Alternative "No Math Required" way to duplicate a bolt circle

[HiltzVW]

Hi there,

I was recently machining a backing plate for my 922 Logan and had to put the bolt circle on it to match the 4 jaw chuck I wanted to mount.

I decided to use a "no math" method of finding simple bolt circles and thought I would share it here.

First, I aligned the part to the spindle so that the X axis acted as a sort of "mirror image" of the part front to back. I did this by using two gauge pins in the two bolt holes facing toward me.

Then I dialed in the part to find its center. I set zeros on my x and y coordinates.

Next, I went back and indicated one of the gauge pins. Since this was a simple 4 bolt circle, the numbers on my read out after dialing in the gauge pin are all I needed. I could then just reverse the x or y number from + to - in order to find the other holes.

I made a video of it in case you are a more visual learner or I am not describing it well enough.

 

[RJSakowski]

Most newer DRO's have a bolt circle feature. All that is needed is the center of the bolt circle, the diameter of the circle, and the number of holes.

The task then becomes determining the bolt circle diameter of the mating part. I would use two gage pins in opposing holes to find the center of the circle. First measure the diameter of the pin and zero the caliper. Then measure the outside distance between the two pins. The reading is the bolt circle diameter.. I have also used two bolts instead of pins. Thread them in as deeply as practical to reduce play and measure on the threads. You can also use the holes themselves and the internal jaws of the calipers for this method.

To drill the holes for a four bolt pattern without a DRO, the new part would be centered in the mill with the opposing hole positions aligned with the axes and the table offset by half the determined diameter from the center position.

One thing that you should be careful of is that you are reverse engineering the mating part design. The assumption that is made is that the original hole locations are located exactly correct. This is often not the case when dealing with some of the offshore chucks so I measure the distance between opposing holes for all pairs as well as the distance between adjacent holes to determine if there were any errors made in manufacturing. If there are any deviations, you can average the measured diameters to get a best fit diameter for your part. Bolt holes usually are sufficiently oversized to accommodate slight misalignment.

 

[WobblyHand]

The assumption that is made is that the original hole locations are located exactly correct.

Making this assumption can lead to heartache. Measure everything, then decide where those holes need to go! Says the guy whose made in USA arbor threads weren't on centerline. One hole, and it was in the wrong place!

 

[HiltzVW]

Making this assumption can lead to heartache. Measure everything, then decide where those holes need to go! Says the guy whose made in USA arbor threads weren't on centerline. One hole, and it was in the wrong place!

Sad, but true. You can't really trust anything new these days. First thing you have to do is inspect everything yourself.

In this case it would be an easy check to move the gauge pin to the opposing hole and see how it dials in. If the number is significantly different, it would certainly be a big warning sign.

 

[HiltzVW]

One thing that you should be careful of is that you are reverse engineering the mating part design. The assumption that is made is that the original hole locations are located exactly correct. This is often not the case when dealing with some of the offshore chucks so I measure the distance between opposing holes for all pairs as well as the distance between adjacent holes to determine if there were any errors made in manufacturing. If there are any deviations, you can average the measured diameters to get a best fit diameter for your part. Bolt holes usually are sufficiently oversized to accommodate slight misalignment.

All of this is very true, although I think in some instances it may be better to follow the "reverse engineered" dimensions to create a mating part that matched "perfectly" the "imperfect" original. Have to take that into consideration for each application I suppose.

 

[compact8]

Not sure if my understanding is right but these holes should not require very high precision because their diameters need be a bit larger than the bolt ( 0.5 mm typically ) in order to avoid interfering with the part used for alignment which should be the perimeter of the relief on the chuck.

 

[HiltzVW]

Not sure if my understanding is right but these holes should not require very high precision because their diameters need be a bit larger than the bolt ( 0.5 mm typically ) in order to avoid interfering with the part used for alignment which should be the perimeter of the relief on the chuck.

Correct. The register is the critical part. The bolts aren’t key to locating the fit. That’s why I chose to dial in the recess where it registers instead of dialing in the OD, or off gauge pins in the holes.


Sent from my iPhone using Tapatalk

 

[benmychree]

So much easier to use transfer screws, easy to make using setscrews, just turn a small point on the end and using a hacksaw, cut a screw slot next to the point, screw into the tapped holes exposing the points the same distance proud on the surface of the chuck, assemble the backplate and whack it with a hammer, leaving an impression on the backplate; Bob's your uncle.

 

[HiltzVW]

So much easier to use transfer screws, easy to make using setscrews, just turn a small point on the end and using a hacksaw, cut a screw slot next to the point, screw into the tapped holes exposing the points the same distance proud on the surface of the chuck, assemble the backplate and whack it with a hammer, leaving an impression on the backplate; Bob's your uncle.

I turn all my registers to a .0005" fit, so it isnt always easy to get the chuck off the backing plate once I set them together. Using transfer screws in that kind of situation doesn't always work well.

 

[homebrewed]

So much easier to use transfer screws, easy to make using setscrews, just turn a small point on the end and using a hacksaw, cut a screw slot next to the point, screw into the tapped holes exposing the points the same distance proud on the surface of the chuck, assemble the backplate and whack it with a hammer, leaving an impression on the backplate; Bob's your uncle.

I did something similar but instead of putting a point on the setscrews I drilled holes in the allen key end. The holes were sized to just fit inside without enlarging the socket. Then I sharpend and cut little rods to go in the holes. Run the setscrew in, install the rod. Make the marks. Needle-nose pliers to remove the rod.