Re: Gear setup

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Posted by BillS on June 01, 2009 at 16:26:11:

In Reply to: Gear setup posted by Sean on June 01, 2009 at 13:21:15:

The FAQ at this site has a complete description on how to get change gears for a Fellows 6A. Here is the link:

The first two gears are G/T, which are the same ratio as your gear/cutter which is 180/135. Since these are too large, you will need to do some factoring to find change gear sizes you have. Just write down:

180 x 3
-------- = 0.4000000
135 x 10

and start factoring out, dividing and multiplying.

Rules? You must obey the rules. Rules keep the ratio from changing...

If you divide a numerator by a factor, you must either (a) divide one of the denominators by the same factor, or (b) you must mutiply the other numerator by the same factor.

For example, let's factor out a 5 from the 180T. This is 180/5 which is 36. Now you have 36 in place of the 180. Life is good...

But wait - you must also use that factor again. Either multiply denominator 3 times 5, or divide other numerator 135 by 5. Hey it's not my fault - blame the rules!

If you read this far, let's do 135/5. That gives us a denominator of 27. Kinda small, but bear with me.

All righty, this leaves you with:

36 x 3
---------
27 x 10

I don't know about you, but I don't like the 3 for a gear (don't care for the 10 either).

You have a number of choices as to what to do next, but I'm gonna multiply that 3 by 3, which forces me to multiply something in the denominator by 3. Sure I could divide the 36 by 3, but what's the point? I don't wanna make the 36 smaller,do I?.

How about multiplying one of the denominators?

If your eyes haven't glazed over, lets multiply the 10 by 3. Here's what's what so far:

36 x 9
-------
27 x 30

Let's multiply the 9 and the 30 by 3. Now let's multiply the 36 and the 27 by 2. Why? no particular reason, except you want gear sizes that you have in the shop and that will "reach" each other once you go to the trouble to mount them on the machine.

72 x 27
-------
54 x 90

Fellows might have played a little trick on you - Wonder why there is something called a "pitch gear"? Well, on Fellows machines I'm familiar with, the shaft (and bore) size is different for that 2nd change gear (PG) You likely have a limited number of sizes with the larger bore that can be used for Pitch Gears.

If I recall, there is a 90T PG with the larger bore. So we're gonna rearrange some terms.

27 x 72
-------
90 x 54

You might be done, but what if you go to the machine, and the 27 is too small to reach the 90? It could happen...what to do?

Just in case, I'm going to divide the 27 and the 54 by 27 (see, I'm just messing with you now).

1 x 72
-------
90 x 2

Alright, we must fix that 1 and 2. But what an opportunity! We can put some big gears here! Well, don't get carried away... remember, whatever you multiply the 1 by, so shall you multiply the 2. I wouldn't multiply the 1 by 60 unless I also have a 120T gear. Hey, you might! who knows what lurks in a pile of change gears?

By now, you should have gone through all your change gears and made a list that you can use back in the office. If you have duplicate sizes, show that. And don't forget the pitch gear, large bore sizes. Believe me, to find a missing size, you only want to go through that pile one time!

Back to the gear set - I know, let's multiply the 1 and 2 by 40 (or 37, or 48, or anything you want, okay?).

40 x 72
-------
90 x 80

While playing with the numbers, remember - you want to wind up with 4 gear sizes that you have in your change gear inventory. Check the 4-gear ratio you wind up with by comparing it to the original ratio (0.4000). This comparison MUST BE EXACT.

40 x 72
------- = 0.4000
90 x 80

If you read the FAQ, you're asking where are the other two gears. There are 6 gears in the change gear set, right?

You betcha, but I'm kinda lazy, and for your setup, I could tell that the ratio could be made up with 4 gears. So the last two change gears (D/E in the FAQ) can be any pair that is equal. If D/E is 1:1 ratio , they won't change the "gotta have" ratio. Also, if meshing is a problem, start swapping sizes.

More rules? Well sure - you want to be able to swap gears around doncha?

RULE 1: You can swap any numerator gear with any other numerator gear.

RULE 2: You can swap any denominator gear with any other denominator gear.

RULE 3: You CANNOT swap a numerator with a denominator. You just can't so don't try it!

What? We're done?

Not so fast - you checked each size against your list, didn't you? You made up the list, didn't you?

One of those denominators is a real-life big-bore pitch gear, right?

Last but not least, the gears mesh with each other on the machine, right? This can be a heart breaker...but don't worry, you'll be able to swap, double, factor, etc. in your head with a little practice. But don't get cocky. Nothing is more humbling that screwing up a perfectly good workpiece. ALWAYS double check your gearset on a calculator. (A calculator and an opposing thumb are what elevates us.)

I don't feel like this was way too simple for such detail. There is no such thing as a way too simple question - I've been known to make the simplest instruction totally obscure - totally :^)

Besides that, you (almost) said it's your first time, right?

A shout out to Don Shattuck - Hey Don (or anyone), do Fellows 6a's still use the large bore Pitch Gear? Or did I dream that?

Let us know if you have any questions.

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