"Drilling" with an end mill?

The problem you have with an end mill is its ability to cut on the periphery. That always leaves the potential for the end mill to cut oversized, especially if you're cutting to center. Drills, especially when not drilling a full hole, will generally drill quite near size, often even undersized. An end mill will remove anything that it touches, and often more, especially when you load one flute and it tries to wing. Given a very rigid setup, an end mill, held in a collet, is likely to yield a better hole than a drill, assuming the hole is not deep, and the end mill is not cutting to center. Again, that's working in a mill. In a drill press, all bets are off.

The question as posed may not have one good answer. There are extenuating circumstances in which what may be good for one scenario may not be good for another. Opening holes in thin material are an excellent example. If hole size is an issue, the rule of thumb is to never drill the hole in one operation. The best policy is to open the hole undersized and then either drill it again, ream, or bore the hole. In the case where you have an end mill, the hole is shallow, and you have already opened up the hole so an end mill is not cutting to center, it may even be a good choice.

A reamer will generally yield a pretty decent hole, but they are prone to cutting out of round holes, although just by a tenth or two. The hole, for all practical purposes, may appear round, but will have a multi-fluted configuration much like the reamer. It doesn't always happen, but it is fairly common. The ultimate hole would be bored, and if ultra precision would be desired, it would be honed or ground. Does this help?

Harold

I recently ran into this problem and was kinda' surprised. What's going on? Is it like a two flute drill making a three sided hole so a six flute reamer makes a seven sided hole? And what is the best way to prevent or minimize it? Are spiral reamers more or less prone to this than straight reamers? My technique is to feed the reamer slowly with lots of lube using the tailstock with a dead center to align it with the drilled hole while the part is still in the machine.

Richard Coke

Yeah, that's the basic principle. An edged cutting tool that cuts holes tends to make a lobed form that has one more lobe than the number of cutting edges.

Except for drills and badly ground reamers, the effect is so slight that you often can't measure it. But there are some animations around that show what happens. I can't think of where I've seen them; they're part of some tool company's sales presentations, or maybe they were some academic study from a university.

Single-edged cutters don't usually do this because of the way they're supported on the back side. It's something that happens when the only support for the cutter in the hole is the cutting edges themselves.

'Don't know the first question. As for spiral reamers, yes, they're less likely to lobe a hole.

Ed Huntress

Less likely because of the spiral. It tends to cancel the ability of the reamer to generate the pattern, but it does not eliminate the possibility. You'll often see the pattern at the beginning of a hole, only to see it disappear gradually as the hole gets deeper. The spiral pattern of a reamer provides positive rake, so reamers of that configuration often cut somewhat better. Running a hone (rigid type, like a Sunnen) will teach you all you don't want to know about what lousy cutting tools reamers are. My personal opinion is that they are not a very good cutting tool, but certainly better than twist drills.

Harold

I'm not sure I have an answer for how to control the phenomenon, but my way is to not use reamers unless necessary. I bore almost always, and for more than one reason. You have complete control over a boring bar, which will yield a superior hole almost every time, and where you want it instead of where it ends up. I tend to look at reamers as a production tool. They are fast, and when properly applied yield pretty decent results, but they leave a lot to be desired if you're doing custom work, where you get one shot at a hole and it has to be right. YMMV. Where reamers are concerned, I don't espouse my shop theory as one everyone should apply, nor endorse.

Harold

I can refrain no longer here.

I need to replace the wrist pin bushings in a motorcycle engine, because the existing bronze bushings have become loose in the rod end.

The rods are not removeable from the crank as it is a pressed up crank with roller bearings.

The wrist pin diameter is just slightly under 20 mm, and the tolerance on the fit of the pin in the bronze bushing is no more than 0.001 loose.

The new bushings I have will be pressed in with about a three thou press fit, and then they will be about five thousanths too small, so will have to be opened up to the correct size.

So my thoughts are, ream, bore, lap, hone?

I am pretty sure that a 20 mm reamer would make the correct size bore, but have for the time being begun a fixture to hold the rod end for boring.

I am concerned that the surface finish might be poor as this would be done on my hardinge horizontal, using a relatively inexpensive imported boring head. The dial on the head is direct reading thousanths, so to hit within a thousanth I have to split a division.

I had thought maybe I might consider boring to within a thou or so of finish, and then fit some kind of split lap in the spindle of the machine and do the rest that way.

I don't have access to a sunnen hone, and the rod won't come off the crank, so doing this would mean handling the crank and both rods at the same time, and keeping the roller bearings in the bottom end clean from the honing oil.

Also, I have no idea if bronze bushings can be honed that way.

Any comments, from Harold or others, gratefully accepted.

Jim

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I'd first check with a dealership to see what their normal procedure is.

If I had the job to do, I'd not hesitate to do the complete operation on the horizontal, Jim. You should be able to hone a nice radius on the boring tool that will take very light passes with fine feed. You can expect a decent finish and you can creep up on the size with almost no difficulty, even with a head that is not graduated in tenths. I've never had that luxury. In fact, I own two boring heads, both of them made by Bridgeport, and they are less than an ideal head, yet they have served me exceedingly well. When working close, I rarely screw with the slide locks. I set them with some drag, than make adjustments without changing them. That way you get consistent feeding with no surprises.

Seems to me the biggest problem you're going to have is in setting up the rods so they are at right angles in two directions relative to the crank, yet are well supported for the boring operation. Could prove challenging, but certainly possible.

I'm of the opinion that if you took your rods to a rebuild shop, they'd probably hone them, but perhaps not while still mounted to the crank. The mass of the crank assembly could influence the wrist pin hole, changing its orientation to some degree. An alignment fixture of sorts might permit honing in the assembled state, however. Dunno. The way a hone works, I'd have a hard time imagining that scenario.

They hone brass routinely, and apparently with considerable success. The big concern is that the brass gets charged with abrasive, but hard stones are used that shed very little, so it's not much of a problem, especially if the hone is equipped with a filter.

I recall that I used to have spindles honed after installing new bushings for the king pins on the old GM cars I screwed around with as a young kid. Honing, when you don't have a lot of weight to contend with, is one of the easiest ways to keep things in proper alignment, assuming you have a mandrel long enough to do both ends (or all the mains, like in an engine) in one operation.

Sorry I'm not more help, Jim. Some things are a bitch and you have to tough your way through them. I think this will prove to be one of them. Why don't you let us know how you resolved the problem when it's done?

Harold

Just as I suspected.

On one of six parts I reamed I could feel a slight "bumpity-bump" as it rotated on a shaft that turned out to have a very slight irregularity at one spot. If the shaft had been smoother I might not have noticed it at all.

Richard Coke

Yeah, that's what I was going for, pretty decent results. This is for gears to make an all spur gear differential for a pedal car I am building for my grandson. Some might argue that it doesn't need a differential but I say it does. Maybe even a planetary two speed

Richard Coke

Ha ha ha. The bike was built in 1959, and most dealers don't even recognize it as a bmw when I drive in. There are a couple of reliable machine shops that work on these in the US but I would rather not send this crank out if possible.

The plan was to create a fixture to hold the rod end - by placing a 3/8 thick steel plate vertically in my vise, tombstone style, facing the spindle. Then I attached another, shorter steel plate to the spindle side of the fixture with four screws, and bored a hole right through the both of them, in assembly.

The hole was to be a thou or so larger than the OD of the bronze bushings, which are long enough to protrude from the rod end about a tenth of an inch on each side. To I could then place the rod end between the two plates, clamp them together using the screws, and the OD (and hence the ID) of the bushing would then be right in line with the spindle axis.

By mis-reading my funny scherr-tumico micrometer I bored the hole exactly 0.010 too large in the plates. My wife said she could hear the cussing all the way upstairs. The good news of course I only wasted a few pieces of scrap steel from my junk box. No damage to anything on the crank.

If it doesn't work I can always press out the bushings and put new ones in, they're only a few bucks each.

I had not thought to put a small radius on the end of the boring tool - right now it's pretty sharp and I can see the ridge pattern no matter how slow I seem to feed in.

I had always thought they simply held the rods by hand and worked them on the stones, in and out. That might be for non-critical applicantions of course.

Will do, I may even take photos if I think of it.

It's a high miles engine, and I was almost tempted to simply green locktite the bushings back in place. But they were so cheap I thought I would have to try it or feel bad whenever I rode the bike after that. I notice you steered around the reamer issue.

Would you recommend against reaming them in this instance, given the 0.001 max clearance issue?

Thanks again, and I *still* owe you a catalog, have not forgotten about it. I seem to be tripping over them for months at a time, until I need an extra, then they evaporate!

Jim

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Wow! Is this one of the shaft driven bikes? Mind you, I'm hardly a fan of them, nor do I know much about bikes in general, but I recall that particular model because of the shaft drive. I'd never heard of a bike being so propelled until the Salt Lake City police department rode them. Sigh! So long ago. A buddy had a single cylinder model, don't recall if it was shaft drive or not, but it was that same time period. Three of us rode it out of camp one night. (Summer camp in the National Guard). Talk about a load for a small bike. One of the things I remember about them, single or twin cylinder, is that they ran so quietly.

Aha! A four stroke cycle engine. Much better! From this I gather that the crank has both rods in line. When one cylinder is at top dead center, it has just exhausted, the other is ready to fire. Correct? That means that the two holes of the rods can be done at the same time. How far apart are the two rods? This just might lend itself to honing. The two rods honed in unison would share the load and prevent losing the alignment I spoke of. As for keeping honing oil out of the bearings on the crank, that's just a matter of keeping the crank higher than the mandrel. A good washing afterwards would guarantee the bearings were not contaminated. No big deal. If there's only a few thou to come out, it goes pretty fast, so you might be able to press the bushings in and hone with no machining at all.

Would you do one rod at a time? Not both of them, somehow held together? Both at the same time presents a few problems in holding.

I think this is one place where my theory of doing it right, spending the extra time, would really pay off. In order for the plates to do a respectable job, the clearance on the bushings would have to be held to a bare minimum. The one problem I see is that the amount of press is likely going to provide two different sizes, one the size of the bore of the rod, the other the size of the bushing before pressing. I recall you mentioned a

3 thou press. Could prove a little tricky getting the dead alignment needed to keep the bushings in the same plane with one another, but honing would automatically address that issue. You may be well served to have your wrist pins in hand and trust the crank to a shop with a Sunnen and a long mandrel the size of your pins.

Plenty done to your ego, though. I'm harder on myself than anyone could possibly be when I make mistakes like that. I generally break into a sweat and my face gets red. Real red. I don't like screwing up.

Hard to argue with that. I think you have a little wiggle room here, so you'll end up fine. You may go through a learning curve, but it will be worth it.

Funny, some time you're too close to a situation to see it clearly. The slightest radius and your bore should turn out fine. Keep it small enough to prevent chatter.

That's exactly how it's done unless they have a power stroker, which does the same thing for you, you don't have to do the stroking. It's important to know how far off the mandrel to take the part being honed, and it must be the same both front and read. The rule of thumb is to hone to the center of the part with the edges of the stones in the mandrel. The rate at which you stroke is somewhat critical, too. It sort of comes naturally once you've been on one for a short time. The whole idea is to use the stone at a constant rate. It's amazing how straight and round a hone produces a bore. It really is!

That would be great! I photo of the crank right now would surely have helped. I'm having to imagine some things. I don't do well with written descriptions. Maybe that's why my posts are so damned long. I try to say things such that they make sense to me. I often run out of space!

Yeah, I don't think a reamer application would be in your best interest, not in this case. Mind you, there are probably a lot of people that might choose to argue the point, but my experience tells me I'd not do it. The thing you have to come to terms with in this case is that a reamer is highly unlikely to give you a straight, round hole. If you could be happy with a hole like that, why not just keep using the one you already have? Because of the limited movement of a wrist pin, they can usually be set up so closely that they won't fall out of a rod by their weight, yet will move freely. You can easily do that with a hone, and you might hit the size with a boring head, but I'll damned sure guarantee you you'll never hit it with a reamer, not ever, unless by accident, and then the hole won't be round and straight, so it would quickly open up anyway. I hate it when that happens.

Absolutely. I'd recommend against using a reamer for any high speed fitting that is critical unless there's simply no other way to get there. . I think my reasons above pretty well represent my personal feelings.

I get the idea you sort of like this bike. I think it's worthy of your best effort, not your worst. Think of how proud you'll be if you can restore it to a like new condition. You'll be proud to tell others how you did it instead of apologizing for choosing a compromise method.

Don't thank me until we find out if any of this works. Then you can heap on the praise!

Regards the catalog, no hurry on my part, and I'm sort of glad you brought it up. When I didn't receive it I didn't want to nag you, yet had you shipped it and it never got here, you'd have thought me rude for never having thanked you. I hope you realize I'm not like that. When and if the opportunity presents itself, Jim. I'm just grateful you care enough to look out for me.

Regards,

Harold

Hey, to hell with them! I'm on your side.

I've always enjoyed gilding the lily, and what better way than to spoil a well deserving grandchild?

You going to show us a pic of your hard work? Sounds real interesting. I'm trying to visualize a spur gear differential.

Harold

It's mate, a 1969 R60US model (has telescopic forks, rather than earles forkes):

That bike got brand new bottom end bearings and rods, with brand new wristpin bushings in them, by Ed Korn of Cycleworks in Madison WI. I was hoping to keep the original rods and bearings in this one (1959) simply because they have given me no cause for complaint at all in the years I've owned it. It's come apart this past winter because there is more rust on the frame than paint, and the seals in the engine were all hard as rocks so it leaked oil badly.

I had hoped to get it back together by this spring, but work's been a bear lately.

Yes, one at a time.

That is why I had left a generous amount in the fixture - because I was going to finish to several thou undersized, and only then press the bushings into the rod ends. That way I could mike the bit protruding out, and finish the fixture for a nice snug fit.

Imagine my disgust when the bushings, held up to the bore in the fixture, dropped through with ten thou or so to spare!

The eyes on the rod ends are finished nearly to exactly the same size as far as I can tell. If I go ahead and do this by boring, I think I'm going to borrow the federal dial bore gages I have at work - they make things so much nicer than using telescope gages and a micrometer.

Jim

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Thanks for the link. It brought back to memory the fact that the cylinders are opposed. So much for the crank with "rods in alignment with one another" theory. I told you it was a long time ago.

Looks like you may have a little time left before spring. From the weather reports we're getting here, it appears you folks in the east have had a brutal winter and it's not warming up all that much yet. . We're basking in temps near the 80's, and no rain. Talk about highly unusual. We should be up to our butts in rain and temps in the high 50's.

Now that I have a clearer image of the project, I think you can do it as you suggested. With a bit of a radius on your tool, you should get a decent finish, and achieving size is just a matter of playing with it a little. The dial bore surely is more positive in approach, so I'd recommend it. It would be all the better if you can get a ring to set it, but a gage block stack will serve well if used with care. I'd shoot for a snug fit, plate to bushing, so you minimize any tilting of the rod.

Good luck, Jim. I have every confidence in you.

Harold

The California Hiway Patrol and a number of other police organizations currently ride BMWs.

Course they also run the Stealth Camaros also.. Only in California.... Also.... Can You Top This Department: With a slim field of six push trucks present for the Midgets, two racing friendly California Highway Patrol officers present used their two black and white CHP patrol cars to push off Midgets during the evening. They give precision-driving exhibitions at Irwindale Speedway periodically on Saturday nights and recruit potential new officers at the track. The push bars in front of the CHP cars used to get disabled vehicles off local freeways served the purpose well as did their lights and sirens which added to the already festive program. The CHP cars launched USAC Midgets from the pits on the backstretch track access road and were visible from the grandstand.

It gave local fans something new in racing to amaze them. Both cars were there with an OK from the CHP brass who saw the lack of push vehicles as a new opportunity for a little direct community service ? Both law enforcement vehicles got their gas tanks filled up after the event to offset the fuel used during the night. The fans loved the seeing the CHiPs work, and it was said that the Midget drivers were vying to see who would push them off, asking their crews to wave one of the big black and whites to line up behind them!

Gunner

"By calling attention to 'a well regulated militia', the 'security' of the nation, and the right of each citizen 'to keep and bear arms', our founding fathers recognized the essentially civilian nature of our economy. Although it is extremely unlikely that the fears of governmental tyranny which gave rise to the Second Amendment will ever be a major danger to our nation, the Amendment still remains an important declaration of our basic civilian-military relationships, in which every citizen must be ready to participate in the defense of his country. For that reason, I believe the Second Amendment will always be important." -- Senator John F. Kennedy, (D) 1960

I'll post some pictures after I get the carrier and cross shafts completed. In the meantime I'll try to describe it. Picture two large gears that rotate about a common axis but are not connected to each other. These gears (the side gears) each have a shaft extending out to one of the wheels and are seperated by a distance equal to the face width of one gear. A carrier holding six pinions, grouped as three meshed pairs surrounds the side gears and rotates independently about the same axis. Three of the pinions mesh with the left side gear and the other three mesh with the right side gear. The pinions only mesh with each other in the space between the two side gears so that the left pinions do not engage the right side gear and vice versa. Power is input via a sprocket or toothed pulley to the pinion (or planet gear if you prefer) carrier and power goes out from the side gears via their shafts to the wheels. For a given gear size this type differential is usually narrower than a bevel gear differential but is larger in diameter due to the cross connection being outside the diameter of the side gears rather than inside. It's appeal for me lies in not having to cut bevel gears since my only gear cutting tools are a Bridgeport and a lot of patience. But I like to make gears, it's even better than cutting threads .

Richard Coke

Richard,

Using gear rings with internal teeth can reduce the diameter of the spur gear differential.

Jim.

That's generally true for planetary gear sets with 2 or 3 planets but in this case with 3 pairs of gears the minimum outside diameter is achieved when the side, or sun, gear has the same pitch diameter as the planet gears. Lay out seven coins or washers the same size to see how it works. In that case using an internal gear instead of a central external gear would actually make the arrangement larger in diameter. The real problem, however, is my lack of any machinery to cut internal gears. If I ever run across a nice small shaper for a good price I would get it just so I could try my hand at making internal gears. I am fascinated by epicyclic gearing but I'm limited right now to what I can do with external gears.

Richard Coke

Richard,

I take your point :-) I must admit that I was thinking of large diameter internal toothed gear rings and (comparatively) small diameter pinions. I can see now that there has to be a limit on what you can place inside a gear ring.

I've actually built a spur gear differential with only one set of pinions, which worked quite well. I was using it to drive a single eccentric valve gear, and by rotating the pinion cage you could reverse the gear.

Jim.