Petrie's Infamous Core #7

Chris Dunn
posted 10-23-1999 17:06

Thank you Ian for your response.

Ref "Drill core No 7". Yes, I have heard this argument before, and phrased in almost the exact same way. Deja-Vu. Let me address it this way. I am sympathetic to those who see a dichotomy with how my explanation of how ultrasonic machining explains the features of the core and, therefore, reject the notion. In fact if there was such a disconnect within my own understanding of the technique described I would reject the notion also. As I tried to explain the technique on the DejaNews sci.archaeology newsgroup several years ago, and now find myself again faced with the same arguments, I will take the responsibility for not explaining it sufficiently the first time and have another go at it.

(I have developed an understanding of manufacturing over a 38 year period, and I sometimes forget that some of the simplest concepts may be foreign to those who have no experience in this field. I should add, though, that I have not had any problems explaining this technique to machinists, toolmakers and manufacturing engineers. Therefore, I would ask those who do not have any manufacturing experience to read and attempt to understand what I have to say without the assumption that I am wrong simply because the conclusion does not fit within their own particular belief system.)

First of all, let us discuss Reid and Brownlee's findings. To tell the truth, I am somewhat embarrassed that I do not have a personal inspection of the core on record. I am caught flat-footed by their assertions that the grooves on the core are not spiral, as Petrie described, but individual grooves that start and end at the same point. With Reid and Brownlee being the most recent observers of this artifact, I will take them at their word and suspend my own speculations of ultrasonic machining until I have had the chance to check their finding. I admit negligence for not verifying the reports of earlier researchers such as Petrie and Lucas before writing my book, and I am quite willing to make any fact-based changes as necessary in future editions of the work.

Though Reid and Brownlee discredit Petrie's observations, on page 88 in Ancient Egyptian Materials and Industries by A. Lucas, we read a description of cores with as many as 17 equidistant spiral grooves (see also The Giza Power Plant page 86). As both Petrie and Lucas made their observations many years before I came along with my "alternate" views, I can reasonably argue that their observations were without prejudice, and they were not trying to prove, or disprove, a point. Moreover, I have personally examined holes in granite at Abu Ghurab that display spiral grooves, so it didn't occur to me to question the observations of Petrie or Lucas. Being remiss for not physically examining drill core 7, I am left with no option but to question Reid and Brownlee's observations along with Petrie and Lucas and prepare for a trip to London in November to inspect the artifact myself.

In the meantime, I am compelled to explain, in detail, why I determined that ultrasonic machining was the only machining method that satisfied the evidence that was described by Petrie.

When Ian writes, "far more significant is Chris' own admission in his book that ultrasound machining operates primarily via a pounding and not a rotational action, which is what makes the supposed feed rates displayed on the core such a red herring in the first place," I read into the sentence an artful and sophisticated rebuttal of a reasonable and simple manufacturing technique. Let us try to understand what this sentence means.

"Chris's own admission in his book that ultrasound machining operates primarily via a pounding and not a rotational action..."

There is nothing to admit or deny. It is the nature of the beast. Ultrasonic machining is a discrete oscillatory abrasive process. Correct me if I'm wrong, but the way I read this, Ian is assuming that if there is a pounding action for the removal of material, there is no need to be looking at or describing a rotational action.

"Which is what makes the supposed feed rates displayed on the core such a red herring in the first place."

My understanding of what these sentences mean is that the feed rate described by Petrie on the core, and which my theory of ultrasonic machining relies on, may be a red herring - a false clue. Before we go further, let us look at another statement.

"Rather than injecting further red herrings, it would be useful if he could elaborate on how he resolves this major dichotomy."

Right. Well, using Petrie and Lucas's described evidence, the "dichotomy", or separate parts to this process, are actually integral and depend upon each other for the process to work. Before continuing with ancient Egyptian artifacts, though, let me describe a modern technique of machining that is little known or used in most machine shops today. Now remember, we are not in ancient Egypt now, so you can accept what I am saying as demonstrated fact. Remember also that there are other readers of these posts who are my peers - engineers, machinists and toolmakers. So I'm not trying to pull the wool over your eyes.

We know that copper can't cut granite, right? Well, how do you get a piece of copper to tap a hole - or cut an inside thread - in a piece of hardened tool-steel?

The answer is electrical discharge machining (EDM). EDM discretely removes conductive material by creating a controlled short between an anode (tool) and a cathode (workpiece). Dielectric oil flushes the waste material away. The copper or graphite electrode actually burns the material leaving behind an impression of its shape. The round copper rod has a helical thread cut into its outside diameter, like a bolt. An assembly with a nut is attached to the ram platten of the machine. The copper "bolt" is then rotated in a clockwise direction. With each rotation of the "bolt" it comes closer to the workpiece. When the "bolt" is a few microns away from the workpiece, electrical discharge occurs. By continuing to thread the "bolt" through the nut while maintaining electrical discharge from the steel, the geometry and feedrate of the "bolt" is burned into the steel, resulting in a spiral thread. Properly done, the EDM "tapped" hole is precise and effective.

I draw on this analogy to explain my theory on ultrasonic machining. The ultrasonic pounding action provides discrete material removal; however, without some force or mechanism to advance the tool into the workpiece, the tool would be pounding only fresh air. There has to be a means by which pressure is applied to the workpiece. I proposed, based on Petrie's observations of an impossible conventional feedrate, that the pressure was provided by the screw and nut method. With only the screw and nut and no pounding action, we will advance the tool as far as the workface and go no further. The elements are separate but dependent on each other; therefore, there is no "major dichotomy" and no "red herring."

"On this note it is also fundamental that Brownlee asserts that modern ultrasound cores tend to be totally free of striations."

 

I heard this argument on DejaNews, too. In a perfect world, all machined surfaces would be free of tool marks. If you were observing holes ultrasonically drilled into expensive optical components for NASA or the ESA, there would be no striations. Most ultrasonic machines plunge directly into the material without rotating the tool. Most EDM machines do the same. There are exceptions to both. There are also exceptions to the requirements for smooth striation-free machined objects. Machinists and toolmakers have been known to "crowd" or force the tool into the workpiece in order to speed production. In some instances, depending on the tool, it is beneficial to "crowd" the workpiece as it has a self-dressing effect. Take grinding, for instance. A grinding wheel can become clogged with the material it is removing which slows down the grinding process. A technique for overcoming this it to force the wheel across or into the material to the point that wheel breaks down and wears. This assures that the clogged material is removed and the tool can continue to cut efficiently.

 

My understanding of the scientific method of inquiry is that a hypothesis must be fact-based and provide answer for all the known evidence. I was looking for a method that explained all the characteristics of the core, which included a taper on the hole and core, indicative of wear of the tool. Clyde Treadwell of Sonic Mill described an ultrasonic process where the tool would leave a spiral groove while being drawn out of the hole, but the tool was spinning, and the groove was the result of the eccentric rotation of the tool. I concede that if a spinning tool can create these characteristics, then there is no need to resort to ultrasonic machining. I was persuaded by a variation of the ultrasonic method because of the report that the "spiral" groove was cut deeper through the quartz than the felspar. Right or wrong I was basing my analysis on my own personal experience with machining materials that were not homogenous in their physical makeup. The harder material tends to be more resistant to tool pressure than the softer material.

 

The foregoing are reasons why I proposed ultrasonics as an answer. I have always maintained that if another method was forthcoming that would answer all the characteristics noted, and that method was simpler, then Occam's Razor, where all else being equal the simplest explanation is probably the correct one, would take precedence.

 

I hope to be in London within the next 4 weeks to examine drill core #7, after which, ultrasonic machining will either be taken off the table or it will remain a possibility. In the meantime, there is much more in my book we could discuss regarding machining. What about the precision and geometry of the contoured blocks on the Giza Plateau? Or the boxes in the Serapeum and Khafre's pyramid. A detailed description of these artifacts can be found in The Giza Power Plant.

I will return with another post to address Ian's comments about the powerplant theory.

 

Best,
Chris

 

Chris Dunn
posted 10-04-1999 20:54

Ian Lawton
posted 10-19-1999 00:06

Chris Dunn
posted 10-23-1999 17:06

Ian Lawton
posted 10-25-1999 07:21

Chris Dunn
posted 10-25-1999 20:24

Ian Lawton By email to Chris Dunn 12/6/99

Chris Dunn Response12/9/99

Chris Dunn Visit to Petrie Museum Part One - Posted 12/12/1999

Chris Dunn Visit to Petrie Museum Part Two - Posted 12/12/1999

Chris Dunn Visit to Petrie Museum Part Three - Posted 12/12/1999

 

Text and Photographs Copyright 1999 Christopher Dunn
Drill core #7 (UC 16036) Copyright 1999 The Petrie Museum of Egyptian Archaeology
University College London.

With Kind Thanks to the Petrie Museum of Egyptian Archaeology, London.