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View Full Version : a perfect 90deg joint - 15mm Alu



dsc
01-01-2013, 09:14 PM
Gents,

I don't have much experience in the field of Ali stiffness and / or load support, so I'm hoping someone here can offer some suggestions. Here's what I'm looking to put together:


7829

and the main question is:

What's the normal procedure for joining two 15mm thick Alu plates to get a perfect 90deg joint? The drawing above shows two plates, which are 130mm wide and two M8 holes for 80mm bolts and a 5mm deep pocket. The load from the top on the horizontal plate will be around 2kg. Will this be enough to handle the load and still maintain the perfect 90deg, or will it flex? I've seen various z-axis designs and some use additional triangle supports on the corner, others don't (even when using rather heavy spindles).

Any help is massively appreciated.

Regards and Happy New Year
dsc.

Web Goblin
01-01-2013, 10:44 PM
If the machine you are making the parts on is set up correctly then milling out the slot and the parts should not be a problem and should give you a straight edge to the horizontal plate and a nice flat slot. When bolted up the flat faces should meet nicely at 90deg. 2KG is not a heavy load on the plate and I would not worry about that bending the plate. If this is for a mill or router then I would worry about the forces the cutting process would exert on the joint. I think these would be more likely to deflect the joint. If this is to hold a spindle then go for two supports.

dsc
02-01-2013, 01:10 PM
Thanks for the reply WG. This isn't for a router although there will be turning forces acting on the horizontal plate in the CW direction. The top 2kg load is static.

I'm also contemplating whether I need the pocket at all, might just be more work for something that doesn't do much.

Regards,
dsc.

m.marino
02-01-2013, 01:25 PM
DSC,

The index slot while not increasing strength much WILL insure that the plates are ate 90 and serve as an index point for mounting removing most if not all possibility of twist (or slant) of the plate being bolted to the main plate. While this does add a bit of machining the benefit both in initial set up as well as any maintenance down the road can NOT be under valued. The little extra added effort will pay you back a huge amount further down the line.

Michael

dsc
02-01-2013, 01:40 PM
Hi Michael,

cheers for the reply, I'll leave it as it is then.

Regards,
Tom

blackburn mark
02-01-2013, 05:51 PM
I get where Michael is coming from but i would be tempted to keep things simple (unless its being re-mounted over and over)

you could see how you get on with it, that way you may be able to order the sizes you need with minimal hassle on your part to get them machined

going so deep with those bolts wont add much if anything to the plates rigidity, id say 40mm would give as much as 80mm.

an advantage with forgoing the slot is you will have a touch of play to correct any tiny errors that you may have built in elsewhere

a couple of simple Ali blocks could be added as buttresses if it turns out not to be rigid enough... it will look a tad utilitarian but I'm guessing the function supersedes the form and your struggling to get them machined :)
if your paying someone then of course get nice triangular buttresses :)

buttresses will ad an ENORMOUS amount of rigidity to the plate compared to just the bolts but with a 2kg load i would think the bolts would be fine, maybe add two shorter bolts between the outer two?

dsc
03-01-2013, 12:52 AM
Hi Mark,

I can add two more bolt holes like here:

7854

Still not sure about the slot, even with out, I doubt I'll get more play in case of other errors, as the four bolt holes will have to align leaving maybe a few 0.01mm of play here and there (although I won't be able to move much if I need to anyway due to the amount of holes involved). As you say, I can always add supports later if I need to.

Also, I've checked and the total static load will be around the 1kg mark, not 2kg like I thought before. I'm pretty sure four M6 bolts can handle 1kg, so it's basically a matter of machining the parts correctly to get 90degs everywhere.

Regards,
dsc.

Jonathan
03-01-2013, 01:35 AM
I'm pretty sure four M6 bolts can handle 1kg, so it's basically a matter of machining the parts correctly to get 90degs everywhere.

Four M6 bolts would probably handle several hundered times that before shearing! As mark said, you don't need the bolt holes to be deep - bear in mind you'll struggle to tap M6 more than about 25mm deep anyway. The strength of the joint depends on the friction between the surfaces, and thus the clamping force and surface finish. Increasing the length of the bolts beyond 20mm of thread wont make much difference to the force you can apply.

If you don't mind me asking, what is the part for? It can help a lot to know since we might be over examining this, or worse vice versa.

JAZZCNC
03-01-2013, 02:08 AM
bear in mind you'll struggle to tap M6 more than about 25mm deep anyway.

Why would that be then.? . . . . I've tapped 100's M6 threads deeper than 25mm.!

John S
03-01-2013, 02:22 AM
He's got short arms and deep pockets.

Jonathan
03-01-2013, 02:51 AM
Why would that be then.? . . . . I've tapped 100's M6 threads deeper than 25mm.!

I meant significantly more. If the tap does not have a reduced shank then clearly the depth it can cut is limited to the length of the tap, which for some of the ones I have is around 20-25mm. Either way it doesn't change the point I'm making.

dsc
03-01-2013, 11:36 PM
Four M6 bolts would probably handle several hundered times that before shearing! As mark said, you don't need the bolt holes to be deep - bear in mind you'll struggle to tap M6 more than about 25mm deep anyway. The strength of the joint depends on the friction between the surfaces, and thus the clamping force and surface finish. Increasing the length of the bolts beyond 20mm of thread wont make much difference to the force you can apply.

If you don't mind me asking, what is the part for? It can help a lot to know since we might be over examining this, or worse vice versa.

Jonathan, I wasn't worried about shearing the bolts, I simply thought that 80mm long bolts will offer more support for the horizontal plate.

As for the use of the part, it will be part of a coffee grinder with these used to crush the beans:

http://www.portafilter.net/uploaded_images/conical_burrs_LL-752918.jpg

This means there will be mostly clockwise turning forces acting on the mount and the carriage, although I don't know what the torque is yet.

Regards,
dsc.

dsc
24-02-2013, 01:31 PM
I'm contemplating a similar problem at the moment, so I thought I'd add it in here rather than start a new topic.

Here's a corner joint, with three 15mm plates (Eco Cast, so should be 15mm consistent across):

http://i1120.photobucket.com/albums/l496/dsc_MT/random/corner_joint.png

Idea is to get the edges aligned perfectly and easily. Plates are held using M5 bolts, with 5.5mm through-holes on the plate faces to allow some wiggle room on the edge threaded holes. This must come up quite often in various CNC machine builds, so I'm curious what you normally use.

So far I've got the following solutions on my list:

- pocket cuts to match the plates against each other

- corner jig to align plates, so no further machining

- locating pins

Dead square box is needed, I'm open to any suggestions.

Regards,
dsc.