Jump to content
Military Firearm Restoration Corner

Drill Size


ken98k

Recommended Posts

My machinerys handbook says the minor diameter is .517" - .527". The calc they use is based on engagement length, but basically:

 

Hole size = Basic major (.562") minus {1.08253 x % of full thread (normally 75%)} divided by threads per inch. I came up with .528". Seems large to me. .500" seems small. I would experiment with 33/64" (.515") it will probably cut .518 or so.

Link to comment
Share on other sites

An easy formula to remember is:

 

Tap drill size = O.D. (of bolt) - pitch (of threads)

Pitch = 1/ threads per inch

 

This gives a decimal size drill. An example would be 3/8-16. 3/8 - 1/16= 5/16 or 0.3125 1/4-20 would be 0.250" - (1/20) or 0.050"= 0.200" a #7 drill is 0.201"

This gives a good fit and easy to remember, works for metric also.

Link to comment
Share on other sites

An easy formula to remember is:

 

Tap drill size = O.D. (of bolt) - pitch (of threads)

Pitch = 1/ threads per inch

 

This gives a decimal size drill. An example would be 3/8-16. 3/8 - 1/16= 5/16 or 0.3125 1/4-20 would be 0.250" - (1/20) or 0.050"= 0.200" a #7 drill is 0.201"

This gives a good fit and easy to remember, works for metric also.

 

.562-24=538?

Link to comment
Share on other sites

I used to remember a real easy one for cutting speeds too but my memory lets me down sometimes! I've got some notes somewhere but I probably couldn't find those either. :lol:

 

I think I got this from Don Markey

 

CUTTING SPEED MULTIPLIED TIMES 4,

DIVIDED BY CUTTER DIAMETER,

EQUALS SPINDLE RPM

For a quick reference you can use the formula of Cutting speed (CS) X 4/Diameter of cutter (D).

cs is around 125 with carbide and low carbon hardened alloy steel. 125X4/.5=1000RPM. If the steel is softer use 300 for a cut speed. Depending on the steel you can go up to 600cs, but if your chips are black or dark purple slow it down until they are amber. Feed rate depends on the # of flutes on the cutter.

 

 

Link to comment
Share on other sites

×
×
  • Create New...