Good Day,
Thought I'd share a little "special" project I do... I'm passionate about building vintage single shot firearms from castings (and scratch) and one of the hangups I've always had is threading these and getting the threads square to the breech block. The common method that most use is a tap, and generally this produces an acceptable job, but I've discovered that the tap has the opportunity to create a threaded hole slightly out of square more frequent than not - but what's a couple thousandths among friends?? Before I go any further though I want to elaborate that this set up isn't really for the novice or those that don't understand the process of dialing in a part using a 4 jaw chuck in a lathe - even for the weekend warrior that spends all his free time in the shop - they understand that dialing in obscene parts increase the frustration level - to some, cutting small internal threads can be a challenge - with practice anyone with patience and a little finesse can be quite successful.
The fixture in my photos is the fourth such fixture I've made. The first fixture I made, (not shown) I used a setup that jacked the action around once it was in the machine making adjustments to the angular run-out then use the chuck to dial in the radial run-out. At minimum, that was a cluster - tightening the screws enough to move and secure the action caused distortion. The last three fixtures I've made are very similar in operation to the one shown, they have just gotten smaller and more manageable, not a lot of need for terrible amounts of mass. My recent design and function allows me to place the action in the fixture, eyeball the part close to center of mass and make it secure by the use of several screws from multiple directions. I generally use pieces of copper between the screws and the action as the copper forms easily to the casting and creates a non slip bond. When all the screws from these different directions are snugged up, none have to be exceptionally tight so the result has been very little distortion to the casting. I've threaded several dozen actions to date and never had an issue with one slipping or moving. Another advantage to threading this in a lathe verses a tap - one can pick the thread. Original design is not a problem, like squares on rollers or Hepburn, or timing like the original Winchesters had...
I mentioned that I can get the threads square to the breech block... most of the castings I build from have the mortise close to size. Applying a little elbow grease and using a made up dummy breech block I can get a good fit to the dummy block in about 2-3 hours of file work. Once the block is in place, the action face can be filed and scraped much like the ways of a machine tool, measuring from the action face to the block confirms this, if the readings are the same everywhere. If the fit in the mortise blues and show 80% or better contact, the action face is good to dial to in its setup.
This step takes some practice but I've students in my classes that have never spent intimate time with a file prior to taking my class and they can brag about their successes.
The fixture does hold the action secure enough if one decides to use the thread milling feature of the cnc mill too. If I had more than one or two of the same action I may consider it, but I don't and I don't do this as any part of my business even for friends or associates.
I've used this fixture to pickup and chase damaged threads on original actions, not much different than many of the jobs in the life of a job shop machinist.
The pictures attached are of the latest one I've built, showing some of the steps making the fixture and threading a Wesson Mid Range #1. The Wesson did present a problem of it's own, the thread diameter is slightly larger than the width of the mortise for the breech block and I didn't want threads showing up in the walls of the mortise, I had to get out of the threads and disengage the half nut about .025" (about half a thread) before entering the mortise. The other issue with the mid-range action, there are virtually no flat surfaces on this action so all but a couple of the anchoring screws were making partial contact. Another action I did and learned from recently was a 1902 Webley Rook that has a round breech block. The original's that I have seen in the pictures, the threads don't appear to come out of the front ring. On mine, I didn't catch that and two threads show ever so slightly here. Need to pay a little more attention to detail.
There's a pdf attached with my build Fixt 4.pdf and a dimensional sketch of the fixture. sketch.pdf Please feel free to comment as there are changes yet to be made that may simplify the process- at the last minute before welding the angle brackets on, I decided to drill and tap the base plate so angular adjustments could be made with the turn of a screw rather than using a pry bar...
Greg
|
   
   
|
|
Reply With Quote
