Reproducing Clock Movements with CNC ScannerApril 13, 2011 by: Andrew Grevstad
PCNC 770 Owner Chris Moller is working on reproduction of a 18th century clock movement. He’s using the Tormach CNC Scanner to image the old pawls, ratchets, gears, etc. to get the correct part profiles of each movement before he cuts them on the mill. This is a perfect job for CNC Scanner, and the scanned image really turned out nicely. Take a look:
More From Chris about the project:
Attached is a picture of the original 18th century chime pawl and a copy I made on the Tormach based on the scanned image. I believe I had sent you the scan. I photographed the part I made upside down, but it does look the same on the other side except the splines are going the opposite direction! The process of going from CCD scan to CAD model to machined part really works. I intend to replicate the entire clock mechanism mostly for fun, but also for learning.
I milled a profile around the exterior splines with a 1/32 endmill taking .020 depth of cut until I went .020 beyond the actual thickness of the part (.108). This was easy because the stock is a piece of brass 3/8s thick and could be held in the step jaws. My strategy was to then turn the part over and face mill it to the correct thickness. By going an extra .020 in depth during profiling you get a cleaner edge, because the outside material falls away well before you stop face milling the back side. To hold the part to face the back side, I drilled and tapped an M3 hole which is smaller than the hole needed to fit the axle. You can see the M3 hole which required a 2.5 mm drill in the picture. I then milled flat two parallel edges of a large thick washer and attached it to the front of the pawl via an M3 screw and a small lock washer. This enabled me to hold the reverse side flat but only on the edges of the flattened washer! This method works but only with very light cuts (less than .025). A heavy cut (> .050) will send the part flying! Goggles are required!! This method only works if the part design has a center hole or if having a hole does not matter. The final diameter of the hole in this case is .125 inches, so tapping a smaller hole has no impact on the finished part. Lastly I engraved the interior disk detail with the engraving tool Eric recommended. It worked like a charm. Next time I think I will mill a .25 inch thick square boss within the area of the disk. This will allow a firmer hold so I take heaver cuts on the back side. Then I will toe clamp the part to the table and mill off the boss. If you know of a better way PLEASE let me know.
BTW, these are the engravers from Harvey Tool that we’ve been getting really good results with lately here at the shop.
Thanks again for sharing, Chris!