bending stainless rod (3/16") to special shape... how to make jig?

[USAeng]

I made a drawing for this hook type shape with 3 different bends total... see attached pic... material is 3/16" 304 stainless rod

I hope I dont sound dumb for not knowing how to go about bending this... do you make a jig or what? how would you go about instructing a worker to make this part? We have to make 6 total

Is there any online instruction somewhere that may help? Thanks for any help.

 

[rb1957]

no pic

 

[vpl]

USAeng

The drawing didn't make it ...

Patricia Lougheed

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[MintJulep]

http://tinyurl.com/3ebu6yh

 

[USAeng]

sorry.... here it is

 

[USAeng]

just had a guy in to do PMI on the material... turns out it is Inconel 600

 

[MikeHalloran]

Normal title block tolerances would be difficult to achieve.
Since there's no title block and no statement about default tolerances, I guess anything goes.
Overall length is not stated or implied or computable.

The part would ordinarily be made in something like a Hossfeld manual bender, probably without any jigs, just measuring with a tape measure and a protractor or angle finder. Most fabricators would start with an extra piece, and try to make all their mistakes on that, then discard it.

Lacking a proper bender, I might drill and ream some holes in an aluminum block, e.g., to accept three steel dowel pins of appropriate size, clamp the block in a vise and beat the wire into shape around the pins with scrap aluminum blocks and a big hammer.



Mike Halloran
Pembroke Pines, FL, USA

 

[flash3780]

That's a tricky part since the bends are so close together. Since your quantities are so small, you might consider making an SLA fixture to bend the tubing in.

 

[racookpe1978]

Piece of cake - Then, after I describe all the shenanigans you gotta go through to get your part, you'll end up tearing out (what's left of) your hair.

Realize this is a wire being bent - NOT a perfectly machined part. NOTHING will move until you pull it (or push it) past the end point through the yield poinit, then let it relax BACK into the (almost correct) final position. Make sense? A little bit like mold making - you have to plan on that 3% flexibility so you always bend a metal past its desired position so you get something close to what you want.

Also: You need to start with a wire much longer than needed to get enough straight length at both ends to clamp or pull the first bend around the form.

But the good news is: 3/16 dia wire is small. You only need 6x parts, and each part is pretty short. Expect to use up 4 or 5 before you get things right, so the trial and error work (bench test and QA) is relatively cheap.

Your bend points will NOT be the tangent where the wire "bends" from the straight but rather your forms (the round steel of radius (0.450-0.188/2 = 0.356) around which you are bending will be at the centers of the three bends.

To test your parts as you finish them, you need a template or go-no-go gage: You cannot reasonably "measure" tangent points of the final product. your drawing should dimension the centerpoints of your parts, not the tangent points. (If you were maching the thing, these dim's could be used, but ...)

But you can only bend one point at a time. Your wire must be completely supported (pushed actually on the outside of the bend) by a straight bar or clamp everywhere except where it curves around the round form.

Pulling the wire as you bend helps - the tension keeps the wire firmly against the form.

You need to dimension the straight lengths of both ends.

The 45 degree tip should be done last.

What are your critical dimensions? What are your tolerances?

27.57 degrees? My (digital) protractors can display that kind of accuracy, but you can't get repeatability to more accurate than .2 degrees in real life. And measuring angles is real tricky - almost impossible! - on small round parts. You've made that part even harder by needing very short distances between the bends. (That's why I strongly recommend making a go-no-go gage first.)

316 bends fairly easily, but repeated back-and-forth bending to try to tweak a wire this thick back to shape will cause a loss of springiness/yield strength/material properties eventually. But a realistic number of bends back and forth won't anneal the wire too much.

 

[rb1957]

you could rout the desired shape into an Al plate as a go-go go gauge. maybe several different shapes so you can verify the big hook bend on it's own. maybe bend the hook around a post, tweek it till it fits, then do the other (simpler?) bends in the routed plate ?

agree with above about angular accuracy

 

[racookpe1978]

Hmmmn.

Couple of other things:

How "round" do you need the final 3/16 wire to be? If you don't mind some distortion of the wire as it bends, you can use the simplest form: a vertical cylinder of the 0.356 radius. The inside dia will flatten, but that may be acceptable to your final purpose.

But if distortion matters of the wire's section, then you'll need an (expensive!) curved shape (like an hourglass) to force the 3/16 wire to stay in its round shape as the inner surface deforms around the form.

Can you afford any "clamp marks" or other surface mars on the final product? Hammer marks?

 

[SnTMan]

The joggle will make it tough. What have you got to work with?

Regards,

Mike

 

[racookpe1978]

3/16 Inconel 600?

Makes dollars go up by 2 or 3 times. But 6x in quantity still isn't too bad.

Degree of difficulty? Goes up by 10x. Inconel is vary stiff.

You'll need a special roller type setup for the bender with the pivot pin and mandrel supported at both ends: a "pin and plate held in a vise" may not be strong enough for Inconel, but you could use the simpler setup to try techniques.

 

[USAeng]

I forgot to mention that there are dimple marks on the inside of each bend on the small bends. I am guessing this was to make the bend easier or maybe from the tool or jig they used to bend.

 

[USAeng]

I have the plans ready for tomorrow using some of the advice from above. Thanks a lot. One more thing- do you think we should just cold bend it? or maybe apply some heat?

 

[rb1957]

sounds like you're reverse engineering ?

i'd've expected to see dimpling on the inside of the hook, but not on the small joggles ...

 

[ornerynorsk]

A DiAcro pin and plate bender, or see "wire bender" on the McMaster Carr website.