dividing head / indexing head question

[Old Salt 65]

I am trying to machine 24 equally spaced 1/4" holes in a piece of plastic 2.550" in diameter.The Machinist Handbook indicated for 24 equally spaced holes to use the plate with the 39 hole ring and to turn 1 26/39 each time. Doing this I came out with the holes approximately .100" apart - Except the last hole was .200" from the 1st hole. I'm thinking that some where I need to use the diameter or the cir***ference of the piece I am working on to get this to be evenly divided around. Am I out in left field?

 

[BRIAN]

Hi Welcome and thank you for joining us.
If you have every thing else correct, it just maybe that you are looking at the data for the wrong ratio. count the input turns you have to do to get one turn of the table then see if the data you are using is for that ratio.

just my 2c
brian

 

[Hawkeye]

Assuming you are using a 40:1 indexing head, the numbers you quote would be correct. Also assuming you have sector arms, lets take a look at the method of setting the arms.

With the pin inserted in the hole of the 39-hole circle on the disk at the starting position, you set the bevel of the first sector arm against the pin, then count out 26 holes in the direction of rotation, not including the first hole. Then set the bevel of the second arm against that hole and lock the sector arms into that angle.

If you included the first hole in the total of 26, you would introduce a small error which would be multiplied by 24 by the time you get to the last hole.

 

[Old Salt 65]

Hi Welcome and thank you for joining us.
If you have every thing else correct, it just maybe that you are looking at the data for the wrong ratio. count the input turns you have to do to get one turn of the table then see if the data you are using is for that ratio.

just my 2c
brian

Thank you for replying

Matt (old salt 65)

- - - Updated - - -

Assuming you are using a 40:1 indexing head, the numbers you quote would be correct. Also assuming you have sector arms, lets take a look at the method of setting the arms.

With the pin inserted in the hole of the 39-hole circle on the disk at the starting position, you set the bevel of the first sector arm against the pin, then count out 26 holes in the direction of rotation, not including the first hole. Then set the bevel of the second arm against that hole and lock the sector arms into that angle.

If you included the first hole in the total of 26, you would introduce a small error which would be multiplied by 24 by the time you get to the last hole.

Hawkeye,

YOU hit the nail on the head - no one ever told me not to count the 1st hole the pin was in & that was what I was doing
Thank you

Matt (old salt 65)