OXA Boring bar holder improvements?

[mikey]

I suggest you use mild steel. 12L14 works well. I use a through slot on one side and a partial slot directly opposite the though slot; there is about 1/16" of material left in the partial slot to allow the sleeve to easily compress. So far, there has been no cracking or failure in any of my sleeves.

Robin Renzetti has an interesting design for sleeves. He uses a 3-lobe design where he mills off some material on three sides and uses a single through slot. I tried to find it but couldn't; its in one of his videos, though.

I think 10-32 is fine for this kind of holder. My holder is just over 2" long and houses 4 each, 10-32 screws. The thing is rock solid and does not allow the bar to shift. Even just snugging the screws will lock the bar solidly so if you can, use 4 screws.

Remember that the tapping drill for a 10-32 roll tap in aluminum is a #17 drill. Try it on a test piece; if it is tight, go with a #16 drill instead.

 

[mikey]

What do you mean by one size smaller? For a final 0.5000 reamed hole, the first hole is 15/32" and the second, pre-reamer hole is 31/64"?

Last question, what's a good way to check for drill straightness? Both the bit itself, and when chucked? Yes it's obvious if the error is large, but not so much when it's small compared to the drill dimension. Can you indicate (somehow) on the chucked drill? It's easy up near the chuck, but how's it done near the business end?

Yeah, whatever drill you use just before the reamer, do all the roughing with a drill one size smaller than that pre-reamer drill. This debulks the hole with a drill that is big enough not to bend or wander when you apply pressure to it. Consequently, the hole is straighter. This staging allows the pre-reamer drill to take off only a small amount of material so it is more likely to cut straight and leave just enough material for the reamer to take off easily.

I don't bother indicating drills. They will all be off enough to worry you. I use an Albrecht chuck in my spindle and a good drill from a good maker will look like a static pin when it is running and that has been good enough for me. My chuck was carefully installed on the arbor so it is accurate. My spindle is accurate. I use sharp drills before a project like this and I use a good reamer with lots of cutting oil. So far, this has worked fine for me.

The real trick, if there is one, is to not use tiny pilot drills that WILL wander. Go straight to the pre-pre-reamer drill to rough out the hole, final drill it a tiny amount with the pre-reamer drill, then ream it all in a single pass. When you rough out that hole, feed so that the drill cuts continuously but don't rush the drill. You should see a continuous flowing chip while feeling a slight resistance to the feed. Do some practice runs (not with the drills for your project) and try to get this feed thing down. Once you get it, you won't forget it.

 

[WobblyHand]

Just didn't feel I could make the part. Lack of confidence. Have been staring at a block of 7075 aluminum for 8 months. This month, decided to just do it. Ran out of reasons why it couldn't be done. Bought some decent drill bits from a decent vendor, so I didn't have to worry about off-kilter holes. After all, I was able to make the knurler, so why not a boring bar holder? So decided to finish it. More or less made per the instructions above. However, I made it with all metric hardware, to be common to all my other lathe tooling. Made a thread on it in Projects. But wanted to come back here to complete this thread.

The only thing which I didn't do is make a cut on the backside of the sleeve. Does this help much? Does it help grasp the bar better? Is there a preferred orientation for the sleeve? Same as the holder? The sleeve thickness is 1/16" now, do I make a slot that's 1/32" (0.031") on the opposite side?

Pictured is a Circle QCBI37560R carbide shank boring bar. Have the 0.002" inserts, need to find some with larger nose radius as well.

 

[jmkasunich]

The only thing which I didn't do is make a cut on the backside of the sleeve. Does this help much? Does it help grasp the bar better? Is there a preferred orientation for the sleeve? Same as the holder? The sleeve thickness is 1/16" now, do I make a slot that's 1/32" (0.031") on the opposite side?
View attachment 375108
Pictured is a Circle QCBI37560R carbide shank boring bar. Have the 0.002" inserts, need to find some with larger nose radius as well.

Not sure you need the second slit, and might be better off without it.

Think about what it takes to clamp the bar. You tighten the screws, and something has to bend to let it close down on the bar. Where is it going to bend? In your case, it looks like the thinnest spot is under the bar, followed by above the bar, followed by directly opposite the slot. So when you tighten the screws, the bottom will lift up a bit, and the top will pull down by a bit less, with an even smaller amount of deflection in the body where the dovetail is.

If you have an accurately reamed hole and your boring bars are also accurate, the block only needs to clamp down 0.001" or so to clamp the bar, and you can probably do that just fine as it is. However, if the hole were oversize or the bar undersize, you need to bend the block quite a bit more to clamp, and a lot of your screw force will go into bending the block instead of clamping the tool. Cutting an additional slot part way thru from either the top or the bottom forms a "hinge line" that allows the hole to close up without requiring as much force. I would never put a second slot on the "back" (across from the first slot), since that would encourage it to flex in the dovetail area which can't be good.

I'd say if you get a good grip on the boring bar with only modest screw tension, then you are good to go. If you really have to bear down on it before it grips, the hole must be a little big an a second slit might help. But keep in mind that the second slit reduces rigidity. In your case, since the bottom is already thinner I'd put the slot down there if it is needed at all. That way the thicker top part retains its full stiffness.

Regarding the sleeve - the wall is thin enough that it only needs the one slit.

 

[WobblyHand]

Not sure you need the second slit, and might be better off without it.

Think about what it takes to clamp the bar. You tighten the screws, and something has to bend to let it close down on the bar. Where is it going to bend? In your case, it looks like the thinnest spot is under the bar, followed by above the bar, followed by directly opposite the slot. So when you tighten the screws, the bottom will lift up a bit, and the top will pull down by a bit less, with an even smaller amount of deflection in the body where the dovetail is.

If you have an accurately reamed hole and your boring bars are also accurate, the block only needs to clamp down 0.001" or so to clamp the bar, and you can probably do that just fine as it is. However, if the hole were oversize or the bar undersize, you need to bend the block quite a bit more to clamp, and a lot of your screw force will go into bending the block instead of clamping the tool. Cutting an additional slot part way thru from either the top or the bottom forms a "hinge line" that allows the hole to close up without requiring as much force. I would never put a second slot on the "back" (across from the first slot), since that would encourage it to flex in the dovetail area which can't be good.

I'd say if you get a good grip on the boring bar with only modest screw tension, then you are good to go. If you really have to bear down on it before it grips, the hole must be a little big an a second slit might help. But keep in mind that the second slit reduces rigidity. In your case, since the bottom is already thinner I'd put the slot down there if it is needed at all. That way the thicker top part retains its full stiffness.

Regarding the sleeve - the wall is thin enough that it only needs the one slit.

I can say the holder hole is bigger than 0.4990" and smaller than 0.50015" because that's the diameter of the pins that fit and don't fit in the hole. Both measured with a Mitutoyo Quantuum Mic. Wasn't planning on additional slitting of the holder.

I was asking about the sleeve. Since it was only 1/16" thick, I wasn't sure if it needed any additional slotting to improve grip. My boring bar measures 0.3746". I reamed the sleeve ID to 0.3750" The bar just barely fits the sleeve (prior to slitting). The dowel pin I have at 0.3754" wouldn't fit the hole. Sounds like things are pretty much ok. I'll be finding out soon.

 

[jmkasunich]

I was asking about the sleeve.

Oops. I wrote a long and detailed answer to the wrong question. Sorry about that.

 

[WobblyHand]

Oops. I wrote a long and detailed answer to the wrong question. Sorry about that.

No problem. Happens to me too.

Holder works great. Bar hasn't budged. I'm really happy with how it turned out. Bores very well.

Pity my mini-lathe just seized a gear bushing/shaft. Need to find out if anything else was damaged. Fortunately both the carbide boring bar and carbide insert are fine.