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01-14-2017, 12:35 PM
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Join Date: May 2014
Location: KBVY Massachusetts
Posts: 1,100
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Countersink Enlarges Clearance Hole - Is that ok?
Hi all,
I'm new to metalworking so today I'm practicing on scrap metal and running into a problem:
I'm trying to drill and countersink for an 8-32 flat head structural screw (1/2" long) in 0.063" thick material.
I'm using a nylon-faced micro-stop.
I start by drilling a clearance hole with a #19 drill. This makes a great hole - no slop.
Then I used a #20 Avery countersink in the micro stop and attempted to dial in the countersink depth.
I started shallow by (it turned out) about 5/1000's and so I increased the depth by 1000th increments to the point where I do not catch a thumbnail on the screw head BUT.....
....the clearance hole is now wider and there's a little slop in the hole.
Is this expected?
Or, should I start with a smaller drill and a smaller countersink cutter and then the increased hole diameter will work out to be perfect for the 8-32?
Please advise.
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Flying RV-8 N880BC
2019 Dues - happily paid.
Last edited by Saville : 01-14-2017 at 12:38 PM.
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01-14-2017, 12:42 PM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,597
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You are transforming your countersink into a router.
What you want is a pilot hole in a piece of metal to backup whatever you are countersinking. Then, the rule is "get in and get out". Creeping up on a countersink tends to have the problem you describe.
Get the depth set on a piece of scrap, then do the product.
Carl
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01-14-2017, 12:48 PM
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Join Date: May 2014
Location: KBVY Massachusetts
Posts: 1,100
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Quote:
Originally Posted by Carl Froehlich
You are transforming your countersink into a router.
What you want is a pilot hole in a piece of metal to backup whatever you are countersinking. Then, the rule is "get in and get out". Creeping up on a countersink tends to have the problem you describe.
Get the depth set on a piece of scrap, then do the product.
Carl
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Hi Carl,
Thanks for the fast response!
To be sure I understand you, I should:
Drill a #19 hole in a scrap piece.
Drill a #19 hole in the piece to have the countersink where I want the screw to go
Stack the scrap piece UNDER the piece to be countersunk and somehow fasten them together (carpet tape?) such that the holes are lined up.
The scrap underneath will prevent the countersink from moving horizontally.
Is that it?
Thanks Again!
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Flying RV-8 N880BC
2019 Dues - happily paid.
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01-14-2017, 01:07 PM
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Join Date: Feb 2013
Location: austin, TX
Posts: 62
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That's it. Or back your piece being countersunk with a wooden bench, drill the pilot hole through metal into wood, don't move anything then countersink. That will hold it in position as well.
Provided it can be placed on a flat surface of course
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Cory Bull
Lakeway, TX
RV-10 , Interior done, Engine and avionics remain.
Planning on (3)10" Dynon HDX, Garmin GTN 650 and PS Engineering Audio.
2017 Dues Happily Paid
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01-14-2017, 01:09 PM
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Join Date: Oct 2016
Location: Up north eh
Posts: 27
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Quote:
Originally Posted by Saville
Stack the scrap piece UNDER the piece to be countersunk and somehow fasten them together (carpet tape?) such that the holes are lined up.
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This is where a few other holes and some clecos come in handy... or cleco clamps... or clamps... carpet tape is kinda bush league
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Brandon Simunac
Prince George, BC, Canada
RV Wannabe. Just soaking up knowledge for now
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01-14-2017, 01:34 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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It might help if you specified what holes you are working on.
There are some holes that are expected to get enlarged when you machine countersink (wing spar flange for tank screws is a primary one).
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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01-14-2017, 02:45 PM
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Join Date: May 2014
Location: KBVY Massachusetts
Posts: 1,100
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Quote:
Originally Posted by rvbuilder2002
It might help if you specified what holes you are working on.
There are some holes that are expected to get enlarged when you machine countersink (wing spar flange for tank screws is a primary one).
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Sure:
I'm drilling and countersinking holes in a battery tray in the aft position of an RV-8.
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Flying RV-8 N880BC
2019 Dues - happily paid.
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01-14-2017, 03:17 PM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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If the head is flush with the surface then it is correct for a portion of the countersunk hole to show.
This is caused by the construction of the screw head where what would be the sharp edge on the flat top surface caused by the 100 degree countersink is removed in manufacture. It can be seen in these pictures -

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Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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01-14-2017, 03:37 PM
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Join Date: May 2014
Location: KBVY Massachusetts
Posts: 1,100
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Quote:
Originally Posted by az_gila
If the head is flush with the surface then it is correct for a portion of the countersunk hole to show.
This is caused by the construction of the screw head where what would be the sharp edge on the flat top surface caused by the 100 degree countersink is removed in manufacture. It can be seen in these pictures -

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Hi Gil,
It's not the upper portion of the countersink showing (i.e. a small "rim" around the screw head) that's the problem.
The problem I'm having is that when I drill the countersink, the original clearance hole diameter is increased. I would have thought that since the countersink doesn't go all the way through the thickness of the metal, that the clearance hole would stay the same diameter.
__________________
Flying RV-8 N880BC
2019 Dues - happily paid.
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01-15-2017, 08:20 AM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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Quote:
Originally Posted by Saville
Hi Gil,
It's not the upper portion of the countersink showing (i.e. a small "rim" around the screw head) that's the problem.
The problem I'm having is that when I drill the countersink, the original clearance hole diameter is increased. I would have thought that since the countersink doesn't go all the way through the thickness of the metal, that the clearance hole would stay the same diameter.
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If it is a #8 screw and the material thickness is less than 0.072 then original hole should stay the same size.
SORRY - BIG TYPO ERROR - should say "more than 0.072"
The screw head spec. is here -
http://glftechnical.com/PDF/MS24693.pdf
If the thickness is only a bit above the minimum you probably need a scrap piece of material with a #19 hole clamped behind the work to act as a guide.
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
Last edited by az_gila : 01-15-2017 at 12:32 PM.
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