Boring an accurate hole.

[jcdammeyer]

I'm asking the question here because it's more about how to use my lathe than about the specific project.
The yellow drive spindle is held in two tapered roller bearings and turns at 1/36th of the grey drive shaft. The purple bearings hold the drive shaft centered in the yellow spindle. The small green disk inside the spindle is pressed in but can, from the threaded end be pressed out pushing the needle pilot bearing with it.

The question is how to bore the 16mm hole for the pilot bearing. It's 70mm down inside the spindle. The relief hole in front of the 16mm hole is 20mm diameter while the spindle itself is 25.4mm outside diameter.

I'm thinking that if the spindle is held in either a collet or 4-Jaw and centered that boring it with an oversize custom boring bar to say 15.5mm and then reaming with a 16mm reamer will create a concentric with the OD hole.

I'd also bore the 35mm hole for the ball bearing at the bottom of the spindle at the same time.

Does that all make sense?
Should I even bother with a reamer?

 

[RobinHood]

Here is what I would do:

At the location of the 35mm ball bearing at the bottom of the spindle, I would do a “trial bore” and pretend that’s where the little 16mm needle bearing lives. Drill, bore to 15.5mm using the custom boring bar (at the extension you need to reach the 70mm) and ream. Check the fit. It will give you a clue about tool deflection and bearing fit. Do the same thing ~ 35mm down the bore, see how things behave there. Now you have ”practised“ twice and should have a good idea of how things will go. Even try just boring to size. You know that your “practice areas” will be bored out to at least 20mm - so no harm done (no scrapped part) and you have greatly improved your chance for overall success.

Reamers like long bores, so even at the final depth, I would not bore the relief to 20mm until the bearing seat is done. The wall thickness of the spindle in that area is only (25.4 - 20) / 2 = 2.7mm. I would leave as much material for as long as possible.

Collet or 4J should work just fine.

 

[jcdammeyer]

Reamers like long bores, so even at the final depth, I would not bore the relief to 20mm until the bearing seat is done. The wall thickness of the spindle in that area is only (25.4 - 20) / 2 = 2.7mm. I would leave as much material for as long as possible.

Collet or 4J should work just fine.

Thanks. My SB lathe has 1" 5C collets.

The 3D printed spindle has a 16mm bore all the way through and when I tightened the plastic nut too much one time I snapped it in half. Glued in a 16mm section of turned down aluminium casting sprue to repair.

But good point about wall thickness. I could use 17mm as I have that size drill bit. but if it's off center at all I wouldn't want the boring bar deflected by that off center pilot hole.

I do have a pair of slightly larger trailer bearings so I could increase the OD a bit. Or move the pilot bearing further down into the thicker section. But there's side load on the cup so I'd prefer the support over a longer length. OTOH, the shaft at that point is only 12mm diameter so maybe it's not such a good idea.

Making the shaft shorter means the boring bar isn't as long. Easier to trial fit a bar turned to 12mm.

 

[Tom Kitta]

If the bearing needs to be very precise to rotational axis I would bore everything to size and not use a reamer. Reamer will create 16mm hole but that hole does not need to be super precise to the actual rotation of the part. I.e. there will be some "wobble" by say few thu. Reason is that your tailstock is probably not aligned with rotational axis of your lathe that well, not just side to side but also up and down (i.e. wear under your tailstock).

It took some precise project and some precision failures to figure this one out.

You don't need a collet, use 4 jaw.

 

[Dabbler]

One of the things not said above - and I'm sure you know it already: it doesn't matter how you hold it, but as many operations should be done in the same setup as possible, including all test cuts, reamed holes, etc.

You can ream the holes if you wish - the reamer will always follow the hole precisely if properly aligned, and will always keep the hole concentric if the amount of removal is small. I usually bore to .005 undersize and ream. Bert used to do this for holes that hat to be on center by 2 tenths, and he hit the mark every time.