Boring bar overhang
Boring bar overhang
(OP)
I am looking for any guidlines or references on bore diameter to length ratios. Our sales dept is having difficulty quoting jobs properly. They take orders for parts that have extreme bore diameter to length ratios. In the manufacturing dept we try to stick with a 3x ratio for steel boring bars and up to a 7x ratio for carbide boring bars. However we are constantly getting orders for parts with up to a 10x diameter to length ratio with .001" tolerance and a 63 finish. Any help or advice would be greatly appreciated.





RE: Boring bar overhang
The ratios sound OK to me. Perhaps other methods would get you to your finish and geometry requirements with less hassle. Hone? Ream? Burnish?
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RE: Boring bar overhang
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RE: Boring bar overhang
The following two sites have some good information and some shops who can do the different work.
http://gundrilling.tripod.com/
http://www.business-seek.com/Business/Industrial_Goods_...
RE: Boring bar overhang
RE: Boring bar overhang
What is the diameter, length, and material?
RE: Boring bar overhang
RE: Boring bar overhang
This sounds like a horizontal boring mill or machining center operation. Is that correct? Could you provide details about the spindle/quill and coolant available?
What are the conditions at the bottom of the hole? Drill point? Flat bottom? Undercut?
Gun drilling coupled with honing would certainly get what you want for depth, size and finish. The only concern I would have would be the straightness of the hole relative to some other datum on the part. The gun drillers and trepan houses I worked with would usually quote a .001"/1.00" depth straightness allowance. This could be in the form of either staight line misalignment or a "rainbow" type effect.
Also, gun drilling can sometimes leave "rings" in the bore where a chip jams between the part and the drill. Trepanning gave me a better hole than gundrilling. This was more than 10 years ago, so my info may be dated. I would NOT say that either process will give you a perfect 63 end to end.
My vote would be to drill the hole about .060 to .080 undersize and then bore it .002 to .004 undersize trying for a 125 finish with no rings in the bore and hone for finsh off machine. Honing a blind bore requires a bit more care, but should be a piece of cake on these specs. Talk to Sunnen.
All of this depends on your HP, coolant, tooling and ability to break chips and get them out of the hole. Best of luck.
RE: Boring bar overhang
If you decide to go the way funnelguy suggests I would talk to Engis.
Quote with a been there and done that. You cannot go oversize on a bore.
"Once set to size, Engis superabrasive bore finishing tools precisely finish the inside diameter of parts in a single pass. The result is bore geometry as good as 20 millionths (.0005 mm), sizing to better than .0001" (.0025 mm) and surface finishes as fine as 4 microinches (.1 micrometer Ra.)."
RE: Boring bar overhang
black_lab_rat@hotmail.com
Any other questions/experiences I'd rather see on this thread.
Nate102/1
RE: Boring bar overhang
The part is a 20deg spur gear. Pretty basic part, just turn the blank, cut the teeth, keyway and setscrew. Need to hold .003" t.i.r. for teeth cutting. Thought maybe about a bore releif with no surface finish and more tolerance. Then just match bore the two ends.
RE: Boring bar overhang
I'm sorry, you've lost me. I thought the part had a 1.689" bore 12.250" deep. I thought the "LTB" designation meant length to bottom.
Now I'm envisioning a gear machined from a hub on a rather long hollow bar whose bore extends through both ends. It also sounds as though only a portion of the 2 ends of the hollow bar need to meet the critical bore dimensions and the center between them could be relieved.
Am I anywhere near close?
RE: Boring bar overhang
Kinda close. First of all this is just one example of a part with a long bore to diameter ratio. That is the problem that started this thread. You are right that this part has a thru bore. The work piece will start as a solid bar sawed long. Then put in a turning center and drilled, bored and turned. Agian, with the long bore it becomes a problem of boring. That is why I suggested a bore relief in the center of the part and match bore the two ends. But, a lot of the time for one reason or another the customer will not let us modify their design. Sorry for any confusion.
RE: Boring bar overhang
If you can get precision with the drilling/boring operation go for it even though it may cost a little more it sure saves in the long run.
If you plan to relieve the center section as stated, the Engis hones are well suited for this type application. If you look at the section on Helical Hones they will even hone with a cut keyway.
RE: Boring bar overhang
The ability to machine most of the part in a single chucking is a big plus. I would definately try to get the bore machined for concentricity in the same setup as turning other critical features. Working the part from 2 ends will almost certainly unecessarily compromise precision and add cost. The only reason to work the part from both ends is to help alleviate the overhang problem. Stop trying to get a 63 finish and +/-.0004 with the bar.
I can see how you might want to face and turn the first end, and drill halfway through the part. Then flip the part end for end, and finish machine the rest of the part. You will have fewer problems drilling.
You are throwing away quality and money and turning center time. Bore for position, hone for finish and tolerance. You will be amazed at how quickly you can achieve your objectives.
A hone is a funny animal. You have to work with one to understand how much money you can make (save) with one and where it may help you in other parts. Similar to CAD, waterjet and vacuum heat treating, IMO.
Find a local shop with a vertical hone and some expertise and pay them to show you the processes' capabilities. Or have one of the manuf's train an operator AND an IE. I wouldn't farm out the honing operation. Better to keep control of the quality and lead times in house. Hones aren't that expensive, are trouble free and easy to learn. You won't need a dedicated operator unless your work rules require one.
Last, I personally have stopped trying to get customer's to produce sensible designs. Producing overdesigned parts with practically no tolerance is what I get paid to do.
RE: Boring bar overhang
RE: Boring bar overhang