An Odd Project Perfectly Suited For A Lathe

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randyc
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An Odd Project Perfectly Suited For A Lathe

Post by randyc » Sat Sep 04, 2010 3:34 pm

Introduction:

This is a "different" project - not likely to be encountered by most of us, LOL. The project may seem trivial but the configurations and tolerances involved in the work are challenging to those of us with old machinery (no DROs - just dials on the cranks and a travel indicator).

A few weeks ago my fourteen-year-old nephew, who is interested in making a "yo-yo", showed me a rough sketch of the shape, including dimensions and weight of his ideal device. This isn't about the old wooden "Duncans", or the newer plastic ones that are illuminated with flashing LEDs. Kurt was envisioning a modern version made of metal, including features like a ball-bearing axis and plastic side pads. Apparently, these are necessary for advanced tricks, and are readily available (although costly).

Kurt did some internet research and purchased a few parts for his prospective design. One was a ball-bearing with an outer race ground in a shallow concave arc, to guide the tether string toward the center of gravity. He also obtained a pair of low friction pads (shaped like washers, made of "Teflon" or a similar low-friction material) that fit into grooves on either side of the hub. (The purpose, I suppose, is to reduce the friction of the string against the hub surfaces of the yo-yo.)

We discussed other considerations that might increase "spin time" of the yo-yo. Most importantly, the yo-yo should be configured like a flywheel, with the greatest amount of mass distributed as far away from the spin axis as possible, maximizing the stored rotational energy. The original concept morphed from a pair of solid aluminum discs (like the commercial yo-yo's) to a four-piece design consisting of thin aluminum hubs attached to rims with more mass. Here is a photo of a commercial metal yo-yo:

Image

The material for the rims needed to be of greater density than the hub material, located as far away from the axis as comfort permitted. The choice of rim materials could include most of the ferrous or copper-based alloys, since these are about three times the density of aluminum (selected for the hub material).

This is the design that we conceived (chamfer/radii of sharp corners not shown). To provide a scale factor, the yo-yo is about 2.300 diameter x 1.625 from rim-to-rim.

Image

There isn't much in the design that is dimensionally critical other than the ball-bearing interface, the low-friction pad grooves and the fit between the hubs and the rims. BUT it's important that the yo-yo be well-balanced, according to my nephew. I arbitrarily decided that non-critical tolerances should be +/- .005 inches which is not normally a difficult requirement. In this case, several angular transitions exist so it's important to account for backlash in the cross-slide and compound-slide. (Determined by measuring the lash in the cross and compound slides with a travel indicator, noting the dial reading error.)

In any situation more complicated than turning to a shoulder or facing - one or several times - the sequence of events should be considered. It's especially important when dealing with structures that aren't orthogonal or rigid - the thin hub sections of this yo-yo project aren't very rigid and far from orthoganal. Diameters of the various parts are too large to fit in a normal collet; the workpiece needs to be held in a chuck or a shop-made mandrel and it's desirable to perform as many operations as possible without flipping the part and losing concentricity, making the center of gravity unpredictable.

"Wobble" is not good - the parts need to be concentric for maximum conservation of energy and "smoothness" of spin, right? I planned a sequence of operations that seemed sensible to me, progressing through the processes of turning/boring/drilling/tapping and of course, de-burring and finish improvement. The hubs are flimsy and deflect easily under turning or boring pressures - this requires very sharp (HSS) tooling and "spring" passes on the thinner sections.

Deflection of the hub was problematical because high spindle speeds caused the O.D. of the rim to centrifugally expand. For example: without moving the cutting tool location and changing the spindle RPM from 400 to 950 produced a significant, measurable difference in the turned diameter - as much as .001 - this was NOT a spring cut - it was a measurable expansion of the perimeter of the workpiece due to the flexible cross section !

This wouldn't normally be a problem but CAN be for precise work. The lesson - to me - is that we frequently change spindle speeds when changing from roughing to finish cuts - usually increasing the speed for the finish cut This could detrimentally affect accuracy in irregular-shaped workpieces with thin cross-section. (Operators of large lathes - those capable of finish-turning the outside blade retaining ring for low-pressure steam turbines or large water turbines - are doubtless well-acquainted with this problem.)

The finish diameter of the aluminum hub was fairly critical for a shrink fit with the rim material. Keeping this in mind, had I spun up the spindle for a finish cut (after roughing at a low RPM), the "expansion" of the O.D. would have resulted in the finish diameter being SMALLER than the design value - the part would have been out of tolerance and the rims could not have been shrunk to fit the hubs. New rims would have had to be made to accommodate the hub O.D. error.

Both hubs were roughed to shape (in a 3-jaw chuck) except for the bearing and friction pad features which were finished to size (a 1/4 square HSS trepanning tool was ground to produce the friction pad grooves in one pass). The next step was weighing the hubs to get a rough idea of the consistency of the two halves. The rough-finished hubs are marred, in this photo, by handling and scratching from caliper points …

Image

FWIW, the lab balance, like the one in the photo, is used by those who load their own ammunition. The scale has the capability of resolving tenths of a "grain", up to 1000 grains. The scale is capable of differentiating weights down to .0000143 pounds or .00023 ounces, at least in theory, up to a maximum of around 2 ounces.

Image

This was an interesting exercise: comparing the calculated weight of an object with a complex shape with the actual weight.

Rather than drilling a pilot hole for the #8-32 tapped hole in the center of the yo-yo halves, the hole was bored with a 1/8 diameter end mill - for maximum concentricity with the other features. Finicky work but this is the type and scale of work for which I purchased my "Compact Eight" lathe some thirty years ago. (Most of my work - prior to retirement - involved workpieces that fit within a MUCH smaller envelope than 8 x 18 inches.)

randyc
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Re: An Odd Project Perfectly Suited For A Lathe

Post by randyc » Sat Sep 04, 2010 3:35 pm

PART TWO

The mandrel is a 2-piece device, a removable section threaded onto a fixed stub that is secured to the spindle (held in 3-jaw chuck). The removable part can be supported by the tailstock center for rigidity. I wanted to perform final finishing operations with the yo-yo completely assembled so that all features would be concentric. The hubs were secured between mandrel parts, separated by the ball bearing and steadied by the tailstock. Making the mandrel was a routine task but a few items are worth noting. The mandrel was made from 1018 CRS, cut to length on the ubiquitous small Chinese horizontal bandsaw.

During sawing - if I need to attend to other tasks - it's not practical to apply coolant to the bandsaw blade (one of these days I'll cobble up a drip-coolant). I devised this cheap trick to lubricate the blade while otherwise occupying myself:

Image

I save old candle stubs (electrical power is sometimes intermittent during the winter) and lay a stub next to the workpiece being sawed. The saw blade picks up enough candle wax to keep the blade lubed. (Of course, the saw shuts down on the completion of the cut.)

Here's the roughing pass on the fixed mandrel, .250 depth of cut on my little 8 inch lathe. A sharp HSS tool combined with sulphur cutting oil has always provided excellent results on this material. Had I the need to remove LARGE amounts of steel quickly, I likely would have used a brazed-carbide tool but going from one inch diameter to .162 diameter only takes two passes with a sharp HSS tool.

Image

The next pass produced a finish diameter of .162 for threading to #8-32UNC-2A, followed by the thread chasing head to produce the threads:

Image

The faces of both mandrel parts are slightly undercut so that the yo-yo hubs are gripped at the perimeter of the mandrel - to minimize flexing during subsequent turning operations. One might question why this would be necessary but the several different facing operations and the many interfaces (between the hubs, the bearing race and the mandrel surfaces) suggest that a problem machining perpendicular and concentric to the spindle axis is possible. Making peripheral contact - as opposed to uncertain contact across a wide surface - is likely to produce the most satisfactory results.

Image

And this is the completed mandrel:

Image

The hubs were mounted in the mandrel and finish-turned to final dimension. At this point, most experienced machinists will be wondering how these very thin-wall parts, turned from solid bar, were not singing like a tuning fork and producing distinct chatter marks.

The answer is that the parts WERE singing like a tuning fork, whenever the boring tool approached certain areas of the workpiece that stimulated the natural resonant frequency. There are techniques for minimizing the chatter marks that this problem produces. A common one is to pack internal cavities of a workpiece with putty or modeling clay - this works well for thin-section workpieces when the clay can be packed in such a manner as to prevent it being flung out by centrifugal force.

In this case, dipping a finger in cutting oil and lightly touching the surface opposite from the cutting tool was adequate to dampen the resonance and produce an acceptable finish. Here is the mandrel plus the two hubs while turning the OD for a shrink-fit with the rims. (This would have been better performed with chip-breaker geometry, eliminating the messy, stringy pile-up caused by a sharp HSS tool.)

Image

Looking through my stock of materials, I didn't find anything suitable for the yo-yo rims. After noting the outrageous prices that the hobby-oriented internet metals suppliers charge for their "treasures" (not to mention shipping costs), I e-mailed another forum member ("Atomarc", who is also located here in Eureka, CA). Stuart has been generous with his parts/materials, his machinery, his time and his years of knowledge/experience as a millwright (retired). Atomarc rang my doorbell a few days later, with 25 pounds of various materials that he considered dimensionally appropriate for the yo-yo project. Many thanks, Stuart, you're a heck of a guy !

randyc
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Re: An Odd Project Perfectly Suited For A Lathe

Post by randyc » Sat Sep 04, 2010 3:35 pm

PART THREE

For density, I selected a "naval-bronze" blank that Stuart provided … the rims were drilled and bored to finish I.D. - a single setup in a 3-jaw chuck - then sawed individually, slightly oversize. I was surprised at how finicky machining the bronze turned out to be - I substituted several different configurations of boring tools while varying spindle speed until the right combination was obtained. Incidentally, this is a situation where large Morse taper drills are very useful. Unfortunately, one requires real HORSEPOWER to take full advantage of these big drills - I had to feed lightly with my small, underpowered lathe to prevent slipping the drive belts. (In the long run, given my horsepower limitations, it probably would have been quicker to bore out the excess material from a 1/2 drilled hole.)

Image

The bored blanks were faced to near finish width but the O.D. was left rough. The bore dimension had been previously determined for a shrink fit with the hub O.D. based on the thermal expansion characteristics of the two different materials and the desired overall dimensions.

Image

A shrink fit is a good way to achieve a permanent joint between rotating parts (all performance parameters being considered). For a yo-yo, not much has to be considered, LOL ! Aluminum has a linear temperature expansion of about 23 ppm/degree C/inch while bronze is about 19 ppm/degree C/inch (ppm = parts per million = 1 x (10-6).

Desiring a simple method of shrink-fitting the parts, I put the aluminum hubs in the freezer and the rims in the oven (on a heavy steel plate to retain heat). The aluminum hub will shrink by about 23(10-6) x 25 x 2.115 (the O.D. of the hub) = -.0012 inches. The bronze rim will expand by about 19(10-6) x 230 x 2.115 = .009 inches … so when the two parts are at their temperature extremes, their interface dimensions will vary by about .010 from the nominal.

For ease of assembly, I made the fit .001 interference; that's tight enough for a permanent joint but allows a generous .009 clearance for fitting the parts together before they shrink to size. The aluminum hubs were placed in the freezer and the rims placed in the oven, heated to 500 degrees F. After 60 minutes, the parts were removed, easily fitted together and then allowed to reach room temperature. The halves were rough-turned and faced to identical dimensions so that their weights could be determined.

After weighing the halves, they were reinstalled on the mandrel. The final lathe operation of the yo-yo was completed by facing the rims flush to the hubs and turning the rim OD to finish dimension. Removing the halves from the mandrel and re-weighing, the two parts were within 5 grains of one another (0.01 ounce). The O.D. of the yo-yo was intentionally left about .125 oversize so that Kurt could test drive it before further modification. (It's always easy to remove more material and lighten the device - the converse isn't as simple, LOL.)

Image

After trying out the yo-yo, Kurt liked the diameter of the rims but felt that removing five or ten grams would be helpful. We decided to lighten the hub because removing material from the hub wouldn't reduce the spin time since the greatest mass is in the rim. At this point, the yo-yo is very difficult to fixture since the walls are quite thin and flex easily. I made a "collet-like" split clamping device from a slice of 4 x 4, bored to fit the O.D. of the yo-yo halves and split diagonally, part-way through, in the table saw. (This could have been held in the 4-jaw on my rotary table but this was quicker than mounting the sixty-pound monster.)

Image

Installing the halves in the split block, clamping in the mill vise and applying some side pressure with a "C" clamp held the thin parts so that 7/16 diameter holes could be plunged in the hubs. The intention was to produce eight holes in each hub but Kurt stopped me at four holes - he wanted to try the yo-yo again before plunging the remainder of the holes.

Image

We reassembled the device and Kurt pronounced it perfect, in terms of size, balance and most especially spin time. Here's the yo-yo prior to de-burring, polishing and coating with shellac - all of which will be performed by Kurt. The teflon washers and the ball bearing have been installed and this thing will spin for a l--o--n--g time !

Image

The project was intended to accomplish the following:

• Acquaint my nephew with a few fundamentals of metal turning

• Produce a unique device - not necessarily better than a commercial product but a custom-designed, hand-crafted product that Kurt would value because of his personal contributions

• Introduce a performance advantage by composite construction: lightweight hub + heavier rims = more stored energy. This resulted in longer "spin" time, allowing the boy to perform more elaborate "tricks"

Cheers, hope you've enjoyed reading about this small project. If the design was successful, my nephew will advise me after his next competition, LOL. I expect his interest in yo-yo competition will last only until he discovers girls - any day now, LOL.

It goes without saying that the project took twice as long as I'd estimated (mainly due to dealing with the thin, easily-deflected aluminum hub sections, resulting in MANY passes that removed only slight amounts of material.)

Randy C and my capable apprentice, Kurt

PixMan
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Re: An Odd Project Perfectly Suited For A Lathe

Post by PixMan » Sat Sep 04, 2010 7:48 pm

Nice work Randy, cool project.

In your first posts for this you show a drawing with a ball bearing in there. How is the string attached to your assembled yoyo? Is it just a slip knot to the O.D. of the ball bearing?

I'd be keen to make one with a small bearing I have. I think it's a 4mm through hole, 8mm O.D. and 4mm thick. I'll be keeping my eyes open for an Ebay deal on a 5C collet step chuck as I'm not so good with wood. ;)

Do you think the device would work any better with a parabolic curve instead of the angles you've employed? I used to do a lot of I.D. contouring like that with manual lathes by mounting a thick sheet metal (.130") template of the bore shape in a drill chuck in the tailstock, then follow the contour of the template with an indicator mounted on the cross slide.

Not that I'd ever engage in any competition, but are there rules about shapes, bearings, overall size, weight or ?? for competition?

randyc
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Re: An Odd Project Perfectly Suited For A Lathe

Post by randyc » Sat Sep 04, 2010 8:55 pm

Hi PixMan,

Some of those questions were posed to my nephew a few weeks ago, he didn't know of any limitations in weight, shape or size. Regarding the parabolic curve, he does have a couple of yo-yos that are radiused in cross-section, I don't know the shape, however. His expressed personal desire was 2.25 diameter x 1.625 wide and 65 grams total weight. We ended up with 80 grams and the thing just seems to spin forever !

Recall that my participation was limited, I suggested the composite structure with heavy rims, made the inertial calculations and served as Kurt's "hands". The shape that we produced (a very odd 37.4 degrees off vertical) was one that he selected based on a commercial product that he felt had good performance. (He has a LOT of these things and they aren't cheap - around $100 each, I think.) The string is attached with a simple slip-knot to the bearing as I recall although I wasn't paying much attention when he attached it.

I have a tracer attachment for my lathe and I have used plywood patterns (with taped edge) on it to copy - exactly as you described using an indicator (moving s-l-o-w-l-y). There is another neat trick for facing arcs that I learned from a NASA publication a long time ago. I've been thinking about posting it here for a while and probably will when there is more free time. (It's a great way to produce both concave and convex shapes - amateur telescope makers might be interested.)

I hope that Kurt will bring the finished yo-yo over sometime soon so that I can photograph it in polished and finished configuration.

Cheers and thanks for your interest,
Randy C

happy boy + toy:
P1010680.JPG

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marcofsiny
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Re: An Odd Project Perfectly Suited For A Lathe

Post by marcofsiny » Sun Sep 05, 2010 8:38 am

I really enjoyed reading this post. Projects like this are why I became interested in lathes. The problem is that I'm starting to realize that I will never reach 1% of the knowledge of the people using these machines, like you've shown,, and i should just sell my lathe.
But really, these are projects I love seeing. That came out beautiful.

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GlennW
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Re: An Odd Project Perfectly Suited For A Lathe

Post by GlennW » Sun Sep 05, 2010 9:07 am

Randy,

Great project, and it certainly looks as though Kurt is happy with the results as well!!
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

randyc
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Re: An Odd Project Perfectly Suited For A Lathe

Post by randyc » Sun Sep 05, 2010 6:47 pm

Thanks, Glen - it was a fun project but I wouldn't want to make many of them. My innate thrift makes me abhor throwing away MANY cubic inches of expensive material in the scrap bin - in this case, the finished 80 gram yo-yo required discarding over 1,000 grams of waste in the form of chips !

Marco, I hope that the comment regarding selling your lathe was meant jokingly, you'd regret it always. Don't get frustrated by lack of experience (I make the assumption that you are new to this wonderful hobby). You would acquire skills as quickly as the rest of us greybeards did if you had the opportunity to work in a shop environment. Working on your own, without guidance, is challenging but lessons learned on your own are rarely forgotten !

Forums like this one are helpful to those who are starting out in metalworking. I've found the members of most forums to be universally helpful although you must bear in mind that there are as many different ways of performing a machining operation as there are machinists, LOL. Listen carefully to what is suggested and select an approach that makes sense to you and that you feel is within your current capabilities.

I mentioned in the original post the value of planning the sequence of operations. (This isn't directed at you, Marco, it's general and something that I feel is critical for developing good habits when working on your own.) There is an overwhelming temptation for eager and curious metalworkers to chuck up a piece of metal and go to it ! And for simple projects, this is fine - not a heck of a lot of planning is required to make an oilite bushing to fit an existing shaft.

For critical parts, it's a real good idea to plan every operation in detail - actually writing down the individual steps. Frequently, reviewing the planned approach will reveal problems and it's WAY quicker to correct problems before the first cut has been made, while the part is still at the drawing stage. In fact, large shops frequently have every operation written out by a manufacturing engineer in the form of a "work traveler", "operations sheet" or similar name. As an example, here's part of the operations sheet that I wrote out for Kurt when planning the yo-yo (it was on the drawing of the yo-yo):
traveler.JPG
traveler.JPG (31.11 KiB) Viewed 4959 times
By reviewing each step of the turning operations, it became apparent that a mandrel was required to keep all of the diameters concentric. A 4-jaw chuck could have been used, indicating the O.D. and adjusting to maintain concentricity when reversing the part to perform operations on the other side. But there's no way to have held the part securely without distortion because of the very thin wall thicknesses.

But there is NO substitute for experience and common sense; one learns by doing - not by reading - at least for the most part. Reading what forum members write about various projects DOES give one insight as to the problems likely to be encountered during certain operations. Keep at it, Marco, make something every week, even if it's not much more than a center punch or a tap guide. Make yourself a nice brass-head hammer with an ash handle, for example … you'll use it and enjoy it for the rest of your life.

Cheers and good luck,

Randy C

samthedog
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Re: An Odd Project Perfectly Suited For A Lathe

Post by samthedog » Thu Sep 09, 2010 1:48 am

That was a great read Randy. I am sure the boy appreciates the yo-yo and will likely pass it on down the line when he has kids.

Paul.
Speak with the circus owner instead of arguing with the monkeys.

randyc
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Re: An Odd Project Perfectly Suited For A Lathe

Post by randyc » Thu Sep 09, 2010 6:31 pm

samthedog wrote:That was a great read Randy. I am sure the boy appreciates the yo-yo and will likely pass it on down the line when he has kids.

Paul.
Many thanks, Paul. It was fun but I wouldn't want to make them on a daily basis.

Randy C

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marcofsiny
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Re: An Odd Project Perfectly Suited For A Lathe

Post by marcofsiny » Thu Sep 09, 2010 6:46 pm

thanks randy for the words of encouragement. Just when I think I found an easy project to practice it says 'now go to your milling machine" and I'm out. Although,, I do like the idea of a brass head hammer, for putting in guitar frets!! I need to find beginner projects, maybe a metal lathe beginners book.

randyc
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Re: An Odd Project Perfectly Suited For A Lathe

Post by randyc » Thu Sep 09, 2010 8:05 pm

Marco:

As one who is interested in guitars and works on them, you may be interested in my book (it's a free download and is linked below). The book concerns the design of vacuum tube amplifiers but also has much information about guitar set-up and adjustment, describing techniques that I've accumulated and devised for the past 52 years. (I own about twenty various Gibson, Heritage and Guild archtops, a couple of Gibson and Fender solid-bodies, several acoustics and a ton of old vacuum tube amplifiers as well as a few solid-state amplifiers.)

http://www.chaski.com/homemachinist/vie ... 11365f1f2e

Additionally, here is another link that describes gifts that I've made for relatives. Some of them are lathe-specific, no other machinery necessary. These projects will give you some experience (and lots of fun). Those to whom you give them will be impressed by your skills and imagination. Not to mention that you will provide your spouse with an explanation of why you spend so much money on tools, LOL. Stress the CREATIVE aspects of the hobby … she will quickly ignore your expenses when you make a few things for her mother.

http://www.chaski.com/homemachinist/vie ... 11365f1f2e

Have fun and work safely,
Randy C

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