HOME SHOP MACHINIST A press fit, also known as an interference fit, is the preferred ways to assemble hubs to shafts when concentricity, balance, and security are critical, or when other means, such as bolting or welding, are less than practical. Depending on the class of fit, from a “light drive fit” to a “heavy” or “force fit,” the interference (the difference in diameter of the shaft, or the larger diameter, being pressed into a hole or bore, the smaller diameter) is typically between 0.0005 inch of interference per inch diameter, to about 0.0020 inch of interference per inch diameter. When delving into machining operations like press fits, or any work that requires accuracy better than 0.001 inch, everyday calipers and micrometers are often not accurate enough. A way to overcome this, at least with interference fits, is to machine a go/no-go gauge right into the workpiece. The trick is to lathe turn the shaft 1½ to 2 times longer than necessary, then machine a short step (the first of what will be a series of steps) until it just slips over

The shaft was turned to have a series of ever-increasing diameters, or steps. The first step should be a close sliding fit. Note: if using a three-jaw chuck, all fitting must be done with the part in the lathe to maintain concentricity.

Bob Hadley 64

KITPLANES August 2018

Interference

(Left) This exaggerated view shows the difference between a light press fit (1.0005), a medium press fit (1.0012), and a heavy press fit (1.0020). (Right) The task: Press this shaft onto this hub, which is a section of 3/4-inch-diameter aluminum with a 3/8-inch hole drilled in the center. Since a 3/8-inch drill doesn’t exactly drill a 0.375-inch-diameter hole, a way is needed to accurately gauge the true diameter.

the hub bore. Then turn the next step 0.001-inch diameter larger and check the fit. If the hub still slips over the step, turn yet another step, and so on, until you

reach the point where the hub no longer fits. You should measure the steps in the shaft with a good micrometer to confirm that each step is, more or less, larger by

The second step measured 0.00075 inch larger than the first. A test with the hub revealed that one end would slip over the shoulder of the shaft, but the other end would not, an indication the drilled hole in the hub was slightly tapered.

Since the hub partially fit the middle step, a third step works for the press fit. It measured +0.0008 inch from the previous step.

Bob Hadley is the R&D manager for a California-based consumer products company. He holds a Sport Pilot certificate and a Light-Sport Repairman certificate with inspection authorization for his Jabiru J250-SP.

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0.001-inch diameter. By machining the steps accurately, the step where the hub no longer fits should result in an interference of between 0.0003 to 0.0008 inch. With a little practice, you can adjust the dimensions as needed to yield interference fits within reasonable tolerance for everything from the lightest to heaviest force-drive fits. While the method described is for fitting a shaft to an existing hole or hub, it can work the other way around, too, by step boring a hub to custom fit an existing shaft. The basic ways to join an interference fit are by force, such as with an arbor or hydraulic press, a vise, or a hammer; or by using thermal expansion (heat) or contraction (cold). Parts that are joined by thermal process are referred to as having a “shrink-fit.” This is often the way propeller hubs are joined to crankshafts. Heating is usually required on the tightest force fits. Steel has a coefficient of expansion of around 0.000006 inch, per inch, per degree Fahrenheit (°F) and aluminum 0.000012 inch, per inch, per degree F. This tells us that when heated to 300° F, most hubs will expand sufficiently to allow even the tightest force fit to slip together easily. If you’re worried about overheating a sensitive part, lower the temperature a bit and put the mating shaft in a freezer. What’s important is the temperature differential. Heating the hub to 225°F and chilling the shaft to 0° F (in a typical freezer) gives you the same fit as heating the hub to 300° F with the shaft at room temperature (77° F). The American National Standards Institute (ANSI) has established definitions for four basic types of interference fits. They are light drive, medium drive, heavy drive, and force drive fits. The following is quoted from the ANSI guidelines: • Light drive fits are those requiring light assembly pressures and produce more or less permanent assemblies. They are suitable for thin sections or long fits, or in cast-iron external members. • Medium drive fits are suitable for ordinary steel parts or shrink fits on light sections. They are about the tightest fits that can be used with high-grade cast-iron external members. Photos: Bob Hadley

A final check confirms an interference and that the parts are ready to press together. The extended stem has a dual purpose: to gauge the hole for the correct interference fit and to align the parts for final pressing.

• Heavy drive fits are suitable for heavier steel parts or for shrink fits in medium sections. • Force drive fits are suitable for parts that can be highly stressed or for shrink fits where the heavy pressing forces required are impractical. As the descriptions point out, the type of fit depends on the application. For connections that need to be permanent, use a medium, heavy, or force drive fit. A good example would be bearing races pressed into wheel hubs. These fits are “permanent” in the sense that they won’t come apart from normal use or abuse. Bearing races, of course, are designed to be pressed out and replaced, so exactly how permanent depends on many factors. A gear pressed onto a driveshaft can almost always be removed with a gear puller or by reversing the assembly forces. A rod or shaft pressed into a blind hole can usually be removed with a slide hammer or by applying heat to relax the grip. Parts that have been pressed together with a blind mechanical interlock can usually be released with heat. Nothing is really permanent if you’re determined enough, though the ultimate solution might prove destructive to one or both parts of the assembly.

An arbor press is preferred for maintaining alignment when driving home interference-fit parts. But a well-placed blow with a soft-faced hammer will also do the trick. An appropriately sized socket is used to support the hub and provide clearance for the protruding stem.

Section view of the pressed shaft and hub assembly after machining off the protruding stem. Note there is no noticeable delineation between the two parts.

There are a number of helpful websites with calculators for sizing parts, estimators for driving force, and holding power (the holding force is pretty much equal to force required for assembly). Case assemblies often have alignment pins press fit into one or both halves to help alignment during assembly, but never both halves. Bolts or screws do the holding in these situations. Don’t mix materials if the assembly will be subject to temperature changes. The difference in the coefficient of expansion of an aluminum shaft in a steel hub could cause the press fit to come loose. J KITPLANES August 2018

65

SLSA max empty weight, sailplane condition inspections, licensing a RANS S-7LS as an ELSA.

By Mel Asberry

Question: Is there a maximum empty weight for an SLSA? I read on a forum about an 890-pound max empty weight, but that’s the first I’ve heard about that. Answer: Yes, there is an empty weight limit for SLSA, but it is not a definitive number. Basically, it’s a formula that says: Add 190 pounds per seat plus a number equal to 1/2 the rated horsepower. Subtract that number from the gross weight. That will give you your maximum empty weight. Here’s an example for a two-seat, 100hp SLSA on wheels: 190 x 2 + 50 = 430. 1320 - 430 = 890 pounds. Obviously a single-seat aircraft, a different horsepower engine, or the addition of floats will provide a different empty weight limit. Experimental Light Sport Aircraft (ELSA) do not have this limitation. This is the reason SLSA manufacturers using a 180-hp engine typically go with ELSA. Question: There are a number of different subcategories of experimental aircraft: amateur-built, R&D, racing, etc. Many foreign factorybuilt sailplanes imported into the U.S. are licensed as Experimental/ Racing or R&D because the foreign 66

KITPLANES August 2018

manufacturer never got a U.S. type certificate. When it’s time for the annual condition inspection, does the inspector need to be an IA, or is an A&P certificate adequate, no matter the subcategory? Answer: The inspector requirements should be listed in the operating limitations for each particular aircraft. To the best of my knowledge, an IA should never be required on these aircraft. The primary purpose of an IA is to confirm compliance with the type certificate. Since these aircraft do not have a U.S. type certificate an IA should not be required. Question: I’m helping a friend sell his RANS S-7LS, which was factory built and has an SLSA certificate, as well as helping the new buyer get situated with the airplane. It’s my understanding that the new buyer can convert it to ELSA pretty easily, which is a huge personal property tax benefit here in Kansas and Missouri (about $50 per year instead of $1,000 to $2,000 per year), in addition to being able to take the training courses to do the annual condition inspections. However, I’ve also read that you have to be

very careful not to do anything that violates the ELSA certificate. What kind of things are allowed? Can you update the avionics, for example? Answer: Yes, the current owner can convert an SLSA to ELSA. The local FSDO or a DAR with the proper function code can perform this conversion. The only physical changes to the aircraft are changing the Light-Sport placard to an Experimental placard, and changing the passenger warning placard to the appropriate Experimental passenger warning placard. After this conversion, the owner may take the 16-hour LSA repairman/ inspection course and obtain the repairman certificate. This will allow him/her to perform the annual condition inspection on an ELSA that he/she owns. Under ELSA, the aircraft may be modified without approval from the manufacturer, as long as the modification does not take the aircraft outside of Light-Sport parameters as described in FAR Part 1.1, General Definitions. J Please send your questions for DAR Asberry to [email protected] with “Ask the DAR” in the subject line. www.kitplanes.com & www.facebook.com/kitplanes

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KITPLANES August 2018

71

The

Creative Homebuilder

Hardened Steel Mandrels for Close-Quarters Dimple Dies

By KITPLANES® Staff

Dissatisfied with using the provided mandrels for Cleaveland Aircraft Tool’s closequarters dimple dies, our Creative Homebuilder uses the hardened steel mandrels from 5/32-inch blind rivets. He feels that blind rivet mandrels are much stronger than those supplied with the dies and do a much better job of actually forming the dimple. When taken to the breaking point, they always load the die faces the same, as they are designed with a frangible stem in order to secure the blind rivet at its design load. This characteristic provides consistency in the dimpling process, so he pulls the mandrels to their breaking point. On very thin material (0.016–0.020 inch), he pulls to “feel and experience.” To date, the Creative Homebuilder hasn’t convinced a blind-rivet manufacturer to sell stand-alone mandrels (and he’s tried!), so he occasionally “harvests” the stems from blind rivets to ensure an inventory that is stored with the die sets in his tool drawer. The holes in the dies may need to be opened up a bit for the new stems, but the holes are still smaller (0.095 inch) than the #40 hole from which the dimple is made. He further notes that this technique does a good job on 0.020-inch aluminum and thinner. J 72

KITPLANES August 2018

The original 5/32-inch blind rivet.

The blind rivet is nested vertically in a vise, ready for harvest.

A light tapping with a hammer drives the mandrel out of the blind rivet shoulder. The harvested mandrel, ready to slip out of the blind rivet and receive the close-quarters dimple dies.

Ready to go! The harvested mandrel with dimple dies installed, waiting for a piece of aluminum to dimple.

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