Author Topic: How to drill a precision bolt pattern  (Read 6629 times)

Offline goodfellow

  • RestroLibrarian
  • Can use hammer
  • *****
  • Join Date: Jun 2010
  • Posts: 9590
  • location: Northern Virginia
How to drill a precision bolt pattern
« on: June 19, 2010, 07:13:48 AM »
Following the milling machine session on accurate edge finding,

http://www.thegaragegazette.com/index.php?topic=74.0

the next logical step for my friend was to learn how to accurately lay out a bolt pattern and drill precise holes. This is not a trivial step. How many times do we need to make an adaptor or flange with accurately spaced and drilled bolt patterns? The more holes we need, the more complex the procedure and the greater the chance for error.

There are only a few ways to do this in conventional machining

1) lay out the pattern manually with a machinists rule, compass, and divider (not very accurate)
2) purchase/borrow a precision rotary table or univeral dividing head (very expensive)
3) use some basic math along with layout tables provided in the Machinery's Handbook.

Every mechanic and DIY guy should own a copy of this book. An older copy is just a few $$$ and is better suited for DIYers because it doesn't include a lot of specialized structural, metallurgical, and CNC information. My copies are from 1946 and 1982. Learning how to read and use the information in this book will pay off "big time".



Within the first section is a long list of tables that address the subject of "jig boring" bolt patterns. There are various "types" of patterns, but the most basic is referred to as a "Type A" central coordinate pattern --- on page 82 is in my 1982 handbook. Central coordinate simply means we start out from the center of the bolt pattern circle at X,Y coordiante 0,0. The "Type A" refers to the layout where the starting hole in the pattern is located (at a 90 degree angle to the "X" axis (or 12 o'clock).

Within this table, are descriptions for many different bolt patterns ranging from 3-28 holes. Just for fun we chose a "Type A"  5-hole pattern with a 3" diameter bolt circle (outlined in red).



The table shows the required coordinate "units" necessary to accomplish this task. However, "units" are not actual measurements. The units must be manually converted into workable coordinates by multiplying them by the diameter of the required bolt circle -- in our case that's 3"

So here we must multiply each of the the table's 10 "units" by 3 (time for a calculator)

for instance, the table units for X1 and Y1 are 0.0000 and -0.5000. Multiply these units by 3 (our diameter) and we get X1=0.0000" and Y=1 -1.5000"  (makes sense since the radius of a 3" diameter bolt circle is 1.5") Since the first hole lies only on the "Y" axis at 12 o'clock, the X axis will not need to be moved (hence it stays at 0.0000")

NOTE: The "-" sign simply means that the mill table must be moved from center (0,0) in the "negative" "Y" direction (towards you) to drill the first hole at 12 o'clock. This sometimes causes confusion because even though the first hole is drilled in the "positive "Y" axis side" of the cartesian plane, we need to move the mill table in the "negative" direction to get the workpiece aligned for the first hole. To confuse matters further, some machinist tables in other books use table movement to suggest direction, while others use hole placement to suggest direction. Machinery's Handbook uses mill table movement in determining direction.

To summarize

Hole 1 coordiantes X1 = 0.0000        Y1 = -1.5000
Hole 2 coordiantes X2 = - 1.4265      Y2 = -0.4635
Hole 3 coordinates X3 = - 0.8816      Y3 =  1.2135
Hole 4 coordiantes X4 = 0.8816         Y4 = 1.2135
Hole 5 coordiantes X5 = 1.4265         Y5 = -0.4635

So, since my friend has already "found" the edge of the workpiece (in the other GH thread) all he had to do was use the mill's X,Y dials to find its center. To do that he used a vernier dial caliper to measure the length and width of the workpiece and from these measurements he found the exact center. He then used the dials on the mill to position the center drill exactly over the calculated center of the workpiece (aka -- coordinate 0,0)

At this point, he again set both XY mill dials to zero and proceeded to dial in the first coordiante X=0.000 and Y= -1.5000, and drilled the first hole, then he used the dials to reverse back to coordiante 0,0 (All holes are drilled from coordinate 0,0 -- so after each hole is drilled you have to get exactly back on center)

Second hole is at corrdinates X2 = -1.4265  Y2 = -0.4635 -- drill the hole and use the dials to get back to 0,0

Third hole is at coordinates X3 = -0.8816  Y3 =  1.2135 -- drill the hole and use the dials to get back to 0,0

Fourth hole is at coordiantes X4 = 0.8816  Y4 = 1.2135 -- drill the hole and use the dials to get back to 0,0

Fifth hole is at coordinates X5 = 1.4265  Y5 = -0.4635 -- Finished!

Done!!! A perfect 3" diameter 5-bolt circle executed in a precise manner



There are several other ways to use these table values and the XY coordinate calculations for more efficient and quicker bolt circle drilling without reversing back to the 0,0 starting coordinate after each hole is drilled. They are designed to eliminate backlash error, but this is the most straight forward way of doing it, and if you take your time, it will be accurate.

My friend is coming back next week to drill the rest of his brackets -- I'm sure it will be a much smoother, and less frustrating effort ;D

For those that of you that don't want to do all the calculations, there are plenty of machinist's helper applications on the net. Modern Digital Readouts will do the calulations and keep track of your XY mill controls to get you easily back to "0,0" (or any other coordinate), while KWIKTRIG will calculate the XY coordinates for you simply by inputting some basic info.

I like kwiktrig http://wildone296.home.mchsi.com/kwiktrig.htm

Update -- kwiktrig has a new home page -- http://www.fracaro.net/

However, using the jig boring tables in the Machinery's Handbook is really a skill to be learned if you're a serious DIYer.








« Last Edit: June 08, 2013, 08:20:23 PM by goodfellow »

Offline Mickey O

  • Hunt and peck king
  • ********
  • Join Date: Jul 2010
  • Posts: 4766
  • location: Chicago, IL
    • Eye On Lombard
Re: How to drill a precision bolt pattern
« Reply #1 on: July 26, 2010, 09:57:50 PM »
Great write up. Years ago I used to do quite a bit of layout work on large aluminum plates (couldn't use a mill) using a compass and a mag base Milwaukee drill, I did pretty good but they were by no means precision. Eventually I got smart and made a template on a mill with a turntable and then used transfer punches. I'm having flash backs of taps breaking in the aluminum plate, I hated that.
   Cheap Tools Are For Chumps 

Offline welderwink

  • Newbie
  • *
  • Join Date: Jul 2010
  • Posts: 42
  • location: Pa
Re: How to drill a precision bolt pattern
« Reply #2 on: July 27, 2010, 02:09:42 PM »
Very good read.  Do you have any particular site in mind to buy that book at?  Thanks

Offline goodfellow

  • RestroLibrarian
  • Can use hammer
  • *****
  • Join Date: Jun 2010
  • Posts: 9590
  • location: Northern Virginia
Re: How to drill a precision bolt pattern
« Reply #3 on: July 27, 2010, 02:23:01 PM »
Very good read.  Do you have any particular site in mind to buy that book at?  Thanks

You can pick up used versions for less than $5 on ebay. The older books are actually better because they don't contain so much CNC-based information and formulas. Any volume from 1940 through 1980 is a good cheap buy. E-bay has pages of them.

Offline welderwink

  • Newbie
  • *
  • Join Date: Jul 2010
  • Posts: 42
  • location: Pa
Re: How to drill a precision bolt pattern
« Reply #4 on: July 27, 2010, 03:32:26 PM »
Thanks goodfellow.

Offline 79pacecar

  • Full Member
  • ***
  • Join Date: Jun 2010
  • Posts: 248
  • location: Wisconsin
    • Dallasj.com
Re: How to drill a precision bolt pattern
« Reply #5 on: July 28, 2010, 09:39:35 AM »
Very good read.  Do you have any particular site in mind to buy that book at?  Thanks

You can pick up used versions for less than $5 on ebay. The older books are actually better because they don't contain so much CNC-based information and formulas. Any volume from 1940 through 1980 is a good cheap buy. E-bay has pages of them.

The machinist's ready reference book is also a good choice, they are quite a bit smaller, But have a lot of the same info (sorry I don't have a pic)
Horsepower is how fast you hit the wall.
Torque is how far you take the wall with you.

The Garage Gazette

Re: How to drill a precision bolt pattern
« Reply #5 on: July 28, 2010, 09:39:35 AM »

Tags: