Bolt hole circle without a mill
 

What is the most accurate and repeatable way to make a bolt hole circle on a round part without the use of a mill? I do have a lathe which I could possibly use to scribe a cicle, but I still need 6 holes evenly spaced around the circle.

Re: Bolt hole circle without a mill
 

Could you possibly CAD a circle split into as many equal segments as you needed, and then print it on an overhead transparency, and then use this along with your scribed circle to place the holes?

Re: Bolt hole circle without a mill
 

Ahhh... fond memories of our early years of putting those bolt hole circles into sprockets that fit those skyway wheel hubs. The way we did it was to CAD a pattern of the bolt hole circle with center point marks for each hole and a center hole. Print out a 1:1 scale drawing, cut it out, glue it on the sprocket centered with the middle of the sprocket (hardest part), use a punch for the holes to be drilled lined up with the center points on the drawing, and drill away!!

On second thought if you use a lathe to scribe the hole circle and then an edge to scribe some lines that bisect the circle into the parts where you want holes, that may be just as easy.

Re: Bolt hole circle without a mill
 

Along with the methods mentioned above, if you have multiple parts to make with the same bolt pattern, invest some time making an accurate template. Then you can duplicate your pattern without as much setup hassle.

Re: Bolt hole circle without a mill
 

This is actually pretty easy to do, and does not require a CAD system, transferring drawings, or anything more difficult than some simple scribing skills. Remember, machinists were doing these types of operations long before CAD systems, DROs, CNC machines, or EDM systems were ever invented. So there is usually a simple way to do a lot of these operations in a straightforward manner. In this case, it actually takes longer to explain than it takes to perform.

Mount a three-jaw chuck in the lathe (if you have a six-jaw chuck it would be even better, and make this whole operation trivial). Place a sharpened tool or scriber in the tool post, and adjust its height very precisely so that the cutting edge is on the centerline of the lathe. Make a short spacer that will fit in between the ways and the jaws of the chuck, so that when the chuck is rotated by hand it will stop the rotation by blocking passage of the jaw.

Coat the face of the part with marking fluid, and mount it in the chuck. BY HAND (not under power!) rotate the chuck so that one of the jaws is up against the spacer. Move the cutting tool in so that it just touches the face of the part. Run the cross-slide across, so that the tool scribes the part. You should now have a horizontal line across the face of the part that runs precisely through the center of rotation. Back out the cross-slide.

Move the spacer out of the way and BY HAND rotate the chuck 1/3 rotation. Put the spacer back in place and run the chuck jaw up against it. As long as there is no side-play in the chuck jaws, this will index the chuck precisely 120 degrees. Move the cutting tool back in, and run the cross-slide across to scribe a second line. Back the tool out, and repeat the whole operation a third time. You should now how three lines that intersect in the exact center of the part, and extend across the face of the part. The lines define six equally spaced radial segments.

Mount a sharp scribing tool in the tool post. Align the point of the scriber with the center of rotation of the part (where the radial lines intersect). Back the cross-slide out the desired radius of the bolt circle you want. Move the scriber in until it contacts the part, and then BY HAND (not under power!) rotate the chuck. This will scribe a circle that is concentric with the center of rotation of the part, and that intersects with the six radial lines. The intersections of the lines and the scribed circle will define the locations of the bolt holes.

When you are finished, take the spacer you made and store it away somewhere safe that you will remember. You will be using it again...

-dave

Re: Bolt hole circle without a mill
 

dave beat me to it by 4 minutes


you should be able to scribe a circle on the part you want the holes on and then figure out where you want the holes...in this case ill use a 6 hole evely spaced bolt circle. First start by scribing the diameter you want the holes at. Find the degree of each hole 360degrees/6holes= 60degrees spacing. on the lathe mark a line then turn the chuck 60 degrees arny direction and mark you next 5 lines in this manner. to get the exact degree you need a backgear with 72 teeth which allows you to remove the pin and advance 1 tooth to rotate the chuck 5 degrees. to rotate the chuck an even 60 degrees rotate the chuck 12 teeth and replace the pin that will be every 60 degrees


hope this helps

Re: Bolt hole circle without a mill
 

Pure old school genius!

Re: Bolt hole circle without a mill
 

you all are making this more difficult than it needs to be. all you need is a compass. scribe a circle centered on the part the dia of the pattern. then DO NOT CHANGE THE COMPASS IN ANY WAY. place the anchor point of the compass on the circle you just scribed. make an arc through the circle. then move the anchor point on the compass to the intersection point, and repeat the process. do this all the way around the circle. wahlah you now have 6 points equispaced around the circle. centerpunch the points where the circle and arcs meet, and drill

Re: Bolt hole circle without a mill
 

There was the word "accurate" in there.

Most compasses I've seen don't come close to even acceptable quality and you still have the problem of finding the center.

Re: Bolt hole circle without a mill
 

A decent compass is as accurate as the person using it. My hand happens to be a bit shakey evidently.

Re: Bolt hole circle without a mill
 

but still not quite the level of accuracy achieved by using (what i assume is) a quility lathe

Re: Bolt hole circle without a mill
 

Dividers can be very accurate IF you can find the center and mark the bolt circle accurately. It's a matter of developing layout skills.

Re: Bolt hole circle without a mill
 

What about on of those compases that have the thread shaft throught the middle and screw pen and closed.....That is if you know what I am talking about.....