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fixerdave
01-21-2012, 11:37 PM
I have a 6" rotary table that I picked up a set of index plates for. Never had a use for the plates until today... wanted to drill a 12 hole bolt circle. Set everything up, went to the plates, no 12... what? It started at 15, then counted up on the plates to every obscure count besides 12, 24, 36, or 48. Nothing would give me 12.

It's no big deal, I just counted off 30 degrees a few times around and the job is done. But, now I'm curious. This is cheap Chinese stuff, both the table and the index, but 12 seems like an obviously useful number to have. So, here are my questions;

1) am I missing something stupid?

2) if anyone has the same cheap Chinese set, how many plates did it come with? Mine has 3.

Paul Alciatore
01-21-2012, 11:46 PM
Most rotary tables have a 90 tooth worm so you need 90 / 12 = 7.5 turns to do 12 divisions. Any even number plate will let you do this.

It is all a matter of prime numbers. 12 consists of 2 X 2 X 3 = 12. You need each of these prime numbers in either the worm or the plate you use. The 90 tooth worm gives 2 X 3 X 3 X 5 = 90. You have one 2 and one three there (two actually, but you only need one for 12) so you only need another 2. As I said above, any even number plate will supply that second 2.

High school math really is useful.

TriHonu
01-22-2012, 12:15 AM
Take a look at the Grizzly 6" Rotary Table Manual (http://cdn0.grizzly.com/manuals/h7527_m.pdf).

This unit comes with 3 index plates (15, 16, 17, 18, 19 & 20) (21, 23, 27, 29, 31 & 33) and (37, 39, 41, 43, 47 & 49).

It has directions on indexing and has a chart on page 17 that will give you the number of turns and holes for divisions up to 100.

fixerdave
01-22-2012, 12:15 AM
Most rotary tables have a 90 tooth worm so you need 90 / 12 = 7.5 turns to do 12 divisions. Any even number plate will let you do this.

It is all a matter of prime numbers. 12 consists of 2 X 2 X 3 = 12. You need each of these prime numbers in either the worm or the plate you use. The 90 tooth worm gives 2 X 3 X 3 X 5 = 90. You have one 2 and one three there (two actually, but you only need one for 12) so you only need another 2. As I said above, any even number plate will supply that second 2.

High school math really is useful.

Ah, yes, I'm missing something stupid. Reduction gear... handle turns round more than once... I'm not indexing 12, I'm indexing the remainder. Got it.

Thank you, and thank you for not being too harsh on what's now an obviously silly question. Some day I might actually learn how to use a few of those tools I've bought over the years.

David...

mklotz
01-22-2012, 10:24 AM
Just yesterday I ran this case on the DIVHEADT program available on my page for a fellow over on HMEM. Hope it's of help to you.

Turns & holes/plate for dividing head with worm gear ratio = 90:1

Available hole plates =
15, 16, 17, 18, 19, 20, 21, 23, 27, 29, 31, 33, 37, 39, 41, 43, 47, 49,

2 => 45 & 0
3 => 30 & 0
4 => 22 & 8/16 or 9/18 or 10/20
5 => 18 & 0
6 => 15 & 0
7 => 12 & 18/21 or 42/49
8 => 11 & 4/16 or 5/20
9 => 10 & 0
10 => 9 & 0
11 => 8 & 6/33
12 => 7 & 8/16 or 9/18 or 10/20
13 => 6 & 36/39
14 => 6 & 9/21 or 21/49
15 => 6 & 0
16 => 5 & 10/16
17 => 5 & 5/17
18 => 5 & 0
19 => 4 & 14/19
20 => 4 & 8/16 or 9/18 or 10/20
21 => 4 & 6/21 or 14/49
22 => 4 & 3/33
23 => 3 & 21/23
24 => 3 & 12/16 or 15/20
25 => 3 & 9/15 or 12/20
26 => 3 & 18/39
27 => 3 & 5/15 or 6/18 or 7/21 or 9/27 or 11/33 or 13/39
28 => a plate with an integer multiple of 14 holes is required
29 => 3 & 3/29
30 => 3 & 0
31 => 2 & 28/31
32 => 2 & 13/16
33 => 2 & 24/33
34 => 2 & 11/17
35 => 2 & 12/21 or 28/49
36 => 2 & 8/16 or 9/18 or 10/20
37 => 2 & 16/37
38 => 2 & 7/19
39 => 2 & 12/39
40 => 2 & 4/16 or 5/20
41 => 2 & 8/41
42 => 2 & 3/21 or 7/49
43 => 2 & 4/43
44 => a plate with an integer multiple of 22 holes is required
45 => 2 & 0
46 => 1 & 22/23
47 => 1 & 43/47
48 => 1 & 14/16
49 => 1 & 41/49
50 => 1 & 12/15 or 16/20