Turning concentric bearing pockets

[Lykle]

OK, I am making a stainless steel cylinder that has two bronze bearings on both ends, inserted in pockets.
Overall size is round 55 mm and length is 60 mm, pockets are 30 mm diameter and 17 mm deep.
There is a through and through hole of 22 mm.

Now, I did the outside and it seems to be nice and clean and straight. Then I kept it in the chuck and drilled and bored the 22 mm through hole.

Now, to keep left and right pockets concentric I decided to mount on a expanding mandrel. So I made one and tested it out, it works and with the bolt tightened, the outside is nice and parallel and 22 mm.

So I mounted the part on the mandrel, held in a collet chuck. What a wobble!
It really is wobbleing (hmm spell check does not like that word, neither do I), but why?

I will go back and dismantle everything and recheck everything but I don't get it. All was straight and aligned when I started. Any ideas or suggestions where to look?

For now I think I will simply mount my normal chuck again and align the shaft. It is nice and thick and not too long, so I should be OK. Brings me to another question I have, but that is the next post.

Lykle

[RussellT]

It really is wobbleing (hmm spell check does not like that word, neither do I), but why?

Try it without the "e".

Russell

[sparky961]

Pictures would help here.  I can't quite envision what you're describing.

[BillTodd]

Best to line-bore this sort of thing:

This is how I bored the motorcycle headstock for the frame of my brother's HL500 replica

[Jasonb]

When you made & tested the mandrel was it in the same collet chuck or did you make it in the 3 jaw and then use it in a collet?

How good a fit is the cylinder on the unexpanded mandrel, if you are having to expand it too much by a bolt a t one end then it will mean teh ctylinder ios flapping around at the chuck/collet end.

[loply]

Normally when I have this situation I would just bore all the way through 30mm, then insert some kind of 'sleeve' in the middle if required to give the 22mm through hole.

This way they are 100% concentric with no real chance of messing up.

[Noitoen]

Since the out side is true, a 4 jaw chuck and dial indicator should do the job.

[vtsteam]

Seems like Lykle's question is, "Why."

A very useful question, but impossible to answer without being there to see what he is talking about. I don't want to post the "....without pics" smiley since it's obnoxious and the thread isn't worthless.

Oh what the heck.....

 

[Lykle]

Hi all,
Sorry, I should have added some pics and later I probably will, but I don't have em yet.

Anyway, the only thing I can figure out is that the central hole of 22 mm is not straight enough.
The part was lined up on the outside and I then drilled and bored to 22 mm. I think the error happened with the boring. I know, it is a boring subject but hey, it's business.

Normally when I have this situation I would just bore all the way through 30mm, then insert some kind of 'sleeve' in the middle if required to give the 22mm through hole.

This way they are 100% concentric with no real chance of messing up.

I cannot bore the full length to 30 mm as I need the meat in the middle to hold arms. Oh OK, pics it is.

When you made & tested the mandrel was it in the same collet chuck or did you make it in the 3 jaw and then use it in a collet?

How good a fit is the cylinder on the unexpanded mandrel, if you are having to expand it too much by a bolt a t one end then it will mean teh ctylinder ios flapping around at the chuck/collet end.

Yep, I made the final cuts on the Collet chuck and the fit was very close. Also, clocking the mandrel when expanded showed it to be concentric and no taper along the length. Was quite chuffed with that result by the way.

Anyway, I finished the part in the three jaw chuck by using the indicator to get it lined up properly. As far as I can measure it is pretty straight and concentric.

Now to the bronze bearings!

This pic shows the part in question. I rendered it slightly transparent so you can see what the structure is like.

 
If you guys really want, I can take some photos of the part and the mandrel, but the part is made now and besides, the wobble will not show up on the photo.