widening brass pinion for microscope?

My son, who is in graduate school, uses a high quality but no longer made microscope for his home studies. Recently, one of the small brass
pinions broke in the focusing unit and he asked me if I could repair it. I was able to disassemble to access the pinion which is on a rod. Unfortunately, since the scope is no longer made, the exact part is no longer available, but I have determined that a 64p 16T 0.4 modulus would fit it almost exactly except that the bore is too small (it is 3mm and I need it to be 4mm... actually a bit wider at #21 drill bit sized). Unfortunately, web searching cannot locate a pinion of the proper bore size (all are either 2 or 3mm bore), so I am thinking of getting one of the 3mm ones and drilling it out to #21 sized. The problem is that this needs to be very precise and if off by even a little will cause binding and other problems in the focus mechanism. So I pose the question: How would I go about drilling this out from 3 mm to #21 accurately using ONLY hand drill, vice, etc as I have no access to lathes, presses, etc? Thanks!
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I'd go to Edmond Scientific site and you can get parts like all get-out! They are an Optical leader for science.
Martin
On 2/19/2016 9:08 PM, Bill Baxter wrote:

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On 2/19/2016 10:08 PM, Bill Baxter wrote:

Your _fundamental_ problem is that drilling is not "very precise" in any context. It's due to the nature of the cutting action of a drill bit. The hole diameter and shape aren't going to be closer than a few thousandths and even the direction will be off, depending upon the ratio of depth to diameter.
That said, "very precise" is not a very precise specification and for a good answer, you will need to give tolerances. But you probably don't have the ability to do that. So I would recommend that you just do it as best you can free hand. There's a fair chance that it will work and if it doesn't you're only out the cost of a pinion and you'll know better what tolerances are required.
Good luck, Bob
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Bill Baxter wrote:

If he's in graduate school, do they have a machine shop somewhere on campus? In general, most campus machine shops will either do tiny jobs like this for free, or can possibly charge his department for the fix. (Or a 6-pack of beer might help it along.)
Take a piece of aluminum, bore approximately the OD of the gear, then slit and grip the gear in a chuck. If they have a collet chuck with rubber-flex collets, then this may not be needed. Then, bore the hole with a tiny boring bar. This should be a 5 minute job for an experienced machinist, or a 30 minute job for somebody in the student shop (but not too many universities have student shops anymore due to liability and disinterest.)
Jon
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Bill Baxter wrote:

What make and model scope, It might have a twin out there.
How did the pinion break? Through the bore? sheared teeth? What is the shaft made from? How is the pinion secured to the shaft?
Does the replacement have to be brass? I'm wondering if a delrin/nylon or similar plastic one would work as well.
Or find a 4mm ream and ream the hole out.
--
Steve W.

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On Fri, 19 Feb 2016 22:08:11 -0500, Bill Baxter wrote:

Given your constraints I would make a split sleeve of soft aluminium and grip the gear+sleeve in the drill chuck - and spin it to check visually if the hole is centred. If not, loosen, shift, check again. If at all possible I would put a pin that fit the existing bore in it; dab of hot glue, check the spinning end of the pin.
Then I'd set up a mirror beside the vice and grab a drillbit in the vice, probably horizontal (my vice has a horizontal V-slot at one end for this; if your does not, put a bit of soft wood on one side of the drill and bed the shank in it when you tighten the vice). I'd get the best drill I had - sharp and evenly ground. Look at it with a magnifying glass.
Then I'd drill it. I like horizontal because I can support the spinning chuck with my fingers and feel as well as see (x2; that's what the mirror is for) if it is wandering.
Don't make the drill free end short. You are hoping it will follow the hole, so let it.
I would VERY much like not to have to do this right the first time, so replacement pinions to modify - especially cheap ones - would be very helpful. I'd bet with five tries I would get at least one good enough to work.
Are you sure it is .4 module?
Because this one
http://www.surplusshed.com/pages/item/m4203.html
is 9mm outside 16T .5module 4mm bore if you trust their measurements.
At 20 cents each the fact that they are plastic becomes less important.
If yours is .4 it should measure 7.2mm outside.
64p 16t .4mm pinions are available in the rc/slot car world but they usually have smaller bores. They are also (relatively) cheap, and some are even aluminium so drilling them freehand will be easier.
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That's a tricky job even with a lathe, since the outer surface of the pinion may not be "precisely" concentric with the pitch diameter which controls smoothness of engagement.
Without a lathe you might at best achieve the accuracy of a clockmaker from the 1700's. http://www.ronellclock.com/Broaches_c208.htm
Our tech was little better than the Greeks' and Romans' until the modern lathe was invented around 1800, enabling the Industrial Revolution. https://en.wikipedia.org/wiki/Antikythera_mechanism "After the knowledge of this technology was lost at some point in Antiquity, technological artifacts approaching its complexity and workmanship did not appear again until the development of mechanical astronomical clocks in Europe in the fourteenth century."
If precision metalworking by hand was easy we wouldn't need the expensive machine tools. Lacking them you could look up the old manual clockmaking and gunsmithing methods. I didn't find very much useful there, though
--jsw
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On Sat, 20 Feb 2016 09:27:24 -0500, "Jim Wilkins"

FWIW (and this will be of little use to the OP), fixturing used for boring center holes in pinions sometimes uses a set of round pins that engage the teeth. The idea is that you want to locate off of a consistent location on the gear flanks, rather than the tips of the teeth. And you want to average those locations, so you use pins on a lot of the teeth.
When I was at Wasino (now Amada Machine Tools), we designed such fixtures for DeWalt, to bore the bevel gears used in their angle-head grinders. The final design actually used wedges -- one for each tooth -- rather than pins, but the idea was the same.
You get much smoother running of the finished gear set that way.
--
Ed Huntress

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As a one-off approach I'd put a snug-fitting shaft in the pinion's center hole and do whatever was needed to make it run true as the chucked pinion rotates, then remove it and bore larger. A reamer or broach might cut straight enough if you spin the work and ensure that the reamer doesn't start to wobble. Bob Engelhardt gave you the sad truth about drill bit accuracy. They also tend to grab and break in brass.
This is the cheapest type of tool I know of that approximates a lathe headstock: http://www.grizzly.com/products/5-C-Spin-Index/G5649
--jsw
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On Sat, 20 Feb 2016 10:20:38 -0500, "Jim Wilkins"

Wow, I've never seen a spin fixture for $50 before. They're usually hundreds, or even over $1,000.
Also FWIW, those DeWalt bevel gears were bored with single-point PCBN boring tools. Diametral accuracy was +/- 50 microinches, and the tendency for PCBN to fail in a very rapid progression, once they reached a certain amount of wear, meant that the tools had to be changed when the air gage said that accuracy was slipping beyond 20 microinches either way. The gears were (and probably still are) iron alloy PM infiltrated with copper.
--
Ed Huntress

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wrote:

The two low-cost spin fixtures I have were machined to $50 tolerances. Lapping might make the spindles more cylindrical and carefully adjusted rounded-end setscrews might take out the play to the housing. They let me move work between the lathe and mill without losing registration, and are good enough to machine keyways and wrench flats. http://www.sherline.com/1045pg.htm
I used a 3-jaw one to hold the split clamp I just made, to face off the bandsawed side. My larger 3-jaw wouldn't fit into the 1.30" ID.
--jsw
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On 2/20/2016 10:25 AM, Jim Wilkins wrote:

Here's a fellow the brought his $50 spindexer up to tighter spec's.

First of 4 videos. Mikek
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On 20/02/16 16:25, Jim Wilkins wrote:

Both the cheap 5C spin indexers I have were not machined on the base sides so that was the first job to do by accurately locating the bore and machining the base sides parallel to the bore. I remembered this as one of the high school shops I learned some machining in had a Suburban Tools 5C indexer (expensive, accurate, US made) and that came as standard with accurately machined/ground sides apparently. The instructor mentioned this and it meant for most applications you could place a 5/8" flat in the BP T slots and place the indexer up against that and you were good to go with the indexer axis parallel to the table axis. Works for me with the stuff I do and the initial machining time is more than offset by the increased ease of set-up.
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wrote:

I also lucked onto a lightly used one of these for about the same price as the spin index: http://www.use-enco.com/1/1/35195-collet-index-fixture-5c-collet-indexers.html It's more rigid and better for milling, limited mainly by work slipping in the collet.
Like you I milled the base parallel to the bore axis, and clamp it horizontally in the milling vise or upright on an angle plate
It costs a lot more new and unlike the spin index wouldn't allow the OP to rotate the chucked pinion while advancing it onto a reamer.
--jsw
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Wouldn't the original hole (3mm) in the replacement pinion be good enough to use as the reference? If the replacement pinion is all wonky well, then that sucks and the microscope runs a bit rough, vs. being broken.
If I had to do this on my sherline, this is how I'd proceed-
Fixture the new 3mm bore pinion in bored out Al scrap with a slit like suggested by somebody, put it in the the 4 jaw chuck, locate the center with an inserted 3mm shaft next to the pinion and inch or so away to make sure it's straight, then drill undersize and then with the #21 using the tailstock.
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On Mon, 22 Feb 2016 22:01:01 +0000 (UTC), Cydrome Leader

Assuming that the original center hole was accurate (and it probably is), then drilling it out will almost certainly make it *inaccurate*. Whether it will be out-of-concntric or out-of-round by enough to matter is questionable, but unknown until you do it.
Conventional drill bits do not drill straight holes, and the holes are always at least a little bit trilobal. The two lips fight each other for dominance; there is nothing to guide the drill bit. The interaction of two lips on a hole-drilling cutter produces, believe it or not, a trilobal hole. If you grind your drill bits by hand, you often can measure the trilobal innacuracy with a dial indicator.
The way to do it would be with a single-lip cutter; ideally, in a lathe or a really good mill, with a boring bar. Then the force is equal all around the cut. A single-lip D-bit will do the same thing.
--
Ed Huntress

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The very first hole I drilled on a lathe was a big fat triangle. Though a 1/4 inch hole in brass would be child's play. Wrong.

It sounds like the part just runs a gear rack on a microscope. How precise does it even need to be in the first place?
I have an overbuilt (it's German) camera macro focus rail that has a helican rack and pinion. It even has a hidden eccentric bushing to be used for adjusting the play between the pinion and rack. Maybe the microscope has this as well to make up for tolerances of parts and wear too.
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On Mon, 22 Feb 2016 23:24:18 +0000 (UTC), Cydrome Leader

The smaller the gear, the more that accuracy matters. But my son's microscope as a spring loading the pinion into the rack, and that probably takes care of some inaccuracy. If it's not smooth, however, it's a real pain to use one.

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Here's how to make a 4mm D bit: http://www.machineconcepts.co.uk/smallpipes/tools.htm "..although I know of makers who use the twist drill, I have found it very prone to wandering and the bore produced is generally less accurate and somewhat rough.."
D bits aren't limited to constant diameter holes. I made one to experiment with tapered convergent nozzles.
Manual methods for precision metalworking do exist, and were passed along during long apprenticeships.
Is the original poster beginning to see why we purchase expensive metal lathes for jobs like this?
--jsw
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On Mon, 22 Feb 2016 18:47:56 -0500, "Jim Wilkins"

That's my biggest interest in machining -- toolmaker's buttons, D-bits, lapping, self-checking gages; the works.
Some of the most fascinating stuff I learned when I was at _American Machinist_, back in the '70s when it was published by McGraw-Hill, was the methods that were used to produce clock-plate master gages to 50 millionths or better accuracy -- in 1900.
And it didn't hurt that I got to have lunch with Dick Moore. He was an inspiration.

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