what is the purpose of this cut in the threads (see red arrow)
https://www.milsurps.com/images/impo...gleqe1in-1.jpg
Printable View
what is the purpose of this cut in the threads (see red arrow)
https://www.milsurps.com/images/impo...gleqe1in-1.jpg
It is a place for crud to ooze into (as shown in your photo) and allow to bolt to go into battery when the rifle is dirty and circumstances prevent you from cleaning it (like when someone is shooting at you!).
I've got a funny feeling that that is the starter for the cutter as it cuts the extractor way. The extractor way can't be cut from the rear so has to be cut from the front as the body tilts to accommodate the shape. Just thinking on my feet but that's why a rifle is ZF when it is damaged externally at that point.
that would be logical if the extractor cut was not on the other sideQuote:
Originally Posted by Peter Laidler
It's a fairly common artifact in (all types of- Marlin lever actions for instance-) receivers, usually from a broaching operation.
I have just examined 6 FAL bodies I have lying around. The groove aligns perfectly with the upper left corner (looking forward) of the squared bolt recess in the receiver ring, so it has clearly been formed as part of the broaching process to form the bolt recess. There is also another similar groove on the opposite side. The extractor recess has clearly been cut from the rear of the action with a special end mill as it comes to a square blind end before the barrel thread, and the axis of this cut is angled upwards towards the rear by about 10 degrees. This extractor cut could only be made by a special fixture and custom tooling in a milling machine.
by your description I assume these are not lithgow produced bodies you have. I have several lithgows and they dont have any groove in the threads on the extractor side but the imbel's and FN's do (must be a metric thing).Quote:
Originally Posted by Woodsy
I wondered if it was a safety feature like a built in fail point because there is really no other practical reason for it to be there. That part of the bolt recess can be completed from the rear just like the extractor recess was done. To me it makes no sense to do the extractor side from the rear and then just finish off that corner from the front and run into the threads while doing it.
It's just the way the machines did it, that's the direction it approached. The thread is not needed at that point. Do you sit up at night and think of things to worry about? Look up the sequence of machine work on their receivers and you'll find the answer...Quote:
Originally Posted by Mr E
my next step in machining is to do this area and I can find no information on these on the www. I dont want to waste time and effort making a fixture to set up to do that cut if its just window dressing but if it is a safety feature then I have to do itQuote:
Originally Posted by browningautorifle
https://www.milsurps.com/images/impo...100_3608-1.jpg
No Mister E. what you have there; a good start!
May the swarf be with you!
I refer to my comments in my earlier post about it being a place for crud to collect. The groove through the thread is a by product of the broaching process for the square bolt face recess. The Lithgow L1A1 and the FN FAL both have this groove. The following photos show that bolt face recess is much larger than needed, and the side of the bolt is cut away. The purpose of all this clearance is to provide a 'crud receptacle' for operators who don't clean their rifles. The last photo also shows the crack from the locking shoulder hole, through the upper ejector block pin hole and into the body pivot recess. This is what happens after about 100,000 rounds (and 5 barrels) fired at deer from helicopters by 'rough as guts' meat shooters. The rifles were seldom cleaned properly and the gas relief sleeve was closed down to keep them running! In the pair photo the L1A1 action is on the right.
Mr E, if you wished to make the bolt face recess slightly narrower to aviod the groove in the threads, it would make no difference to the operation of the rifle in civilian terms. The groove is from a manufacturing process. It is a not a designed 'weak point'. I have seen a lot of horribly abused and damged FAL's and none have given way in the barrel ring, despite the best efforts of some rough b**stards!:)
While I don't profess to know exactly what the thread clearance channel is, I wonder if I might be so bold as to slightly disagree with woody about the slot being a clearance channel of sorts. That's because to be a clearance channel for crud, oil brass shavings etc etc you'd HAVE to have an exit point otherwise it'd just fill up with crud in short order and you'd be back to square 1. And STILL no means of clearing it out!. And there is no exit point here as the 'opening' is covered by the barrel when it's screwed on.
When you think of clearance points, think Sterling SMG barrel.breech area where crud can be pushed forwards and under the breech area
Peter, I did not mean that the groove itself was designed for crud accumulation, but that the width of the bolt recess, coupled with the scallop on the side of the bolt, was for this purpose. The groove in the threads is because of the width of the broach required to cut the recess.
Yep. got that woodsy after reading it again........
best answer I have had so far was from a tool maker (never worked on guns) who said the reason that is cut that ways is because thats what was on the drawings.
Might it possibly be a gas escape path for ruptured cartridge?? Is there any communication with the barrel chamber face there?
-TomH
Perhaps you're right MR E, after all, as Peter has mentioned before about all design changes from the original Fal drawings needing to be paid for by the Commonwealth.Quote:
Originally Posted by Mr E