# Bridgeport Series II Interact 2 back gear ratio

I need to know the exact back gear ratio for Bridgeport Series II Interact 2. I need to know it exactly, because I need it for rigid
tapping.
I know that it is between 8.2 and 8.42 from just looking at my dial, but I need to know the exact value, not approximate.
The specs on my spec sheet say 8.3:1 ratio, but I am not totally sure if this is this ratio exactly or approximately (as in teeth ratio).
http://igor.chudov.com/projects/Bridgeport-Series-II-Interact-2-CNC-Mill/specs.jpg
Thanks
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On 12/29/2010 12:52 PM, Ignoramus18879 wrote:

I still think this whole idea of measuring the input shaft rather than measuring the actual spindle position is a mistake. I know it is harder to do this, and it isn't easy to do on the 1J head, either.
You will need to tell what head you have for people to help. Most Series-II machines have either a 3J or 4J head.
If nobody knows, you may have to open it up and count teeth on the gears to be entirely precise.
Jon
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The 8.3:1 is the decimal approximation to a rational number, and probably isn't quite accurate enough. There is no real alternative to counting teeth. Unless you have enough encoders to accurately measure the ratio between input shaft and output shaft rotation, and have access to both shafts. I bet counting is quicker.
Joe Gwinn
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Rational, or irrational, Joe? If it's rational, he can calculate the exact value, with enough precision. If it's irrational, technically he cannot, but still, within the few revolutions a tap makes, it would still be accurate enough, with enough arithmetic precision.
LLoyd
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On 2010-12-29, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

All reductions done with gears, are rational.
i
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But you didn't clarify that it was _actually_ gears. You seemed to wonder if the ratio wasn't somehow variable. We were still wondering how a "teeth ratio" could be "approximately"... ;)
LLoyd
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:

In mathematics, a "rational number" is defined as the ratio of two integers (zero divisors being excluded). Such as counts of gear teeth. It's mathematically impossible for an ordinary gear train to have an irrational speed ratio.
By contrast, irrational numbers are those that cannot be expressed as the ratio of integers. Standard examples are Pi and Sqrt[2]. In mechanical terms, belt drives (excluding toothed timing belts) can achieve any speed ratio, not being limited to rational ratios.
As a mathematical brain twister, there are infinitely more irrational numbers than rational numbers, even though there are infinite numbers of both kinds of number. It turns out that infinity comes in sizes, a shock to all. When Georg Cantor published this in the late 1800s, there were riots in the streets (of university towns anyway). <http://en.wikipedia.org/wiki/Georg_Cantor
Joe Gwinn
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All understood and approved, Joe... I was just sort of needling Iggy (ineffectively, apparently) about the fact that he seemed to think a "teeth ratio" could also be "approximate".
It's one, or the other...
The joke failed...
LLoyd
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On 2010-12-30, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

That 8.3 ratio looks a little suspect to me. It could be a 83:10 pair of gears, but maybe this is another gear ratio that just is close enough to 8.3 to put that number into the spec sheet for for simplicity...
i
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Could be 106/20 or 212/40 (might be a more likely combo) or it could be two steps (like in a conventional back-gear relationship).
I kind of like to think that Bridgeport engineers (back when that mill was made) were _engineers_, and would've blanched at the thought of putting something "approximate" in the specs when it was an integer ratio.
LLoyd
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On 2010-12-30, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

Maybe they were engineers, but by now no one is left there.
i
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"Lloyd E. Sponenburgh" wrote:

If the head on his mill is the same basic design as the 1J, it is indeed a double reduction with the first stage being a timing belt and the second stage being gears.
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:

I wondered, I must say, but decided that deadpan was safest, if more annoying. And Iggy is still being serious.
Joe Gwinn
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On Wed, 29 Dec 2010 12:52:56 -0600, Ignoramus18879

I had to do this on my excello. The trick is to get a very large sample. Can you get encoder counts off your spindle encoder? You sure should be able to. Then go a large number of turns on your bottom spindle in back gear, say 100 revolutions. Then it only takes a second in an excel spread sheet to find the exact ratio.
Karl
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Yes, I have a counting tachometer, I can do it to a good accuracy, say 10,000 turns (just five minutes at 2k RPM). That ought to be good enough.
i
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On Wed, 29 Dec 2010 16:46:16 -0600, Ignoramus18879

My backgear ratio was 92:15
What did yours come out?
Karl
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Spec sheet says 8.3.
i
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Wait! If it's _gear_ driven, and the "teeth ratio" is 8.3:1, then you can be pretty comfortable that the ratio between the input shaft and the output shaft will be 8.3:1, too (unless it skips teeth once in a while).
(or did you not mean to apply "teeth ratio" to the word "approximately"?)
LLoyd
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