2011 machines great as it comes off in little chips usually and its soft. 6061 gums up and galls to the tool tips + makes giant sharp ribbons that you have to cut off the part/chuck occasionally. It was definitely unpleasant to machine some of the larger parts as a result. But it is quite a bit stronger.

2024 would have been awesome. Seems to machine just as good as the other 2000 series aluminiums plus it is far stronger. Just tricky to get here.
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quote:

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Originally posted by Glen:

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I Guess it would depend on the part. This was just to hold the pulley to the hub so nothing too strenuous. However the same method is used in taperlock bushes which are very widely used in industry.

Failure modes would likely be rolling out the hole of one part, which would seem highly unlikely given large enough bolts + enough of them, or the entire part shifting upwards caused by pulling out all the threads on one part. Again, would seem unlikely especially given both parts are a tight fit together anyway.

I'd use it again
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I learned that trick from my machinist instructor at college. I use it on my 1lb ant weight bots to hold the pulley onto the shaft and it works quite nicely.

There is the CAD model of the join and the dovetail. Counterweight is steel, possibly stainless, and the main bar is G5 Ti. Thoughts?

[/img]

EDIT 3: Finally got it working

Last edited by Eventorizon-GB on Wed Dec 10, 2014 9:27 pm; edited 3 times in total

Sorry, the link isn't working on this side of the planet. This forum generally only likes jpeg images.
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Updated ^

I don't think that will last very long; Its not the screw fastener, its the acute inside angles in the dovetail. They are huge stress risers and will lead to fracturing in short order. I would go for a design like NST's teeth or my own Scissorhands.

The key thing is to avoid inside angles or to round them off with a generous radius. making the counterweight from stainless steel is not going to help - I once had a design a bit like this and the titanium bar fractured at the base of the dovetail after a couple of events.
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Yeah, I expected that was going to be a problem, just sticking with the traditional dovetail angles and methods.
Adding radi to the insides off all the corners isn't an issue.

I will dig through your thread and look up Scissorhands, see how that is done. It also explains why Ray used just a basic slot on Last Rights for its Alu Bar rather than a dove tail or slot of any kind.

You'd be much better off just bolting one piece to another. That has a huge amount of stress points and almost no support, particularly the placement of the bolt weakens an already high stress area without adding any useful strength.

Dovetails are good if they're used correctly, such as with NST as Nick pointed out. Sam machines large radii into all the corners and doesn't put a dovetail all the way through so that the remaining material supports the tooth whilst the dovetail keys it nicely into position and stop most lateral force. The bolts aren't taking any real force then.

Any better?

Slight tangent, Nick (and Grant given you have eye on) do you still think this design for C3 is a good idea?

http://www.fightingrobots.co.uk/attachment.php?attachmentid=4708&d=1408708301

I am getting quote for the design and I'm still worried about the threaded holes. It now has 5mm radi on the shafts and on the inside of ever corner too.

Here is a short-cut for you:



This is broadly equivalent to a dovetail; the Ti bar is waterjetted while the steel tooth is CNCed with a 1/2" corner radiused end mill. The lobes cut in the parts have a 1/2" diameter that made accuracy very easy. The radius on the end mill is important; it takes out all the stress risers at the bottom of the pocket in the steel. I had to finish the edges of the Ti bar with a Dremel too the match the end mill's radius.

There is still a sharp inside radius where the straight part of the Ti meets the lobes - according to stress analysis, because the lobes are enclosed inside the pocket, the stress forces bypass this area.

I chose this design over a dovetail for several reasons:

* The steel tooth can be slightly heavier for the same outside dimensions.

* I was worried about the accuracy of the CNC and waterjetting not matching up. Completely unfounded!

* There were some concerns that the dovetailed parts would wedge together and be hard to separate - even more unfounded.

As a long-time woodworker, I can tell you the differences between different angles on dovetail joints: A very shallow angle with little taper will lock together more firmly, but will also tend to exaggerate any tolerance problems. In Scissorhand's bar design, that may have caused alignment problems between the holes in the parts. It can also mean the parts lock together so tightly that they are hard to separate.

A dovetail with a steeper angle will not lock together as tightly and the parts will release more easily. The parts will align more accurately and even if the softer Ti part distorts a little, the counterweight will release more easily. Steep is a relative term - I would go with a 5 to 7 degree taper on the dovetail.

Update: I just saw the new CAD - The new rounded dovetail shape is much better, but I am not sure what the purpose of the extra bit at the lower left serves.
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That is brilliant nick! Thank you!

As for the 'extra bit' it is required to balance the bar vertically as the main position shifts the COG up. If the bar were symmetrical that wouldn't be an issue but since it is single tooth it needs it to get it back in balance.