Friday, March 21, 2008

Chasing threads

R finds the stuff. Jan takes it apart.

This is the mechanical part of the oak desk chair. After dis assembly, I used paper and tape to protect the machined areas during the next step which was sand blasting. Once all the parts were sand blasted and the loose grit removed with an air gun I inspected all the parts. One of the long threaded bolts had damage to the threads. I had several options for restoring the threads. One was to use a thread chaser which is similar to a file but has four sides with different spacing between the teeth. Or I could use a die and run the bolt through the die to clean up the threads. Either method requires that I use a thread pitch gauge to determine the pitch (threads per inch). Here's a closer look at the damaged threads and the thread pitch gauge.
Here is the button die that I used to clean up the threads. The long file shaped tool is the thread chaser. When using a button die or tap always use a lubricant. I use tapping compound but you can also use a 3 in 1 oil if you don't have tapping compound. Depending on the severity of damage, you may need to back up (reverse), clean out the debris from the die and start again. After I chased the threads I re sand blasted the bolt to remove any residue so that primer and paint will adhere to the bolt.
Here is everything reassembled and ready for primer and paint. At one time someone must have broken the spring and replaced it. The spring that is currently there is the wrong size spring. It is the correct length but the diameter of the spring is too small and does not seat itself correctly on either end. R will have to shop around and get the correct one.

This weekend we will glue the wood part of the chair where the individual pieces are loose. I'll wait to stain and clear coat when I am using the stain on something else. For those interested...the chair was manufactured by the Colonial Chair Co. in Chicago.