Reaming bolt holes

[jtrom]

Can someone explain the process of drilling undersize and then reaming a bolt hole? Is this done on wood AND metal?.....what exactly is a reamer?

[Neil]

A reamer is a tool that is used to precisely size a hole. It is not designed or intended to be used to remove large amounts of material but as a follow up to a drilled hole. Unlike a drill, the reamer will not bend and can be used to true a drilled hole that has wandered off just a bit. Best used in machine tool applications (milling machines,
lathes, drill presses) it is possible to use in a hand held drill and some are intended for use by hand when held in a tap handle. They are usually purchased in specific sizes but they do make adjustable reamers which are handy if you would like to add just a little extra size to a fit for whatever reason. There are all sorts of specialty reamers and job specific reamers but you don't really need any of those in aircraft building. (Generally speaking. There is always an exception.)

Most builders will need some of the standard size reamers. 1/4", 5/16", 3/8" will cover most needs. You will need a reamer for the AN3 fasteners as well. An AN3 is NOT 3/16". An AN3 is a #10 screw. A #10 screw is .190 diameter. A 3/16 reamer will make a .1875" hole. An AN3 bolt will not fit. You will need a # sized reamer and they are sort of hard to find. A #10(.193") or #11(.191") reamer is used for an AN3 bolt.

Reamers are best run at about 1/3 of the recommended speed of the same size drill bit for a given material. They are not intended for wood but I have been guilty of passing a reamer through a wood spar assembly. Most of my reamers are what is known as Straight fluted machine reamers. That is what I would recommend that you buy.

It is much easier to get a good round hole with a reamer. Drills should be used to get you close then finish the hole with a reamer. Larger reamers (over 3/8") will take out 1/64" with ease. Smaller reamers should be limited to less than that. Reamers should always be used with a cutting fluid.

[Neil]

A reamer is a tool that is used to precisely size a hole. It is not designed or intended to be used to remove large amounts of material but as a follow up to a drilled hole. Unlike a drill, the reamer will not bend and can be used to true a drilled hole that has wandered off just a bit. Best used in machine tool applications (milling machines,
lathes, drill presses) it is possible to use in a hand held drill and some are intended for use by hand when held in a tap handle. They are usually purchased in specific sizes but they do make adjustable reamers which are handy if you would like to add just a little extra size to a fit for whatever reason. There are all sorts of specialty reamers and job specific reamers but you don't really need any of those in aircraft building. (Generally speaking. There is always an exception.)

Most builders will need some of the standard size reamers. 1/4", 5/16", 3/8" will cover most needs. You will need a reamer for the AN3 fasteners as well. An AN3 is NOT 3/16". An AN3 is a #10 screw. A #10 screw is .190 diameter. A 3/16 reamer will make a .1875" hole. An AN3 bolt will not fit. You will need a # sized reamer and they are sort of hard to find. A #10(.193") or #11(.191") reamer is used for an AN3 bolt.

Reamers are best run at about 1/3 of the recommended speed of the same size drill bit for a given material. They are not intended for wood but I have been guilty of passing a reamer through a wood spar assembly. Most of my reamers are what is known as Straight fluted machine reamers. That is what I would recommend that you buy.

It is much easier to get a good round hole with a reamer. Drills should be used to get you close then finish the hole with a reamer. Larger reamers (over 3/8") will take out 1/64" with ease. Smaller reamers should be limited to less than that. Reamers should always be used with a cutting fluid.

[Neil]

Don't know how the double post happened.

[bwilson4web]

A reamer is a tool that is used to precisely size a hole. . . .

Most builders will need some of the standard size reamers. 1/4", 5/16", 3/8" will cover most needs. You will need a reamer for the AN3 fasteners as well. An AN3 is NOT 3/16". An AN3 is a #10 screw. A #10 screw is .190 diameter. A 3/16 reamer will make a .1875" hole. An AN3 bolt will not fit. You will need a # sized reamer and they are sort of hard to find. A #10(.193") or #11(.191") reamer is used for an AN3 bolt.
. . .
I need to drill two, AN3 holes for tube-in-tube, aileron bell crank mounting. I've assumed the #12 is .189". Since it is important to eliminate or minimize slop in the control system, would a #12 reamer be the right choice to finish the hole for an interference fit?

I have assumed an interference fit requires a clamp with a block and a hole to press the bolt home. However, if the threads clear the hole and the AN3 shaft needs a little bit to be pulled through, could a bolt and washers be used to pull it through and make a better, interference fit?

We have a calibrated torque wrench but would the bolt still be properly torqued if there is an interference fit?

This is replacing a defective part so the original bolts were removed. The outer, aluminum tube has the original hole. I have new bolts to replace the originals so there is always the question of whether or not the outer, aluminum tube hole will still be tight . . . we used to call this an 'engineering tolerance' problem.

Thanks,
Bob Wilson

[Neil]

If I had a #11 and a #10 I would use the #11 reamer as the first attempt and see how that works out as the bolts tend to wander a bit on size and all you will do with the #12 is scrape all the cad plating off the bolt if you get it in at all. The cad plating is a functional part of the bolt and care should be taken to retain the plating. For the most part the #10 reamer is the best over all choice for the typical builder.

I don't know if you are following the wording in a manual when you are trying to produce the interference fit, but in most cases it is best if the bolt slides in easily with no detectable looseness. Since the bell crank is aluminum I'm sort of guessing this is not a 400mph turbine powered whiz banger so the clamping force of the AN3 bolts on the tubes with only a couple thousandths clearance should be sufficient.

[martymayes]

I need to drill two, AN3 holes for tube-in-tube, aileron bell crank mounting. I've assumed the #12 is .189". Since it is important to eliminate or minimize slop in the control system, would a #12 reamer be the right choice to finish the hole for an interference fit?

An AN-3 bolt has a shank dia of .186" - .189", (.1875 +/- .0015) the threads are 10-32 x .406" A #12 reamer will provide a Class II fit or "free fit" which is exactly what I would shoot for.

[bwilson4web]

Thanks!

It makes sense to have all three sizes and start with the #12 followed by the others as needed to make the hole fit the bolt.

As for the structure, it is steel tube inside an aluminum tube of the aileron. The bell crank is a welded, steel plate to a steel shaft bolted to the steel tube. This end is not the part I'm worried about. Rather the aluminum tube that is part of the aileron that is the area I'm . . . interested.

Thanks,
Bob Wilson

[CraigCantwell]

One thing that will help you make the best of it, is to run the reamer in under power and stop the drill motor and withdraw it while turning backwords by hand. You can also do just as well, if access allows, to simply unchuck the reamer after it's thru the hole and pull the shank on thru. Unless you have lots of practice with using reamers in your hand drill, or are using rigid tooling, it's very easy to allow the reamer to cut additional materal on the withdrawal if you continue to run the drill motor. Use some solid type lube on the reamer before you insert it and turn at the appropriate speed for the toughest materal in the stack. Also remember that the flutes are very sharp and can slice your hand easily.

[Neil]

It is generally considered bad machine tool practice to turn a reamer backwards but unchucking and drawing the reamer out the back of the work is a good suggestion.

All the "book" numbers say that a #12 reamer would be ideal, but in actual practice I don't find it to work out. If you have all three give it a shot.

[Eric Witherspoon]

Reality check. Reamers mostly aren't necessary (100% not necessary for the designs I've built). I would agree, reamers would be necessary if the application calls for a bolt loaded in shear - you want the hole very close to the bolt size as the intent is for the sides of the holes in each piece being joined to bear up against the bolt shank. Well, AN3 bolts are NOT close-tolerance bolts. As in, the bolt size itself is allowed to "wander around" enough such that it just isn't a close-tolerance fit. If one was to try using AN3 bolts in a close-tolerance shear-loaded condition, one would need to measure each bolt individually, determine where in the allowed tolerance band it fell out, then have a selection of reamers throughout the tolerance band and individually ream each hole for each individual bolt. Well, homebuilt designs (that I know of) don't call for this.

AN3 bolts are loaded in TENSION. As such, where the sides of the hole are doesn't matter as much - there is intended to be a little gap there. The way a non-precision-fit bolted joint works is the preload on the bolt (generated via the torque applied), puts the items attached together into compression - yes, they actually deflect a little into the direction the bolt force is pressing on them. Failure of this joint is defined as applied load exceeding the preload. There is separation of the materials at this point.

So how are there so many joints in aircraft, made with AN3 bolts that are loaded in tension, but it looks like the force on the parts is sideways through the bolts? How does that work? Well, the friction generated by the preload keeps the parts from sliding relative to each other. The bolts aren't loaded in shear. Of course, in reality, there's forces and loads and combinations of things going on all over the place, but that's the general theory.

Back to reaming - I wouldn't worry about it unless your plans/kit designer has specifically called for it. First airplane I built, the designer recommended #11 drill (.191) for all AN3 holes. This I did. I sold it before I flew it, but it had well over 1000 hours on it before I lost track of it. So I'd say it works. Second airplane, which I built and completed (same design), I again used the #11 drill across the board for AN3 bolts. 150 hours and 3 annuals so far, and no problems.

To the point that a drill won't make a round hole - true, a 2-fluted drill bit will make a 3-lobed hole - in thin material. Generally, if it's going through a stack of parts, approximately the drill bit diameter's thickness or greater, there's enough material there to support the drill bit (around the helix of the flutes) to stabilize it and produce a round hole. A reamer could be useful in achieving round holes through thinner stacks. But if you're starting to think, I've got 10,000 rivet holes (meaning at least 20,000 holes through material), and most of these material stacks are 1/2 of the drill bit thickness, so maybe I should ream 20,000 holes - no, probably not. A rivet shank expands in the hole, so perfect circularity isn't even required. A rivet IS a shear fastener (generally, there are some tension rivets, but this is not common in a small/simple airplane design). As such, the SIDES of the rivet are loaded against the SIDES of the hole. So the fact that a rivet expands to perfectly fill a less-than-perfectly circular hole is just what is needed here.

Another issue to consider - if the idea is to ream such that the bolts "tap fit" or "hammer into" the holes - this is a BAD idea for at least two reasons.
1. In hammering a bolt into a hole, the protective cadmium plating on the AN bolt will be scraped off. So you've just lost the corrosion protection built into the bolt.
2. This will introduce HUGE stresses around the perimeter of the hole. Aluminum can not handle this. Cracks will start at the hole.

So be careful about this reaming subject. I would contact the designer / kit / plans supplier and see what they require, and not necessarily follow along with advice presented here and start to believe that reaming is necessary for a "better" product. The holes may be rounder, but maybe they don't have to be. The fit may be tighter, but that may be introducing stresses that wouldn't be there otherwise. See my signature line.

[CraigCantwell]

I agree with you Neil, that normally turning a reamer backwards is bad form, but there are times that it is the best practice when using hand tools. Most homebuilders don't ever have the need to ream large quantities of holes so they never get a chance to develope the knack for keeping everything straight and not cutting on withdrawal of the reamer from the hole. When I was doing production work, it became a skill, as we did thousands of close tolerenece holes every week. You could always tell holes that we reamed by someone that hadn't quite got the skill down yet as the entry to the holes were generally belled a few thousandths. Fortunately, most of those holes had to be either countersunk, or have the edges relieved slightly so they were still good holes. It's one of those skills that takes practice. I can drill close tolerence holes in Al, Ti, 4130 and various composites without problem, but can't weld 4130 worth a flip.....That's the next a/c skill I need to master.

[Neil]

Not trying to make machinist of any one here but after 40 years as a Tool & Die Maker/ Special Machine Builder in the auto industry I have learned the importance of a good fit and not one that involves a hammer. Now retired I have a fun job working at a facility that builds airplanes and offers builder assistance. The number of homebuilts that come to us that have sloppy control systems due to poorly drilled holes or systems that bind because the parts of a system are all drilled and not properly line reamed is appalling. A little machine tool knowledge would have made all the difference in nearly every case. I can't immagine any design that wouldn't bennefit from smooth, clean straight, slop free holes provided by the use of reamers over a hole finished with a drill bit. Not all holes need to be reamed. True. But there are those that should be.

[bwilson4web]

You'all have given me a lot of ideas I'd never considered. A close fit does not have to be a 'forced' tight fit. Just have a set of reamers for my AN-3 bolts. Also the AN-3 bolts are not so uniform so it is OK to check the inventory and pick a bolt that best matches the existing aileron tube hole:
http://hiwaay.net/~bzwilson/dragonfly/2012/wing_400.jpg
The aluminum tube hole is the one I'll use to fit-test the AN-3 bolts. No, I won't be reusing the original hardware. The line is a felt-tip pen mark I made when removing the steel tube.

It is difficult to see but the bellcrank has two holes, 90 degrees apart. Although the forces involved are not that heavy, I just don't want to fly on a part that has 'extra' holes. One of our chapter members joked, 'Oh, just a lightening hole.' . . . well maybe in his plane, not mine.

So the assembly scenario will be:

Select AN-3 bolt that best fits in the existing aluminum tube.
Measure AN-3 bolt and drill a hole smaller than the bolt.
Ream the hole to match the bolt
Loose fit the bolt, steel tube in the aluminum tube
Mark where to drill the bellcrank hole
Clamp bellcrank and tube per marks (is it OK to drill both parts together?)
Separate the parts and ream the holes to match the AN-3 bolt
Reassemble parts and torque to spec
I don't know if drilling the two parts with one bit at the same time makes sense. Alternatively, I could just drill the bellcrank and use its hole to mark where to drill the steel tube and drill it separately. Regardless, I'll use the reamer to size and shape the hole.

One last question is drilling a steel tube with a hand drill strikes me as being risky. I'm thinking a wood base on the drill press table with a "V" on top to hold the steel tube clamped in place makes a lot of sense. I'll be able set, check, and check again to make sure the hole is where I want it to go. Then I should be able to swap in the reamer for the same hole.

Sorry if I seem a worry wart about this but it is an area I don't have a lot of experience . . . yet.

Thanks
Bob Wilson

[Neil]

If at all possible the parts need to be drilled and reamed as an assembly. Drilling and reaming separately is near impossible in most home shop conditions. It will be quite a task to reuse an existing hole that is already to size but it can be done. Good luck.

[bwilson4web]

I just ordered a set of carbide reamers . . . ~$35/ea. Their catalog had them listed at ~$25-30/ea. According to the salesman, China is making these rare. . . . Get'em while you can.

Bob Wilson

[ekimneirbo]

If at all possible the parts need to be drilled and reamed as an assembly. Drilling and reaming separately is near impossible in most home shop conditions. It will be quite a task to reuse an existing hole that is already to size but it can be done. Good luck.

While this thread contains a lot of good information, Neil probably made the most important point of the whole thread when he said the parts need to be assembled before drilling and reaming. When you ream parts it is a precision fit but also a precision alignment of the parts. It will be virtually impossible to get the parts to work together with something near a slip fit if they are not machined while assembled. Remember to deburr the parts after drilling and reaming them to remove the sharp edges and any potential shaving that can jam in the now
precisely sized and located hole.