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Skybolt 15/32" holes

R

rightrudder

Well Known Member
Hi guys,

How are you achieving the finished holes for the Skybolt V-Loc grommets? I've spec'd out my Unibits and none has a step with this 15/32" dimension. Get it close with the smaller step, then sand a little with a Dremel sanding drum? I really want the grommets to fit snugly in very uniform holes.
 
rightrudder said:
Hi guys,

How are you achieving the finished holes for the Skybolt V-Loc grommets? I've spec'd out my Unibits and none has a step with this 15/32" dimension. Get it close with the smaller step, then sand a little with a Dremel sanding drum? I really want the grommets to fit snugly in very uniform holes.
Click to expand...

There is no easy way to get the hole that large, tight, and in the perfect position without some extra tools.

Here is my post to this effect. I will check the drill size but I experimented and I think it is smaller than 15/32. I drilled all the holes by the method in the post, and then wrapped some sand paper on a dowel until it fit and then sanded each hole, they came out tight. I will have to do it again after paint, but so what - they will be tight.

http://www.vansairforce.com/community/showpost.php?p=894624&postcount=2
 
Skybolts

I bought shouldered bushings (metric as I recall) at the local hardware store that the OD fit the Skybolt flange holes. Also found brass tubing that fit the inside of the shoulder bushing. I machined the shoulder of the bushing very thin so the bushing could be installed in the flanges under the cowl. Looked like this:





Installed in the flanges. Used a bit of tape in the flange hole to take up a teeny weeny bit of clearance.





I worked symmetrically doing a couple holes each on opposite sides of the cowl. Shined a light thru the back of the hole and marked rough location on the outside of the cowl. Used a step drill to reach 7/16", checking each couple of steps to stay centered on the shoulder bushing/brass bushing combo underneath. Knocked the brass bushing out the backside (into the cowl) and increased the hole size from 7/16 to to just under 15/32 with a needle file. Only took a couple of minutes per hole. Finished with a reamer. Reamed thru the bushing from the inside/out where I could and went from the outside/in where the engine mount and other obstructions were present.

http://www.yardstore.com/browse.cfm/4,5205.html

The grommets were still tight so I touched up the holes just a few thousands with the needle file.

If you can figure out a slower way to do this, please post so I won't officially be the slowest Skybolt installer in the history of the world. :D

That said, every hole came out dead nuts.
 
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Hand tapered reamer to upsize the hole until the grommet just fit...tight enough that after paint, the grommets wouldn't go, necessitating a little bit of sanding with a dowel.

Easy peasy.
 
DaAV8R said:
I bought shouldered bushings (metric as I recall) at the local hardware store that the OD fit the Skybolt flange holes. Also found brass tubing that fit the inside of the shoulder bushing. I machined the shoulder of the bushing very thin so the bushing could be installed in the flanges under the cowl. Looked like this:

Installed in the flanges. Used a bit of tape in the flange hole to take up a teeny weeny bit of clearance.

I worked symmetrically doing a couple holes each on opposite sides of the cowl. Shined a light thru the back of the hole and marked rough location on the outside of the cowl. Used a step drill to reach 7/16", checking each couple of steps to stay centered on the shoulder bushing/brass bushing combo underneath. Knocked the brass bushing out the backside (into the cowl) and increased the hole size from 7/16 to to just under 15/32 with a needle file. Only took a couple of minutes per hole. Finished with a reamer. Reamed thru the bushing from the inside/out where I could and went from the outside/in where the engine mount and other obstructions were present.

http://www.yardstore.com/browse.cfm/4,5205.html

The grommets were still tight so I touched up the holes just a few thousands with the needle file.

If you can figure out a slower way to do this, please post so I won't officially be the slowest Skybolt installer in the history of the world. :D

That said, every hole came out dead nuts.
Click to expand...

You are a better man than I am, sincerely. I could not get a light to shine and determine precisely where to drill the hole. My engine is mounted and just too much stuff under there. Also, keeping that cowl down tight was an issue for this solo builder. If you look at my older post, even the 1/8 holes were off somewhat, and I wallered them to get a tight cowl fit. You should get a good chuckle at my slow method, but it allowed for some errors to be easily corrected in the next steps. It turned out well, perfect in fact, which is the only goal. I think I win the slow method though. ;) We can meet and toast at the next RV Social!
 
I think I found a quick and dirty method for these holes that's very simple. The pics are of my worst pilot hole, which I really buggered up with a runaway Dremel tool.

Once I modify the pilot hole enough to get the cleco into the machined aluminum spacer behind, I just trace around the base of the cleco with a fine-tip Sharpie. This circle is still a little undersized for the final grommet, but it'll give an excellent, centered baseline hole that I can enlarge with the tapered reamer.

I was lucky that I still have enough material on this hole to make the final hole without re-glassing.



 
rightrudder said:
I think I found a quick and dirty method for these holes that's very simple. The pics are of my worst pilot hole, which I really buggered up with a runaway Dremel tool.

Once I modify the pilot hole enough to get the cleco into the machined aluminum spacer behind, I just trace around the base of the cleco with a fine-tip Sharpie. This circle is still a little undersized for the final grommet, but it'll give an excellent, centered baseline hole that I can enlarge with the tapered reamer.

I was lucky that I still have enough material on this hole to make the final hole without re-glassing.



Click to expand...

Doug, This elongated hole is exactly the reason I made the drill guide out of a piece of scrap steel, also each hole was identified with an arrow to tell me which side of the hole was correct to displace the alignment pin. At this point, you should go back and look at my post in this regard. Practice on a scrap piece. If you can make a good hole with a miss-located guide hole you are better than me. Entirely possible. Hole tools wander in fiberglass.

I bet I could have used all fixed receptacles.

As always, YMMV.
 
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Thanks, Bill. Good advice about practicing on a scrap piece. On my way out the door to buy Harbor Freight's finest $3 T-handle tapered reamer! :):)

The pilot hole is grossly elongated, but the hole I traced around the cleco is centered, so I figure I should be OK enlarging out to the Sharpie circle. I'll do this using the Dremel (with a far less aggressive bit!) and then take over with the reamer.
 
Done! At least the upper cowling, and the fit is spot on. I didn't even need sliding receptacles for the corners. What I really like about this method is that the pilot hole needn't be all that precise, just less than the diameter of the cleco body. In fact, a little bigger is sometimes better, to make sure that the pointy end of the cleco is not exerting even the slightest side load on the cowling.

Once the holes were traced, I used a rat tail file to open the holes up to the Sharpie circles. Then, with baseline holes established, a Unibit to just under final diameter, then make the grommets a press fit with the tapered reamer.

Lower cowl should be far easier to mount with the aluminum strips rather than the receptacle plates, as match drilling will get everything centered up nicely.

HF's reamer works far better than it should for the price, but the loose T-handle is annoying. I fixed mine in place with some J-B Weld.





 
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Skybolt Fasteners 15/32 Drill

The Unibit-1 has a 15/32 step...last step before the final 1/2 step.

Typically, before composites came into being and everything was aluminum, the 15/32 hole (.4687) would allow the grommet to clear the retainer ridge that is .471 to .472 wide by slanting the grommet when you insert it into the panel. The typical grommet has a groove/undercut to accommodate the retainer. We have high shear grommets made specifically for the Vans series aircraft, however, there remains the groove for the retainer.

All this works great on .032-.040 aluminum or composite. Thicker panels (aluminum or composite) do not allow enough insert clearance, thus, a .500 hole (.472 to be exact) might be required....depending on how tight you want the grommets to fit. The standard undercut allows some float in the NS or OS style grommets. By eliminating this float, we are allowing a tighter fit with reduced cowling shift and hole wear over time but we are also required to have aligned holes.

Like everything, perfection comes at a price.

May I add that Skybolt is promoting a 20% sale on Complete Vans RV Fastener Kits from April 1st through April 26th (end of Sun N Fun). Look for us to be back in 2016 as a vendor with plenty of kits and advice, including cowling mock ups for the RV series. Skybolt provides patented interlocking support plates, our DiamondHead series studs, high shear grommets, extensive how-to instructions. The DiamondHead stud are super hard with a machined facing to prevent galling such that they will look as clean after thousands of cycles as they did in the bag they came in.

Call us or go on line....1-800-223-1963 www.skybolt.com If you have questions, give us a call.

Ned Bowers
RV4, RV8, B17, B24, B everything else, L39, Beech everything, Cub.....more Vans...time for a 9....older.
 
Wow, very thorough explanation, Ned! You're right about the Unibit, but I guess I was just a little scared that I would open up the holes too much, so I hand-reamed them for a press-fit of each grommet. That's a very high quality Unibit, incidentally....cuts fiberglass effortlessly.

I'm really liking the Skybolts. The cowling sort of snaps in place before you give the 1/4-turn cinch, as the fasteners locate in the plate holes.
 
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