What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Tips for countersinking spar flanges?

missile29

Member
Does anyone have any tips for how to countersink the flanges of spars, specifically the horizontal stabilizer spars? I’m looking for tips or even pics of a jig or something that (ideally) allows you to run the whole shebang through a drill press. That’s probably a lofty goal, so I'd settle for some kind of clever jig or attachment that helps hold the countersink cage square to the spar flange, and keeps the whole thing from torquing over in your hand and cutting too deep.

Maybe the answer is “just do it” but the problems I’ve had on the stringers (that I DON’T want to repeat on the spares) are that I can’t hold the countersink cage square to the surface I’m cutting, and the single cutting edge bit I’m using bites hard and digs out all the material at once, making it impossible to go slow. It also, as mentioned, twists in my hand and on some of the holes on the stringers ended up going too deep. So now I'm terrified about doing the spars.

Thanks in advance!
—mgm
 
MGM,

E is best option.

To remedy.
Use 1x4 wood cut to size under project piece.
Use scrap piece of metal same size of piece, fasten all to table, drill in place.

Will keep all square and level end out so cage will not tilt.

Boomer
 
MGM,

E is best option.

To remedy.
Use 1x4 wood cut to size under project piece.
Use scrap piece of metal same size of piece, fasten all to table, drill in place.

Will keep all square and level end out so cage will not tilt.

Boomer

Thanks! I followed your advice (using a piece of wood and a spacer) on a lot of the holes on the stringers and it definitely helped. For the spar, are you suggesting clamping the 1x4 so that it sticks off the end of the table (so the web and other flange of the spar can hang below? or something else?) I toyed around wit a setup like that in the shop but was too chicken to drill yet. I think it should work, but I need to get my wife to help hold everything stable. I also set up a little backer+spacer out of two pieces of HW store aluminum that I might give a try.

I'll probably also do it in multiple passes and work my way down to the depth. Sure is a lot of holes to have to make two or more passes each, but that's better than screwing up a spar.

Thanks!
--mgm
 
MGM,

E is best option.

To remedy.
Use 1x4 wood cut to size under project piece.
Use scrap piece of metal same size of piece, fasten all to table, drill in place.

Will keep all square and level end out so cage will not tilt.

Boomer

Bruce,

I am also approaching this point. Can you expound a little on your remedy? If I understand, the 1x4 goes on the bench, the scrap on top of that, and the flange attaches on top of that with the web approximately perpendicular to the bench. What is the purpose of the scrap piece of metal?

Thanks!
 
You need to have a backup piece of some sort because the pilot of the countersink bit needs to be supported in some way. With thin material the pilot being unsupported will result in messed up countersinks. You can even use a block of wood and move it along as you go, but it will get enlarged over time. Then you need to drill another pilot hole and start over. As a result, using a scrap strip works a little better, but the main thing is for you to know the pilot needs to be supported.
 
Change cutter

mgm, I'm wondering if a different cutter will give you better results? I've only ever had and used the 3 flute cutters and they have worked very well with no tendency to grab or cut too fast. It's possible to shave off the tiniest amount by using light pressure. They can however chatter if the material is too thin and a backing pilot hole and steady pressure helps with this.
There's going to be many more holes to countersink in the project so I think it would be worth getting the cage to work right! :)
 
Totally agree, and that's exactly what I did on the spars. I was determined to come up with a workable and repeatable solution before I screwed up the (much more expensive) spar subassemblies. I switched to the 3-blade cutter and was able to go more carefully and slowly. Just to be safe, on the rear spar (which I did first) I only set the cutter to flush depth and did all the holes like that. Then I came back and went to +0.007" in a second pass. MAN was that a LOT of countersinking! My wife (who was kind enough to hold the other end of the spar for me the whole time) was about ready to jump out the window! But it turned out great and I got into a rhythm and got comfortable with the tool, which was what I really wanted.

I think the single bladed cutter can make nice cuts, but it's probably better to be used in an electric drill or drill press, where you can go very slowly with lots of low speed torque and have a lot of control over the progression of the depth. Or maybe a pneumatic drill is fine, as long as it's someone with more skill and finesse than me ;-)
 
Which model are you working on? I believe the HS spar FLANGES are dimpled, while the bars that go into the WEBS are countersunk. On the wings, it is in fact the FLANGES that get countersunk, I’m just making sure you have the terminology correct, flanges vs webs..
 
You need to have a backup piece of some sort because the pilot of the countersink bit needs to be supported in some way. With thin material the pilot being unsupported will result in messed up countersinks. You can even use a block of wood and move it along as you go, but it will get enlarged over time. Then you need to drill another pilot hole and start over. As a result, using a scrap strip works a little better, but the main thing is for you to know the pilot needs to be supported.



What Ray said.
 
Which model are you working on? I believe the HS spar FLANGES are dimpled, while the bars that go into the WEBS are countersunk. On the wings, it is in fact the FLANGES that get countersunk, I’m just making sure you have the terminology correct, flanges vs webs..

This is for an RV-10. The HS spar flanges are countersunk to accept the dimples in the skins. Not sure what part you're referring to when you mention "bars" that go into the webs, but on the -10 there are indeed quite a few AD426 rivets that attach the spar doubler to the spar web that also require countersinking into the doubler. I don't specifically remember any countersinking done on the spar web directly, but I'm going from memory at the moment so there could be.

Thanks,
--mgm
 
FWIW, I'll attempt to describe the solution I came up with. It's similar I believe to what Boomer described:

I used two 4-foot long pieces of 2" wide by 1/8" thick steel strips (HD hardware section) as "hangers" to support the spar by the flange as I hung it off the edge of my table.

Picture these two steel strips laying on my table, with the ends protruding past the edge by a couple of inches. This allowed me to hang the spar by the underside of the flange that I was about to countersink, with the rest of the spar (web, bottom flange, mounting brackets, etc.) dangling below the edge of the work surface.

The steel strips were heavy enough in their own right to support the weight of the spar without clamping, but for good measure, and because I would be pressing down with the drill, I clamped and added additional weight to hold them in place.

I prepared the ends of the steel strips first by rounding the 2" edge off and deburring it a bit so it wouldn't scratch the spar. I also wrapped a thin layer of masking tape to further protect the spar.

I drilled a 1/4" hole in one of the steel strips, right near the end, to allow the pilot of the countersink cutter to protrude through the underside of the spar flange as I cut without hitting the steel.

I used thin strip of scrap aluminum (also from HD) double-sided taped to the same piece of steel that I put the relief hole in, and aligned with the edge of the spar flange when it was hung in place for drilling, to help support the part of the C-Sink cage that hung over the spar flange so the cage wouldn't tilt.

With this setup I was able to align each hole in the spar flange with the hole in the underlying steel, countersink the hole, then slide the spar a tad to align with the next hole. I used a small spring clamp mostly as an extra hand to hold stuff still while I drilled. I had my wife stationed by the other steel hanger, which was just to support the end of the spar that I wasn't working on, in order to keep it from accidentally falling off as I slid the spar from one hole to the next.

For the front spar, I had to adjust the strips of aluminum supporting the cage (add another layer) when I did the holes that also include spar caps under the spar flanges. And for the rear spar, the steel strips needed to protrude past the edge of the table enough to allow the elevator hinges to not hit the table itself.

All in all, this method worked great. Not saying it's the only way or the best way, but it worked ok for my purposes, held everything stable, and allowed me to get into a rhythm and have consistent results.
 

Attachments

  • IMG_4787.jpg
    IMG_4787.jpg
    381.7 KB · Views: 105
Last edited:
You need to have a backup piece of some sort because the pilot of the countersink bit needs to be supported in some way.

I built this jig for the center 9 + 9 holes that you have to countersink into the flanges and caps of the HS front spar on a -10.

This worked really well to support the countersink cage along the edge. There's a spar cap there so elongating the hole wasn't a concern, but seems this approach could make countersinking flanges in other areas a little less hit or miss and would provide a pilot behind thiner material too.

Definitely some extra time clecoing from hole to hole but it worked. Eventually only used 2 clecos for each set up and it stayed put.

(Ignore the extra holes. They don't do anything. This was scrap from a botched doubler :))
 

Attachments

  • IMG_5128.jpg
    IMG_5128.jpg
    348.5 KB · Views: 74
  • IMG_5131.jpg
    IMG_5131.jpg
    273.9 KB · Views: 75
  • IMG_5132.jpg
    IMG_5132.jpg
    330.9 KB · Views: 63
Last edited:
Back
Top