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Early Lyman Flip-up Sight Design


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#1 Bob B

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Posted 24 October 2004 - 04:24 PM

U.S. Patent 1,408,276 describes a sight mechanism identical in most respects to the one with which we're all familiar - with one notable exception. The patent describes a spring-loaded pushbutton locking device for holding the aperture slide assembly at the desired position on the leaf. When the button is depressed, the slide is disengaged from the leaf serrations and is free to move to the desired position; when the button is released a pawl engages, providing a positive lock. In this original design, the serrations are located on the inside of the left leaf leg.

In contrast, the sight used on the Thompsons I've seen utilizes a flat spring "friction pawl" that is always engaged with the leaf serrations. In this version, the serrations are located on the outer edge of the left leaf leg. This arrangement can be quite stiff making the aperture slide difficult to move and adjust. (The flat spring can of course be carefully bent to increase or decrease pressure, and the pawl tooth rounded somewhat to make it less "sticky", but this is at best a compromise.)

The patent illustrations below show the original design. The pushbutton is identified as "24" on the drawings.

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The patent describes the adjustment mechanism as follows:

"The slide S comprises a body 17, side members 18 and 19, contacting with the outer sides of the legs 12 and 13, and flanges 20 and 21 which contact with the rear sides. The upper part of the slide (Fig. 6) has a horizontal chamber formed therein and in the chamber wall, adjacent the legs, is provided a slot 23. A stud 24 fits in the chamber and has a cavity 25 therein in which lies a compression spring 26. This spring by its pressure against the end of the cavity urges the stud outwardly from the chamber. A tooth 27 projects from the stud 24 through the slot 23 for engagement with the serrations 15. The stud 24, tooth 27 and associated parts form a catch for securing the slide to the leaf.
...
The slide may be adjusted by depressing the catch through pressure on the stud 24, thereby releasing the tooth 27 from the serrations 15. The slide, being freely movable on the leaf, may be set for any distance by aligning the lower edge of the eyepiece E with the proper graduation 38. Releasing the catch secures the slide through the engagement of the tooth 27 and one of the serrations. If it be desired entirely to remove the slide from the leaf, the stud 24 is depressed by some pointed instrument, such as a nail, into the chamber 22 until the tooth 27 is in a position to pass through the clearance notch 16 of the bar 14."


(Dontcha just love the suggestion that a NAIL be used?) rolleyes.gif

As a matter of historical interest, was the version described in the original patent ever produced and used on the prototypes and/or early model Thompsons, or did the change to the subsequent flat spring configuration occur prior to first production? Was patent # 1,408,276 ever superceded by a new patent showing the revised design?


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#2 WILSON

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Posted 25 October 2004 - 12:57 PM

Bob- Check out my site:

THOMPSON REAR SIGHTS

I do believe that the DOVETAIL SIGHT has the button that you are talking about. It has been a while since I had it in my hands but I will take a closer look tonight.

I am very impressed that you take the time to actually read and attempt to understand the patents. May I recomend "The Complete Book of Thompson Patents" as a good bedtime read (in case you have trouble sleeping). I usually dose off immediately after the patent #!
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#3 Bob B

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Posted 25 October 2004 - 04:03 PM

Thanks for the link, Wilson. Very good pictures and you're right - the "dovetail sight" has EXACTLY the slide-lock arrangement shown in the Eickhoff patent. Interestingly, the patent application and issue dates are the same for both the "dovetail" and the "adjustable", so I imagine it's one and the same patent.

The use of this slide on the dovetail sight, however, puts even more of a point on the original question - since they obviously had done the tooling for the pushbutton lock, why didn't they use it on the "adjustable" model? Admittedly, either sight would be very expensive to manufacture, but if the only nod to economy was that the pushbutton lock was omitted in favor of the flat spring type they really couldn't have saved much. There MUST have been something other than (or maybe in addition to) cost that factored into the decision
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#4 SecondAmend

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Posted 25 October 2004 - 04:13 PM

Bob B,

In the '276 patent there are 13 claims. Each claim "stakes out" (i.e., claims) an embodiment of the invention. See, for example, claims 5 and 12 for embodiments that implement the spring loaded stud. Modern patents are written a little differently primarily for cost reasons.

If you have not done so already, to gain a fairly reasonable understanding of patents from an easy to read book, check out "Patent It Yourself" by David Pressman from your local library.

Kudos to you for digging into these from me also!
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