Milling a long and large radius

[avenues]

Hello everyone,

I just brought home a LMS 3990 and can't wait to make some parts. One of my hobbies is amateur rocketry and I would like to make some aluminum fins that have a radius machined on the root edge that matches the outer diameter of the body tube (4.020") they will be attached to. I have attached a quick sketch showing the radius I'd like to machine, the fin length is 10" which is under my x axis travel length on my mill but I dont know how I would machine this radius. Only thing I can think of is to hold the fin vertically and use a boring bar along the root edge of the fin in the z axis. Any thoughts?

 

[ttabbal]

Another rocketeer! I think I would try a boring head. Another option is a fly cutter. Both would need the fin vertical or the ability and willingness to tilt the head, I'm not sure if the minis can do that.

A large diameter ball nose could work as well, but I don't think I've ever seen a 2" ball nose end mill. Since you are working aluminum, a carbide router bit might work as well. But be careful with that one. Take light cuts and go slow at least to start.

 

[savarin]

I did a 90mm dia curve into an end plate for my telescope on the lathe.
The long stick out was a little disconcerting but keeping the cuts within reason was not a problem.
I have since done this in steel with bo problems.
Check out from post 119.

80mm long focal length refractor

Oh yeah! That's the ticket!

www.hobby-machinist.com

 

[benmychree]

If your mill has a head that can be inclined in the direction of table travel, you can use a smaller radius cutter and incline the spindle until the resul ting cut approximates the radius you need.

 

[avenues]

Thanks for the input guys, unfortunately, the head on my mill does not tilt. I do have access to a large lathe so Savarin's technique might be implemented. That or I convert the mill to CNC.

In the past, I've done a lot with composites but looking forward to some metal working!

 

[C-Bag]

Very interesting project. Even if the head did tilt I don't think either the mini mill or 9x20 would have enough travel for a single pass, when the tail stock and tool overhang on mini is taken into consideration would they? Sometimes I can't fit the whole thing in my pea brain until I try and do it.

I agree that some kind of line bore like what Savarin did would be the best approach. I love seeing how these kinds of things are worked out. That thread of the telescope Savarin made is a masterpiece.

 

[derf]

You are over thinking this......use a ball end mill or a router bit and do it in several passes figuring the step over and z depth. It is just over lapping cuts to make a larger radius.

 

[C-Bag]

You are over thinking this......use a ball end mill or a router bit and do it in several passes figuring the step over and z depth. It is just over lapping cuts to make a larger radius.

Obviously I never would have thought of that..

 

[mmcmdl]

Simple job for a 90 degree head and fly cutter . The head is a great tool to have in your arsenal for other applications also . Options …. hand grind an end mill with the correct radius , mill flats in the rocket body and leave your fins flat . Your sketch is lacking dimensions on the width if the fin landings .

 

[Dabbler]

If using a 4" diameter fly cutter on your small mill doesn't turn your crank, you can set up a spreadsheet for all the offsets on your curve, and manually dial to each coordinate. to do this in a reasonable fashion, you blue the part first, and scribe the line you want to cut, then cut to the offsets. Having the layout helps prevent mistakes.

By far the easy way to do this and measure it is to cut a circle using your boring head out of a piece that leaves you a whole circle and measure your result. Then use the body of your rocket as a plug gauge when you get really close. Your boring head is now set to the correct radius, and you can now do the machining to that size. (save this test piece as a future radius gauge)...