An OXA boring bar holder

[WobblyHand]

Played around in CAD and made a drawing. Think I know the answer, but why is it that I should not do this in my mill? To do this in my lathe, I will not be able to make the piece as exactly, since I have no edge finders nor fancy DRO's. I will need to eyeball the dimensions, which I have already done.

I have a 15/16" & 31/64 drill (or will on Tuesday) and a 15/16" ER32 collet. Don't have a 31/64" collet, could I use a 4 jaw and indicate the drill bit? Or should I shop for a 31/64" collet? Not sure if I should clamp down a 1/2" collet that far (about 0.016") I heard is is bad for concentricity. Also have a 0.500" reamer and the matching 7/16" collet.

You do spy a too short 15/16" drill bit in the lathe. Unfortunately I don't have a quality jobber length bit in that size, only HF. I have learned not to use HF bits for anything critical, having been burned 3 too many times.

Is there anything I need to especially watch out for when drilling this out?

 

[mickri]

Do you have a typo on the size of the drill? Shouldn't it be 15/32. Not 15/16.

The clamping range for an ER collet is typically 1 mm (.0393"). You would need to look at the clamping range for your 1/2" collet to determine if you can clamp a 31/64 drill in your collet.

You should use the lathe because using the lathe the hole will be inline with the spindle centerline. Using a mill to drill/bore the hole will not give you concentricity with the lathe centerline. The hole will be off. By how much will depend on how careful you are with your set up and drilling/boring the hole. I guess it gets down to how much time do you want to spend trying to dial it in on your mill to maybe come close to what you will achieve with the lathe.

A lathe will always drill a hole concentric with the spindle centerline. Even if there is run out the hole will still be concentric. It will just be slightly bigger than the drill by 2 times the amount of the run out. For example if you are using a .500 drill in a 3 jaw chuck with .003 run out the hole will be .506.

Also the hole for the boring bar needs to be a skoosh bigger that the boring bar. What is most important is that the hole is inline with the centerline of the lathe.

 

[WobblyHand]

Yes, typo on drill size. 15/32" and 31/64".

These are not "quality" collets, but they are ok. Guess I'd be asking the collet to compress down from nominal by 0.4 mm.
25.4 * ( 1/2 - 31/64 ) = 0.4 mm. Runout on many of the collets is under 0.0005", although I have not measured the entire set, due to a lack of reference pins. Think I will use the 0.500" collet.

Appreciate the comment on the hole needing to be larger than the bar! I'll have to mic the boring bar tonight. If I need a different hole size, it's good to know, AHEAD of TIME!

 

[mickri]

You want the boring bar to be a close slip fit into the tool holder. Not loose or sloppy.

If you have an imperial .500 collet the clamping range should be .461 to .500. That should hold the 31/64 drill with no problems but would be marginal on the 15/32.

 

[WobblyHand]

Have a by 32nds imperial ER32 collet set. Good thing I checked the carbide boring bar. It's 3/8" not 1/2". My original plan, now that I think about it, was to bore the holder to 1/2. I have a 0.5000" reamer, so the hole will be 0.5000" + runout. It will be fine.

I'll have to make a sleeve for the bar. The bar actually measures 0.3746", so my 0.3750" reamer will be fine for the sleeve, especially if there's a wee bit of runout. Sleeve won't be too hard to make, I've made them before. At least this time I have a slitting saw. Last time, I used a hacksaw. Wasn't beautiful, but it worked.

 

[WobblyHand]

Finally got the longer drills. Drilled a hole through the block, although had a false start. Someone, ahem, didn't really tighten the compound. It started to rotate when using a center drill. Fortunately, hadn't proceeded very far, so made it square to the spindle and really locked things down. Making the first hole (15/32") was slow. A mini-lathe carriage crank doesn't have that much mechanical advantage, so I was really pressuring it to get it to start cutting and hoping the tool post wouldn't rotate. If I didn't pressure it, the drill just made fairy dust and nothing was coming out of the hole. The second hole (31/64") was easy. Reaming wasn't difficult, just maintained some cutting pressure at very low RPM and slowly went through the bore. The reamer was kind of long, so part of it was inside the spindle. When extracting the reamer (not rotating) it left very light tool marks, where the cutting edges were, wasn't expecting to see that.

Must not have much runout, on this 7/16" ER32 collet, or something magic happened. Reamer is 0.5000". A dowel pin which I miked at 0.50015" wouldn't fit. It's close but won't go in. A piece of 12L14 rod which was 0.4990" slipped in nicely. So the hole is greater than 0.4990" and less than 0.50015".

Next step is to pull it off the QCTP and do the milling work. (Drill holes for screws and slit the piece.) Once that is done, back to the lathe and make a 12L14 spacer for my carbide boring bar. I was thinking of putting the spacer in the boring bar holder, tighten the screws to clamp it in place, then drill it out. (Two drills then a reamer.) More or less, just like what I just did above. When that is done, I'll make a brass adjuster nut.

 

[mickri]

Try using a smaller drill first then step up to the larger drills. Even if the smaller drills would go all the way through.

 

[WobblyHand]

Didn't feel like fiddling with the mill this evening. Made the brass nut instead. It is made of 353 brass, which I have never worked with. Kind of a chunky design, but it works. Went a bit heavy on the knurler, forming points, but none tore off. Not going to take off the points, as I'd like to be able to easily grasp and adjust the nut. Been a good day.

 

[WobblyHand]

Got it done today. Finally! Went to the mill and drilled a 4.2 mm hole through the piece, a 5.2 mm hole part way through ( to the slit ) and an 11/32" end mill for the counterbore. This is for an M5 screw. 4.2mm is the diameter for tapping. 5.2mm will allow the tap to drop to the 4.2mm section. 11/32" is just about the right size to allow a M5 SHCS through. I remembered from the last time I used this end mill that it was a little bit undersized when using an R8 collet in the mill. So I decided to use the end mill in the drill, hoping for a tiny bit of runout to open up the hole. It worked! Not best practice, but, it did work!

Did the tapping, but didn't bother to take a photo. Then I did the slitting. Somehow I managed to set the height wrong, so I raised the blade a half blade width and did it again. This is a tedious operation. Get greedy and the blade jams. So it was a little bit at a time. Eventually got it all done.

Now that the holder is basically done, back to the lathe to bore out the sleeve. Using a 0.4990" pin in the spindle, was able to line up the boring bar holder. Then cut off some 12L14 of about the right length and put it in the boring bar holder and clamped it down. Drilled out an 11/32" hole, then a 23/64" hole. Finally ran a 0.3750" reamer through the hole.


And a fit check. What a nice sliding fit!

One more trip to the mill to slit the sleeve. Set up was a little dicey, but it is done. The big block on the left side of the vise plus some assorted spacers was to balance out the little sleeve I was clamping. Used a shim of a pizza box to make it all work.

Wow. Can't believe it's done. Need to celebrate tonight with a beer. Been wanting a decent bar holder for a year, but wasn't quite capable enough to do it. Now I am! Another good day, and a project complete!