But I've done this 1000 times

epanzella

epanzella

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It's amazing how I can screw up something I've done countless times. EVERYBODY knows tap drill for a 5/16x18 hole is an F drill at .252" or thereabouts so why look anything up. I've done this countless times. AFTER breaking the tap off in the piece I crack open the machinery handbook. It seems that the drill size changes with material thickness, DUH. As I'm going into .500 plate the recommended tap drill is .262" to .268". Five more fingerprints on my forehead. I'm running out of room.
 
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mmcmdl said:
Only time the minor diameter changes is what class you need . ;)
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The machinery handbook calls for larger tap drill sizes as the hole gets deeper. The chart is broken up into how many diameters the hole is compared to the tap. I never even tried looking that up before because I never broke a 5/16 tap before.
 
benmychree said:
I have used the F drill for many years on many parts without a problem, perhaps the tap was dull?
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Yeah, me too. It was a HSS taper tap and relatively new. I followed the chart in the machinist handbook (.265 tap drill) and got the holes tapped but it still took considerable effort even with a brand new spiral flute 3 flute tap. Maybe this junk yard steel is not your run of the mill variety.
 
epanzella said:
It's amazing how I can screw up something I've done countless times. EVERYBODY knows tap drill for a 5/16x18 hole is an F drill at .252" or thereabouts so why look anything up. I've done this countless times. AFTER breaking the tap off in the piece I crack open the machinery handbook. It seems that the drill size changes with material thickness, DUH. As I'm going into .500 plate the recommended tap drill is .262" to .268". Five more fingerprints on my forehead. I'm running out of room.
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9/16" bit for 5/8x11. 7/16" bit for 1/2x13. 5/16"bit for 3/8x16. 1/4"bit for 5/16x18. No. 5 bit for 1/4"x 20 Never had a problem. Use a 82 degree counter sink on the lip to help the tap start on whatever size hole and if your braking taps buy new American made ones. Use lub. or spit when tapping. Use 4 flute taps.
 
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.257 F drill is the standard size regardless of depth . This is within the limits of all class threads . What size tap did you use and was it dull ? A GH3 should tap with absolutley no issues . A GH7 is .002 larger but should still tap easily . My guess is the tap was dull . :dunno:
 
mmcmdl said:
.257 F drill is the standard size regardless of depth . This is within the limits of all class threads . What size tap did you use and was it dull ? A GH3 should tap with absolutley no issues . A GH7 is .002 larger but should still tap easily . My guess is the tap was dull . :dunno:
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I agree with the probability of a dull tap being the culprit.
 
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