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Advice on boring tapered holes

valensdiesel

Plastic
Joined
Apr 3, 2020
Hey all,

Hopefully you can give me a bit of advice on how to tackle this particular problem. I've got to place 24 tapered holes around the circumfrence of a circular object, (a wheel for a rotary punch). The taper is designed as a JT1 to accommodate some insertable punch tools which I will also have to make. This will be a one-off setup, but if you've read this far, you've probably already realized the tolerances involved are...fun. I think I've got a pretty good handle on the location issues (serendipity is a wonderful thing), so my question at this point is, how would you make the holes themselves?

I am fortunate enough that I own a Tree tapered boring head, so my plan is currently to drill then straight bore with a tenthset to the small diameter of the taper, then come back in with the tree boring head and cut the taper in a single pass. The "socket" is actually shorter than a standard JT1, so the difference in large to small diameter is only 0.018", but the snag is this is going through either 17-4 PH or Custom 465, if I can get it in an appropriate size.

I found a vendor in PA that makes JT taper reamers in M2 HSS, and I know some others here have had good results with his tools. Given that I can get a JT1 reamer, would you follow the tree taper bore with the reamer to clean it up, or do you think I should make a lap and clean the bores that way, or do you think I can get away with a single-shot with the tree head and still have good location and retention with the punch tools? The tree boring head typically makes very nice holes, but the material gives me pause, as the tree head's feed is fixed at 0.005" per rev and I think those grades of stainless are run with less feed. Also, recommended material for custom 465 is carbide (so possibly also an issue with that HSS reamer), which tends to like heavier feeds than it's going to get at the end of the hole, so I'm worried about the effect of rubbing at the bottom of the taper.

I'd appreciate any advice y'all can offer. You guys are a wealth of information and experience.

Thanks in advance!
 
It all depends on time vs money, whick do you have more of? I doubt you're going to get the needed finish from from the boring operation to jump straight to lapping. As far as the reamer, HSS should hold up fine at low speeds with proper cutting/ cooling fluids. A good sharp hss tool doesn't need (or even like) to be fed fast or hard, which is why many reamers are "hand reamers", I don't know about others, but my hands are not exactly metal hogging machines.

Sounds like an "interesting" job, good luck!
 
If you can get a good reamer I'd just bore the hole to a little under the small end size then use the reamer. A tapered lap won't stay straight along its conical shape for long. A tapered reamer used well ought to achieve a far better finish than boring also.
 
The Tree would do it, if it is still tight.
Otherwise over a "long" depth it might be a little bellmouthed.

You note that the reduction end to end is only .018"/side, so not sure how many passes, 2?
If you don't want to bump the holder out (swivel it a little more, or nudge the bit, if HSS) to accommodate, you already know that as the hole gets wider, the bar has to go deeper in the remaining travel. So there is the set up accommodation of that fact. If you can do all the roughing around once, and all the finishing around with a final tool position set, the depths should be fine.

However, I think boring close and faint reaming would be the least troublesome to hit profile and depth.

17-4 is easy and consistent.
I'd use a Borite pos insert, and make my own holder or adapt one of the Tree short holders if necessary.

I do have and use a tight Tree. But not very often. :)

smt
 
The Tree would do it, if it is still tight.
Otherwise over a "long" depth it might be a little bellmouthed.

You note that the reduction end to end is only .018"/side, so not sure how many passes, 2?
If you don't want to bump the holder out (swivel it a little more, or nudge the bit, if HSS) to accommodate, you already know that as the hole gets wider, the bar has to go deeper in the remaining travel. So there is the set up accommodation of that fact. If you can do all the roughing around once, and all the finishing around with a final tool position set, the depths should be fine.

However, I think boring close and faint reaming would be the least troublesome to hit profile and depth.

17-4 is easy and consistent.
I'd use a Borite pos insert, and make my own holder or adapt one of the Tree short holders if necessary.

I do have and use a tight Tree. But not very often. :)

smt

Yes, this was basically my thought process. For straight reaming, the best order of operations seems to be drill -> bore - > ream, so I figured that would be the thing to do, since the tapered reamers are available, and I can bore a tapered hole. Kinda figured that pre-boring after drilling would put less load on the tree head while it was doing its thing, and if I aimed for slight undersize in a single pass, the reamer would clean it and bring it to size with relatively little work being required of the reamer. In my mind (a scary place...) this seemed like the best way to get the most accurate result. I would rather do one positioning operation and then finish to size before moving to the next location rather than trying to circle back around for each (manual) toolchange.

That said, as I am sitting here thinking about it, if I go with the tenthset bore directly to the taper reamer, I can probably do it without actually needing to move the knee provided I chuck the reamer up at a reasonable length. I would guess that not having to move the setup at all would produce the most accurate location, and then it just becomes a question of whether the reamer can clear the 0.009" depth of cut it will need to at the top of the hole. When I ream small holes, I usually go with a
drill size down from the final diameter (so a D for 0.2500, for example). Machinery's handbook lists recommended step drill sizes and depths for taper pins (which are less tapered than the JT) and the recommendation seems to be you can avoid step drilling at all if the depth is less than ~1 inch (for tapers in this diameter range), which is equivalent to a diameter change of ~0.030-0.040" (much more aggressive than my usual procedure, and less than required for this job). Not sure how that applies given the difference in taper steepness, though I would think the cutting would be easier on a steeper taper since there is less surface area in contact with the flutes per unit length. This is of course why I seek help from the experts ;)
 
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Greetings from NZ
I was looking around on the Webb for where I might be able to purchase a JT1 reamer and came across this forum. I have bought a small Colette chuck for my mini milling machine. However, (unfortunately) the internal taper in the Colette chuck needs enlarging to allow it to sit further onto the quill spindle.

I see 'valensdiesel' in your original text, that you have acquired the above named reamer, and purchased it from a vendor in PA (Pennsylvania?). I'm wondering if you could supply me with the contact details of this vendor.
 
Hi valensdiesel:
How good do you believe the tapered holes need to be?
I ask because every JT taper I've ever seen has been ground.
That doesn't mean they're all made that way, but I believe self locking tapers like this have to be geometrically very good to function properly.
I would not be surprised if they have to be better than what a reamer can do, but I don't KNOW that...I'm just guessing.

The roundest, straightest most accurately positioned holes I can make with my equipment is to sinker EDM them if they are blind, and wire EDM them if they are through holes.
Are those processes an option for you, even if you have to farm the job out to do it that way?

I've never run a jig grinder, so I don't know if there is a magic way to jig grind tapered bores to tenths...I can't see how it might be done, except with a conical wheel which has a large contact area with the bore and might not work well at all.

I don't believe you can truly lap a cone...if you drop in a tapered lap and spin it, you scarf rings into the bore because the workpiece and lap don't translate relative to one another, which is necessary for true lapping.
However automotive valves are "lapped" this way and seem to work well enough for sealing...I do not know if this strategy could produce a good enough geometry match to make a self locking taper work.
I would guess you'd have to match the tapers and "lap" them together to have a prayer.
God help you if you mix them up by accident.
Also, I wonder how likely it would be to have a mandrel grab in its mating bore and weld itself in there while you're trying to lap it?
I say try before you commit...your experiments will tell you how to proceed.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 








 
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