Boring bar sizing

[wildo]

I would like to bore a very small hole. I want to press fit a .1875 ball bearing (±2.5 micron) into brass as an interference fit. Engineer's Edge shows that I would want a max of .0014" and a min of .0002" for an interference fit, though it doesn't state the material. I would think that it would matter if it was steel vs brass vs aluminum, etc. So I guess I'd want to bore a hole to .1865" (Honestly that doesn't seem like much of an interference to me, but what do I know.)

I found this boring bar on ebay that states:

Extended Reach Micro Boring Bar Uncoated 0.0600in Diameter 1/8in Shank 0.250in LOC 1 1/2in OAL Series MBE (MBE-0600.250)

Can you guys help me understand what some of those terms mean? Can I assume that .060" diameter means that the cutting edge is probably half that- .030" in order to cut that radius? The shank is .125" so how can you cut a hole smaller than the shank? Wouldn't the bottom of the shank interfere with the hole as you cut? And finally- are these boring bars meant to plunge in, or does one drill a small hole first?

Thanks!

[EDIT] - since this is a manual bench lathe and not a $50000 super rigid CNC lathe- probably a boring bar as close to nominal size as possible would be better in order to not instantly break it, yes? If I'm understanding the diameter measurement correctly, then perhaps this .1800" diameter boring bar would be a better option. Really, the question remains on if the diameter spec means what I think it mean.

 

[RandyM]

Personally, I'd think you'd be better off reaming the hole rather than boring it. That is a really small hole to bore accurately. Are you doing this on a lathe or milling machine?

 

[wildo]

On a lathe- and it will be a blind hole most likely. The hole depth will be just slightly more than the bore diameter- around .190" or so.

 

[RandyM]

The .180 diameter bar is going to be way to big for you to use. The ad states it is .180 in diameter, you still need to allow for the protrusion of the cutting edge. Is there any way you can just drill it oversize and glue the ball in, or even just to size?

 

[wildo]

Well that's what I'm trying to learn. If I need to bore a hole .186, then it's unclear to me why a .180 bar wouldn't work. A #14 drill bit is .182" so I would think you could drill the #14 and then use that bar to bore to .186. Is my thinking wrong? I don't know how the terminology works for boring bars, so that's what I'm asking about.

I need the ball bearing perfectly on center. Drilling oversize and gluing will not work for my application. Actually, I'd prefer an interference fit and no glue.

 

[mikey]

Can you guys help me understand what some of those terms mean?

The shank is 1/8" in diameter; this is the part that must fit into your boring bar holder. The "0.0600" part should be the size of the cutting tip. You will need to drill a hole larger than this to begin boring. I should think a #45 or #46 drill should be large enough. Its hard to tell from the pic but this looks like a positive lead bar, which means the end of the bar is angled so that the tip leads the way; this allows you to cut to a flat bottom without rubbing, among other things.

This tool has a neck 0.250" long so that is your limit for the deepest bore you can make. The overall length of the tool is 1-1/2".

You will need a sleeve to use this in your boring bar holder. It is solid carbide so it should be sharpenable with a diamond stone.

Another brand to look at is Micro 100 - their bars are top of the line and is the brand I prefer for these tiny bars.

 

[wildo]

Mikey- that is exactly the info I was looking for! Very helpful- I can now find the appropriate sized bar with this info. Much appreciated!

 

[12bolts]

Yeah the part of the bar on that ebay listing that is round is probably only 040"-050" in dia. Want to be careful using that tool. A reamer would be my choice

Cheers Phil

 

[wildo]

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.



I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.



Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544

 

[wildo]

Oh- I guess I have misunderstood the shank diameter a bit. In my illustrations, the circle would represent the necked-down part of the shank, in which the necked down portion is less than the cutting diameter. The shank would be the area that fits into the tool holder. Ok, pretty sure I got this. The main point is that with the cutting edge on center of the actual shank, then the necked portion can be nearly as large as the drilled hole, not half the size as I improperly thought in the first illustration.