Drilling "Tight Fit" Holes
Posted: Sun May 17, 2020 4:31 pmThe Sonex plans call for drilling "tight fit" holes in several locations (wing attach blocks, forward fuselage angles, fuselage splice plates, horizontal stabilizer mounts). Sonex tech support has further defined a tight fit as requiring thumb pressure to press the bolt into the hole. Obtaining this fit requires closely matching the drill bit diameter to the bolt diameter.
The specifications of an AN bolt require the final diameter of that bolt to be within a fairly tight tolerance range (typically 0.186" to 0.189"). Although authentic AN hardware will almost always meet this spec, drill bit diameters are much less consistent from brand to brand. Holes drilled by a sampling of typical drill bits might vary in diameter by much more. This has the potential to create a loose fit, and in critical areas a loose-fitting fastener has less strength than intended.
Methods:
There are several acceptable methods of creating a well-fitting hole for the specified fastener: trail and error, precision drill bits, and final-fit reamers.
Trial and Error: This method is the simplest and may require no special care. Select the specified drill bit and drill a test hole (using good drilling shop practices) in a piece of scrap material that closely replicates the actual part. This hole can then be test fitted against the fastener to assess the fit. If the fit is adequate (requiring thumb pressure to press into the hole), no further action is required and the actual part can be drilled using that drill bit. If the fit is too loose that drill bit can be rejected, and other drill bits can be tested and assessed.
Precision drill bits: An accurate hole diameter can be drilled by using high-quality "numbered" or "lettered" drill bits that closely match the required hole diameter. Even so, be sure to test the bit first on scrap prior to drilling the actual part. These bits can often be purchased from aircraft supply and/or industrial supply stores (Aircraft Spruce, Brown Aviation Tool Supply, etc).
Reamers: A common and repeatable way of producing accurate hole diameters is to drill the initial hole slightly undersized and then use a reamer to enlarge the hole to the specified size. Reamers are typically produced to very exacting tolerances, and thus are highly repeatable. The initial undersized hole can be drilled with the next smaller fractional drill bit size (e.x. use a 11/64" bit to pilot drill a 3/16" hole), or by selecting the next smaller numbered drill bit (e.x. a #13 bit to pilot a 3/16" hole). AN3 size bolts are actually made to fit in #10 screw holes (0.190") rather than an exact 3/16" (0.1875"), so you'll need a #10 (0.193") for most of your "general fit" holes. For "tight fit" holes a #12 (0.189") reamer is ideal. 3/16" and #10 reamers are easy to find at the normal aircraft tool suppliers, while #11 or #12 reamers can be challenging to locate.
Good shop practices: Hold the drill firmly and steady so as not to oval or wobble the hole. Ensure the drill bit is perpendicular (e.g. 90 degrees) to the surface so the hole is square with the parts. Use fresh, sharp bits with lube. This is especially important for thicker material. Unibits are convenient, but test them first as they are often oversized in diameter. Do the final reaming right before inserting the fastener.
Conclusion:
With proper care and a drill bit that you've tested for size in advance, it's easy to create good quality holes. For the most critical applications a reamer can be used for the last stage to ensure a more precise fit. Reamers add a small amount of time to the process and add some additional cost, but are durable and will last the life of your project. In any case, make sure you drill small to start and test as you go. It's easier to enlarge the hole later rather than correct a too-large hole already drilled.
Recommended Tools:
AN3 bolts and AN525 washer head screws - "tight fit":
Pilot with a #13 size bit (0.185")
Ream to final size with a #12 (0.189") reamer or drill bit
(Alternately, use a #11 size bit (0.191") as the final AN3 hole bit if the #12 is too tight)
AN3 fasteners - "general fit":
Pilot with a #13 size bit (0.185")
Ream to final size with a #10 reamer (0.193")
AN4 size fasteners (0.246-0.249"):
Pilot with a #D size bit (0.246")
Ream to final size with a 1/4" reamer or drill bit
Common Suppliers:
Drill Bits:
Yard Store - https://www.yardstore.com/drilling-and- ... obber-bits
Brown Tool - https://www.browntool.com/Listview/tabi ... fault.aspx
Aircraft Spruce - https://www.aircraftspruce.com/catalog/ ... tdrill.php
Reamers:
Aircraft Spruce - https://www.aircraftspruce.com/catalog/ ... mer011.php
Brown Tool - https://www.browntool.com/Listview/tabi ... fault.aspx
Cleaveland Tool - https://www.cleavelandtool.com/collections/all/reamers
MSC Direct - https://www.mscdirect.com/product/detai ... m=02318129
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Keywords: tight fit hole, tight-fit hole, slip fit, loose fit, drill bit size, reamer, ream to size
The specifications of an AN bolt require the final diameter of that bolt to be within a fairly tight tolerance range (typically 0.186" to 0.189"). Although authentic AN hardware will almost always meet this spec, drill bit diameters are much less consistent from brand to brand. Holes drilled by a sampling of typical drill bits might vary in diameter by much more. This has the potential to create a loose fit, and in critical areas a loose-fitting fastener has less strength than intended.
Methods:
There are several acceptable methods of creating a well-fitting hole for the specified fastener: trail and error, precision drill bits, and final-fit reamers.
Trial and Error: This method is the simplest and may require no special care. Select the specified drill bit and drill a test hole (using good drilling shop practices) in a piece of scrap material that closely replicates the actual part. This hole can then be test fitted against the fastener to assess the fit. If the fit is adequate (requiring thumb pressure to press into the hole), no further action is required and the actual part can be drilled using that drill bit. If the fit is too loose that drill bit can be rejected, and other drill bits can be tested and assessed.
Precision drill bits: An accurate hole diameter can be drilled by using high-quality "numbered" or "lettered" drill bits that closely match the required hole diameter. Even so, be sure to test the bit first on scrap prior to drilling the actual part. These bits can often be purchased from aircraft supply and/or industrial supply stores (Aircraft Spruce, Brown Aviation Tool Supply, etc).
Reamers: A common and repeatable way of producing accurate hole diameters is to drill the initial hole slightly undersized and then use a reamer to enlarge the hole to the specified size. Reamers are typically produced to very exacting tolerances, and thus are highly repeatable. The initial undersized hole can be drilled with the next smaller fractional drill bit size (e.x. use a 11/64" bit to pilot drill a 3/16" hole), or by selecting the next smaller numbered drill bit (e.x. a #13 bit to pilot a 3/16" hole). AN3 size bolts are actually made to fit in #10 screw holes (0.190") rather than an exact 3/16" (0.1875"), so you'll need a #10 (0.193") for most of your "general fit" holes. For "tight fit" holes a #12 (0.189") reamer is ideal. 3/16" and #10 reamers are easy to find at the normal aircraft tool suppliers, while #11 or #12 reamers can be challenging to locate.
Good shop practices: Hold the drill firmly and steady so as not to oval or wobble the hole. Ensure the drill bit is perpendicular (e.g. 90 degrees) to the surface so the hole is square with the parts. Use fresh, sharp bits with lube. This is especially important for thicker material. Unibits are convenient, but test them first as they are often oversized in diameter. Do the final reaming right before inserting the fastener.
Conclusion:
With proper care and a drill bit that you've tested for size in advance, it's easy to create good quality holes. For the most critical applications a reamer can be used for the last stage to ensure a more precise fit. Reamers add a small amount of time to the process and add some additional cost, but are durable and will last the life of your project. In any case, make sure you drill small to start and test as you go. It's easier to enlarge the hole later rather than correct a too-large hole already drilled.
Recommended Tools:
AN3 bolts and AN525 washer head screws - "tight fit":
Pilot with a #13 size bit (0.185")
Ream to final size with a #12 (0.189") reamer or drill bit
(Alternately, use a #11 size bit (0.191") as the final AN3 hole bit if the #12 is too tight)
AN3 fasteners - "general fit":
Pilot with a #13 size bit (0.185")
Ream to final size with a #10 reamer (0.193")
AN4 size fasteners (0.246-0.249"):
Pilot with a #D size bit (0.246")
Ream to final size with a 1/4" reamer or drill bit
Common Suppliers:
Drill Bits:
Yard Store - https://www.yardstore.com/drilling-and- ... obber-bits
Brown Tool - https://www.browntool.com/Listview/tabi ... fault.aspx
Aircraft Spruce - https://www.aircraftspruce.com/catalog/ ... tdrill.php
Reamers:
Aircraft Spruce - https://www.aircraftspruce.com/catalog/ ... mer011.php
Brown Tool - https://www.browntool.com/Listview/tabi ... fault.aspx
Cleaveland Tool - https://www.cleavelandtool.com/collections/all/reamers
MSC Direct - https://www.mscdirect.com/product/detai ... m=02318129
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Keywords: tight fit hole, tight-fit hole, slip fit, loose fit, drill bit size, reamer, ream to size