Need help with dividing head...

B

Brad125

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Hey guys,
Tonight while trying out my dividing head using the dividing plates for the first time instead of using the degree marks to cut 4 equally spaced flats on a 3.5” piece of steel, it didn’t not come out equally spaced. 3 of the flats look ok but the 4th is way off. I used the correct plate and the correct number of revolutions and holes per half turn. In my case it was a plate A 20 hole and it states to crank the handle 20 + 10/20 to get 4 equally spaced flats. However, this was not the case. I am stumped, each time I try and start a 0 degrees and turn the handle it always ends up short of where I started???? Something doesn’t seem right, I have watched many videos and haven’t seen this problem. I started with the rotary table set on “0” just for reference and none of the flats line up with the degree marking I was using for reference, in my case 0, 90, 180, 270, 360. I have no clue what’s going on. Can somebody please help...
 
Ok, let's start out with the basics:

How many turns of the handle does it take to rotate your table one full turn? At a guess, I'd say it's 90:1, which is fairly common.

Assuming I'm correct:
90/4 is 22.5, so for each position you need to turn the handle 22.5 times. You can use any even numbered plate pattern to accurately advance the handle half a turn, but in this case, it's 22 full turns of the handle plus 10 holes (half the holes on the plate) to turn the table 90 degrees.

I suspect you're getting confuzzled about the 22 turns plus ten holes. Each move needs to be 22 turns plus 10 holes from the last move.

That all being said, I'd index 4 by using the graduations on the table and hand wheel.
 
I have a rotary table and have only used it a few times but I can't seem to grasp why dividing heads are used as much. To me it seems easier to us the degree marks on the side of a rotary table instead, no need for counting turns or nothing. Just crank from 0 degrees to 90, 180, 270, 360 to do the four flats. I don't know how more simple that could get, but mind you I am not a guy with 20 years experience either
 
Yep, graduations are fine for simple indexing. As are spindexers. It's when you want index stuff like 7, 19, 52 or suchlike that you'll be breaking out the plates. I suspect OP was trying out the plates as a learning exercise on a simple job, which is a great way to start.
 
Not so much to answer the question, more a show of sympathy. . . I struggled through the seventh(7) grade, unsuccessfully, with fractions. Never finished the eighth. I answered questions on tests by converting to decimal, figuring the answer, and converting back to the closest answer. That's the way Pop showed me, he couldn't do fractions either. I was well into my 50s when I first dealt with an indexing head. Fractions suddenly made good sense and I've not had a problem since.

The indexing plates are called "Fraction Plates" for a reason. I have a 40:1 and a 90:1 indexer. There's also a 72:1 rotary table, but it doesn't have any fraction plates. For 4 sides, the 40:1 is a matter of 10 turns, no fraction. The 90:1 uses 20 turns plus 1/2 turn. That 1/2 turn is the key, it can be 10 turns on a 20 hole plate. Or one half rotation of any even number plate.

Fraction plates may well be an "old school" expression. I don't know. But using off the wall fractions fits the work very well. I must appreciate the indexing head as a learning tool, albeit a bit late. It did something that schools couldn't do. And in my old age, I now have a good grasp of arithmetic. It ain't "math" until you get into algebra. . .

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I tried the 20 turns + 10 holes out of 20 on a 20 hole plate, It didn’t come out right. In theory if I started at “0” degree mark for reference then the 20 turns + 10/20 should have been right on the degree marks 0,90,180,279,360 but they were not, appears it needed more turns to line up where it should have been right. But then why does my book say A-20, 20+10/20? It should have been right. And when double checking my revolutions and 10/20 it never comes back to the same starting place? Doesn’t make sense.
 
Your manual is probably wrong. You should see how many turns a full revolution takes. I suspect Lo-Fi is correct and you have a 90:1 ratio
-Mark
 
If you have a 90:1 head, I believe you should be doing 22 turns and 10 holes on a 20 hole plate
 
In that case, there are a couple of answers. First, and most likely, is that you are missing a count. I'm prone to do that, especially if I'm interrupted. The other is not very likely, but still a possibility. Rotate the head one full turn, starting at a known point. Count the number of turns, making sure to count each one. And make sure the manual matches the head. My 40:1 isn't too much of a problem, the 90:1 takes a lot of concentration. Make absolutely, positively sure that there are ninety counts per revolution. If it's 89 or 91 or some such, take the supplier to task. Be nice, insert a few inocuous words along with the cussing.

EDIT: It occured to me, after the fact naturally, that half of 90 is 45, of which half would be 22-1/2. I would say the manual is incorrect.

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Walk through the maths:

20 turns on a twenty hole plate is 400 holes. Plus 10 holes is 410 holes the manual claims for a 1/4 turn. Multiply by 4, you get 1640 holes for a complete revolution of the table. All according to the manual, remember.

Divide that by the number of holes in the plate to get back to revolutions of the handle, you get 82. 82 turns of the handle for a table revolution. Very unlikely. Count out how many turns of the handle for one revolution of the table... 40:1, 60:1, 90:1 and sometimes 72:1, I've seen. Never 82:1

Applying Occam's razor: if you're making the moves correctly as per the manual and not getting back to zero after the last move, the manual is wrong.
 
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