Hole sizes in wing brackets

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

birdus

Well-Known Member
Joined
Apr 3, 2012
Messages
185
Location
Tacoma, WA
I'm building a Super Baby Great Lakes and am currently working on the wing brackets. In some places, the plans say to ream a .187" hole. In other places, they say ream a 3/16" hole (which is .1875"). Is this just an inconsistency and I'm supposed to ream a 3/16" hole in both instances?

By the way, if you're interested, you can follow my build at:

Building the Super Baby Great Lakes

and

https://www.youtube.com/channel/UC1uN0DvRAjYEgmn-O4aPFyw

I have a bit of catching up to do with the blog, but will try to do so soon.

Thanks,
Jay
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
16,140
Location
Memphis, TN
Some parts need to be installed close tolerance and others don't. That is probably what the plans are after. I love biplanes so I will be checking the build out.
 

Turd Ferguson

Well-Known Member
HBA Supporter
Joined
Mar 13, 2008
Messages
5,885
Location
Upper midwest in a house
I'm building a Super Baby Great Lakes and am currently working on the wing brackets. In some places, the plans say to ream a .187" hole. In other places, they say ream a 3/16" hole (which is .1875"). Is this just an inconsistency and I'm supposed to ream a 3/16" hole in both instances?
I agree, a fractional dimension is not as critical as a decimal dimension that specifies ream to size.
Thanks for the link. I built one half of a Baby Lakes back in the 80's.
 

birdus

Well-Known Member
Joined
Apr 3, 2012
Messages
185
Location
Tacoma, WA
Sounds like maybe the .187" should be press fit vs. slip fit for the 3/16". The reamer set I just got has a set of over and a set of under reamers, which I believe are for slip fit and press fit, respectively. Hopefully, it will become clear as I go along. Thanks for the thoughts.
 
Last edited:

birdus

Well-Known Member
Joined
Apr 3, 2012
Messages
185
Location
Tacoma, WA
FYI, there doesn't appear to exist a .187" reamer. I'm experimenting with different reamers and seeing how the bolts fit.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
10,161
Location
USA.
Adjustable reamers are nice also. Dan
 

birdus

Well-Known Member
Joined
Apr 3, 2012
Messages
185
Location
Tacoma, WA
I know 3/16" reamers exist, just not .187". The 3/16" over is a tiny bit loose and the 3/16" under too tight (the threads on the bolt damage the metal on the way in and it won't go through without forcing it). I'm going to pick up a standard 3/16" today and see how the bolts fit. At this point, I'm thinking the designer just intends for it to be reamed 3/16" and is using .187" interchangeably with that fraction.
 

Turd Ferguson

Well-Known Member
HBA Supporter
Joined
Mar 13, 2008
Messages
5,885
Location
Upper midwest in a house
Oh, an AN-3 bolt has a tolerance of .1860" to .1890" (-.0015 to +.0005) so if an standard AN bolt is called for, you many find some bolts that fit better than others.

The purpose of reaming the hole may be simply to get a precise surface finish, and not so much an exact size.
 

BoKu

Pundit
HBA Supporter
Joined
Aug 15, 2013
Messages
3,002
Location
Western US
One thing to consider here is that, contrary to popular belief, the nominal shank diameter for a #10 fastener such as an AN3 bolt is not 3/16" (0.1875"). The UNF spec actually calls for a nominal shank diameter of 0.190". Accordingly, the AN3 spec calls for a shank diameter of 0.186" through 0.189" (-0.004" to -0.001"). So when you do manage to drill and ream a perfect 3/16" hole, you will find that many if not most AN3 bolts won't go into it without some persuasion, and that when you remove the bolt its cad plating is compromised.

Here's what I usually do:

* For most fastenings that will be assembled and disassembled repeatedly in the course of development and construction, I drill #11 (0.191"). This is my standard for control system pivot bolts subject to clamping torque.

* For structural fastenings that will be rarely disassembled, I drill #12 (0.189"). That yields a tighter joint that is still kind of sticky, but one that can still be disassembled with relative ease.

* On rare occasions I will drill #13 (0.185") to yield a light drive fit for good fatigue properties in structural attachments that will never be disassembled in service. The one example of this that comes readily to mind is the set of NAS4703 screws that join the spar cap splice plate to the inboard and outboard spar caps in the HP-18 sailplane.

Thanks, Bob K.
 

Mad MAC

Well-Known Member
Joined
Dec 9, 2004
Messages
819
Location
Hamilton New Zealand
The point about the diameter of a AN3 misses the point that it isn't a close tolerance bolt, an AN173 or NAS6303 is. What hardware is called out to fit the hole, does it install a close tolerance bolt, then yes it is a close tolerance hole.
 

birdus

Well-Known Member
Joined
Apr 3, 2012
Messages
185
Location
Tacoma, WA
Do you not ream any holes, Bob?

Another thing I just started to wonder. When I prime the internal wing fittings, and prime and paint the ones that will stick out of the wing (e.g., for the interplane struts and wires), should I re-ream those holes to clean up the paint? Of course that would remove any primer in the holes, which might be bad (i.e., I probably want all the meal primed). The way my holes that I test reamed and the bolts fit together now, the bolts would fit almost perfectly into the holes with a few molecules of paint in there.

Thanks,
Jay
 

ekimneirbo

Banned
Joined
Oct 31, 2014
Messages
1,009
Location
Deep South
Do you not ream any holes, Bob?

Another thing I just started to wonder. When I prime the internal wing fittings, and prime and paint the ones that will stick out of the wing (e.g., for the interplane struts and wires), should I re-ream those holes to clean up the paint? Of course that would remove any primer in the holes, which might be bad (i.e., I probably want all the meal primed). The way my holes that I test reamed and the bolts fit together now, the bolts would fit almost perfectly into the holes with a few molecules of paint in there.

Thanks,
Jay
First off, 1/2 of a thousandth or .0005 or 5/10,000ths won't make any difference in a hole thats designed to accept a bolt in your wing assembly as long as you actually get a
decent fit. Even when using a reamer, hole size can vary a little due to lack of a ridgid setup or inability to stay perpendicular while doing the reaming. But, a person can only do so much to
try to insure good round holes are made in a correct size. Thinner metals are harder to hold size than materials with some depth to them.A two flute drill bit will not make a round hole in thin
metal. A reamer has more flutes and tends to round the hole back to shape. Three flute drill bits are harder to find but will make rounder holes. There are also core drills which work much like
a reamer and make rounder holes. In all cases you have to drill a primary hole and then finish it to size. If you are really needing to make a snug fit for something the best way is to mic the
fastener to be sure of its shank size and then use an adjustable reamer or a hone to finish the hole. One thing to remember is that if the assembly that the bolt is going in to has movement as
part of its function when installed.........too snug a fit can cause galling and immobility of the attached part.

You can put paint plugs in the holes prior to painting, but I would still run the reamer thru any holes that may have paint residue in them.
 

BoKu

Pundit
HBA Supporter
Joined
Aug 15, 2013
Messages
3,002
Location
Western US
Top