[asifjahmed]

Hi guys -

I am looking for any tips and tricks for locating the center of a a hole for purposes of drilling a hole patter.

I have a chain sprocket (55 tooth) and it has a large hole in the center. I need to drill a 5-hole pattern in the sprocket, and this hole pattern needs to be as perfectly centered as possible. The hole pattern is 5-holes, or 72 degrees apart from the center.

I have a Sieg SX2 that I converted to CNC.

I plan to set the sprocket between 2 123 blocks and clamp it down.

Any help is much appreciated!

[fizzissist]

Buy a dial indicator.

[asifjahmed]

I have a dial indicator. I was more asking for tips/techniques, etc. Thanks.

[fizzissist]

.... Then what's the issue??
Clamp the thing down. Put the indicator in the spindle. Run the spindle to what looks to be the center of the hole, bring the indicator down and adjust it till it's touching the inside of the hole.... rotate it around while at the same time adjusting your X & Y till the indicator doesn't move....

The spindle is on center with the hole when the indicator don't move when rotated around the hole's ID.

Zero your X, zero your Y.

Trig out the locations of each of the 5 holes, and using the center of the hole as 0,0 you can run the table over by the values calculated and drill your holes. Use these values for your input into your code for a CNC program.

That's how the big kids do it.

http://www.penntoolco.com/images/catalog/583_b.gif

(don't turn the spindle on while the indicator's in it)

[asifjahmed]

That is helpful. Thanks!

[fizzissist]

In the alternative, and sometimes you have to do it this way.....

Clamp the part snugly, but not tight.... zero the X & Y, and with the indicator in a 0 or 90 deg position, tap the part till the indicator reads the same at 0 and 180, and 90 and 270..... sweeping the indicator back and forth and tapping the part until the indicator's needle doesn't move as you sweep around the periphery of the part.

You can indicate off of an OD or ID this way. And do it this way if you've got clamps in the way.

Sometimes either of these methods takes 3 minutes, or 10 minutes. Or 20. Check your horoscope prior to starting the process.

[handlewanker]

Hi, I would have done it on my rotary table with a 4 jaw chuck, holding it on the outside of the boss (if it has one).

Many people fit a 3 jaw chuck to both dividing heads and rotary table.

Poor fools.....if the 3 jaw chuck runs out you cannot get the job to run true.....ever.

I fit 4 jaw chucks to my div heads and rot tables...this way you can make a job run true to a bore PERFECTLY, well just about bang on maybe, depends on how you work the chuck key.

There is a weeze you can employ when you come up against a 3 jaw chuck mounted on a dividing head or rotary table that runs out just a thou or two.

For some reason well beyond me, people fit 3 jaw chucks tight to the spigot on the back, thus making any truing exercise impossible, unless of course you make the register that fits the spigot a loose fit, at least .003" loose.

This will enable you to grip your work in the 3 jaw and "adjust" the run out by slackening the holding bolts at the back of the chuck and tapping the body with a soft mallet, so making the 3 jaw chuck into a Griptru type chuck, which is adjustable for run out.

When you do a gear cutting operation, the gear blank MUST run perfectly true before you attempt to cut the teeth, and if you have the gear blank mounted on a mandrel held in the 3 jaw chuck, instead of between centres, 10 to 1 the chuck will run out, so will your gear blank teeth to the bore if you can't true it up.
Ian.

[RICHARD ZASTROW]

3-jaw "Adjustru" type chucks. They have clearance between the back plate and the mating bore on the back of the chuck. (4) adjusting screws @90 deg. let you tru it up within .0001ths".

That said, I'd still prefer a 4-jaw chuck.

Dick Z

[handlewanker]

Hi Dick, that's interesting, the convenience of the 3 jaw with adjustable features would be the way to go for a dividing head/rotary table, probable costing 1/3 the price again.

I've just invested in a Vertex BS-1 dividing head and also a 4 jaw chuck bought while there was a special offer on at out local tool dealer.

I already have a Cincinnatti dividing head I bought at an auction, but it's a queer set-up.....it points to the left when on the right side of the mill table....can't be swung over all the way 180 deg to point to the other side as the chuck body fouls the table bolt, and as I always use the head on the left side pointing to the right I decided to sell it and get a " proper one"...LOL.....anyway, it only has one index plate, out of a set of 4 I think as supplied by the makers, so there is a limitation there.

I realise the Cincin head was designed to do spiral milling etc and be coupled to the Cincinnatti mill table at the right side, but it's an odd set-up for normal work.

The right angled spiral drive mechanism was removed too sometime in the past,why I don't know, so it's unuseable in that mode.
Ian.