GrahamC

For anyone who has successfully drilled out the crank pin on a Cox crankshaft -

The cranks are hardened - are they hard all the way throw or are they just case hardened?

What did you use to drill it with? Carbide? Cobalt? or some other exotic material?

cheers, Graham

JCT049

Carbide is the way to go. I get carbide drill bits from MSC Industrial Supply Co.
Check out the Norfolk Aeromodelers website; http://control-line.tripod.com/ . I have been drilling the OS LA 46 crankshafts and installing tungsten to help with balancing.

JCT049

GrahamC

Thanks for that.

I was thinking carbide but hadn't tried it yet - no sense reinventing the wheel if someone has already been there.

I had a look at the web site you mentioned and the mods for the LA46 - no doubt to get a better stunt run.

cheers, Graham

JCT049

We found out with less vibration the fuel will not foam up in tank as bad. We tried the Armor All trick for awhile. The Armor All helped with the foaming problem but the glow plug would foul out after about 50 runs.
JCT049

jetpack

My Feedback: (1)

When using carbide, always use a steady stream of air across the hole to remove chips as your drilling. Do not let it powder inside the hole.

kersplat

What you really want, ideally is carbide center drills. They are stubbier and less likely to bend. At best start with a center drill and finish with the straight drill. As jetpack says, don't let the hole fill up, the dust will dull the drill.[:@] The best way to do it is center the crank pin in a four jaw chuck in a lathe, (the rest of the crank shaft will be off center quite a bit). Then you will drill the crank pin true with the pin journal.

combatpigg

My Feedback: (3)

In the medical field, the #1 question that is asked is, "Is the reward worth the risk"?

gkamysz

My Feedback: (19)

There are literally millions of replacement cranks available. Why not take a chance at learning something?

combatpigg

My Feedback: (3)

Do a before and after tach reading and let us know how it turns out. A little bit of fuzz on the tip of a 5x3 prop will have 10 times the effect on momentum that a micro gram's worth of metal has that is located 1/4" from center.
A Cox TD in good condition with a solid pin should be able to spin a 5x3 at 24,000......anything above that mark and the ball sockets will predictably crumble within minutes. So at that point it became fruitless to look for any more performance. Since that time, shaving the pistons has improved the redline, but that is the best possible area to remove weight.
One thing is for sure, once you invest the time and effort to get set up to drill 1 pin, you might as well drill 50.
To answer Greg's question directly....have at it and have fun . I'm just voicing my thoughts about it.

kersplat

CP is right, it is do able if you like to tinker in maching. It will not make ANY difference in the RPM though. If you do this in conjunction with the other Killer Bee crank mod, it may make the engine run smoother. I have three BW's that I use for Mouse racing. I did just the KB crank cut aways and the engine ran smoother, but the RPM didn't increase. This as well as the Paul Gibault Mouse mods too.

Honker1

My Feedback: (4)

Could someone explain what exactly is being drilled out and why? I think you are trying to hollow out the crank for less rotaional mass but not sure. I am getting close to the breaking point on rpm's with a Sure Start, this is a blast to keep modifying and testing. Lighten the piston, polished crank, trimmed piston for SPI, enlarge intake 3/16" or as large as you can go. What can be done to cylinder, can a tool be used to expand the liner such a nurling? (if I spelled that right). Then rebored for a taper fit? I am getting close to 23K now with a 5X3 rubber ducky prop.

Thanks,

Honker1

kersplat

If you exceed 23K on the SS, the crank pin is at serious risk of breaking.[:@]

proptop

My Feedback: (8)

I was wondering about that...hollowing it ain't gonna help in the strength dept.

Might be an interesting experiment, but as CP said, is it worth it.
After all these years, and so many knowlegable guys who have previously done (probably ) virtually all there is to do to these things...ain't nuthin' new under the sun.

But what the heck...

I would think anything you could do to improve high r.p.m. balance would reduce drag on the crank bearing surfaces, and give a bit more top end...but if the crankpin snaps off...[X(]

GrahamC

Always! Nothing ventured, nothing gained and all that. Even if the reward was a loss in RPM or strength as you would now know what not to do or how far to go the next time.

Lightening and balancing where not foremost in my mind rather I have been thinking about trying to make a rear rotor mod RR-1 much like what Tim Wiltse has done; the drilled crank pin to take an extension pin that in turn drives the rotor.

cheers, Graham

combatpigg

My Feedback: (3)

Ah-So....now I see the method to your madness! That would be a most honorable project. The only trouble with such a cool looking engine is you would never want to cowl it [8D], always run it in the open like a hemi in a deuce coupe.

proptop

My Feedback: (8)

As Emily Litella would say "Ohhh, that's very different...nevermind"

To those who don't remember or are too young, Gilda Radner played Emily Litella on Sat. Nite Live in the mid 70's...(dating myself there, huh!? )

longdan

I read what was said about the reasons for drilling the crank pin - to insert a pin to drive an intake rotor.
But I was wondering if there was actually any advantage in lightening the crankpin. Was too much metal taken off the counterbalance weight at the factory?

Also - when drilling hardened steel - step drill it. That means, start with a smaller drill and open up the hole until it is the size you want. Don't try and get it to size with the first drill. And use the solid tungsten drills, not the tungsten carbide coated ones. They are a waste of time. They might look cool with their gold colouring, but they are a gimmick. And as has been already mentioned, keep the swarf out of the hole, either by air, or with a good blast of coolant. I'd recommend the coolant over the iar, but thats just my opinion.

GrahamC

The reasons are two fold.

First is to reduce the rotating mass, a drilled out crank pin is wee bit of weight and the best place is in the piston itself hence the effort sometimes expended to thin the skirt of the piston. I have even heard tell of attempts to lighten the connecting rod itself. Reduced mass then the less energy expended in just moving the internal parts of the engine and potentially more available to turn the prop.

Secondly and often a side benifit of the first is improved balance with the same end benfit as the first in an attempt to reduce the amount energy used by the engine just keeping it running and instead make it available to turn the prop.

And, is it worthwhile to shave wee bits of grams and here and there? Of course it is! But in the end whatever difference you have made may or may not be immediately evident or easily measured. Improvements may be seen in simply how the engine behaves and are often more qualitative than quantitative.

In any case, it's the journey that is interesting and educational even if you don't end up where you intended to go.

cheers, Graham

kersplat

Those "Gold drills are either Titanium nitride or Cobalt drills. Neither will work on a Cox Crank pin. Also, If you use a pure Carbide center drill, you will not need to step drill because the drill is rigid. I use these Carbide center drills in aircraft structure repair in the Airforce and they work well on monel rivets and Hi-Loks without step drilling.

combatpigg

My Feedback: (3)

Wouldn't a gunsmith soften, then drill, then reharden the part? Is it just not necessary with the diamond tools nowadays?
I can't see how any twist drill bit could plow through steel that is thoroughly hardened. Especially without wandering. I know an old guy who worked at Boeing when titanium was first introduced. They screwed up a lot of it before discovering feeding freon into the hole.

GrahamC

Something like what is detailed here?

http://www.precisiontwistdrill.com/t...drills_2-4.pdf

I usually get speciality stuff like this from KBC tools so I will have a look at their catalog.

I have also looked at some of the solid carbide PC board drills but I dont' think these are suitable - cheaply priced however but you do get what you pay for.

Thanks for all the ideas,

cheers, Graham in Ottawa Canada

gkamysz

My Feedback: (19)

Annealing and hardening a crank without bending it or ruining the bearing surfaces would be tricky at best. I was watching some videos on youtube of lathes cutting parts, and was surprised at the kind of feed rates and depths of cut possible today with modern insert tool bits. Extremely hard material is still tough to deal with, but what are Cox cranks made of? I haven't attempted to cut one yet. I also have an RR type project on the books for one of these days. That will probably after several other Cox projects.

GrahamC

I thought of trying to locally anneal and then re-harden the crank pin but much of what Greg is true. I decided it against it (so far) as it is likely to end up distorted and the pin might then end up needing to be re-ground to size and might even end up being off concentric with the main shaft.

I read somewhere that most of the steel used in the Cox engines was 12l14 (aka ledlloy, leaded steel, free machining steel) and where needed was case hardened (nitrided?). I know the cyclinder is 12l14 as is the piston. Whether the crank was or not I am not certain. Peter Burford describes how he makes his little diesels and he uses a lot of 12L14 in their construction; the crank being something else but I don't recall what but was indeed a "tougher" steel than 12L14.

One other option through all this if drilling the crank pin becomes too much of a problem is to simply make a new crankshaft but there are lots of cranks still available and that seems too much like re-inventing the wheel even if I end up ruining a couple cranks in the process.

I think I will try drilling one first - Tim Wiltse has done it (and I looked for but couldn't find some of his old posts where he talked about this). And besides, maybe it is only case hardened and once started the hole making will get get easier.

cheers, Graham

kersplat

If you anneal the crank to drill it, in the process of annealing some of the carbon would precipitate out of the steel weakening it lowering the ability to get a good hardness on the Rockwell scale.
Drilling Titanium is easy. I have drilled Titanium jet engine parts with just cobalt drills. The secret is using Bees wax and low drill speeds.

combatpigg

My Feedback: (3)

I think some of the old KillerBees has hollow pins, so Tim W might not have needed to drill.