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Dimples - the new chequer plate?


simonr
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Judging by the number of odd dimples appearing on new trucks - I thought I'd better not be left behind!

With a fairly scrappy (nasty to machine!) lump of 50mm bar and a spare half hour this evening, I made this:

dimpledie.jpg

A single die which can put a dimple in a 25 or a 35mm hole! There is a 16mm hole up the middle which is either used with an M16 bolt to put in a dimple in situ or as a guide on a press.

Tried it with my fly-press on 3mm (above). The covered hole on the right was made by tightening an M16 bolt with the rattle gun. It was less controlled and if anything pressed a bit hard - but it proved it works OK.

Si

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Nice stuff. Are you going to be marketing them?

I doubt it - I don't think there is enough of a market - and to make properly (heat treated EN8 grade) would cost too much.

Also - if my truck gets covered in dimples - I don't want everyone elses to be the same!

Si

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I quite like the dimples, and they're good for strenght arent they? something about same strength as the sheet being the full thickness of the dimple.

How about a replacement for Chequer wing tops, loads of little ones of these on the sheet? with the dip side facing down to get grip on the surface. X-Tops maybe?

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Simon, Would you be able to put up a simple 2D sketch with some dimensions? I fancy turning one ;)

Err...for once, I just stuck a lump of metal on the lathe and turned the handles until it looked about right!

I decided to go for 25mm and 35mm because I have a 35mm hole saw and my biggest drill bit is 25mm.

It has a straight bit which is a snug fit in the hole to allign it, followed by a 45 degree taper (compound slide set to 45 deg) followed by a flat bit to give it a sharper corner between the dimple and the flat sheet - and to minimise deformation of the rest of the sheet. Not based on any evidence other than it felt right.

The other half of the die is just the inverse shape and forms a snug fit on the other half.

The 16mm hole up the middle hyst has to be a good fit on whatever bolt you use otherwise the dimples will be skewed.

You should be able to figure it out from that.

Si

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So the 2 halves fit together perfectly, you dont have to allow a 2mm gap or so to allow for the sheet thickness?

That would probably be ideal - but will vary depending on the material thickness. I figured I'd give it a go as a snug but not precise fit - and it seemed to work. A gap would probably give you a sharper angle on the outer lip - but I quite like the curved look of it!

If I make any more - they will be the same!

Si

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First attempt, knocked up in about 10mins. Just to see what the outcome would be.

DSC00106.jpg

Second attempt, made so that they flatten the plate out on full compression. Rather than calculate the angle finish point to then flatten the material i used the witness marks on the first attempt to determine the dims.

DSC00154.jpg

DSC00155.jpg

Using 3mm plate, tried 4mm too. Worked well.

DSC00114.jpg

Used EN24T for the material. I use this material more often than most when machining at work. Good tough material that work hardens in time. Made up 1" 3/4 and 1" As that was the hole saw i had.

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What do the dimple's do? Are they functional or just to look nice?

Holes are put into a fabrication to reduce weight. The dimple puts the strength back. Sometimes it can add to the strength at that point by transfering bending loads to a stronger part of the fabrication.

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The hole obviously removes material and thus lightens the plate. Putting a lip or dimple on around the hole locally stiffens the plate which compensates for the loss of material. It will probalby also generally add to the stiffness of the plate. Don't know how much it stiffens or compensates for material loss, but I am sure a grown up will be along shortly....

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Guest diesel_jim

What practical application do they have (apart from asthetics).

if a given piece of flat plate is "weak" from having a hole in it, would not making said plate thicker do the job? or am i getting it wrong :unsure:

actually, i seem to recall seeing some of the aluminium (and steel i think) PSP with dimpled holes, is this the sort of thing dimples were invented for?

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It is weight saving. If you made a thing with the right strength material and then needed it lighter you might put holes in it. In the right place a hole may not affect the strength of the item but in other places it would. In sheet materials in particular a hole would weaken it allowing it to bend across the hole more easily. A dimple adds a fold that increses stiffness and adding some, or all, of that strength back.

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is this the sort of thing dimples were invented for?

IIRC They were/are used in the construction of steam boilers as a means of stiffening where a plate has a tube passing through it. Metal aeroplanes are also absolutely ridlled with them. In that application you get the maximum stiffness with the minimum weight.

I've used them for years in diy fabrication. If you haven't got a lathe you can form them by hand over a tube or even a large socket. Takes ages but it looks the business. I've always known them as 'swaged holes'.

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