[fastturbovet]

We have an application where I'm cutting .020" 6061 AL sheet. We are doing it on a Bridgeport CNC which can go up to about 3500rpm or so. We're taking a piece thats 9" x 2" and I am cutting out a bunch of channels with a 1/16" endmill. The item is a heatsink and I need the channels cut out so that it will not touch any exposed traces on a circuit board when its installed, so they need to be accurately cut and with as good and "square" a cut as possible.

I am having a couple of problems. The first is that its extremely hard to hold down the sheet. Because I am cutting it, there ends up being small peninsulas of metal which are not very rigid. Any tips on holding it down? Problem is it will be bonded with epoxy to a PCB later, and if I use tape it can leave residue which is tough to get off (since the sheet is thin you can't rub it hard without bending/deforming it a little). I thought about making a custom adapter plate for a vacuum clamp? I also thought about taking a thicker piece of material (maybe steel) and cutting out the same channels in maybe 3/64 width and clamping that down over my AL sheet before milling. We also thought about stacking the sheets maybe 3-4 thick and clamping them down with a top-plate to help with the problems. Any other ideas? Which of these ideas sounds best?

Second problem - getting a pretty bad finish on the cuts. There are tons of burrs and hanging bits of shredded AL all along the edges of the cut. Is this a vibration problem? A tool deflection problem? A feed/speed problem? Or maybe a combination? I am cutting dry with no coolant, and the last time was full depth of cut (.02"), about 3,000RPM with about 4ipm feed. That was a "WAG" )wild-assed-guess) so I could be way off. What could I do to help with this? Different feed/speed? Fixture the part better? Carbide endmill? Less depth of cut? Is my DOC OK doing the full .02 at a time with a 1/16" endmill? Or should I try to take it easier?

And last but not least, I am not necessarily tied to 6061AL nor to .02" material (but I'd like to stay somewhat close). If I went to .05 or .06" thickness AL would it be easier to machine and fixture? I am also not tied to 6061AL. I can get 7075 in the same sheet size, so thats an option too. Being more brittle I wondered if it might flake easier and not burr up so muhc?

Any tips welcome - trying to get this figured out in the next day or so

[harvard5]

Try the 7075 we use it all the time repairing aircraft it finishes alot better than 6061. I don't know much about holding it down but I have used a vacum table to hold down balsa wood to use a router on it and it worked great!
Bill

[cbass]

Hey, you guys are tied for posts

Sorry, couldn't resist (its late, I'm tired etc..)

[Spezzaferro]

Harvard5 is right, 7075 is a lot better to machine than 6061
I cut thin sheet a lot and I use a double sided vynal adhesive that I pick up
at a sign shop supplier, It's quite pricey but it works great and leaves no residue.comes off all in one shot. I use a Carpet roller to roll on top of the sheet to ensure good adhesion.
Also I use a sacrificial piece of aluminum underneath and . with .020 material
i would go .050 deep, but cut your sacrificial first,

[mxtras]

This seems like a tough one, that's for sure.

5052 is an extremely common .020" coil and cuts well - another option, maybe.

How many of these do you have to make?

I think I would try a clamp plate arrangement to trap the material between two plates. You could run the part profile in the top plate with a larger diameter tool for clearance, then maybe even relieve the bottom of the top plate to increase the local clamping pressure around the windows.

Vacuum doesn't seem like an option, but I don't have a clear image of exactly what you're doing.

Scott

[little bubba]

For the burrs I would go with a carbide endmill, your probably bouncing all over the place. Are you climbing, conventional or full width? Generally climbing will keep you almost burr free, if your going full width is it possible to take a .002" or so clean up pass? You didn't say what quantity your making but if its only a few I would go with the double sticky tape, just drop it into a shallow pan of acetone after if there is some residue. If you have to do a whole bunch, a steel template over the top sounds like a good idea, a vacuum chuck sounds like even more fun.

[fastturbovet]

Thanks for the feedback. I am going to a local metal shop tomorrow to see if I can't pick up some 7075 and also some 1/8" stock to make a top-plate.

I have to make quite a few of these. Its for an electronics doo-dad we're making and we will probably need about 100-150 a month to start, and once we convert some other electronics products over to this new style we will need maybe 200-300/mo or so. Sticky tape would probably work but it gets expensive and its difficult when you consider how many we'd be doing.

I am going to go with the top clamp and 7075 and see how it does. The problem is, imagine if you were taking a 12"x12" square, and cutting (from the center) 5" one way, moving up 1/2", cutting 10" back, going up 1/2", cutting 8" back, etc, etc. You would have these thin slivers of material that were *extremely* flexible. Thats the problem I have - once the machining is done, the part is flexible as a sheet of jello. I will try the double-sided tape and use acetone to remove if the clamp idea doesn't work as hoped

Thanks!

Mike

[miljnor]

when making a sandwich style set up its best not to cut the channle in the top plate first... just sandwich the material inbetween to plates of material and cut everything at once. if your top plate and sub plate are thick enough and the sandwich is nice and tight this will also eliminate your burrs.

The other method is wax but you said you didn't want a residue. and you would still have to melt the wax off. the lower melting point wax melts in boiling water.

[Karl_T]

Considering how many you're making, look at sending this out to a laser. Tolerance on laser cut is around 0.010 may be a shade better, might be a problem for you.

Karl

[ViperTX]

If the backside of the aluminum doesn't need to be flat and in the quantities you're speaking off....I would look at some type of forming rather then machining....tool & die comes to mind as well as hydroforming....

[miljnor]

the company i use for laser can keep +_ .002 on that thin of material

[skippy]

As vipertx says, I would definately look at a tool and die set up. Considering you have a mill to make the tooling and the product quantities are not large, a manually operated set up would take no time to punch out 100 - 200 units. I guess first compare costs with contracting out to a laser guy.

[take a cut]

not knowing all the info makes it hard to say although if the quantities are as you say i would probably build a double stage punch to punch this out possibly. The first stage springloaded to clamp and strip the plate and the second power stage to punch??

[Scott_bob]

2 more cents...

With the quantities you're into, vacuum tooling would definetly be the way to go.
Also, when trying to finish the edges of thin material, this is the one time you'll want to try conventional milling. I say "try" because you almost always want climb cut. The problem with milling the thin material along an edge is the material wants to peal up into your cutter, I think this is what you have described...

Try conventional cutting, it tends not to pull the material up during milling. Not knowing your shape, it is hard to advise clamping. If you possibly can, go for a vacuum clamping method. Can you post a quick image of your shape?

Sincerely,

[mxtras]

If the laser cutting route is not attractive to you and you are committed to cutting these, you might also want to get your hands on reverse helix end mills to push the material down during cutting. Chip extraction will then become an issue - but it's another option.

Vacuum? I can't see it. Not for what you have described. The thin webs don't offer enough surface area to provide enough clamping force.

Actually - the laser route seems the most logical.

Scott

[fastturbovet]

update...


Here is a pic of what I am trying to machine (with a quarter next to it for size comparison - the piece is about 8" long and 2" high, give or take, and I upped the thickness to .032 on 6061 and .050 on 7075 which seemed to help a lot since it was more rigid)



Well, I tried machining out of 7075 and it worked very well. I also tried a carbide endmill and both 6061 and 7075 worked very well. The only problem now is that its taking 30-45min to cut one. I am using 2" dense foam backing and using double sided tape to adhere it. The tape is a PITA to get off though (although soaking in MEK or Acetone helped a LOT).

I talked to a few places about getting it laser cut. I called 4 shops... one said $45/ea, the other said $29/ea, one took my info and never called back, the fourth I could never get them to give me any info on actually getting it done. Given this, I gave up. My budget is about 1/100th their quotes Its part of an electronics doodad (the plate is epoxied to a circuit board and there are little metal "ears" around the edge that connect the piece but are cut off after its bonded to the circuit board, to make a bunch of small but seperate 'heat sink pads'). The laser shops I talked to said it would take a long time to machine because the laser is very small so each cut above would actually be a loop on the laser, whereas with a CNC mill I can use a 1/16" mill (the size of the tracks) and do it in a single pass.

I was thinking it may be a good candidate to get stamped? I was thinking of talking to some stamping shops and seeing what they say. From what I know of stamping, a die is made which is $$$ but the per-part cost after that is very low, in quantity. It needs to be flat but thats so it lays flat against the circuit board. If stamping would curl the edges a litlte, I could get it staped from the back and at least the non-flat part would be sticking up (which would be OK if it was off just a little bit)

I need about 100 a month but if the price was low, I'd be happy to buy them several hundred or even a thousand at a time.

Any more ideas/comments? My plan at the moment is to just fixture as large a sheet as I can on my CNC machine and let it run and make these unattended, then leave them in an acetone or MEK bath to dissolve the tape and wash them off afterwards. I would prefer to get them made for me, but really the budget is less than $1-2/ea since anymore would mean I'd have to raise my product price, and I have contracts in place with distributors so I can't do that for 18mos or more.

[skippy]

Mike,
(A) I went to your website and I have seen what you are trying to do. I am heavily into hi-brite and ultrabrite LEDs also and I am 100% convinced that you do not even need this heat sink! Why? Because the only time you'll be driving the LEDs at their full capacity is when you turn on the indicator (blinker) function or stop light and in the case of the stoplight that's for about 3 seconds maximum. Yes, heat dissipation is an important issue with this style of led BUT only when running constantly at full capacity in high temperature situations. Take your situation (not many leds, not being driven at full current for more than few seconds in the case of the brake, generally in the wind due to being a part of a motorcycle) and now compare that with the example of a fixed, permanently on, speed sign (mph) using 300 LEDs all mounted in a black metal case, at midday in Kansas in summer. Get my point?
For further info log into http://forums.linear1.org/ You'll find Rob very knowledgeable and happy to answer any specific questions you may have regarding cooling. Hopefully you have chosen to use groups of LEDs that are wired in series. Generally about 4 LEDs to a group depending on the mA chosen but you can use the array calculator for that. Each group has the required resistor and groups can then be paralelled together. This is much more efficient than each led running it's own resistor.

(B) Second issue is that heat dissipation is achieved via the pins/legs. If I understand correctly you want to have a circuit board with LEDs mounted in the usual manner except to fit the heatsink on the non copper side of the circuit board. Even if heat dissipation were an issue, which I believe it's not in this case, having the heat sink on this side of the board where there is no heat won't achieve anything. My advice to you is to use the thickest FR4 board material you can find (for stability against the vibrations of the bike) with the thickest copper layer you can find to dissipate the heat. From memory, 0.124" thickness with 1oz laminate thickness is available but you'd have to do some more research on that. At least you're in the "anything is possible all it takes is money country" which is a good thing.

(C1) Alternatively, buy straight FR4 board (without copper laminate) in +/- 2mm thicK and also 1mm copper sheet. Get the FR4 and the copper guilotined to just over 8" x 2". Bond the 2 pieces together. Modify your circuit board layout so that all you are doing is cutting the separator tracks and nothing else so as to leave the maximum behind. Make a vacuum hold down arrangement specific to your job which should be very simple, fast and cheap to do. The heat sink and circuit board will be one and the same with this method. Route it all out followed by assembly. You could even put some grooves in the copper to "supposedly" add cooling.

(C2) As per C1 but get one die to punch out external shape of FR4 board, another die to punch out 1mm copper shape as per your picture, bond both together, again using copper as the circuit board and away you go soldering/assembling the board with your router only being required to do the hole drilling.

(D) If you feel you need to have some type of visual gimmick such as a heatsink to give you a "perceived" advantage over any competition you may have, think about a "dumby power management chip" Doesn't work but looks hi-tech. I'm not having a dig at you here as I have enough commercial background to know that sometimes it can be advantageous to have a gimmick of some kind. Sorry I've raved on so much.

[SRT Mike]

Edit...

[fastturbovet]

Mike,
(A) I went to your website and I have seen what you are trying to do. I am heavily into hi-brite and ultrabrite LEDs also and I am 100% convinced that you do not even need this heat sink! Why? Because the only time you'll be driving the LEDs at their full capacity is when you turn on the indicator (blinker) function or stop light and in the case of the stoplight that's for about 3 seconds maximum. Yes, heat dissipation is an important issue with this style of led BUT only when running constantly at full capacity in high temperature situations.

I am 100% sure I need the heat sink . I am not using SuperFlux LED's on this board, I am using a different type of LED. They *require* 1sq-in of supplemental heat sink per LED. Also, they have a very low R theta j-a of about 100C/watt, and a max junction temp of 125c. They run at 150mA and they will toast themselves in about 2 seconds without a heat sink. Also, it's critical that the LED's be able to be run at 100 duty cycle for extended periods of time without burning out. I have about 6,000 dealers of our products in the USA so if I send something out that even fails in 1% of cases, I am (as we say) "screwed" . Furthermore, the LED I am using has such a low max junction temp that it cannot even be soldered to a circuit board, it must be mechanically attached to a metal substrate which provides the mount as well as the heat sinking.

Take your situation (not many leds, not being driven at full current for more than few seconds in the case of the brake, generally in the wind due to being a part of a motorcycle) and now compare that with the example of a fixed, permanently on, speed sign (mph) using 300 LEDs all mounted in a black metal case, at midday in Kansas in summer. Get my point?

I think the bikes are much hotter... most new motorcycles (at least sportbikes) have undertail exhausts. Its very common for the light to be less than 3" from the exhaust pipe, and we've seen temps of well over 150 degrees in testing.

For further info log into http://forums.linear1.org/ You'll find Rob very knowledgeable and happy to answer any specific questions you may have regarding cooling. Hopefully you have chosen to use groups of LEDs that are wired in series. Generally about 4 LEDs to a group depending on the mA chosen but you can use the array calculator for that. Each group has the required resistor and groups can then be paralelled together. This is much more efficient than each led running it's own resistor.

Thanks for the info - I checked out the site. I'm pretty familiar with the subject though We are working directly with the engineers at Lumileds on this project - we sell many thousands of lights per year and this new design is a complete departure from what they normally do, so everyone is pretty psyched about it. We have about 80 tail lights we manufacture but this is the first one using the new LED which requires the metal substrate, and its a smaller volume product so its a good test-bed. The LED's are run each on an individual circuit, because we use a microcontroller and PWM to modify the brightness level of the light, but we are also using a switcher regulator which is pretty efficient and precludes the need for beefy resistors at each LED. The problem with stringing too many LEDs together is the variance in brightness due to fluctuations in vehicle voltage, but the switcher solves that.

(B) Second issue is that heat dissipation is achieved via the pins/legs. If I understand correctly you want to have a circuit board with LEDs mounted in the usual manner except to fit the heatsink on the non copper side of the circuit board. Even if heat dissipation were an issue, which I believe it's not in this case, having the heat sink on this side of the board where there is no heat won't achieve anything. My advice to you is to use the thickest FR4 board material you can find (for stability against the vibrations of the bike) with the thickest copper layer you can find to dissipate the heat. From memory, 0.124" thickness with 1oz laminate thickness is available but you'd have to do some more research on that. At least you're in the "anything is possible all it takes is money country" which is a good thing.

No, the LEDs are not mounted in the traditional way. A lot of work has gone into the determination of what conditions the light will be used under as well as the exact amount of heat-sink material required. The LED's will be mounted to this heat sink and this heat sink will then be mounted to a PCB. The heat sink is the conductor and the heat dissipator. The copper on a normal FR-4 isn't nearly enough to dissipate the heat from the LEDs in this case.

(D) If you feel you need to have some type of visual gimmick such as a heatsink to give you a "perceived" advantage over any competition you may have, think about a "dumby power management chip" Doesn't work but looks hi-tech. I'm not having a dig at you here as I have enough commercial background to know that sometimes it can be advantageous to have a gimmick of some kind. Sorry I've raved on so much.

no insult taken, but I can assure you the metal plate is very functional - no gimmicks! We're kind of on the bleeding edge of LED automotive applications at our shop, so there is a whole bunch of thought and engineering in this product. The method being used currently is the best approach. The LED's require the heat sink to function correctly, and we move tens of thousands of lights per year, so the product must be 100% reliable, even if used at 100% duty cycle in the desert in the middle of july for 30 minutes straight full-blast brake light.


Still interested in feedback on whether stamping would be an appropriate method of having the aluminum heat sink made? Anyone?

[mxtras]

Stamp it. Be done with it. Get some sleep.

I think you are going to kill yourself any other way - you already proved that to yourself, I think. Machining - hold down and efficiency issues. Laser Cutting - too expensive. Casting - uuh....no. Extrusion - no. Praying to God that you wake up and find a thousand of these things under your pillow tomorrow AM?

I think that you should get through your always painfully slow prototyping process and then stamp em out. Until then - did you try machining more than one at a time using the clamp plate? Just curious.

Keep us posted. You have my curiosity now.

Scott