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True helical machined locking lugs

Pat C.

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Something that seemed to allude most M14 receiver makers. I captured in these series of photos the final shaving of left and right locking lugs on the true .200" lead one turn right hand.
Both lugs were first cut flat on the lead angle 3 degree 8 minutes .Several thou shy of final gage location 1.272" -.002

Final shave is done in the lathe set to cut 5 TPI .200 LEAD
I'm turning chuck by hand ,leaving thread lever engaged throughout, back lash is taken out by going past start .
First step I zero tool against right lug and start the cut ,lugs are blackened to visually see.
Tool steel indicated to true perpendicular first pass tool clears radius at bottom of right lug.
Then backing off to clear bore ,receiver is rotated until tool can enter left bolt passage .Going to zero I set the left lug is shaved.
Each pass I take.0005" until desired gage location is met and lugs are cleaned.

Watch how the lug faces transform from flat to helical. The left lug cleaned up quick two passes ,then the right you notice how it cleans up.
Although tooling and machine are different this is exact process USGI receivers were done ,except they cut both lugs at same time two tool.

When complete you have locking lugs that mirror image the bolt requiring no lapping.Although heat treat can sometimes cause lugs to change slightly.
 

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Pat C.

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Because of the very light shaves and super sharp tool steel kissed on diamond wheel ,surface finish is excellent.
Fresh parked bolt about 20 strokes with 1700 grit quick lap to check the results. 100% contact has been achieved.
 

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Pat C.

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The receiver is chucked in four jaw side pad, heel bottom with block and two sides. 1/8 copper flat bar between jaws and receiver.
Simple as indicating mandrel that fits bolt / barrel passage in.
No problem what So ever on hang out, just shaving .0005 pet pass.

Same set up initially was used to machine barrel thread bore, bridge passage ,and timed threads.
 

Pat C.

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Originally I rough milled the lugs so they would be very close and on correct lead angle. In the pictures you'll notice the tool steel is upside down because I have lead screw in reverse so I can start on the right lug and end on left.

Because of back lash you have to go past start and come back . And because both lugs were milled seperatly the cleaned up at different times.

The M14 and M1 locking lugs are basically part of a square thread of 5 TPI
We debated this years ago on Hawks forum that if lugs were milled flat but on correct lead angle its good enough.
Yes but proof is in these photos , you can see how many .0005 cuts it took the right lug to go from flat to helical.
 

Pat C.

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Yeah its harder for me to explain than do the job. I've tried to illustrate the actual principle on how these lugs were cut originally . In machining there are always more than one method or machine that can produ've same outcome.
Today's CNC shops making receivers choose a different direction by using the side of an endmill to inturpulate the lugs .
But the access points and clearance does not allow for exact machining to reproduce USGI spec.

The lathe is just one method where the exact precision can be duplicated.
Requiring no lapping other than quick check after heat treat .

To cut a true helical surface either the part or tool has to be rotating around the centerline with single point of contact precisely at the center axis.

M14 and M1 receivers were roughed out and this shaving was all that was done to receivers.
USGI bolts were ground on a thread grinder after heat treat to qualify them to a plus zero minus .002 " spec.

That's why USGI receiver /bolt combos fit so nice . Actually at one time SAINC M1A's were finished like GI receivers ,possible on old GI tooling. Not sure what their doing today.
 

Pat C.

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If you look close two of the pictures I caught the tiny shaving coming off the tool steel.
Super tough because I was holding Camara and turning chuck by hand to shave each lug .

The radical clearance grind to the back of tool steel allows tool to enter the bolt slot under the left lug .

Basically , the cut starts by turning the chuck in reverse by hand using wrench as it turns the tool steel is feeding at .200 " per revolution .
It shaves the right lug first clearing the radius at bottom .

Once cleared the cross slide is cranked to bring tool to center thread bore to clear.
The chuck is turned until tool clears bolt slot at left lug ,then cross slide is cranked to zero and the lug lug shaved to complete one complete cycle.

Once cycle is complete the tool is cranked back to center and chucked turned to bring tool beyond right lug again going past to clear back lash.
The thread lever remains engaged at all times.

The tool steel is exactly perpendicular to centerline (indicated) and .0005 per pass and couple free passes finishes the lugs
perfectly.
 

Pat C.

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If you look at most commercial receivers the left lug clearance area is grossly oversize because they are using a small endmill to interpolate radially the cut.
When you use an endmill to cut an internal radius it leaves a radius at lug corner ,that's why they go deeper.
 
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