Bolt hole pattern with only a drill press

[sanddrag]

What is the best way to make a bolt hole circle/pattern on a small hub (~2" diameter) with only a drill press? Mainly, I need the most accurate way of marking it. The only thing I've come up with so far is drawing it out in CAD with center marks then printing it 1:1 scale to glue/tape to the part, then center punch on the center marks.

I'd like 3 or 4 holes at 120 degree or 90 degree intervals respectively. The material is 6061 aluminum.

Any other ideas?

There is dikem and scribing but I don't know how I'd scribe the circle or get the angular spacing right.

[Geof]

Pair of dividers with sharp points. Center punch the center of the circle. Using a good metal scale set the dividers to 1" for radius. Scribe circle from center punch mark. The aluminum is soft enough the divider tip will make a mark. Now center punch lightly anywhere on the scribed circle. With dividers still set at 1" use this center punch mark to scribe two marks on the original circle. Lightly center punch these marks. Use each of these new center punch marks to scribe another two marks on the circle. Again lightly center punch and use these two marks to scribe on the circle. These final scribe marks will fall in exactly the same place if you have done everything nicely.

Now you have the circle divided up into a hexagon so use every other mark to drill a hole.

[ErnieD]

Hi,

Use tool makers buttons. You can mic them into adjustment and then tram each one prior to drilling. You can get the holes within .002 thou if you are careful. Google tool makers buttons and maybe you can get a detailed description of their use.

ErnieD

[mxtras]

This may help (?) -

http://www.sparro.com/KwikByte4&5.htm

Scott

[BobWarfield]

It's divider work all right. For your 2" diameter, 3 holes want to be 2 * pi / 3 or 2.094" apart. 4 holes want to be 2 * pi / 4 or 1.571" apart. If you have an inexpensive digital calipers, you can dial in those distances and use the points as your dividers to scribe out the circle as Geoff describes.

My version of "measure twice, cut once" is to use some layout dye so I can see the scribed marks and wait until I can go around the circle and have the last scribing line up exactly before I hit it with center punch.

You will also want to take great care that your hole actually comes out where you've punched and that you have punched in exactly the spot you wanted. You may want to use a center drill to start the hole as they are shorter than most regular drill bits.

BTW, early in my career I went through this same exercise making a backplate for a lathe chuck. It worked out okay, but I learned that even drill presses need tramming for best precision.

Enjoy!

[Geof]

Originally Posted by BobWarfield .... For your 2" diameter, 3 holes want to be 2 * pi / 3 or 2.094" apart. 4 holes want to be 2 * pi / 4 or 1.571" apart....

Actually this calculation is not correct for using dividers. This gives the distance between holes going around the circumference but for dividers you need the straight line distance. The link mxtras gave shows you the straight line distance.

[BobWarfield]

Originally Posted by Geof Actually this calculation is not correct for using dividers. This gives the distance between holes going around the circumference but for dividers you need the straight line distance. The link mxtras gave shows you the straight line distance.

Yep, you are absolutely right and the silly thing is I had noted it down on my web journal for my own future reference:

http://www.thewarfields.com/MTLayout.htm

There's also some handy stuff there for laying out dovetails and other things.

Best,

BW

[Geof]

Bob;

I looked at your tips for measuring bolt circles and remembered one of the most embarassing times in my whole career as a Machinist. I deviated away from the trade for a few years to get a couple of university degrees. I was working summers in a shop doing mostly lumber mill maintenance and of course because I was also a university student most of the guys were trying to catch me out. One night I had to make a replacement leg mounting flange for a straddle truck. I was real pleased to have it ready to send to the mill first thing the next day so off it went. Then when I arrived at work that afternoon I found I had put eight holes in a ten hole flange. Didn't live that one down for a long time.