DIY skid plate: is a punch the right tool for the job?

[Pnutbutterpie]

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!

 

[Hector_Fisher]

+1 on 'get a drill press'

I use my drill press way more than I thought I would before I bought it- like more than once a week. It spends most of its time with a 2 1/2" drum sanding head on it. I've used it for making wooden longbows & recurves, light duty milling, and even actual drilling once in awhile.

Its certainly on the list. Amongst debating which is the most useful shop tool at home, a drill press is up there. I've thought about starting a thread to get input about "tiers of tool buying". Basically, what do you need right off the bat, what do you need within a year of being in the hobby, then 3 years, what you need to do DIY stuff, etc. I'm sure there might be a thread already though.

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!

Awesome input. The annealing and tips on the tool material types especially. I had considered getting some 3/16 stock as the "die" to hold the shape, like you mentioned. I'm thinking I'd need to match the die hole diameter with the opening of the countersink in the chassis, maybe ever so slightly larger on the die to account for curve of the bending on the divot. I think you bring up a valid point about drilling after wards. Thinking about it, if i did drill before and then created divots after, the diameter of the underside would likely be larger than the topside as it bowed down. So I might need to drill small before hand, then finish drill with the right through hole size.

Another think I considered was doing divots and that countersinking on the top anyway, just to get good contact from the screws.

I know most of those in this thread tried to talk me out of doing divots, but I'm secretly still considering trying it because I'm stubborn...! Version 1 will probably be flat bottomed, but V2 will have divots for sure!

 

[Pnutbutterpie]

I had considered getting some 3/16 stock as the "die" to hold the shape, like you mentioned.

Yes, but be sure it's tool steel. Regular 3/16" mild steel bar will just squish (known as upsetting in blacksmithing terms) under the force required to make the divot. You could use a 3/16" drill bit as your stock, just dremel or die grinder with an abrasive disc to cut off a length of the shaft. Just don't try a hacksaw, the bit's too hard!

Don't know why I didn't mention this at the front end, but there are dimple die sets available that are purpose built for this process. They're typically more suited for a press or vice fixture though.

ever so slightly larger on the die to account for curve of the bending on the divot.

Exactly, curve of the bend and thickness of the material, 2.5x your hole diameter should be about right for 7/64" plate

Thinking about it, if i did drill before and then created divots after, the diameter of the underside would likely be larger than the topside as it bowed down.

Bingo!!! There's a metal worker in your brain somewhere

Another think I considered was doing divots and that countersinking on the top anyway, just to get good contact from the screws.

This is a great idea, little kiss with the countersink bit would ensure a good tight fit with the fastener.

I know most of those in this thread tried to talk me out of doing divots, but I'm secretly still considering trying it because I'm stubborn...! Version 1 will probably be flat bottomed, but V2 will have divots for sure!

Hey if you have a vision, go for it! It might not be worth it to most to form their own sheet metal, but that's why we each do it our own way. More power to you!

 

[Hector_Fisher]

Yes, but be sure it's tool steel. Regular 3/16" mild steel bar will just squish (known as upsetting in blacksmithing terms) under the force required to make the divot. You could use a 3/16" drill bit as your stock, just dremel or die grinder with an abrasive disc to cut off a length of the shaft. Just don't try a hacksaw, the bit's too hard!

Don't know why I didn't mention this at the front end, but there are dimple die sets available that are purpose built for this process. They're typically more suited for a press or vice fixture though.



Exactly, curve of the bend and thickness of the material, 2.5x your hole diameter should be about right for 7/64" plate



Bingo!!! There's a metal worker in your brain somewhere



This is a great idea, little kiss with the countersink bit would ensure a good tight fit with the fastener.



Hey if you have a vision, go for it! It might not be worth it to most to form their own sheet metal, but that's why we each do it our own way. More power to you!

To clarify, are you saying to use a 3/16 drill bit for the punch? Or the die? I figured i would use flat bar for the die underneath (sourcing tool steel will be the challenge). Have any recommendations for countersink bits that could hold up to hard steels? Most of the stuff i'm finding is intended for wood use only.

 

[Pnutbutterpie]

To clarify, are you saying to use a 3/16 drill bit for the punch? Or the die? I figured i would use flat bar for the die underneath (sourcing tool steel will be the challenge). Have any recommendations for countersink bits that could hold up to hard steels? Most of the stuff i'm finding is intended for wood use only.

Yeah, tool steel only necessary for the punch. Your die can just be a lightly deburred hole in some flat bar if you have that laying around. A regular HSS (high speed steel) 5 flute countersink should do just fine