Dividers: How to Lock Your Legs 

                                                   Part 2: At the Hinge Joint

In Part 1 of this topic we looked at the most common way of locking divider legs in place by means of a traditional wing. In this post we’ll look at the less common technique focusing on locking the legs at or very near the hinge itself. 

A most interesting and early example of locking the joint is pictured below. It was manufactured by the English firm of R. Timmins & Sons, Birmingham and is so marked. The model is shown on a plate from a Timmins Pattern Book, 1800 -1845 showing this design (Tools For The Trades and Crafts, Kenneth D. Roberts, 1976). This tool appears to have been manufactured at the earlier end of the range.

                                                

 

The remaining examples in my collection involve locking the legs at (or slightly above) the joint. These are mainly machinist calipers made from flat stock. The two leaf hinges lend themselves to being squeezed shut.                     

                                                 

Both are L.S. Starrett, Lilja’s 1885 patent, marked

            The larger pair are inside calipers with an interesting fine adjustment mechanism     

 

                           

J. Stevens A&T (marked) outside calipers. No. 56A “Vise Clamp”

    

  

                                                    

McGrath, St. Paul (marked) 

               

Since examples of this locking method are limited, I’ll add two other interesting examples that don’t fall into one of the three categories I’ve outlined. Both appear to be user made and possibly “one offs”, but certainly crafted by a creative mind!   

                                                  

                                                         

Based on the overall design and noting the “ram’s head” thumb screws, these dividers possibly date from the late 1700’s. Unmarked.

 

                                             

      Certainly, a creative design. One problem may be that the legs don’t lock very well. Marked “H” on one leg and one brass arm. This is often done to keep parts of the same unit together. If so, this argues that more than one was made.

The final post on this topic (Part 3) will focus on dividers and calipers where the legs are locked by a mechanism above the joint.

                                            

                             Dividers: How to Lock Your Legs 

                                                      Part 1: Below the Hinge Joint

In order for dividers to function the legs need to remain fixed once they have been set to a particular spacing. In friction dividers a balance is struck at the hinge between the need to move the legs a given distance and the resistance provided by the hinge to keep the legs from easily shifting. The larger the dividers and the less friction in the hinge the more difficult this balance is to maintain. Early on in the development of this tool a practical addition was devised to hold the legs in a fixed position and to give added stability to the joint: the wing  and so we have wing dividers. 

The earliest image of a pair of wing dividers I have found comes from an article in the encyclopedia Britannica showing a woodcut dated 1245 AD. 

                                             

The wing typically adds two additional benefits to the tool, added friction where the wing passes through one of the legs and most importantly the ability to lock the wing to the leg, usually with a thumb screw passing through the leg and contacting the wing. Occasionally you will find dividers with a wing, but no locking thumb screw. As we’ll see in a bit, there are a host of other fascinating variations that achieve the same effect. The added advantage of fine tuning the leg separation involving the wing will have to wait for a later post. The rest of this post will look at some of the “variations on a theme” in my collection for locking divider legs. Dozens of U.S. patents were issued over the last 150 years dealing with this element of divider construction. I’ll leave it to the reader to delve into the patent descriptions of how each of these patents supposedly achieves its goal. The “Directory of American Tool and Machinery Patents” (DATAMP) website is an invaluable source of information. Many of these are variations on the traditional wing dividers in use for 800 years and  often were applied to both dividers and calipers.                            

           

Pfleghar and Schollhorn’s 1866 patent    

 

      

Wm. Bernard, 1900 patent 

            

 

                                                                                                                                                       

             Both of the above were produced by Wm. Schollhorn & Co., New Haven  

                                                        

        Marked “Monument Brand” English 

Marked “C.B.M.S. No. 23257” on wing

Three of these examples have the possible advantage in construction of not needing to create a hole in one of the legs to accept the passage of a wing. 

In some examples the wing is enlarged into a “plate” which is attached to the hinge joint as well as one of the legs. This certainly provides added support.

Unmarked Caliper

                                                                 

                                                               E. Selvk & Co. Scores lines on wallpaper and linoleum

If the legs are made to cross each other, the intersection provides the opportunity to lock them. 

                          

                         

McDonough 1876 pat., Star Tool Co.

                                                        

Tjurnlund 1929 pat. Reiner & Campbell “Quickset” 

       

                                                                              

Unmarked Inside Caliper 

     

In the next leg locking post (Part 2) we’ll look at examples of locking mechanisms involving the hinge itself. Part 3 will deal with mechanisms locking the legs above the hinge.