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Turning Wheel Flanges

There are a few reasons why one would like to turn down NEM or those visually obtrusive AC flanges. Visually the Proto:87 is the best but can be troublesome achieving good running characteristics. Proto:87 require near perfect laid track and is not practical for the everyday modeler, so, we will focus on NMRA RP-25 specifications which is visually acceptable and also provides good running characteristics. The second important criteria is the actual cost of replacement wheel sets. In the coming paragraphs we will discuss step by step how to turn down wheel flanges, add insulators to AC wheels, and also weathering them. .

Wheels are typically cast from soft metals such as brass, Aluminium, white metal and then nickel plated or blackened. It is recommended not to turn wheels that are composed of a metal rim with plastic spokes or core. These spokes will tear if they are not clamped properly with the all the cutting force distributed over the entire wheel. Proper tool selection is very important to work with such materials. Typically cutters should be HSS or carbide tip. Also the appropriate drill and reamer sets matched to the required axle diameter is a must. Wheel axles range from 1.5mm to 3mm. Typically driver wheels have a axle diameter of 2mm - 3 mm in increments of 0.5mm. Here is a list of tools and machinery needed...

• Mini Lathe

• 1/4" LH and RH HSS cutter

• RP-25 profiled HSS cutter

• #1 or #2 center drill

• Min 0.1 mm undersized (to axle diameter) HSS drill bit

• Min 0.01 mm undersized (to axle diameter) HSS reamer

• Wheel holder set for flange turning

• Collet-type wheel holder for facing and boring

Step 1: Trimming The Flange - Ensure the work piece (wheel) is 3^o offset to the lathe bed or tool travel. This is very important as the tool should travel parallel to the wheel tread which is approximately 3^o . This can be achieved by  either rotating the head stock by 3^o to the lathe bed or using a compound slide tool holder and making the tool travel at the same  3^o taper to the bed. Trim the flange using a regular right hand (RH) facing tool to about 0.8mm flange depth. At this point you will have the flange cut down with a square edged profile. It is important that you do not attempt to cut the flange and profile it in one step with the RP-25 tool. This tool was not made to remove so much material in one pass. Use the right tool for the appropriate job at hand and you will extend the life of the tool without having to grind and sharpen the tools often. Also ensure the right speed for the depth and material being cut. Typically the metal shavings should look like long thin coils. If chipping, flaking or chatter occurs adjust spindle speed or tool feed speed.

Step 2: Shaping The Flange Profile - The RP-25 profile is achieved by simply shaping the face and rear of the square flange using the RP-25 profiled tool and barely removing any material if at all from the wheel tread. Remember the wheel tread is typically at 3^o and hence we positioned the job in step one to accommodate this. The RP-25 cutting tool is ground  on two sides with the appropriate profiles to shape the face and rear sides of the flange.

Steps 3 is only needed for Märklin locomotives or AC wheels being converted to 2-Rail. Omit if you are already working on a 2-Rail locomotive or run on 3-Rail system.

Note: For step 3 the job or tool travel should be set back to 0^o to the lathe bed. This is very important and do not forget.

Step 3: Wheel Boring -  The wheels have to be bored to accept the insulating plug. This plug production is not depicted in this illustration but is easy to manufacture using readily available nylon or Delrin rods. Mount the wheel in a collet-type wheel holder. This collet will clamp on the wheel-tread and will center the job to the tread. Do not clamp the wheel from the flange as the locomotive rides on the tread and we want everything from now on to be true to the tread.. You may have to face any spacer already cast into the wheel (spacer to prevent the wheel from scraping into the chassis) to be in line with the back face of the flange. Mounting an insulating plug on this spacer will add to the over all wheel thickness and have undesirable effects by altering the minimum turning radius of the locomotive.

Typically for Märklin (and most other manufacturer's) driver wheels the required bore is 3.2mm or 3.6 mm depending the axle and wheel type. This bore can be cut directly as the existing axle bore will act as the pilot. Make sure the insulating plug diameter is at least 0.6 mm larger than the required axle diameter.

The insulating plug is slightly oversized by 0.10 to 0.15mm compared to the bore. The insulating plug is press-fit into the wheel and this tight fit is all that is needed to keep this in place under normal stress and running conditions.

Once the plug is set into the wheel it must be bored and reamed to the appropriate axle diameter. For details on drill and reamer sizes for axles see table 1.0.

I recommend cutting a pilot/center into the plug with a center drill. Directly drilling with such fine drill bits into rather roughly faced surface at this stage can have undesirable consequences as the drill may catch a ridge or burr and tend to wander and result in a bore that starts slightly off center and flare the start of the bore. It is extremely crucial for the final bore to be true center and parallel, so take no chances. Center drills are specifically designed to drill a small diameter pilot hole followed by a 60° countersink which provides a bearing surface for the 60° point of the lathe center. Just mildy cut the start of the bore (no more the 1 mm deep, this is just enough to provide a perfect center for a regular drills to start the bore

Drill the bore for the axle about 0.15 mm under size compared to the required axle diameter. Again I emphasize on setting the appropriate spindle speed and cutting speed to prevent chatter and over heating. Do not feed the drill too fast as it might snag the plug and pull it free off the wheel.

Set the speed to about half of the drilling speed. Now using a reamer (a min of 0.01mm) undersized compared to the desired axle diameter finish the bore. The reamer is used only to remove very little material and get a very smooth finish to the bore. Again do not skip this step. A smooth bore is critical and drilling alone cannot achieve this. Keep in mind the axle is also press fit into the bore so make sure the final bore size is at least 0.01mm under the required axle diameter for a good snug fit

Step 4: Touchup, Blackening & Weathering - Touching up chipped paint and blackening the wheels is a must but weathering is only for those who dare to take their priceless possessions and dirty them up to look very real. I personally prefer weathered locomotives compared to the clean shiny unrealistic look. Here we will discuss restoring and weathering only with respect to wheels. This is also the perfect time to weather the chassis, body and other components when they are completely disassembled. See Weathering locomotives for details.

I recommend a two step process..

a) Degrease the wheels using standard industrial degreasers (do not use paint solvents). Do not touch with bare hands after this. Blacken the bare metal using A-west's Blacken-It  for about 15-30 seconds. Thoroughly cleanup with warm water and soap, dry, and reload the wheels on the lathe and polish with a soft cloth. This polishing step is a must and do it thoroughly where ever the wheel may come in contact with the rail or contact wipers.

b) Touchup the scuffed paint and wheel jig marks including the rim. Use RAL 3002 and once the paint is dry then use a very simple black wash to weather the wheels and chassis. The black wash step is optional. Mix about 1 teaspoon of India Black Ink in about a pint of rubbing alcohol. Adjust the India black ink based on how much darker or lighter you prefer the end result to be. Dip the wheels in this solution and place to dry naturally on some paper. Assemble the wheels and touchup exposed axle & wheel hub with paint and let it dry. Gently pour a few drops of black wash solution with a pipet or eye-dropper on the hub and let dry.

Table 1.0

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