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Thread: Looking for fraction chart
09-30-2010, 07:52 PM #1
Looking for fraction chart
I am looking for a chart that shows uncommom fractions to decimals.
I am trying to set up gearing on my gear hobber. It says to look in the book of decimal equivalents for these fractions.
Is there a book of these numbers published or is it online?
09-30-2010, 08:05 PM #2
Offhand, I'd say it should be in Machinist's Handbook. If you don't have one, you would serve yourself well obtaining a copy.
Here is the first online fraction/decimal calculator I found with Google.
Fraction to Decimal Calculator
From the same site, a printable spreadsheet up to an inch.
09-30-2010, 08:07 PM #3
09-30-2010, 08:23 PM #4
It seems to me that you could make a spreadsheet. I take it you are looking for a certain gear pair to get close to a certain decimal, right? Should be straightforward. There! I just did it.
I just have a row with numbers 1 to 130, and a column with the same range, and I divide the row by the column numbers and create a big matrix of decimal ratios. Voila! You've got your chart.
I made it a bit fancier: I have a cell you can put a ratio into, and I use conditional formatting to turn any cell that is close to that ratio yellow. So if I'm looking for a specific ratio, I can find the driving/driven gear teeth.
09-30-2010, 08:28 PM #5
I'm really trying to not sound like a butt here, but is there any reason a calculator just won't do?? It sounds like you have fractions and want decimal from that. I may very well be missing the point. Or did you actually want to go from decimal to fraction? Many calculators do that too. Or if you just want papers with detail, guess you could do the excel route as was suggested
10-01-2010, 02:33 AM #6
The advantage of a spreadsheet is that you can print your own look-up table and pin it to the wall. If you want fractions in your Excel spreadsheet, right click on the column, row or cell you want the fraction in, then select:
format cells > fractions > [select from list]
It will accept fractions as input, or generate fractions as output. There are standard options, or you can do pretty much anything with the custom option. For instance, if you want your input column in the form 1/87, 2/87 etc, do this:
select the column by right clicking at the top > format cells > custom > highlight the contents of the Type: box > type in ##/87 > OK
You don't have to type in all those fractions. Do the first two, highlight both cells , click on the square in the bottom right corner and drag it as far south as you want. The column will fill automatically.
If you go beyond 87/87 and the improper fractions offend you, then format the cells with # ##/87 and it will give them as proper fractions.
Incidentally, those who have been reading the 'Let's go metric but how?' thread and feel driven to invent their own tradition for formatting the date - for instance putting the year in the middle - can do it with the custom option.
10-01-2010, 01:26 PM #7
I have never set up a hobber, but I have set up helical and differential indexing setups. There are tables in Machinery's handbooks that help with finding the required compound gear setups to get particular lead and division numbers. Whether there is some way to engineer the number facts in those tables for use on a hobber is the question.
It really does not do a lot of good to convert a fraction to a decimal when you are looking for compound gearing. What you need to do is expand the numerator and denominator (of the final ratio) with factors until you get some magic combinations that exist in your set of change gears.
That number 87 in the denominator doesn't look good for factoring, but you can, of course expand it with factor multipliers.
What we'd need to know is what is the default ratio of the hob to the work spindle at some given ratio of gears already on the machine. That would only be a starting point.
10-01-2010, 02:10 PM #8
A TI-30X calculator will go from fractions to decimals and back and forth, and will do calculations in fractions or decimals.
About $10 at back to school prices.
10-01-2010, 02:14 PM #9
Back in the day I used a slide rule to reduce ratios to gear pairs. VERY quick. There's also methematic factor tables.
Last edited by Forrest Addy; 10-01-2010 at 04:37 PM.
10-01-2010, 02:24 PM #10
10-01-2010, 05:04 PM #11
10-02-2010, 04:49 AM #12
Send a list of the gears you want ratio'd, and I'll make up an EXCEL based chart. I'll send the info back to you in the form of a spreadsheet and a pretty power point chart you can laminate and hang up next to the machine.
10-02-2010, 05:40 AM #13
I appreciate the offer. I have a friend that is putting together a spreed sheet on the computer for me. If it does not turn out the way I want I may call on some help.
10-02-2010, 05:58 AM #14
I am surprised that your book says "look in a table of decimal equivalents.
The last thing you want is a decimal equivalent of a required gear ratio.
As HuFlungDung pointed out what you want are factors, or whole numbers as nominators and denominators which when multiplied together give the required ratio.
Its similar to the calculation needed for a dividing head .
For example to find the turns of the handle on a dividing head with a 40 to 1 ratio to make 7 divisions to cut a 7 leaf pinion for a clock.
Sometimes an engineering student would come to me puzzled saying
"I know I need to divide 7 into 40 and the calculator gives the answer 5.7142857, but how do I use that to determine which plate and how many holes do I pass with the crank pin to get the 0.7142857's worth of a turn ,after 5 full turns.."
The answer is of course that he must throw the calculator away and pretend he is back in his junior school . 40 divided by 7 is 5 and a remainder of 5 = 5 and 5/7ths.
So the handle needs 5 complete turns and then 5/7ths of a turn .
There is no plate with just 7 holes, so any plate with a multiple of 7 in a row of holes will do. For instance a row of 21 . So 15 holes on the 21 plate is the same as 5/7ths.
Similarly 20 holes on a plate with a row of 28 holes .
One clever dick came up with a calculator that actually had a function to give answers in fractions, but I still encouraged the schoolboy method with pencil and paper.
I have Googled "hobbing tables" but as yet have found no hits.
10-03-2010, 06:28 PM #15
10-03-2010, 08:02 PM #16
Thanks for the info on the book. I have already ordered it.