It looks great. But it isn't round. One can feel a lengthwise ridge in it, even when simply handling. And when it's rotated by the mill, the ridge feels like a mountain ridge. One can almost see the ridge.
It's 0.004" out of round, measured with a V-block and a dial indicator while rotating the rod as it rests in the V. This is a far cry from the stated tolerance, -0.0000" to +0.0005".
Back it will go.
Wonder what happened, though. A divot taken out of a roller in the centerless grinder? Coupled with take-no-prisoners quality assurance?
What happened it that some happy worker in that productive paradise did not set up the centreless grinder properly.
If you check the diameter of this piece with a regular micrometer it would probably meet the stated tolerance.
This lobing is quite insiduous and can drive unsuspecting people to tear their hair out.
One might find, for example, that a steel bushing has a bore of
1.0000" when measured very carefully. The mating pin, a commercial item, may very well measure .9995" diameter when measured with calibrated micrometer. Yet the pin will not fit the hole! This happened to me during my apprentice days and my supervisor, hearing me cuss profusely, pointed out the likely problem.
As you stated the error is easily identified with a V-block and dial indicator. To actually measure the amount of lobing would require a V- block with an included angle of 120 degrees for a 3-lobed pin. 5 and more lobes are rare but possible.
Although quite precise, V-blocks are pretty confusing with irregular shapes, so I mounted the irregular drill blank in a collet in the mill, spun it very slowly, while measuring runout with a dial indicator.
It's largely one ridge and one valley. The pattern is
0_-0.002_0_+0.003_... with minor wobbles in the _0_ regions as well.
I'm of the opinion that one of two things occurred. The part was tipped into the grinding wheel at one point, or it wasn't straight and was ground irregularly as it left the centerless on the final pass. That's a common occurrence with inexperienced operators. A centerless is very capable of making crooked parts dead straight, but you have to know how to do it-----otherwise only a portion of the error is removed-----and your part can look like it.
One wild possibility-----it part was run without enough coolant----which often yields a hot spot where the expansion pushes one side into the wheel. That condition cascades almost instantly---especially in stainless. Does you part have a discolored look?
No. It's all nice and shiny. Although the reflections are deranged because it isn't even close to a true cylinder. So the second alternative looks unlikely. Unless they have a post-grinding polish step.
I talked to MSC Customer Service today. They will replace it, and pay the return shipping. They did a "bin check", and all but the one I have are OK: it seems that I was unlucky, and got the bad one. The "tech" I talked to today sounded like he knew what he was doing. He made the decision to accept the return.
I just received the replacement drill blank, one that was supposed to be OK. It is not even close to round, although it measures 0.6250" on all diameters tried. The variation from a cylinder is about 0.003", in a complicated pattern, and the reflection on the rod varies oddly as the rod is spun by the vertical mill.
I don't think that the people at MSC realize that a micrometer is not enough. Time for some schooling.
I'll be calling MSC tomorrow, this time for a refund, not a replacement
Don't feel bad, I ordered some E8 Kennanetal brazed carbide tooling for cutting some 2 tpi threads. We were making some big thread gauges for head nuts used on rock crushers. I screwed up the thread because the tooling was not properly ground and I missed it when I checked it with the fishtail gauge. The sides were bowed about half way back I should have ground my own but I figured that Kennametal could make a better grind than I could. Drove down to the MSC warehouse in Jonestown PA and checked the other ones they had in the box. Most of them were bad. They would have been ok for a finer thread but not for a 2 tpi. thread.
That smacks of an object run on center, or very near center. That's an invitation to exactly what you've described, and well documented by Cincinnati in their operating manuals for centerless grinders. I'm of the opinion that they are ground by someone with little or no experience.
I also won't discount the possibility that the parts aren't straight, and crowded when ground. As I stated earlier, you have to give the parts a chance to become a true cylinder before crowding the grind.
Indeed! But don't expect them to understand what you're going to tell them.
Well ... the right *kind* of micrometer would do. There are V-anvil micrometers -- different ones made for measuring either three/six flute miling cutters or five-flute ones. Those should show the problem. And, IIRC, MSC does sell those. :-)
It could be. The blanks have saw-cut ends, as if they were cut from a much longer centerless-ground rod. The piece I have is too short to really tell if it the original rod was straight or not.
I also notice that the typical good drill; blanks in my collection all have pointed ends, and so must have been ground individually, rather than in a longer rod that's suibsequently cut into individual blanks. Are they actually ground over centers?
True, I fear. But Don Nichols had a good suggestion - ask them to use a V-anvil mic, which MSC carries. They will probably do it just to humour me, and then be flummoxed when the ordinary mic and V-anvil mic disagree so violently. It may be a teachable moment.
Well, it did *not* go well today. I again talked to MSC's Customer Service, who informed me that MSC does not guarantee anything they sell, not being the manufacturer, and that the Import stuff in particular is not expected to meet any sort of tolerance. When I pointed out that the online catalog in fact does call out a tolerance (+0.0000, -0.0005"), she pointed me to the paper Big Book, where it is claimed that all responsibility is disclaimed. Sure enough, on page 35 only the made-in-USA stuff has a stated tolerance. So I asked her if that meant that the online catalog could not be trusted, and so should be taken down as useless. She didn't agree, but simply repeated the mantra. I was getting madder and madder and she evaded all over the landscape.
I have not yet looked, but she also said that the front matter of the big book states the no-guarantee policy for imported stuff. The stated reason was that MSC has no recourse against non-USA manufacturers. This cannot be true - money talks, and one can always cut a bad supplier out.
As a sop, she did give me a credit on the sale, so it won't cost me anything. I did say that I wanted to return the rod, being two-for-two (having returned the first rod), but she did not want the rod back, saying that the return shipping would cost MSC more than the rod was worth. Not quite true, but OK.
She infuriated me far more than the issue really deserved, and I'm still figuring out exactly why.
After anger has turned cold, a letter will be written. MSC has wasted a lot of everyone's time by their failure to be clear. Simply put, if it had been clear that there was no tolerance guarantee on the drill blank, I would have bought the made-in-US one, for $8 more. But I had no reason or way to know that it mattered. Nor is infuriating customers considered good practice.
How the ends are prepared isn't a sign of good or bad drill blanks, but when they're ground certainly does. If you feel yours were cut to length after grinding, it's almost assured that the material wasn't straight when ground, and was never allowed to get straight. You'd have to see a centerless grinder to have a better understanding of what I'm saying, and the importance of having things straight.
A centerless grinder doesn't use centers, thus its name, but how a part relates to the centerline of the grinding and regulation wheel is everything. If you grind on, or very near center, the machine tends to generate parts that are tri-lobed. Move above or below center and it will generate perfect cylinders, given the opportunity. Sounds nuts, but when you think about how the grinder works, it makes sense. It's much the same as a rigid hone (think Sunnen). The mandrel has two shoes that are opposite the stone, and are spaced such that the stone is on the centerline between the two shoes, and parallel to them. Because they don't oppose one another perfectly, they have a tendency to remove highs, and not cut where there's lows. Doesn't take any effort to round up and straighten a hole with that design. It's much the same with a centerless grinder. As long as the object is larger in diameter than the space between the grinding and regulating wheel, the difference will be removed from the part, but no more. Underneath the part there's an angled blade (HSS, carbide, or even Meehanite) that supports the part and keeps it in relationship with the centerlines of the wheels. Typically, parts are run above center, maybe
5/16" or a little more. On rare occasions, or when you may wish to run a part that is smaller in diameter than the blade width, parts can be run below center by a similar distance. The regulating wheel is angled and has a variable speed control, so that generates motion that propels the part through the machine, riding atop the blade. At the point of contact, part with the wheels, the grinding and regulating wheels must be almost dead parallel----so there is a sparking out of the part once it passes the edge of the wheel on the feed side. The succeeding parts push the previous parts out the opposite side. If the wheel is still grinding the part when it exits the machine, you get feed marks on the part.
Hope this Reader's Digest version of how it works helps you understand a little.
It's hard to tell which came first, sawcut or cylinder grind. The edges between end flat and cylinder are pretty sharp, with no burr.
The picture in the catalog shows conical ends, but the ends on the received blanks are flat.
I know that; my question was if in fact some drill blanks are ground over centers, using the conical ends to hold the blank.
Militating against this possibility is the fact that the cones are not machined (unlike the cone tips on some threading taps). And the fact that centerless grinders with continuous feed are likely to be far more productive, as individual handling of blanks is reduced.
I think I've answered my own question.
It does, actually. A lot. My summary:
If the piece fits between the wheels, a constant-diameter piece of arbitrary length will be generated, not necessarily (or often) a cylinder, as curvature is not controlled, only diameter.
If the piece instead fits against but not between the wheels, a constant-curvature surface will be generated, as this is the only surface that can contact both wheels at once. The only surface having constant finite nonzero curvature in one direction and being a straight line in the orthogonal direction is the cylinder.
I always wondered how the Sunnen hone worked as well, never having met one in person. It's not quite the same principle as the centerless grinder, I think. I've seen this principle of broad support opposite a single-edge cutter in deep-hole drills. The broad-support area rides the wall of the hole opposite to the cutter, so the cutter is forced to bull through high spots, removing them. (This from the Lindsey book on deep-hole drilling.) Likewise the one stone in the Sunnen hone.
Anyway, the symptoms I have seen in the two drill blanks imply that the rod is almost but not quite between the wheels, not far enough away from the between position to generate a proper cylinder.