Boring a shallow cut in a deep hole (helicopter skids)

[jmarkwolf]

I'm fabricating new skids for my helicopter. See below.

The front end of the skid sometimes carries a 30LB ballast weight, and I need to insert a steel sleeve approx 24" into the aluminum skid to support the bend at the front of the 1.75" OD thick-wall skid. I need to remove about 0.010" on the ID of the aluminum skid.

Don't want to remove any OD on the steel sleeve, it's relatively thin-wall chromalloy.

My little 11" x 26" Grizzly lather doesn't have the spindle bore capacity, and I'm wondering if there is some hand-drill version of a boring head that can make this cut. Doesn't have to be particularly smooth bore.

Any ideas

[vpt]

You need to remove the .010" for the whole 24" inside the aluminum tube?

Sounds like a job for a hone on a stick with a drill to me.

[jmarkwolf]

Yes, 0.010" removed, 24 inches deep.

I would think a hone would load up, not to mention take forever.

[Rustybolt]

An expandable reamer with an extension. Use cutting fluid. Go slow. Don't try and take too much out at one go.

[tylernt]

You could make what looks like a counterbore tool from a lump of O-1 and then harden it. I wouldn't bother to temper it, and in aluminum it should last the whole job without too much sharpening.

You'll want a little pilot spigot on the front to just clear the existing ID, then two or more cutting edges. It needn't be as complex as this one (you don't need helical flutes), but will give you the general idea:

[sbmathias]

Can you make the steel sleeve smaller in OD instead of the skid larger in ID? That would undoubtedly be much easier.

[Boostinjdm]

Put the sleeve in the freezer, helicopter in the oven......

[vincemulhollon]

You won't like this answer but from both a labor cost and airworthiness perspective you're far better off buying properly sized stock.

Most of the time you put a weight on a long moment arm at the front end of the skid because a single pilot no pax, maybe a heavy fuel load, CG way in the back, has to shove the cyclic "too far" forward in a hover for comfort/safety/efficiency. Or heli-towing a really heavy banner (or towing it way too fast). Another answer you won't like because it wastes expensive avgas (and limits carrying load, probably right when your fuel tanks are full..) is strapping heavy sacks of sand into the seats. If you're the pilot you already know that and your A+P isn't going to rig something homemade up. The manufacturer usually gets all out of whack at homemade structural redesigns of a certified design... Do it yourself merely means when the A+P finds out at the next annual its gonna get red flagged until the skids are replaced and brought back to official certified design.

From a pilots perspective if the weight falls off in flight it doesn't necessarily mean doom, but it might not turn out well. Suddenly not being able to maintain forward airspeed gets exciting if the density altitude is high, probably not an issue in N.A. this time of year but in the summer maybe in the mountains its not fun to be surprised when you're busy doing other things. If you're the A+P mechanic you might want to ask the pilot what he thinks about the flight dynamics of the skid cracking and the weight falling off in mid flight. It is probably survivable but... From a mechanic perspective this whole discussion probably comes from the bird coming home with the end of the skid bent after a flight with a ballast weight, well the bird's skid was spec'd by the mfgr to easily handle that weight under "normal" flight loads so that means the only way the skid came home bent is either the skid's damaged (maybe internally, replace it before it fails) or the pilot/weather was way outta whack (icing, crazy winds, who knows) and "exceeded flight G limits" in which case the bent skid is the only thing you know about so far ... so take a look at whatever else can get bent in extreme conditions like the boom bent, powerplant overtorque, etc. This brings up the next big issue in that snapping the front end of the skid off sucks but is not as big of a deal as beefing the skid up until the entire leg snaps under load. Landing with partial skid is probably no big deal if it snaps off completely and cleanly, but landing with no skid at all means rotor strike / total loss of the bird unless the pilot gets lucky/hyper creative (land on a stack of bricks or a highway divider or something) The designer probably intentionally designed the overall system to snap off the end of a skid before it snaps off an entire side, so strengthening up just the tip of the skid might result in a pretty bad outcome.

If its an experimental craft where you're the pilot and mech, I'd say be really careful. Sticking a tube halfway down the skid makes a big stress riser at the end of the tube, so if you ever break a skid, its gonna break at the end of the tube. So the tube should probably spend some lathe time getting a smooth taper and edge put on it. I'd definitely design it to snap the tip off before the leg, you don't want to lose an entire leg...

Another thing you're really not going to like hearing about is corrosion. 4130 chomalloy like you're probably using is almost the worst possible (common) thing to electrically connect to aluminum outdoors. Ask some ham radio guys for a laugh about aluminum antennas and chomalloy masts. You can hot dip it and that will help but it'll still electrolytically corrode anyway, and it'll go nuts with even the most microscopic scratch (like when its rammed into a skid...). And when it corrodes it'll expand making it impossible to remove and hoop stressing the skid till it splits. If this is what the designer specd, well trust them not me, but it sounds crazy to me, so there must be a lot more to the story...

This is before you hit the metalurgy. Aluminum is stiffer than chromoly so the aluminum will crack first. Also the aluminum is stronger. The chromoly does have a higher tensile strength but if you're pulling a skid apart lengthwise you're probably doin something wrong. In other words shoving a plug of steel into a tube of aluminum will not strengthen it as much as you'd hope, and definitely not as much as a plug of aluminum. You "can" buy cruddier aluminums than chromaloy but thats probably not what your skids are made out of. What I'm getting at is both aluminum tube within aluminum tube and replacing the whole works with thicker aluminum tube will be stronger/lighter than aluminum tube with steel inside. So you're better off from a performance perspective not doing the aluminum over steel. Again if the mfgr / designer directly orders you in writing to do it, well, do it then, I'm just some goofball on the internet, but it sounds weird.

Now a nice thick tube of aircraft aluminum of about the same alloy as the aluminum skid and the correct OD with permission from the mfgr or at least the designer... now you're getting somewhere...

The whole scene of pounding an oversize steel tube into an aluminum skid tip is just bad news all around.

[derekm]

Investigate filling the aluminium tube with 2 part polyurethane foam. The compression strength of the foam which will self bond to alloy, will make a significant difference to the bending resistance of the assembly.

[tylernt]

its gonna get red flagged until the skids are replaced and brought back to official certified design.

To be fair, the OP does say "The front end of the skid sometimes carries a 30LB ballast weight" which I take to mean that the chopper was engineered to carry a weight there from the get-go. So presumably the engineer knew what he as doing when he specified chromoly be slid inside the aluminum.

[rowbare]

To be fair, the OP does say "The front end of the skid sometimes carries a 30LB ballast weight" which I take to mean that the chopper was engineered to carry a weight there from the get-go. So presumably the engineer knew what he as doing when he specified chromoly be slid inside the aluminum.

There is nothing in the original post that specifies what material the ballast weight is made from. My read is that he is re-engineering some new skids and wants to insert the chromoly for reinforcement.

bob

[jmarkwolf]

For clarification.

This is an experimental aircraft but I'm not breaking any new ground here.

The chromalloy steel insert inside the aluminum skid tube is the way the skids have been manufactured for approx 30 years, with no failures that I'm aware of. I've been using this stock arrangment on my own aircraft for 15 years, as are hundreds of my flying compadres around the world.

I'm just saving about $600 over factory price by buying and bending the tube myself.

And I wasn't planning to pound the steel tube into the skid, rather slide it in, then mandrel bend them 30 degrees together.

The 30lb ballast weight slips over the front end of the right skid, when needed for CG purposes (pilots sits in left seat), and is pinned through the skid and steel insert with a 1/4" pin.

The places I've looked for chromalloy tube didn't have a standard wall thickness that would provide a slip fit.

I had thought about turning the OD of the steel down by 0.010" but am a machining newbie and don't know how to go about about turning an OD on relatively thin-wall tube and get a continuous cut without a steady. Perhaps this was a better question to the group?

The insert will extend all the way back to the landing gear mounting shoe which has two 1/4" aircraft grade AN-4 through bolts.

I'll admit, however, that I hadn't considered the stress riser at the end of the bore cut, however.

I'm going to re-think boring the skid in favor of a better fit between the ID of the skid and the OD of the insert..

Thanks for the input everybody.

[tylernt]

I had thought about turning the OD of the steel down by 0.010" but am a machining newbie and don't know how to go about about turning an OD on relatively thin-wall tube and get a continuous cut without a steady. Perhaps this was a better question to the group?

You might be able to get away with doing it without a steady if you have enough bed length for tailstock support. I would consider making two flanged plugs -- the plugs would be a close fit into the ID of your steel tube, with a slight flange to ride on the tube's faces to keep them from being pushed all the way inside the tube. One plug would have a center hole for a tailstock center, and the other plug would have a small spigot to chuck on to. Superglue the plugs to the tube so they don't slip. The superglue can be dissolved with acetone when done machining.

With a long, thinwall tube, tool pressure will have to be low to keep it from bowing in the middle. Sharp, high-rake HSS (or perhaps even a vertical shear tool) and small DOC should do it.

[Paul Alciatore]

I am not an aeronautical engineer or even a mechanical one. But having done a little bit of work getting a heli mounted device certified, I have a bit of appreciation for the sensitivities of the FAA and the design engineers. You say the chromalloy steel insert inside the aluminum skid tube is a standard way of doing this. OK. But, do they use a tube that has an OD that fits or do they also bore the aluminum? The stress riser at the end of the bore is probably the major point of concern.

Assuming that boring the aluminum is acceptable, there are ways of reducing this stress riser. One possibility would be a tapered section instead of a square step. You could make a boring bar from a steel round that just fits the ID of the aluminum. Then, install one or more inserts about an inch or two from the end of this boring bar so that they cut at a shallow angle, producing a tapered section. The first inch or two of full diameter bar would act as a pilot.



The shallow angle of the insert would produce a tapered section from the original ID to the new ID and there would be almost no stress riser. Adding a slot and a set screw would make it adjustable. My sketch is rough and by no means to scale. The details would need to be adjusted. You probably want to make the pocket bigger than I show for more chip volume to accumulate.

I am not sure why an insert is needed. Is it to strengthen the aluminum? Does it extend past the first bend in the skid to reinforce that bend? Is it to provide a wear resistant pocket for the weights? Or what?

If the insert is for strength, would turning the thin wall tube down be acceptable? It would seem that even a 0.005" reduction in wall thickness would have a significant effect. And what about the stress risers that turning it would produce. I would assume that the tube is either drawn over a mandrel or welded from flat (rolled?) stock. In either case, the finish would be relatively smooth. If you turn it down, this would leave a series of circumferential micro grooves, each of which would be a stress riser when the tube is subjected to bending stress. This would even further reduce the strength of that tube. Would a rolling technique be a better way of reducing the OD?

[goodscrap]

Would it be acceptable to expand the alloy tube, if so pressing a ball bearing through it would be an option there was a thread on here recently regarding out of round tube.

Brian