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      Board Status   07/20/2018

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TarheelRick

How do I get better fishmouth joints

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When building roll cages and frames I like to fishmouth the ends of the rods and tubes for a cleaner fit and a better glue joint. However I have a difficult time getting the fishmouths centered in the ends of the tube (.080 and .100 size). I also have a difficult time getting each end fishmouthed on the same plane, in other words they are quite often a few degrees off from being parallel with each other. I use a small round jewelers file for this work. Does anyone have any ideas on how to get better, more even fishmouths?

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Good question.  No answer for you although I was thinking using aluminum tubing and a hammer to cut the fishmouth.  Getting the right angle might be more difficult - at least for me.

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Posted (edited)

To center a round file in the end of a piece of round stock for fish-mouthing, I usually use a diamond file to cut a shallow groove first.

Then your round file will stay on center as you work into the groove.

I also hold the very end of the piece I'm working on between the tips of my index finger and thumb, and use them as an additional guide to keep the round file centered.

Far as getting the angles right, on models, it's mostly experience. It's fairly straightforward to measure them on real cars, but there's such limited working room on models, it can be tough. I sometimes have to cut more than one piece to get it right.

That said, I'll usually mark the center of where I want the fishmouth to be with a fine black Sharpie line BEFORE cutting the centering groove.

Get one good fishmouth on a too-long piece of stock and fit it up to where it goes. Mark the end for a straight cut just a tad longer than the finished dimension.

Mark the angle of the fishmouth relative to the joint, and centerlines as above.

Then slowly work the end of the piece, fitting often as you file, until it fits correctly.

DSCN0847_zpsz5aqfa3h.jpg

Edited by Ace-Garageguy

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I'll have to remember your idea on cutting the Fish Mouth for may next build.

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I do it like Bill does, but I skip the initial groove. I found I only need to use my fingers as a guide. I eye ball all my angles.

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Not a practical solution for most, but I build  jig for my Sherline mill.  It uses an angel plate and a v grooved scrap of aluminum.  I was doing birds mouths on brass tubing and it worked very well and the angle was repeatable.  You could use the same idea with a round jewelers file to keep everything aligned.  Put your tubing in the groove to hold it steady and then align the file in the groove and file at the desired angle.  you will file away the groove a little but it will stay centered.  

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See if Pete J can cut a bunch of 22.5/45/60 and 90 degree fish mouths for you. If one isn't exactly what you need, use a round file to get those extra few degrees. 

If it's tubing and not rod, cut two pieces to get the length you need, then use a smaller plastic rod as a glue-in coupling to get the right length piece. Clean up the glue joint in the middle.

Also works with anyone else who has a mill or lathe. (Put the piece of tubing in a vise on the lathe cross slide and put the end mill in the chuck)

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