages 38/39 of the Lanston Plate Book. There are two sizes of screws, identified as part numbers 2239 (smaller) and 2235 (larger). These are not to be confused with drive-belt shifter parts coded 6E1 and 8E3 which use the same part numbers but from a different numbering system. Unfortunately, the Plate Book does not generally identify fastener sizes, and being a Lanston mould it is not shown in the English Spare parts List (which does give fastener sizes). For similar anglo-centric reasons they are not listed in the fastener list hosted at the Alembic Press.

I managed to remove one of the #2235 screws intact so I could measure its thread, but even this was difficult. Because the screws are so short, there is some uncertainty in measuring the thread pitch, and the diameter (about 0.190″) also didn’t seem to match any common screw size (close to #10). For the smaller #2239 size the best I could do was try various taps in the hole, carefully by feel distinguishing between the tap trying to remove remnants of the old screw and the tap trying to cut the wrong thread in the hole.

I was having so much trouble that at one point I was considering just drilling the holes larger and retapping them for a modern screw size, but the hardness of the metal made that pretty much impossible. I had already gathered a collection of twist drills blunted by this job, and one broken tap. Removing the stub of the broken tap is almost a tale in itself which required drilling out (and replacing) one of the brass liners where the passage connects between mould parts to provide access to the broken tap end.

Despite this I somehow managed to fit some #8-32 brass setscrews to some of the smaller (#2239) holes, using plenty of thread sealant. These passages carry little pressure so it doesn’t take much to seal the plug screws.

It was, however, clear that this was not actually the correct screw size, partly due to the fact that the machine design somewhat predates the modern numbered-size North American fastener system. Other fastener sizes in Spare Parts List, even the tiniest, are given inch size designations. Some of the tiny fasteners did indeed fit the tapped holes in the mould, allowing me to finally identify #2239 as being ⁵/₃₂″×40 and #2235 as ³/₁₆″×40, both about ³/₁₆″ long.

I also know from experience that the Monotype fasteners do not use the modern standard of 60° for the thread form; I suspect them of being 55° which is an older standard still used for Whitworth (BSW and BSF) fasteners. These particular diameter-and-pitch combinations are not ones used by Whitworth, though. It turns out that the 55° Whitworth thread form also lives on in so-called Model Engineering (ME) threads, most frequently encountered in model steam engines, and this is available in various diameters with either 32 or 40 threads per inch, including the two I need.

Model engineering is a bit of a niche market so it is still a bit of a problem to find quality taps and dies. Many are listed on Amazon but they appear so cheap (often not even cleaned of machining chips before being photographed) as to be not worth considering. I ordered some taps and dies for these two thread sizes from Stuart Models in England, and the total cost was not nearly as horrendous as I feared it might be. I also ordered from them a diestock for the dies, which were smaller than any dies I already had.

One of the dies in the diestock. This is the larger one, ³/₁₆″×40, for screw number 2235. These are split dies, which allow the diameter to be fine-tuned by adjusting the three retaining screws of the diestock.

For stock to make the new screws I used round brass rod which can be purchased at most hobby shops. In this case I’m making a #2235 screw from ³/₁₆″ rod. I cleaned up the end of the rod in my lathe and used the drill chuck in the tailstock to hold the die square to the rod to get well-formed threads.

The rod ready for threading

Starting the die. Although this was in the lathe all threading was done by hand rather than under power.

The finished thread. For all these screws I was only estimating the length required.

To cut the slot for the screwdriver, I used my Sherline mill. Finding small slitting saws, suitable for use in small machines and at a reasonable price, also proved difficult. Fortunately the slot width is not a critical measurement, and I managed to find a thin (about 0.040″) saw in a set of three otherwise junky saws for rotary tools at Princess Auto. I have since found (but not tried) some suitable saws on Amazon.ca under the brand Hegebeck which come in a variety of thicknesses but need a ½″ arbor rather than the small one this blade uses.

Cutting the slot, arbitrarily chosen to be 1/16″ deep.

The finished screw head

I found that trying to use a parting tool in my lathe to part off the finished screw from the stock did not work well, being very chattery and grabby. Perhaps the parting tool geometry was wrong for brass, or the chuck jaws are a bit bell-mouthed providing insufficient support for the work. Instead I used a DCMT insert tool (55 degrees) to both chamfer the ultimate tip of the screw and also neck it down to a small diameter, followed by a fine hacksaw to finish parting off the screw.

Three screw plugs. An original on the upper right, first home-made try lower center including dirt from test fit, second try upper left, all with dime for scale.

The result is a screw with a nice chamfer but a bit of a central nub left by the hacksaw. For the smaller #2239 size, rather than using different brass rod stock I just turned the ³/₁₆″ rod down to ⁵/₃₂″ and made one screw using the same steps:

I installed all the screws in the appropriate holes using a thread sealant. I used Permatex 59206 but I expect almost any thread sealant will do, just make sure it is a sealant and not a locking compound as these screws are difficult enough to remove as it is. The original screw actually did not go into its hole deep enough at first, so I ran it through the die to clean up its threads, and then it went in the hole almost flush to the surface, with a few strokes of a fine file to remove the protruding part. Rethreading the screw produced noticeable chips so I still wonder if I have the correct thread form…

Installing the original screw at the left side of the sub-base.

The original screw installed with excess sealant wiped off. The damage to the slot occurred during removal. The screw had to be filed a bit to make it flush to the surface.

The coolant passages should now be clear as new. The next job is the reassemble the mould and hope for no leaks and adequate alignment. I will have to think about what aspects of the alignment are critical; the sub-base itself maintains the alignment between the blade and the two type blocks, so it seems the alignment of the sub-base is to ensure the pump nozzle seating is consistent with other moulds and the type carrier can clear the type blocks yet not fumble thin types.