Chief Delphi - Tapped Holes vs Bolt an Nut

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- - Tapped Holes vs Bolt an Nut (http://www.chiefdelphi.com/forums/showthread.php?t=138039)

SerpentEagle

28-08-2015 19:45

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by jman4747 (Post 1494476)

That's what we don't know.

Are you using 1/4-20, 1/4-28, 10-32, 10-24, 8-32, etc ?

Also to Adams point, if you went with through holes that block could just be a spacer and wouldn't need the extra flanges on the top and bottom with the holes in them.

Im using 10-32.

And true it could just be a spacer if I used nuts instead.

MaGiC_PiKaChU

28-08-2015 19:47

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by SerpentEagle (Post 1494499)

When you say tapped do you mean bolted together? How thick was the aluminum?

yeah sorry english is my second language...
canon was 1/32 sheet and frame was 1/16 if i remember correctly

we had 1 bolt each 3/4

SerpentEagle

28-08-2015 19:51

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by Mr V (Post 1494482)

The general rule of thumb when tapping is that if the 2 materials are the same you need the thickness of the material that is being tapped to be 1x the diameter of the bolt that it will be used with. Coarse/fine thread it doesn't matter.

Now if you are using a hard/strong bolt into a hole tapped into a soft/weaker material, say a steel bolt into a piece that is made of aluminum the thickness of the tapped material needs to be 2x the diameter of the bolt. Again it doesn't matter if they are coarse of fine threads and there is no rule that says you need X threads.

Both of those cases are based on achieving a strength the same or substantially similar to what would be obtained by using a nut. If you don't need the ultimate strength of the bolt then you can get away with tapping into thinner material but you'll need to use a lower torque to avoid stripping the tapped material.

If you look at bolts and their corresponding standard nut (not nylocks, or jam nuts) you'll find that the height of the nut is equal to the diameter of the bolt.

Things get more complicated when you are going through a tube. The problem is that when you go through both walls of a tube you usually end up with a situation where the tube starts to crush before you reach the ultimate clamping strength of the fastener. Again it does depend on whether you need the ultimate strength of the fastener being used. But if you go through both walls of a tube then you will need to use a lower torque when tightening the fastener in some cases.

I would look at how to make that connection without a bracket and without the need for a spacer, ie move one of the components if possible so that you could attach the two pieces directly. Another option that I would consider is the use of two pieces of angle and rivets to attach them. It would again be best without a spacer but it would increase the number of fasteners and distribute the load over a greater area. Net weight should be lower and the strength should be greater, with high strength rivets.

Thanks for the rules of thumb, they will definitely come in handy.

Im not actually going through both walls of the tube, just the one on the inside. Is this a bad thing?

SerpentEagle

28-08-2015 20:11

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by Travis Covington (Post 1494489)

If you are trying to duplicate what we did on 254, I would heed Adam's advice and simplify things greatly. We make a lot of complicated parts seem easy because we do some unique 3D milling and tabbed machining and have extensive experience doing so. You have changed some of the parts in our design to actually make them more complicated than they were originally. We also do a lot of things design wise on our robot purely from an aesthetics perspective. We could do them simpler, but chose to make them more complicated because we feel that looks cleaner and more elegant. Take a step back to analyze the problem and your chosen solution to decide if you need the complexity you have in there.

Regarding your original question - We tend to use nylon lock nuts in locations where we absolutely do not want something to come loose and do not have a piece of material behind it which we can tap to 1-2x thread diameter. We never use non-locking nuts except for jam nuts on cylinders. 1D (D = thread Diameter) thread depth is about the smallest I would feel comfortable with. We prefer to go thicker where we can. 1/4" thread depth is about as thin as we tolerate for #10 screws, for example.

I do have to admit I definitely used many design ideas from your guys' 2015 robot. Those guides on the lift in particular caught my eye, and alot of what I saw is reflected in what I have here.

I have no cnc milling experience whatsoever, so I don't really know what the machine is capable of and not capable of. Is a 3 axis cnc machine not capable of creating concave fillets as I have in the model?

SerpentEagle

28-08-2015 20:16

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by AdamHeard (Post 1494494)

I'll reiterate you can probably reach your top level design goals with making all of these parts simpler 2d parts. They are much easier to get made, and allow much faster iteration.

I think your definitely right about more simplicity on the lift guides, but Im still not sure how the spacers between the tubes can be made 2d other than just turning it into a solid block with 2 holes.

SerpentEagle

28-08-2015 20:20

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by MaGiC_PiKaChU (Post 1494504)

yeah sorry english is my second language...
canon was 1/32 sheet and frame was 1/16 if i remember correctly

we had 1 bolt each 3/4

Oh ok. From what I see they were used instead of rivets? How did this work out for you guys?

GeeTwo

28-08-2015 20:22

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by SerpentEagle (Post 1494500)

I am using an aluminum block. Is it really that bad with aluminum? I've seen teams use aluminum blocks with threads before, and we've done it before ourselves without many problems.

The coupling nut in the block is an interesting idea. Have you guys tried it before?

We haven't put one inside of a metal block spacer like this, but we did use 1/4-20 coupling nuts with machine screws on both sides to hold some flat angle brackets (used as gussets) alongside some 1-1/4" square x 1/8" thick c-channel for a practice chassis last summer. If we were building this today, we'd probably go with two or three times as many holes and rivets - you can achieve similar strength with much less weight that way. Actually, as I look at your picture (which I can see now that I'm at home), coupling nuts wouldn't help you much as they usually run about 4x the screw diameter, or 20 coarse threads.

Even though I can now see the pictures, I'm not sure what the left horizontal bar for (nor any of the others) or how the loading will proceed. That said, securing the two long horizontal holes to the vertical tube will not add very much to the situation. With the two holes above and the two below, the two in the middle will only contribute strength if things are being pushed right to the edge on the outer four. Therefore, I would suggest tapping the center holes in the aluminum block and putting bolts into those threads only from the horizontal tube side. Unless you have some unusual loading, you can probably get away with tapping all six holes, running four bolts from inside the vertical tube for the upper and lower holes, and two longer bolts from inside the horizontal tube shown in the first pic.

Finally, FWIW, five threads and the nominal shaft diameter are essentially the same number for coarse threads; there are usually six or seven threads per nominal shaft diameter for fine threads. I suspect that maximum safe loading is actually somewhere between the number of threads and the number of diameters. This is because the frictional forces increase for finer threads, but the shear strength of the threads themselves is greater for coarse threads.

There is a loss in strength for a tapped hole vs a lathed hole, but just based on geometry considerations, it's probably more like 15-25% than 50%.

Mr V	28-08-2015 21:52

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by SerpentEagle (Post 1494506)

Thanks for the rules of thumb, they will definitely come in handy.

Im not actually going through both walls of the tube, just the one on the inside. Is this a bad thing?

Going through only one wall of the tube is a good thing assuming that you can actually get tools in there to tighten the fastener.

Keep in mind everything is relative. A lot of it depends on the type and magnitude of the loads that will be seen on that part.

For example tapping a piece of 1/8" wall tubing for a #10 bolt will result in a far lower strength that the bolt is capable of. However it may still far exceed the loading requirements for a given application. For example if you were using that to mount say an encoder or a motor controller you would still have more than the strength needed for the job.

The same thing applies for going through both walls of a tube. A low applied force means that you could stop before the tube starts crushing and have more than adequate strength.

The relative strength of the materials also is a factor. Use a grade 8 5/16" bolt to go through a 1/16" wall tube and you'll certainly be able to crush to tube flat. On the other hand use an ungraded #6 bolt to go through 1/4" wall tube and the fastener will probably fail well before you could do any crushing of the tube.

For applications like the one in your picture where the fastener is at the end of a tube I like cutting the end of the tube at an angle when it doesn't affect anything. That makes it easier to put a tool on the fastener and to get the fastener installed.

MaGiC_PiKaChU

30-08-2015 18:00

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by SerpentEagle (Post 1494511)

Oh ok. From what I see they were used instead of rivets? How did this work out for you guys?

worked like a charm! We didn't use rivets because we knew we might have to open those parts during events

BenGuy

30-08-2015 18:13

Re: Tapped Holes vs Bolt an Nut

Shashank, after reading everyone else's analyzations, I think you'll be fine to just tap them.

InFlight

31-08-2015 11:18

Re: Tapped Holes vs Bolt an Nut

I generally prefer bolted connections when possible as they are less labor intensive.

When tapping soft items like plastic or aluminum you need to use need to use a smaller tap drill that will leave 75% of the threads remaining. Most standard tap & drill charts are for steel which is designed for 50% thread remaining. Make sure to use a tap chart that has both aluminum and steel drill sizes.

It is a very good investment to purchase better high speed steel Taps in the few sizes that you use. You will also need a full set of numbered and fractional drill bits or at least the ones to support the size of taps you intend to use.

RonnieS

31-08-2015 15:18

Re: Tapped Holes vs Bolt an Nut

Use 2D for this:D
But when going through two walls of tubing with a bolt...avoid if possible. My preferred method is to put 45s on the ends of tubing so that you have a nice open face to work with. We used this on 314 often and I believe 973 does this as well.

-Ronnie

Pretzel

31-08-2015 20:26

Re: Tapped Holes vs Bolt an Nut

Quote:

Originally Posted by SerpentEagle (Post 1494509)

I have no cnc milling experience whatsoever, so I don't really know what the machine is capable of and not capable of. Is a 3 axis cnc machine not capable of creating concave fillets as I have in the model?

That depends on the specific machine and tools you have access to, as well as the design of each of the parts. I can tell you with certainty though your pieces that reach around the side of the blue tubing to the other side and attach to the block will require at least 2 different setups with the way you have your fillets designed in.

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