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Countersink Basics, please help

RV7AJeremy

RV7AJeremy

Well Known Member
Another embarrassing question that I should know the answer to.

I attempted to do my first countersinks today and I followed the plans in the manual and I know the results cannot be correct. Section 5E reads ?Use the appropriate rivet or screw as a gage when you machine countersink. When the rivet/screw is flush, you are done. If there is going to be a dimpled skin riveted onto the machine countersunk surface, adjust the machine countersink tool a few ?clicks? deeper than flush. This will make the rivet/screw sit about .005? below flush when dropped into the hole.?

I did all this, tried it on scrap material, measured with my caliper, made sure it was all correct, and when I place the dimpled piece over the countersink they do not lay flat against each-other. I am using a DRDT2 and Cleveland dies. What am I doing wrong, do I need to go deeper and how far is too deep?

IMG_4390.jpg


Also, I realize that I need some local help and I have already contacted my local EAA buddy but Have not heard back yet. There are not too many local people in Columbus MS building RV?s

I also searched the forums but did not find anything this basic.

Thanks in advance.
 
Jeremy,

A couple things. First, it may not always appear that the dimpled material is going to lay flat in the countersunk hole. Test a few pieces by actually riveting them together, and they may pull up.

My suggestion to get this right is to countersink a hole clearly too shallowly and slowly adjust your CS cage down (2-5 clicks at a time) until you have the hole perfect. If its for a rivet, then put the rivet in the CS hole and you should not be able to feel it sticking up (hole too shallow), nor should you see any shiny ring in the aluminum around it (hole too deep). If you are putting a dimpled sheet into the CS hole, you want to see a thin ring of shiny aluminum around the test rivet - that is, put the test rivet in the CS hole and there should be just a bit of a ring around it, so that the CS hole is ever so slightly larger than what would fit a perfect rivet. Once you think you are getting this depth right, then test rivet a couple pieces of scrap together to make sure that the pieces mate (i.e., there is no gap between the two sheets of aluminum.

Be careful to keep the CS cage flat on the material you are countersinking. If possible, I try to do this using a drill press, or I will kind of slightly wobble the cage around on the aluminum. The only way you can get the CS too deep (using the cage) is if you are using thin aluminum and you push way too hard on it. Usually aluminum that thin is not appropriate to CS anyway.

Hope this helps. It's mostly practice and trial and error, thus a good idea to mess with scrap material first. And you are right that a 15-minute session with an experienced builder is better than an entire book worth of written description.

cheers,
greg

p.s. no question too silly here - that is one of the points of this forum.
 
I agree - local help is good, it is much easier to learn this stuff with someone saying, "yep, go deeper" etc...

However, since you are working on scrap you can try to experiment a bit. I bet you need to go a bit deeper than just three clicks past where the screw sits. When I was building I dimpled a piece of .032" to use as a gauge when I knew I'd be putting a dimple into a countersink.

You can even try countersinking your scrap, putting the dimple in, then a screw and then a nut (or rivet as appropriate) to see if everything is all fitting flat. I think you'll find that getting good results requires a few more clicks than you are doing.
 
Thanks, I kept trying and I had to go about 30 clicks to get the piece to lay flat, and the hole looked way to big. It seemed like a lot when the directions said three clicks. I also tried driving a rivet and the pieces did not pull together.

thanks for the help, any other pointers on this topic would be greatly appreciated.
 
I think I have figured it out with the help of the other thread. My dimples were not crisp enough, I readjusted the DRDT2 and really waled on it this time, the dimples look much better and not the parts are sitting flush with each-other. Thanks for all the help.
 
Glad you got it figured out.

I was in Amory over the last weekend. A little earlier on this thread and I would have come down for a visit.

Keep up the work.

Also, a comment on the DRD2. When properly dimpled you will see a very slight circle around the dimple where the outer part of the die contacts the metal. This is known as a witness mark, and is a good indication that the die has fully contacted the metal.

This is not an indentation, but a halo around the dimpled area. I will see if I can find a photo.
 
kevinh said:
When I was building I dimpled a piece of .032" to use as a gauge when I knew I'd be putting a dimple into a countersink.
Click to expand...

We actually made a complete set, in thicknesses from .016 up to .040, with both -3 and -4 rivet size dimples. That way, no matter what thickness the piece that's being riveted is, we can check the countersink depth very easily to make sure it's correct. Also have one for tank skin dimples made with the tank die, and several for the 120 degree dimples/countersinks for blind rivets.

Takes about 30 seconds to make these little gauges...and they've now been used on two airframes.
 
simple method

I also use a scraps of aluminum sheet dimpled and marked at various sizes: 3/32, #6, #8 etc . When I lay a sheet against the dimple in the machine countersunk hole, I don't only look to see if it lays flat, but also if it "wiggles" side to side. A hole that is too shallow will not wiggle, but the two surfaces don't lay flat. A hole which is countersunk too deeply wiggles. In a hole countersunk just right, the test scrap lays flat and doesn't wiggle.

Not very sophisticated, but it works for me
 
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