Pnutbutterpie
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Outlining designs for a DIY steel skid plate.
Looking to put a formed counter sink into 7/64 inch thick (~2.77mm) metal flat bar. Countersink on the bottom (street) side, recessed against the chassis to match the existing countersinks on the chassis (on right side of image).
My plan is to drill 4mm thru holes and then use some sort of shaped punch to indent the outer rim of the hole to contour to the chassis countersink. I'm not a machinist by trade, so i'm not expecting to get millimetrically perfect results, but would this tool do the job?
https://www.amazon.com/dp/B000936R1O/
I don't have a machine shop, so this would just be myself in my garage with some 2x4's, a 4mm drill bit, a mini sledge and the metal flat bar. Would that work well enough?
Hey there! I'm a long time metalworker and have an easy suggestion for you that you pretty much already came up with, just some refinement.
Get a flat ended punch that is about the diameter of your fastener thread (or sand the tip of one like you linked to be blunt), this will be the flat spot at the bottom of the divot you're forming. Use a nut, glued together stack of washers, or drill a hole in a chunk of steel/aluminum/thick delrin that is about 2.5 times the diameter of your punch and de-burr the edge a little, this will be your forming die. Punch sits above your material, forming die below, then force applied accurately will do this exactly. Take care to center your divot spot carefully over your die, then give the punch a solid whack; or better yet, if you have a vice line it all up and slowly squeeze your part to desired depth. If you're going the vice route, I'd strongly advise cutting down the length of your punch so that if there's a little misalignment it doesn't launch across your work space. It still might.
Drill the actual thru hole after forming the divot, otherwise you'll probably just tear out the hole with the punch and drag a little material back without really making a nice punched countersink. The punch needs to be a hardened tool steel, but the die doesn't really. The die needs to be a good bit harder than wood, otherwise the die itself will deform and not give you the defined divot your looking for, but more of a blobby dent. Deburring the punch might be a good idea, just so it doesn't tear your plate while forming. I can draw things if it helps!
Also if you can't make it happen cold, anneal the plate with a kitchen torch by holding the flame on the spot until you start to see temper colors (yellow, straw, brown, blue, purple, etc not glowing) and then wait for it to cool so you can handle it. This will make the area more malleable. Have fun!