Contour Milling

J

Jaylab

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I would like to make a couple of flat plates using .125 thick sheet aluminum to serve as a base for eyepiece holders on a binocular.
The prism housing is from a 7x35 instrument and has a number of blended curvalinear shapes, and I would like the pieces I fabricate to have this same outline.
I could scribe the outline and cut and file to fit, but I was wondering if there might be something method analogous to following a contour on a lathe?

I have a Rong Fu 30, 9-in Trokye rotary table and some misc. precision slides.
I'm wondering if anyone has any ideas?

Thanks! -Jay-
 
Well, you could build a pantograph. There are some videos on Youtube that may give you some ideas. Here's one from Joe Pi :


Ted
 
Sounds like you are interested in tracing or pantography. Without a photo or diagram or even a description of the curves, it's hard to say. Each type of shape/curve has its own set of equations, and how those relate determines how they can be compounded and predictably followed. Also, the level of precision and tolerance for error are important to disclose. Is approximation okay, or are exact curves needed? There are lots of ways to make repeatable curves on a manual mill, just depends on what the constraints are.
 
Joe's setup looks incredibly shaky, and time consuming to make; pantograph engravers such as my Gorton 3-U go commonly for around $1,000 and are quite a useful and capable machine, I have done some contouring work with it and made several steel stamps in addition to name and memorial plates, and it does not scream like Joe's pencil grinder ---
 
Thanks very much, that panograph idea is probably just the ticket, and maybe a dremel would work instead of a die grinder. The shape I'm trying for can be seen at the eyepiece end of porro prism binoculars, and if I could generate the parts +.025" to 0 tolerance it would be quite serviceable.
I did enjoy Joe Pi's video, I've seen a few of his productions in the past, but this one featured a rather shakey rig.
-Jay-
 
I have seen some creative applications of machines like the Gorton 3U, which is a 3 axis machine. One example is performing cherrying operations on mould dies. The machinist used a half round wood profile to generate the shape, used pantograph scaling to fit to the part, and eyeballed the work. Another used it as a sort of partially-constrained die grinder for freehand work, that was again surprising as a finished part.

I have seen many Gorton panto tables for $500. The 3-U can go for way over a thousand, but @Dave Bonzo just passed one up for cheap, and I couldn't justify the drive to get it myself because fuel would be more than the machine. Turn key CNC killed the pantograph industry, but for backwards folks like us, they are still really cool tools.
 
I should add that some pantographs like the Gorton 3-U are not capable of performing 1:1 traces. IIRC, that machine must do 2:1 minimum. No 1:1 means it would be hard to replicate parts without building a 2x scale tracing form first.
 
Thanks for the information on the Gorton panograph, I'll search for some on-line material on this tool.

I have not felt the need to adapt CNC, it is rare that I make multiple items, but if I were to invest heavily in this project I can see this has promise and might have broader applications than an expensive mechanical tool.

Although I need 2 endplates they are symmetrical so I could concentrate on the one outline.

The parts I need are small and the closest thing I have to CNC are a couple of 3D printers. I supose I could make a model to test the geometry and use this as a panograph master.

Thanks for the ideas, you have helped me think out of the box!

-Jay-
 
Somewhere, I have a Craftsman pantograph for a router. I bought it around fifty years ago but don't recall ever using it. Cutting a profile in 1/8" aluminum is quite a bit more demanding than engraving a name plate.

It is possible to cut a profile of intersecting curves on a manual mill using an RT. It requires repositioning the work so the center of each arc is located at the center of rotation of the RT. The easiest way to accomplish this would be to mount an XY table on the RT. This would allow for precise repositioning of the work. It also requires a fair amount of coordinate transformation which is quite tedious without a CAD program doing the math.

Another way to accomplish this would be to drill pilot holes at the center of each arc and reposition the work, using a dowel pin in the mill spindle to locate the work. Then offset the mill table to the arc radius and cut the arc. Not as precise as the above but with care, it can produce a complex profile requiring minim blending by hand.

In the past, when I had complex profiles on two or more parts that needed to be matched, I have fastened them together and machined them as a set. I found that his worked well even if I was doing a lot of hand filing or grinding. After cutting the parts, I would reverse the order of the stack which will reveal any inconsistencies and a little hand work will produce identical parts.
 
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