Greg Arehart

Greg Arehart

Well Known Member
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
 
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.
 
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....
 
GEAR LEGS

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.
 
jrs14855 said:
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.
Click to expand...

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
 
Last edited:
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
 
If you have good equipment and it sounds like you do,

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.
 
gear

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.
 
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.
 
I just drilled my legs for my RV6

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 [email protected] 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,
 
Last edited:
Well, it turns out that the mount holes are already 5/16” holes, so there is no room for error unless I go a size larger for the final bolts. I'm not too thrilled about doing that, as the edge distance would be below minimums. I’m now thinking that I will just ignore those predrilled holes and drill & ream new ones in both the engine mount and legs, with the legs in place and weight on the airplane to check for alignment prior to final sizing (actually probably will do this from the beginning).

Does this seem reasonable? I can't think of any reason why the hole location cannot be moved around the leg by 30 degrees or so (I'm looking for someone to tell me why not to do this!). OK, went to the airport, looks like the geometry precludes a hole in a different location, so back to plan A.

thanks,
greg
 
Last edited:
Greg Arehart said:
I can't think of any reason why the hole location cannot be moved around the leg by 30 degrees or so (I'm looking for someone to tell me why not to do this!). OK, went to the airport, looks like the geometry precludes a hole in a different location, so back to plan A.

thanks,
greg
Click to expand...

Hey Greg, I just started the process of drilling the gear on to the -3 this afternoon, and spent quite a bit of time trying to figure out exactly where I wanted the bolt holes centered so that the head and nut would not interfere with welds or tubes. Pretty much found what you did - there is a very narrow window (around the circumference) where they will go. I piloted those spots today, and will begin lining everything up for the major drilling tomorrow or next week. hoping to be on the wheels by next weekend, but I am not going to rush this step!

Paul
 
Drill jig

I dunno if this helps or not, but I seem to recall talking with someone who had a jig like this made up at a local machine shop. The plan was to use some band clamps to clamp it to the side of the tube socket and drill through the tube and gear leg as an assembly:

drilljig.jpg


Good luck,
 
drill jig

I have a drill jig that I built for the Wittman Tailwind. I don't think it would work on the RV but the concept would. I also have an idea for a jig for the predrilled mount undrilled gear leg. Will try to post something on that later today. I don't have a way of doing pictures or drawings so it will all be text.
 
drilling

This is for Greg and anyone who has a drilled mount and undrilled gear.
Obtain a piece of flat steel, at least 1/4" thick, 3 3/4" x 2". Cut an opening in this to make it look like a U. 1 3/4 wide centered on the 3 3/4 dimension and 3/4 deep along the 2" dimension. You can do this with a hacksaw and file but a trip to the local welding/fabrication shop would be easier.
Take a piece of 4130 tubing 1/2" od x .095 wall x 4" plus long. Ream each end to 5/16" x 1 1/4" deep.
Clamp both pieces to a piece of scrap flat stock, with shims as necessary so the tube is more or less centered on the opening cut in the first piece in all directions. If you have a TIG or MIG four generous tack welds on each end. If not a trip to the welding shop. A oxy acy torch will cause too much distortion.
Cut out the center of the 1/2" tube in line with the cutout in the flat stock.
Make a pin about 2" long from 5/16 round bar or a long 5/16 bolt. Make sure one end is square.
Cut a piece of 5/16 x .065 x 2" 4130 tube. Ream 5/16.
Place this assembly over the end of the gear socket. The 5/16 pin engages the aft hole in the socket. The 5/16 od bushing engages the front of the socket. Make sure both are engaged in the respective holes in the socket.
Drill a 3/16 pilot hole approx 1/16 into the gear leg as described in my first post.
The difference will be that you will jig the gear leg in the machinists vice, bolted to the milling machine or drill press table and drill 3/16, 19/64 and .311 ream WITHOUT MOVING THE GEAR LEG OR THE TABLE.
With the gear back in place, if you're lucky, everything will line up. If the aft hole is misaligned just a couple of thousand's, I would just line ream and leave it alone.
If the rear hole is badly misaligned, the easiest fix is a doubler washer welded to the aft edge of the socket. This can be formed from flat stock or 1 3/4 x .058 tubing. The od of the washer should be at least twice the diameter of the bolt. With the gear in place, two very small tack welds on the washer with a mig or tig, letting everything cool completely between welds. If done properly this will NOT harm the gear leg. Remove the gear leg and finish weld the washer. The washer does NOT need to be welded all around. A second option is the oversize bolt that GAHCO has posted about. These will require a special reamer. The drill jig idea is not mine, it was in Sport Aviation about 40 years ago.
If any one needs a jig and does not have ready access to material or equipment, I could make you one. It will be about 10 days before I could do this.
 
Oversize

Greg,

You can get over sized bolts that are AN5 just slightly larger, they are close tolerance and come in and "X" and "Y" oversize. I just picked some up for my gear legs from Genuine Aircraft Hardware. Tom is on this site handle "GAHco", you can look up posts made by him and learn a lot about gear leg bolts. I did ream my legs to .3115 but the AN5 standard bolt just drops in, after reading on this site I decided to go to the first oversize, which is "X" and I have adjustable reamers now so I will sneak up on the fit so that the bolt requires a bit of tapping to install. Plan to get to this real soon!

Go to the Genuine Aircraft Hardware site and they have all the information on specifications. The close tolerance bolts are not sized the same way regular AN bolts are so read up or call them to get the sizing.

http://www.gen-aircraft-hardware.com/


Greg Arehart said:
Well, it turns out that the mount holes are already 5/16? holes, so there is no room for error unless I go a size larger for the final bolts. I'm not too thrilled about doing that, as the edge distance would be below minimums. I?m now thinking that I will just ignore those predrilled holes and drill & ream new ones in both the engine mount and legs, with the legs in place and weight on the airplane to check for alignment prior to final sizing (actually probably will do this from the beginning).

Does this seem reasonable? I can't think of any reason why the hole location cannot be moved around the leg by 30 degrees or so (I'm looking for someone to tell me why not to do this!). OK, went to the airport, looks like the geometry precludes a hole in a different location, so back to plan A.

thanks,
greg
Click to expand...
 
bolts

I am somewhat fanatical about a light/medium drive fit on these bolts. Get a quality micrometer that has a ten thousands scale. Get a bunch of extra bolts and check the sizes. There may be some variation in the same bunch of bolts from the same supplier. If the bolts vary significantly, mark the diameter with some tape and arrange them smallest to largest. Take the smallest bolt, chuck it by the threads in a drill press, and reduce the diameter by .001 by polishing with fine emery. This will be the gauge to properly size the hole with the expansion reamer. This undersize bolt should slide in with just finger pressure.
I use a brass drift or plastic hammer held against the bolt head. Strike the drift/hammer with a medium size ball peen hammer. I use a bolt with a taper ground on the threaded end and the head cut off as an alignment pin. Drive this thru the hole ahead of the "good" bolt. Protect the firewall so you don't dent it if the taper pin flies out.
If you wind up with a hole that is just a bit loose, look for NAS bolts. These MAY be .001 or more larger than AN.
If you MUST use a non adjustable reamer and your reamer cuts oversize, chuck it in a lathe and turn it backwards while holding a fine machinists stone against the flutes. Drill some 19/64 holes in a piece of scrap metal and try the reamer-trial and error until you get the fit you want. I think the proper reamer size is .3105. I don't believe this is a standard size.
 
reamers

Did a quick check on mscdirect.com start with reamers, straight flute, high speed steel. .310 part 82133109 at $31.53. .3105 part 02040160 at $14.05. These are the only ones I looked at, first is probably US name brand, second foreign. they stock 5787 items under chucking reamers straight flute.
 
OK, so here is my (I think) final solution. I aligned the gear legs properly so the axles are parallel, everything sitting level etc. Chucked my 5/16 bit in the hand drill and used the front hole as a jig to mark the center of the hole with the bit. Turns out the aft hole is slightly undersized, so I used a 19/64 bit and hand-turned this in the aft hole since the drill won't get back there. This marked the aft hole center. Scribed these center marks around to the top of the gear leg. Took the legs to the mill, and centered a hole on these two marks and drilled to 19/64, starting from the front mark. Back to the airport, installed the legs and got everything set up again. The aft engine mount holes were not quite centered, but almost. So to finish this up, I will probably drill and ream the holes to be slightly oversized and use the NAS oversized bolts from GAHCO. Should be able to get a good tight fit using an adjustable reamer.

Thanks to everyone for their comments and ideas. A rather nerve-wracking day, but it looks like everything will work out. Now if only the wheels fit onto the axles....

cheers,
greg
 
You must log in or register to reply here.