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Cutting a concave radius on the end of a round bar

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Robin Graham18/03/2019 22:30:42
691 forum posts
161 photos

I want to make something like this (apologies for carp drawing, I'm no artist!):


Material will be stainless steel and my idea is to make silver soldered butt joints between the 1/4" 'beams' and the diagonal struts.

I would need to make concave radiuses (forum software doesn't like that, radii is OK though, sigh) on the ends of the struts to conform to the 1/4" beams. My first thought was to make a jig from a block of mild steel with 3/16 and 1/4" drilled holes intersecting at 45 degrees - I would put the blank for the strut in the 3/16" hole, then drill though 1/4" from the other hole. My second thought was that when the drill hits the tougher strut at an angle it might deflect, start chewing the MS jig, jam up and everything would go horribly wrong.

Am I worrying unnecessarily? Or is there a better way of doing this? I expect so!






Edited By Robin Graham on 18/03/2019 22:31:32

Farmboy18/03/2019 22:54:32
128 forum posts
1 photos

I think I would make a jig similar to what you suggest, but to fit on the lathe topslide and hold the 3/16" pieces horizontally at centre height at the 45 degree angle, then put a 1/4" milling cutter in a collet in the headstock to cut the concave. I'd be worried, like you, about the drill wandering in your proposed setup.


Jeff Dayman18/03/2019 23:01:46
1789 forum posts
45 photos

Could you make the centre piece from 1 rod, with alternating 90 degree bends? That way the bends would give a large area soldered joint where they meet the 1/4" rods, as well as being far less cutting and fitting work to prep the joints to make the structure. Some welded steel architectural beams and rafters were done like that years ago, but used two angles welded on each side rather than the 1/4" rods.

Hopper18/03/2019 23:26:10
4379 forum posts
92 photos

Posted by Robin Graham on 18/03/2019 22:30:42:.

Or is there a better way of doing this? I expect so!


I think so. Not worth making a jig for such a small number of parts and such a non-precision application.

I would cut the pieces of rod to length with the 45 degree angle on each end. Then hold each piece in the vice at 45 degrees from vertical. File the radius on the end using a small round file. A chainsaw sharpening file might be ideal. They are about 10" long so easy to hold and manipulate and they are about 1/4" diameter so would give the right radius. Otherwise any small round file would do. You are only going to be soldering these joints so they don't have to be a perfect radius and form. A 10 minute job.

You'll be putting the hand back in handcraft. smiley

JasonB19/03/2019 07:09:03
17821 forum posts
1948 photos
1 articles

Just hold at 45deg in a mill vice and feed straight into the side of a 1/6" cutter. If no mill then holed in lathe toolpost and hold cutter in the spindle, no jig needed.

That's how these were done

not done it yet19/03/2019 10:53:12
4477 forum posts
16 photos

Set up on an angle plate on the mll and use either a rotabroach or a suitably sized end mill.

Circlip19/03/2019 11:22:48
1086 forum posts

Do Rotabroach's go down to 1/4" dia.?

Regards Ian

not done it yet19/03/2019 13:17:21
4477 forum posts
16 photos
Posted by Circlip on 19/03/2019 11:22:48:

Do Rotabroach's go down to 1/4" dia.?

Regards Ian

I have absolutely no idea. If you really need to know, I suggest you either consult the internet or ask Rotabroach.

The alternative of using an end mill would work just as well - they are certainly made in that particular diameter. My reply was a rather generic solution. Personally, I would likely use a rota broach for a larger diameter pipe/bar and an end mill for smaller ones 'Orses for courses as usual.

Martin Connelly19/03/2019 18:02:23
1211 forum posts
146 photos

Rotabroach mini cutters go down to 6mm. They mount on an arbor that can be held in a chuck or collet. Part numbers for mini cutters start RCM arbors are RA116 or RA118 (the latter for cutters over 20mm up to 25mm diameter) . I find them useful for counter boring for cap head screws amongst other uses.

Martin C

Neil Wyatt20/03/2019 21:38:58
17703 forum posts
697 photos
77 articles
Posted by Robin Graham on 18/03/2019 22:30:42:

I want to make something like this (apologies for carp drawing, I'm no artist!):

And I thought this forum was immune to the legendary fish-pun thread.

John Reese20/03/2019 23:05:08
835 forum posts

Make a jaw plate for your vise with a V groove at the correct angle. Use an end mill to notch the end. I would first try feeding against the side of the cutter. If that didn't work well I would try plunging with the end of the tool. You can contrive a work stop for the end of the part from a piece of round. That would assure the parts were of equal length and assure the notches were aligned properly.

60 or so years ago I had to notch a lot of 1 7/8" dia. tubes as weld preparation. My boss provided a a beat up 10" lathe and a 1 7/8" shell reamer and arbor. I fit the arbor to the headstock taper and used a center to support the end of the arbor. I made a clamp to hold the tubing that mounted to the top slide. I hand fed the work onto the end of the shell reamer, taking about 10 to 15 thou per pass. It worked but took forever. It was difficult to orient the tubing so the notched in both ends were properly aligned.

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