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-   -   Chassis Questions (http://www.diyaudio.com/forums/construction-tips/225201-chassis-questions.html)

keantoken 7th December 2012 06:30 PM

Chassis Questions
 
For the first time I will be able to buy a chassis for my poweramp projects! My amps will use 2 pairs of MJLx302/x281 type outputs per channel stereo. The power amps are of my own design, and it will also have an attenuator/buffer I designed. I have a 350VA transformer which I can use. Here is the chassis I'm looking at:

Enclosure RE4312H 430x120x314 - Boards | Kits | Components | Modules | Tools

It looks well built and seems reasonably cheap.

The one big hurdle I have may be in drilling and threading the heatsinks. I have to do it right the first time, and I've never done it before. I initially thought I would drill chamfered 2.5mm holes and screw in hex socket M3 screws and let them thread themselves. I'll bet some of you have done this before and can tell me how it goes.

So, any advice?

GOR3 7th December 2012 06:45 PM

I have not tried it, but I don't think it's a good idea. With the heating and cooling of the device, eventually it will work loose. It's not hard to tap the holes yourself. There's got to be a kindly diyAudio member near you who has the tap and is willing to show you how to do it. Were you near Connecticut, I'd be happy to help.
Regards,
George

keantoken 7th December 2012 07:17 PM

I thought we used spring washers to keep them from working loose. I don't see why a tapped thread would make this less so. Not that I really know anything about it.

GOR3 7th December 2012 07:51 PM

The device/heatsink interface actually is more complicated than that. It's not that the device just has to stay on, but that it has to stay on under the correct pressure. Too much torque on the fastener can damage the device; too little torque will increase the device/heatsink thermal resistance. The hole into which the fastener is screwed has to have the proper geometry to hold the fastener under the proper torque. That's what the tap does; it creates a hole with the proper geometry. The method you're suggesting is, essentially, a self-tapping fastener, which many device manufacturers specifically denounce. Take a look at Recommendations for Assemply of Infineon TO Packages (available at www.infineon.com) or TI's Application Note (available at http://www.ti.com/general/docs/lit/g...Number=snoa460)
Regards,
George

yup 7th December 2012 09:27 PM

If you line up your holes so they are in between fins, you can drill through the heat sink (from inside to outside) and install small black socket head cap screws from the outside. This gives a clean look and if you damage the threads, just throw that screw away and replace...
Use plenty of WD-40 and clean the chips out frequently. If you break a bit in the aluminum it is all but impossible to get it out without making a mess.

dchisholm 8th December 2012 06:38 AM

Wow, a decent chassis is certainly expensive! I have to admit it looks like there's a lot of metal working in that particular one.

If you're going to put that much money into the chassis, then give serious consideration to properly locating and tapping the holes you need. For manual assembly by a home hobbyist I'm not sure the self-tapping screws are any less likely to ruin the threads, seize up, or break off in the material, than a real tap. Manual tapping of holes is something of an art and individuals develop the necessary finesse and craftsmanship through practice over time. In the early stages of learning, a failed attempt is a genuine learning opportunity; later in life, a failed attempt is almost always a reminder of some guidance that you chose to ignore in the name of expedience or speed.

My advice echoes the first reply from "GOR3": find somebody to teach you. It doesn't look like your task is beyond the "Beginner" level of difficulty. In the "Good Old Days" (formerly known as "These Trying Times", before hiring a better Public Relations agent) you could often get that help from your neighborhood school. I think the decline of the U.S. economy can be traced to the day they took shop classes out of the Junior High and High Schools . . . but that's for another discussion. Since St Louis Missouri is closer to Texas than Connecticut is, I'll make the same offer that "GOR3" extended.

In the meantime, I posted a hobbyist's procedure for manual tapping in the thread "Hand Tapping Tool?" at http://www.diyaudio.com/forums/const...ml#post3189390 Don't be intimidated by the length of the post - it's deliberately quite detailed, since I expected that somebody with little or no fabrication experience would stumble across it and put it to use. Perhaps someday I, or somebody else, will expand on that post by adding pictures annotated with circles and arrows and a short paragraph.

Dale

keantoken 8th December 2012 08:58 AM

Hi and thanks for the info. I don't balk at terribly long webpages, not anymore at least. I seem to have partly conquered the wall of text phobia.

Bolts rather than thread seems to be the way to go. Need to find myself a torque driver, to get an idea how tight things should be. I think torque could be specified as a half-turn or so after encountering resistance, but this would change depending on the washers used so would have to use exactly what's specified.

I thought of using carriage bolts, but M3 carriage bolts seem difficult to find.

I guess I will probably end up drilling straight through between the fins and using a screw and nut. However I don't like to have important screws exposed, since some children really like them. I may also just commission a local metalworking shop to do it.

I think what I can do is drill and chamfer a hole, and try self-threading the screw, and if that doesn't work just drill it out again with the M3 drill bit and use a nut.

CarlyBoy 28th December 2012 11:27 PM

K, I confess I'm having difficulty visualizing what you want, but I think I can give a little insight..

Re tapping holes. Drill through the material with the diameter of the shaft (not the threads) of the machine screw you want to use. Then get the appropriate tapping bit and work your way through, back and forth. Patience is the key. No more than a half turn in then back out. Clean the hole with a Q-tip dipped in rubbing alcohol. I did a lot of research on tapping holes. Best advice ever: rubbing alcohol instead of any lubricant. Dip the tapping bit in it with every extension to the hole. Works beautifully and the bonus is there is no clean up necessary.

Re correct tension a bolt/nut option. Torque to the nut to the setting you've decided is appropriate (lots of options for small torque wrenches - bike tool suppliers like Park and others have nice ones) and then carefully hold that nut in place while securing another nut against that first nut. This will go nowhere and you haven't applied excessive force to the delicate part that needs securing.

CarlyBoy 28th December 2012 11:41 PM

K, I confess I'm having difficulty visualizing what you want, but I think I can give a little insight..

Re tapping holes. Drill through the material with the diameter of the shaft (not the threads) of the machine screw you want to use. Then get the appropriate tapping bit and work your way through, back and forth. Patience is the key. No more than a half turn in then back out. Clean the hole with a Q-tip dipped in rubbing alcohol. I did a lot of research on tapping holes. Best advice ever: rubbing alcohol instead of any lubricant. Dip the tapping bit in it with every extension to the hole. Works beautifully and the bonus is there is no clean up necessary.

Re correct tension a bolt/nut option. Torque to the nut to the setting you've decided is appropriate (lots of options for small torque wrenches - bike tool suppliers like Park and others have nice ones) and then carefully hold that nut in place while securing another nut against that first nut. This will go nowhere and you haven't applied excessive force to the delicate part that needs securing.

dangus 29th December 2012 11:27 AM

I picked up the the basics of metal work by reading through a high school metal shop textbook. Try the local library, or used bookstores.


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