Making model engines requires frequent rounding over of the ends of linkages, clevises, etc..
I made this jig to simplify this operation.
Top view. The part to be rounded pivots on a central pin which can be changed to suit the size of the hole in the part. The two stops are absolute essentials since the cutter (For safety, I use a rotary file rather than an endmill.) has a tendency to grab the work and pull it out of one's hands. The main bar of the stop is repositionable and can be swung into rough location. Setscrews in the end of the main bar then allow the secondary bars to be slid in and out for fine adjustment.
A view of the underside. The big block is grasped in the milling machine vise. Through the big block is a threaded rod with a hole into which the various pivot pins fit. Tightening the nut on the threaded rod locks the pivot pin into a V-groove in the main block to keep the pivot pin vertical.
Another view of the bottom showing the pivot pin holding arrangement.
In use, the fixture is clamped in the mill vise. A rotary file, held in a collet, is lowered into the slot seen in the other pictures. The y-axis is used to advance the work into the spinning file a few thou at a time. After each advance the work is swung back and forth between the stops to remove material.
If you make one of these, exercise care in its use. Your hands will be close to the spinning collet and rotary file. For extremely small pieces, I solder the part to a larger piece of metal to use as a handle or else use a small hand vise to grip the workpiece.
Last edited by mklotz; 07-06-2017 at 11:32 AM.
Home Shop Freeware
Adding a few details.
This method is quite prevalent, because it works. It works well with endmill that has multiple flutes, generally 3 or more. The rotary file is a milling cutter with differences in the cut geometry for hand use.
1. Use a radius about 1.05x that of half part width, so the 'apex' diverts from side of part without creating dwell (divot) mark in side. There is little mechanical or cosmetic call for a full obround (radius = 1/2 part width). That demands accurately locating centerline lengthwise.
The fixture leading this post uses a positive adjustment system, even when the end is wider than the arm, like a connecting rod. Not always centered, lots of linkages have offset ends.
2. Cut conventional (against) rotation, not climb cut. Retract the spindle, rotate part back to start point. Save some effort, file or grind the corners off away from scribed layout.
3. Get the RPM up, using applicable chip control lube or air to keep flutes clear.
4. Rounding over looks very good when it is 2,3, even 4x part width. Calculate that with cutter radius, radius desired and swing point 'centerline' of part. Same thing works with circle jig in a bandsaw or sander. A large radius is easier when the part rides on a holder and allows any swing point.
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