A little advice needed on miter joints

[crazypj]

It's an issue when you have parts heating and cooling (motorcycle engine covers).
Last several years I've used plumbers Teflon paste, works better than anti-seize if your not going over 400f. Apparently, copper based anti-seize helps galvanic corrosion with stainless steel screws in cast aluminium engine blocks. The high temp nickel based stuff is about the same IIRC? As I 'play' with 40+ yr old 'imports' I would like to think I'm helping keep them around at least another 40~50 yrs

 

[woodchucker]

Yes, I thought about that approach, but my objective was to have flat sides. Obviously, a clearance hole would not allow that. I think your idea would work fine if weren't for that little detail.

Thanks for responding.

Regards

you can still have a clearance hole , a clearance hole allows the screw to go through with out rubbing Your countersink would still prevent the screw from lowering into the side.

Looks well made, as a woodworker that joint was very familiar to me. Would never have thought to use that for metal working. But it sure do look purty.

 

[tjb]

Set the depth stop on the drill press and they will look very professional! Make the head just slightly under the surface.


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Thanks.

 

[tjb]

Because flat head screws have a short cylindrical section, there would be a small groove on the circumference of the screw head if the screw was set flush. There would be two work-around's. Use a countersink the same diameter as the screw head or very slightly larger or set the screw proud and machine them flush after the box was assembled. Another technique would be to use a low profile socket head cap screw and set it below the surface and follow with a plug. With finishing, the plug would virtually disappear into the wall of the box.

It may be sufficient to use aluminum rivets. Two cross drilled holes from each side should prevent the joint from opening. A swab of LocTite inserted into the hole before driving the rivet will help to hold it tight. make a small scratch on the side of the rivet to permit any trapped gas or liquid to escape. After seating the rivet, machine it flush and finish for a virtually invisible fastener.

When I was making aluminum cases for custom electronic enclosures, I would, on rare occasion, put a hole in the wrong place. To repair the hole, I would countersink from both sides and make a short aluminum plug. I carefully peened over on both sides and finished flush. then I wire brushed with a fine wire wheel which resulted in an invisible repair. It was a bunch of work and enough of an annoyance that I was more careful not to repeat the error but it saved a case that I had considerable time invested in.

Thanks, RJ.
Those are good suggestions, but for this immediate application, any sort of countersink approach or cap screw won't work. Using 3/8" stock and a miter joint doesn't leave much room for the countersink or a plug. My approach is essentially identical to your other suggestion of setting the screw proud and then machining flat. Only difference is I milled and tapped the hole to 3/4" and used a 1" screw, thereby machining off the screw head plus a little bit of thread. That made for a very secure joint with a nice appearance.

I like your idea of using rivets. Never thought of that and, since the finished joint will not be subjected to a lot of torque or pressure, that may be sufficient. I'll look into it.

Thanks again for responding and the thoughtful suggestions.

Regards

 

[tjb]

If you were making them from two different metals (eg, brass and aluminium) woodworking 'box joints or dovetails would look the nicest. As it is, if properly finished, you wouldn't see much of any joint so you 'need' the different light reflection from machining marks A small dovetail cutter ain't very expensive but mounting bars upright may be a clearance issue? There are plenty of tutorials for woodwork dovetails, some VERY EXPENSIVE wood planes are made with steel, brass or bronze dovetail plate bodies (or combinations of same) Even a Norris' style kit isn't cheap
http://www.ebay.com.au/itm/Norris-type-steel-dovetail-17-jack-plane-kit-/172856457356

No doubt about it: those joints would be beautiful with two different metals. Such an application is something I considered. One thought I had was to make the sides out of aluminum and the pegs/screws out of brass. That would look pretty nice - assuming, of course, the finished product is not painted.

Thanks for responding.

Regards

 

[tjb]

Two ways to stop the drill from walking sideways.

1. Use a 2 flute endmill smaller than the tap drill size. Plunge down until you have a full diameter flat landing spot in the lower piece. Tap drill and tap.
2. Make a drill bushing guide plate that you clamp on top to guide the drill.

Drill bushings are hardened steel cylinders that guide the drill so it cannot walk.

Good suggestions.

#1 sounds like a good strategy. The drill bit is not actually what 'walked'. The bigger issue was the vertical section of the miter joint wanted to wobble slightly as the drill bit transitioned into it from the horizontal piece. It took some serious clamping effort to keep that from occurring. I suspect using an end mill first would minimize or totally eliminate that problem.

Thanks again for responding.

Regards

 

[tjb]

Try this with scrap...

Build a fixture to hold parts and that is easy...Place angle iron on flat plate with bolts or welds then with end mill make true and cut both x and y at same setup so mill table accuracy is transferred to fixture.

Make another fixture for holding at 45 degree angle.

Both fixtures hold material with standing on small side and lengthwise along the table.

Last fixture is a chunk of metal that fits over the end or corner and has holes pre drilled as guides.

Next make all lengths correct and square.

Using top fixture place parts in but the corners are clamped tight in butt joints.

Place the last fixture over the end on the outside part and drill holes through the outside of that part then into the end of other part.

Witness mark both parts.

Now swap orientation of parts and drill opposite holes.

Now tricky part.

Mount 45 degree fixture and cut the miter with standard end mill.

If you can get all done well and the miter are not over cut the predrilled holes now will line up and miter perfect.

You also could just use the drill guide with the first fixture to hold the fully machined parts then drill the holes.

Given you are grinding off screws and powder coating then the pins can be anything that can be soldered so you could solder instead of screws.

Last option is taper pins which would require modifying the reamer for shorter length or drill deeper holes or both.

Then smacking the taper pin in would be a tight fit without any gaps.

A drop of lock tight insures no movement.

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Wow! That's a very thoughtful suggestion. I had to read it several times to visualize it, but I think I've got it. (Do you have such a setup? Any pix?) I see what you mean about milling the miter being the 'tricky part'. That step would definitely require a lot of precision.

Thanks for the suggestion. Making the fixture would be a challenging and fulfilling exercise in itself.

Regards.

 

[tjb]

I just set the stop on the mill or drill press to set them flush. And use stainless as the look nice.
If you don't have a good stop on your drill press, you can make one of these for an 82dg center drill.
https://www.harborfreight.com/7-piece-drill-stop-set-38336.html

Thanks for the link.

Regards

 

[tjb]

Aluminium has a particular affinity for stainless steel and will corrode over time. For a tray it may not be a problem but making over-length aluminium pins then shrink or force fitting them (plus cut and polish after fitting) would give a just about invisible joint. It will probably be a lot more work making pins but could easily end up with a 'How did he do that?' piece

Good suggestion. And I LOVE 'How did he do that?' responses!
Regards

 

[tjb]

you can still have a clearance hole , a clearance hole allows the screw to go through with out rubbing Your countersink would still prevent the screw from lowering into the side.

Looks well made, as a woodworker that joint was very familiar to me. Would never have thought to use that for metal working. But it sure do look purty.

Thanks for responding, Woodchucker. Countersinking on this particular application simply won't work because I'm only using 3/8" flat stock. That doesn't allow much room for a countersink on a miter joint. That's a good suggestion if I perform a similar exercise on thicker pieces.

Thanks for the compliment. You can't imagine how meaningful it is. My objective was actually to replicate a woodworking-style miter joint in aluminum. It's encouraging to receive a compliment from a real woodworker. Thanks!

Regards.