| Re: Since no one asked...PSA: Drill Size Chart [message #70917 is a reply to message #70911] |
Sun, 08 January 2012 10:17   |
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Wayne Parham
Messages: 18786
Registered: January 2001
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Illuminati (33rd Degree) |
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That's helpful, good link, thanks.
Here's a sorta kinda related story:
When I was developing the cooling plug for the 12π's, my prototypes had plugs fitted to the woofers, but after a few sample woofers I realized there was too much variation in the pole piece for that to be a good way to do it in production. So instead, I opted to hone the pole piece.
What I decided to do was to bore the pole piece to within a few thousandsths of an inch and then hone it to an exact fit, just like engine cylinders. I looked at charts like the one you posted for (both metric and imperial) drill sizes in the 1.230" to 1.250" range. A 31mm drill bit is 1.220", 1-15/64" is 1.234", 31.5mm is 1.240" and 1-1/4 is 1.250". So we use a 1-15/64" or 31.5mm bit to start, then hone to finish for exact dimensions of 1.251" (+0.02"/-0.01") throughout the bore.
The original cooling plugs were 1.235", because the LAB12 woofer pole piece vent has ID of approximately that. We still stock those but we moved to a 1.250" plug a few years back and now machine the pole piece for a precise fit. All new production since 2007 has used the 1.250" plug.
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