Drilling gear legs

[Greg Arehart]

I'm in the process of putting new main gear on my 9 (which used to be a 9A). The new gear legs arrived undrilled from Harmon Lange. I'm looking for advice on aligning and drilling these legs. My current plan is to align the axles using a long straightedge, and then either mark the holes or start drilling them in place. Once marked or started, I will take the legs to a machine shop to assure that the holes are drilled through the center of the leg and perpendicular to the leg. I would think that this should make them fit the mounts properly, but if anyone has a note of caution or better way to do this, I would like to hear about it. I don't like the idea of making a mistake with such an expensive part.

thanks,
greg

[Vlad]

You are one true experimenter Greg! While I can not offer any suggestions how to drill the legs I will be following your progress with great interest. Please post the outcome.

[Ironflight]

Wow Greg - I must have missed the announcement that you were changing sides in the great debate! Last I heard, you were just getting the engine rebuilt....

[jrs14855]

These gears of course date back to 1953 on the Wittman Tailwind. There are two schools of thought, drill to final size in one shot, or pilot drill. I have done both and prefer the pilot drill. I use 3/16 for the pilot drill.The gear is best held in position by clamping the axles to a straight steel angle that spans the width of the gear. I spot drill thru the socket and about 1/16 into the gear leg, in place on the airplane. Remove the gear leg and set it up in the drill press. The end of the gear leg is jigged perpendicular to the drill chuck. This can be done with a level.Make some kind of stand to hold the axle end. With the gear leg in a good drill press vice, put a drill bit in the chuck upside down. Bring the drill bit in contact with the spot drilled hole. Trial and error, measure between the drill bit and vice until you have exactly the same reading between the drill and the vice, measuring with a good dial caliper. Diameter of upper gear leg minus pilot drill size, divided by 2. Jig the gear in place again and repeat the process with a 19/64 drill, spot drilling as above and then with the drill press. Ream to final size in place on the mount with an adjustable reamer.About .3105 will give a light drive fit on a AN5 bolt. Use the best quality drill bits. Some suggest carbide, but I have always used good quality hardware store bits. A word of caution on drilling-use slow r/m and lots of pressure. Try to drill all the way thru in one shot. Use a generous amount of a good cutting lubricant. You will probably need an r/m around 300 for the large drill bit and reamer. If your drill press will not go this slow, take it to a machine shop. If you work harden the gear with too much speed and/or too little pressure you are in trouble. I have done several of these on the Wittman Tailwind and never had a problem. Just take your time and triple check everything before drilling. I don't know what the specs are on the RV but the Tailwind uses 1/16 toe in in the length of the axle. The theory being that this becomes zero with the airplane loaded. 1/16 shim clamped between the front inboard end of each axle when spot drilling and reaming.

[vlittle]

Originally Posted by jrs14855

These gears of course date back to 1953 on the Wittman Tailwind. There are two schools of thought, drill to final size in one shot, or pilot drill. I have done both and prefer the pilot drill. I use 3/16 for the pilot drill.The gear is best held in position by clamping the axles to a straight steel angle that spans the width of the gear. I spot drill thru the socket and about 1/16 into the gear leg, in place on the airplane. Remove the gear leg and set it up in the drill press. The end of the gear leg is jigged perpendicular to the drill chuck. This can be done with a level.Make some kind of stand to hold the axle end. With the gear leg in a good drill press vice, put a drill bit in the chuck upside down. Bring the drill bit in contact with the spot drilled hole. Trial and error, measure between the drill bit and vice until you have exactly the same reading between the drill and the vice, measuring with a good dial caliper. Diameter of upper gear leg minus pilot drill size, divided by 2. Jig the gear in place again and repeat the process with a 19/64 drill, spot drilling as above and then with the drill press. Ream to final size in place on the mount with an adjustable reamer.About .3105 will give a light drive fit on a AN5 bolt. Use the best quality drill bits. Some suggest carbide, but I have always used good quality hardware store bits. A word of caution on drilling-use slow r/m and lots of pressure. Try to drill all the way thru in one shot. Use a generous amount of a good cutting lubricant. You will probably need an r/m around 300 for the large drill bit and reamer. If your drill press will not go this slow, take it to a machine shop. If you work harden the gear with too much speed and/or too little pressure you are in trouble. I have done several of these on the Wittman Tailwind and never had a problem. Just take your time and triple check everything before drilling. I don't know what the specs are on the RV but the Tailwind uses 1/16 toe in in the length of the axle. The theory being that this becomes zero with the airplane loaded. 1/16 shim clamped between the front inboard end of each axle when spot drilling and reaming.

Good Advice. I used a similar starting technique to drill the titanium gear for my Rocket. I started with a 1/8" index hole through the engine mount and drilled about 1/16" into the gear leg to mark the alignment for the machine shop. I took just the gear legs directly to the machine shop, where they were drilled 1/64" undersize.

I then enlarged the 1/8" hole on my engine mount to a 1/64" undersize, inserted the gear leg and lined up the holes with a drift pin. Then, using a 1/32" undersize drill, I bored through to the other side of the engine mount, followed by the 1/64" undersize. Thus, I had match drilled the mount to the gear leg (no drill press involved, but use a good quality 1/2" electric drill). Finally, using a reamer, I bored through the assembly to the final size desired (tight fit).

This technique is pretty foolproof, and it can be done with the engine mount on the airplane. Cost $$ at the shop, though. Note my undersize drill sizes may be off, but the principle is sound.

Vern

[Greg Arehart]

Jim, Vern,

Thanks for the good advice and insights. My engine mount already has holes predrilled (but not full size), so I'll use whatever size those are (I think either 0.1875 or 0.25) for my pilot holes and go from there. Fortunately I do have access to a machine shop (and a buddy who knows how to operate it better than I), so should be able to get this done effectively.

I think I'll measure the toe-in on the existing legs before removal and duplicate that number to account for the gear flex.

Cheers,
greg

[Richard@Langair]

If you have good equipment and it sounds like you do, the best way we have found is to drill a slightly undersized hole and ream to size.

We use either a 19/64 or 9/32 carbide drill and a 0.311 reamer. The machine has to be nice and rigid with good control over the feed rate to use the carbide drill. If it is not controlled well enough, it can catch and break the carbide drill. If the machine is not rigid enough, or you are feeding it by hand, I would suggest to use a Cobalt drill.

If you use a cobalt drill, you should probably spin it about 300-400 RPM, definitely no faster. Also make sure it is nice and sharp. No dull tools allowed here.

My best suggestion is to use a cobalt drill as the carbide drills are finicky in this material. The edge needs to be prepped just right as well as the angle the drill is sharpened at needs to be just right. Unless you are familiar with using carbide in this material, it takes somewhat of a learning curve to figure out.

A HSS reamer can be used succesfully as long as it is not spinning too fast or run dry.

You should plan on using lots of cutting oil (Not coolant) with both the drill and reamer.

If you need any other help, you can contact me via email. It is richard at langair dot com.

[jrs14855]

I agree with Richard except that I prefer to use the 1/64 undersize drill and the adjustable or expansion reamer. MSC Supply is a good source for the drills/reamers and probably cutting oil. In the southwest McFadden/Dale hardware in Vegas and LA has cutting oil, drills and at least some reamers. MSC will have the decimal size if you decide to go that route. Another option is to ream to say .310 with a straight reamer and then open up the hole with the expansion reamer. If you get the holes perfect this will last forever. Less than perfect you will be fixing it at around 300 hours. The reamers should only be around $15 a piece.
One step I neglected to mention in my previous post is to rejig the gear in place and drill thru the second wall of the socket at each step.

[rocketbob]

I would NOT drill to final size until you have the airplane back together with a typical flying weight on the gear (3/4 fuel and the pilot). At that point you will want to check for zero toe-out/toe in and adjust accordingly. Your toe in check can be done with the engine off with bags of water softener salt on the wings to simulate that weight. If you do this your tires will last forever and the airplane will behave nicely on the ground. I helped a friend do this on his rocket (have also done it to mine) and he got 600 hours out of a set of tires and they weren't worn all the way. The check is done by clamping a 4 ft piece of angle to the brake calipers while measuring fore-aft.

[RV6AussieNick]

Hi Greg,

I just did the same for my RV6, except I was given an already drilled set of legs. I emailed Vans to check if I needed any toe in, they said to set the axles up parallel to each other.

To guarantee a proper job I made a drilling jig out of 6mm angle and a solid 6in alloy block to support the new hole alignment, mounted the engine mount to the drilling frame. I then set the legs up in the engine mount, aligned the axles using a long piece of 6mm steel angle and scribed the position of the new holes. I took my legs to work and with the help of one of our expert machinists we dragged and opened the holes for a freeze plug repair made out of the material from my old legs. With the freeze plug repair we had to maintain a min .050 wall thickness. We then turned up 2 plugs with .001" oversize tolerance, shrunk the plugs in liquid nitrogen for a shrink fit that locks the plugs in position when they normalise in temp.

I then reset the let legs in the engine mount and drilling jig, using a .311 drill bush I pilot drilled the new legs to a #13 drill, then opened the holes up to .309 using a gun barrel drill. With the legs removed I reamed the holes up to final size of .311.

I have some photo's to help show how the drill jig worked, send me your email and I'll send threw any photo's that might help. Or email me at nicholaspurcell@qantas.com.au if you have any questions. This is just a quick run down, i can expaind on the process further. Have a look at my Blog listed below, i have some photo's there.

Cheers,