OXA Boring bar holder improvements?

[WobblyHand]

I have an OXA boring bar holder that I'd like to modify the design. Just bought a 3/8" QCBI carbide boring bar any I'm a bit leary about using the stock 250-004 holder. One day I'd like to get a 1/2" bar, and I know I wouldn't trust the holder with the bar. The 250-004 clamps the bar in two little spots which isn't good. It would put a very high local pressure on the carbide bar. In the picture below, the bright edge protrudes into the 1/2" bore, thereby clamping the bar.

The only way I can see to fix this is 1) make my own holder, or 2) modify what I have.

Under #2, my options are limited. The holes are counter bored on only one side. The bottom hole is 9 mm ID, the top is clearance for 5 mm with a counter bore.

One thing I could do is: make a lipped threaded bushing to fit in the backside, out of 1/2" hex stock. By milling a 1/2" slot, say 1/8" deep, on the bottom, it would prevent the nut from rotating. Then I could run a slitting saw down the side. The holder may not quite long enough to handle 3 screws, especially if I mill the 1/2" slots, as I'd only have 0.15" of steel between the 1/2" nuts. Are two 5 mm bolts enough to hold the boring bar tightly?

Are these offshore tool holders hardened? Do I need carbide cutters?

For the 3/8" bar I'll sleeve it, as the 250-004 is bored at 1/2". It came with a crude 3/8" sleeve made of some weird, poorly machined steel. Might make another one, just because.

Does this seem like a reasonable modification, or do you see disaster ahead? This is a spare holder, I have two of them.

 

[mattthemuppet2]

slitting it and drill/ tap for 2 screws to pinch the halves together will work well, or drill and tap for set screws to hold the top of the bar directly. I made all of mine and used 4 set screws per holder - these work really well with indexable boring bars as they all have a flat on the top to set the cutting edge at the right angle. Long term I would make them for the size bars you have, short term a collapsible sleeve will work fine with either the pinch style holder or set screws, not sure about the style you have though.

When I sell my Atlas 618 I'll see if the new owner wants the left over 0XA holder blanks (dovetails cut but nothing else). If they don't I'll let you know and you can have them for postage.

 

[WobblyHand]

@mattthemuppet2 thanks for the offer. Please let me know.

I think I can easily get another 5 mm screw in the center. I'll just thread the bottom half directly, no need for a nut. It will be weird, but still functional. The holder I have doesn't seem to be hardened. Just filed off a tiny bit on a bottom corner without much effort. Guess I know what my afternoon project is...

 

[mikey]

Just an opinion but if I were in your place I would make a new holder from 7075 aluminum. It is light, won't corrode, is strong as mild steel but easier to work/machine with the tool commonly found in a hobby shop. Use a roll tap for your threads and it will be as strong as you would need.

My personal tool holder for my Sherline is made from 6061 and it has held up fine for decades. It will hold a 3/8" carbide bar at full extension taking deep cuts so I know aluminum will work. The largest bar it will hold is 3/8" but I have no doubt the holder itself can handle a 1/2" carbide bar but my little tool post cannot.

 

[WobblyHand]

@mikey Appreciate your opinion. Since I do have some chunks of 7075, I could do that. That is, after I buy a dovetail cutter, some dowel pins and a roll tap. I'm thinking a 3/4" dovetail cutter would be the right size. Can you confirm that cutter size is ok?

In the meantime, I can modify this one (250-004). It won't be pretty, but I think it will work with a sleeve. Maybe it will keep me off the streets awhile.

I'll make the 7075 holder with a 3/8" bore to fit the bar. Is the bore super critical? 2.1" is a deep hole relative to the diameter. 5.6:1. Don't have a boring head for the mill, but do have a lathe. Is just drilling ok, or do I need to ream it? (Don't even remember if I have a 0.375 reamer...)

 

[mikey]

@mikey Appreciate your opinion. Since I do have some chunks of 7075, I could do that. That is, after I buy a dovetail cutter, some dowel pins and a roll tap. I'm thinking a 3/4" dovetail cutter would be the right size. Can you confirm that cutter size is ok?

In the meantime, I can modify this one (250-004). It won't be pretty, but I think it will work with a sleeve. Maybe it will keep me off the streets awhile.

I'll make the 7075 holder with a 3/8" bore to fit the bar. Is the bore super critical? 2.1" is a deep hole relative to the diameter. 5.6:1. Don't have a boring head for the mill, but do have a lathe. Is just drilling ok, or do I need to ream it? (Don't even remember if I have a 0.375 reamer...)

The dovetail cutter I use is Japanese; very sharp, tough, accurate. In a hobby shop where we aren't on the clock, I don't think the size of the cutter is critical. As long as the cutter has enough depth capacity to form the proper dovetail you need then you should be good to go.

If I were you, I would make the bore in your holder large enough to fit your largest bar. In your case, 1/2" would be good. Make reducing sleeves to take your smaller bars so that one holder will handle all your bars.

The finish of the bore in the holder is critical. A poorly finished bore in your holder leads to chatter in the work piece so do a good job here; it will affect every single work piece you bore in the future. The bar holder hole has to be as straight as you can get it and it must have a finely finished bore so a reamer is important to use. My personal approach is to drill in the mill, not a drill press. I stand the holder up vertically and indicate it in so it is dead vertical. Then I spot it and drill it with a drill that is one size smaller than my pre-reamer drill, followed by the pre-reamer drill, then the reamer. I do not use pilot drills because they will wander. When you ream, use a lot of lubricant and feed so the reamer is cutting continuously. Make only one single pass down through the bore and shut off the machine, then withdraw the reamer with the machine off. This will give you the best accuracy and finish possible with a reamer. Don't forget to lightly chamfer both ends of the bore.

Don't forget that the length of the boring bar holder must be at least 3-4 times the OD of the largest bar it will hold. If your largest bar is 1/2" carbide bar then that bar can extend out about 5". For a typical 1/2" carbide bar, total length will be about 8" long, plus the head. This leaves about 3" that can be held in the holder so I would make the holder 3" long.

 

[WobblyHand]

At the moment, the largest boring bar I have is 3/8" diameter in steel. Friday I should receive my 3/8" QCBI and hopefully some 0.002" radius inserts. Strangely enough, I have a 0.5000" reamer, and a 0.2500", but not a 0.3750". Guess it will be 0.5000", then. What degree of care is necessary for the sleeves? Have to ream that hole as well?

 

[mikey]

Yup, ream the sleeves, too.

 

[WobblyHand]

Should the sleeve be the same material? Or can it be 6061, or something else?

For screw size on an OXA in 7075, what do you recommend? I have a 10-32 form rolling tap. The 'stock OXA' uses 5 mm. 10-32 good enough? They are basically the same diameter (and thread). I was surprised I could run a 10-32 in a 5 mm tapped hole. Slightly loose, but not binding at all. Thought I'd do 3 screws.

 

[WobblyHand]

Then I spot it and drill it with a drill that is one size smaller than my pre-reamer drill, followed by the pre-reamer drill, then the reamer.

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?