Accurate location of two concentric cylinders 6

[JediJew]

I'm in the process of designing a small bar end grinder. The device has two casings: one which holds a gearbox and another which holds the bar to be ground. These could not be machined from one piece of material and so I need a way to fix them together so that they do not move relative to one another (i.e. thread is inappropriate)

I've uploaded pictures of the parts. The outer housing has an outer diameter of 150mm, and an inner diameter of 140mm. These are not fixed but I'd prefer them not to be too much larger for weight saving purposes.

Any suggestions appreciated

 

[NomLaser]

You could use a press fit or a shrink fit

 

[drawoh]

JediJew,

How accurate must they be?

How heavy can they be?

JHG

 

[JediJew]

The whole device weighs 10.5 kg and I'd like to keep it below 11. Unfortunately this connection also carries all of that weight in a cantilever as in the picture below. The bar is fixed and a collar connects it to the blue casing. Everything else is left hanging. Because of this a simple press fit won't do.

 

[JediJew]

Oh yes, for the accuracy the two center lines will have to accurate to within less than half a degree.

The reason for this is that this machine is to be used to grind the surface of a Hopkinson Bar test rig which relies on stress waves to produce data. So if the surface finish is not accurately perpendicular, then the stress waves go crazy and the data becomes unreliable.

And by the way, thanks for the feedback! I really do appreciate it.

 

[NomLaser]

I think that a tight shrink fit will still work. You might have to beef up the wall thickness a bit though. For the finish you could leave .005"-.010" then shrink them and come back and bore them together to get your ID alignment.

 

[JediJew]

Yeah, shrink fitting might work, but its beyond my means. I don't have any way of heating the cylinder, and even if i did, i would have no way to assemble it while hot. I'd like to find a purely mechanical solution if possible. Good idea though. Thanks anyway

 

[electricpete]

Maybe with somewhat looser fit you could use Loc-tite?

for the accuracy the two center lines will have to accurate to within less than half a degree

I can't picture what that means. I'm picturing concentric cylinders, the centerline (axis) has no angle orientation?


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[NomLaser]

Yes, in a perfect environment you would have the axes without angle orientation.

It might be possible to weld the cylinders and have them fit with a free running fit or a very small press fit that you can tap in with a hammer. You would then place a deep chamfer on the back edges where the two cylinders meet. You could finish it off in a lathe, like I mentioned before, to keep the ID's concentric. I would worry about warping during welding but if it doesn't need to be balanced then you would just need to keep enough on the roughing passes to allow you to finish it on a finishing pass. Is this a one off part or part of a production setup?

 

[MikeHalloran]

Have it investment cast in one piece.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Actually, I don't see anything you couldn't do in a sand casting, but the investment casting gives you a better finish and the option to maybe eliminate some machining.

With either process, you could put sunken reliefs in the thick parts (think egg crate), reduce the weight, and make the machining go faster.

Low volume? Have the investments made direct by SLA, and just burn 'em up.



Mike Halloran
Pembroke Pines, FL, USA

 

[arunmrao]

Mike has a good suggestion about casting it. You can use the cheaper and quicker sand casting route and then machine it all over. The whole process of development done in a very short time. Similar fixtures are used by us in flexible shaft grindrs. However, we have a generous tolerance and are not very particular as long as the internal shaft does not create friction heat by rubbing.


If you think education is expensive, try Ignorance.
- Andy McIntyre


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[JediJew]

Thanks for all the suggestions. Unfortunately I'm only going to be making a couple of these so casting them would be too expensive for my budget. I have turners readily available who can do the work for no charge, thats why I'm taking this route. Also (sheesh I'm being really picky here, sorry) I need to be able to get inside at a late stage to maintain the grinding tool, so I can't weld it.

I am planning to have a press fit but i need some way to ensure that it stays aligned during use. A suggestion I've been given is to taper the two surfaces and then bolt via external flanges. That way they will stay aligned and firmly held in place. However I think my turners may hate me.

What do you think? Any other suggestions welcome and appreciated!

 

[tygerdawg]

Could you machine matching tapers on the mating parts? I've done something similar with good success to easily match two parts with close concentric tolerance. There's a variety of "standard" taper designs out there.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[Compositepro]

How about press-fit and then pin them together?

 

[JediJew]

I'll have a word with the turners, but they didn't have a problem with some of the more complicated things i requested, so it should be fine. Thanks guys, Very much appreciated.

I'm still keen for other suggestions though.

 

[electricpete]

Did you consider Loctite?

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[MikeHalloran]

Both of the parts depicted appear to require a lot of stock removal to make from billet, whether bar or plate. I'm amazed that you're able to get that much free lathe time, for free.

I'm also not seeing the detail that makes it so essential that the part be made in two pieces and then permanently joined. You might ask your 'turners' if they can conjecture a way to make it in one piece.




Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

JediJew,

0.5° is not a very precise angle. Based on your picture and assuming the length is around 200mm, your centreing allowance is almost 2mm.

Investing casting from a rapid prototype is very feasible. Welded, heavy gauge sheet metal could work too.

JHG

 

[JediJew]

Its true that 0.5 of a degree doesn't seem very precise, but this is the level of accuracy required on the surface that the machine will be grinding. Obviously more accuracy is always best, but 0.5 is Worst case.

As for why I need to get inside, An offset epicyclic gear is installed in the widest cylinder. The ring gear has a diameter of 120mm, where the overall diameter is 150mm. Installing this ring gear is the problem. Additionally The grinding tool will need to be replaced at some stage so it needs to be disassemblable.

Yes its going to be made of huge chunks of material, but unfortunately little can be done about that and the turners are willing to provide the time (they're just nice people!)

As for locktite: I presume that you use locktite together with a thread. As far as I understand locktite, it increases the friction between the two surfaces preventing the connection from coming loose. I'll do more research on this.

Thanks again guys and keep the ideas flooding!