[millminer]

Help.
I have a dividing head on which is bolted an aluminium disk.
I am trying to make an index plate, I divided 360 x 60 to get 60 holes around the disk.
The pitch is 6 degrees, so having drilled 60 holes I find the last hole is 5 degrees from the first hole.

I have been drilling with a center drill, winding on to the next pitch, lock the table, drill, zero the handle, wind on 6 degrees, lock table, drill..........

How can I get rid of the cumalative error?

Help, most appreciated.

Gordon.

[easymike29]

You might do better to calculate the hole coordinates and drill by the numbers. My spread sheet will help.

Gene

[millminer]

Many thanks Gene. I have found your way is error free.
Though I am 74 it is still a learning curve, BIG GRIN.

Gordon.

[easymike29]

Though I am 74 it is still a learning curve, BIG GRIN.

Gordon.

I'm 80 and am not satisfied unless I learn something new every day. Keeps the brain active. Composing spreadsheets is one way to keep me occupied.

Gene

[handlewanker]

Hi, a dividing head is 40:1 ratio, as opposed to a rotary table which is normally 60:1 ratio.

Dividing heads are usually used for horizontal axis work such as gear cutting etc and rotary tables for vertical axis work such as making a large disc with a number of holes in the face, but either can be used for both vertical or horizontal aspect work.

The rotary table lends itself better to direct indexing when you are working in degrees as 360 divides better by 60 than 40.

So if you want 60 (6 degrees) holes using a dividing head the formular is 40 divided by 60, which breaks down mathematically to 4 over 6, and multiplying both sides by 5, gives you 20 on the top and 30 on the bottom, which means you need a hole plate on your dividing head with a row of 30 holes to drop your index pin in.

You cannot get degrees by direct indexing a 40:1 dividing head, not without employing the 40:1 formular and a set of hole plates.

Now if you set your index fingers to the 20 spacing and drop the pin into the 30 hole circle at each 20 hole spacing you'll get 60 holes drilled each time you move the handle through 20 holes.

You might have missed a hole if you haven't set the index fingers correctly.
One hole in the 30 hole plate missed each index position will accumulate to some fantastic figure, probably 58 holes output instead of 60, or if you added just one hole somewhere in the indexing sequence you'll probably get the 5 degrees you ended up with.

If you haven't got a set of index fingers (vital for any indexing) on the dividing hole plate, you'll most likely miss or add a hole somewhere every time.

The other most important thing is to only go round in the same direction, without moving back otherwise the backlash in the worm/wormwheel will give innacurate indexing.

If you wanted to make holes, say, spaced at 5 degrees, then 360 divided by 5 gives you 72, so now you have to calculate 40/72 (dived both by 8) which gives you 5/9, and you then have to multiply the top and bottom numbers to give you a bottom figure that will divide into the hole pattern, whichever, on you hole plate.

The hole plates are standard and the Machinery's Handbook will list all dividing combinations using the standard hole plate patterns.
Ian

[RICHARD ZASTROW]

Nice to see somebody remembers what us old farts were taught (way too many years ago). LOL

Dick Z

[handlewanker]

Hi Rich, I'm into my 70's now but still need to know how to turn the handles otherwise you just gotta get rid of the gear, and that would break my heart.

I wonder what the problem was with the 5 degrees at the end of the indexing for Millminer? He hasn't replied yet. Probably "flew off the handle" and missed a hole or two going round the dial....LOL.

When all else fails, we can always refer to the Machinery's handbook, any year will do, as they list all the turns/hole circle combinations without having to do the maths, Ok for lazy bu##ers.

I wouldn't dream of suggesting compound indexing or differential stuff for trying to get degrees out of a 40:1 ratio head, that's masochistic, and anyway you can't connect a rotary table to the leadscrew for spiral gear cutting unless it's specifically designed to do that.

One thing I did notice with rotary tables and that was they usually have the degrees marked round the edge of the table without any means of exact positioning of the handle, ok if you just want to go to an exact degree position, hopeless for actual dividing proper.

I've got a small 150mm (6") rotary table and it has degrees round the edge of the table with the handle having a collar marked in minutes of a degree, one turn of the handle gives about 6 degrees from memory and the divisions on the collar give you minutes, but that's it, no real dividing as such.

There is a set of hole plates available, (for a charge), with the index fingers to make the current Chinese made horizontal/vertical rotary tables more or less able to do dividing as per regular dividing heads, but you're still stuck with the 60:1 division factor, (not a problem if'n you can do the maths which is very basic), however those type of tables might also come in 40:1 and 60:1 ratios, I don't know.

They're too cumbersome to be used in the vertical mode as a divinding head, even with division plates and a chuck mounted on the table, but can be handy if that's all you've got or can afford.

The real division in the two comes when you want to do holes, whatever, at an angle to the axis, the rotary table in that case needs to be mounted on an adjustable angle plate, you do get there but very tedious, compared to swinging a dividing head over.

It's a case of horses for courses, and I'm told that a donkey once romped home in the Grand National some years ago....LOL.
Ian.