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Accurate location of two concentric cylinders
6

Accurate location of two concentric cylinders

Accurate location of two concentric cylinders

(OP)
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

RE: Accurate location of two concentric cylinders

You could use a press fit or a shrink fit

RE: Accurate location of two concentric cylinders

JediJew,

   How accurate must they be?

   How heavy can they be?

               JHG

RE: Accurate location of two concentric cylinders

(OP)
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.

RE: Accurate location of two concentric cylinders

(OP)
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.

RE: Accurate location of two concentric cylinders

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.

RE: Accurate location of two concentric cylinders

(OP)
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

RE: Accurate location of two concentric cylinders

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

Quote:

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?
 

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Accurate location of two concentric cylinders

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?

RE: Accurate location of two concentric cylinders

Have it investment cast in one piece.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Accurate location of two concentric cylinders

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

RE: Accurate location of two concentric cylinders

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


_____________________________________
 

RE: Accurate location of two concentric cylinders

(OP)
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!

RE: Accurate location of two concentric cylinders

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
www.bluetechnik.com

RE: Accurate location of two concentric cylinders

How about press-fit and then pin them together?

RE: Accurate location of two concentric cylinders

(OP)
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.

RE: Accurate location of two concentric cylinders

Did you consider Loctite?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Accurate location of two concentric cylinders

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

RE: Accurate location of two concentric cylinders

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

RE: Accurate location of two concentric cylinders

(OP)
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!

RE: Accurate location of two concentric cylinders

What about a sleeve with a tight sliding fit, then drill and press some shear pins into them at assembly?

James Spisich
Design Engineer, CSWP

RE: Accurate location of two concentric cylinders

No, but you'll need a thread and wrenching features to jack it apart.

I _really_ don't like the idea of disassembling the gearbox housing for a change of abrasives... which guarantees proximity of, and probably contamination with, abrasive grains.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Accurate location of two concentric cylinders

(OP)
Ah. I haven't been clear. Forgive me.

The gears are separated from the grinder by a plate. The grey cylinder (in my first image) holds the gearing and is sealed on both sides by a plate. The grinder protrudes from a hole in this plate. The blue cylinder is used to hold the bar stationary and act as a guide during the grinding process, so separating the two cylinders will not cause the gearbox to be exposed.

hydtools: I had a look at that website and, as MikeHalloran mentioned, separating the cylinders after bonding will be tricky.

Jspisich: I like the idea of shear pins! My only concern is that after disassembly it may be tricky to line up the holes again. But that is a minor concern. Thanks!

Keep the ideas coming guys. I really appreciate it!

RE: Accurate location of two concentric cylinders

JediJew,

 I'm designing a hopkinson bar and I'd appreciate to share some information about it.  Hence, I can't understand the usage of the gearbox you say in hopkinson bar experiment. Could you clear up my mind?

thanks
lfmarini

ITA - Instituto Tecnologico de Aeronautica
SP, Brasil

RE: Accurate location of two concentric cylinders

(OP)
lfmarini

I can understand why you're confused. I was designing a machine to grind the surfaces of the bars themselves. A hopkinson bar test rig wouldn't require any sort of gearbox.

Sorry to confuse you

RE: Accurate location of two concentric cylinders

I would look at using a collect and collect chuck for you application.  If you can vist a mill supply store or get hold of tool catalogue I'm willing to bet that you can find a combination that will work.

http://www.jtsmach.com/jtswebshop/Workholding/CH038.asp

What is your material and sizes?

There are numerous companies that can provide you with very precision ground bar stock

http://www.bostoncenterless.com/AccuRod.htm

http://www.dowelspinsshafts.com/catalog2.html


 

RE: Accurate location of two concentric cylinders

How about a keyless bushing, like unclesyd above mentioned? I've used the Fenner Drive brand with good success:

B-Loc high-torque couplings:
http://www.fennerdrives.com/keyless_bushings/b_loc_keyless_bushings.asp

Trans-Torque
http://www.fennerdrives.com/keyless_bushings/trantorque_home.aspx

Also have a look at these CMT Maxi-Torque split bushings. These can be used over a key, I believe. We use a keyless one and it works great.

Keep in mind that all the taper-locking couplers have a hard time repeating axial location since they move axially when you clamp them.

If that doesn't work, how about just a cylindrical alignment boss to hold concentricity, then a bolt flange around it? The bolts will hole rotational alignment pretty good.

Beat to fit, paint to match.

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