Page 1 of 2

Holes with interrupted cuts / offset holes, how to cut?

Posted: Fri Dec 28, 2018 2:59 pm
by Bentworker
One of my projects has been drilling some aluminum alloy wheels for a different bolt circle. Where I am drilling there is an existing 1/8” (roughly) dia hole about 1/2” deep that was used to secure the cap.

The drill for the new hole has it’s web entirely in uncut material, but the existing hole is in the diameter of the new hole. Clear as mud? You can see it not so clearly in the picture.


Using an edge finder and some math I fool with the wheel until I get it clocked on the mill table where the existing lug pattern is about as square as I can get it within sanity (less than 1/4 degree).

Then I clamp the daylights out of it and check it again.

Here is what I am doing with decent results, are there better / faster ways of achieving the same or better results?

Using math & DRO I go to all 5 coordinates and complete one step, then change tools.

1) Peck drill the through hole (about 1 1/4” deep) using a 9/16” drill. The cut is interrupted for the first 1/2”
2) Rough drill about .570” deep with a 1” drill for the counterbore.
3) Finish the through hole with a 5/8” end mill, raising the knee to cut.
4) Finish the counterbore with a 1 1/8” end mill raising the knee to a depth of .600”
5) Cut the taper .200” deep with a 60 degree countersink.

So far I have had satisfactory results with the finished hole centerlines +/- .002” radially from the centerbore.

What tricks are there? Anything else besides using a boring bar?


Thanks for your time,
Peter

E8BAAD46-F2D7-4701-BC0B-FA5F8A574A23.png
D8B1CD36-3E3A-483F-AAC4-CD7B8DEF546C.png

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Fri Dec 28, 2018 3:32 pm
by John Hasler
Predrill with a bit small enough not to intersect the existing hole.

Surely it's only the taper that needs to be precise, as long as the through hole and counterbore are close enough to clear the bolt shaft and head.

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Fri Dec 28, 2018 9:17 pm
by GlennW
Bentworker wrote:
Fri Dec 28, 2018 2:59 pm
Anything else besides using a boring bar?
I'm curious about the boring bar comment.

Do you not have one?

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Fri Dec 28, 2018 10:40 pm
by Bentworker
I have a boring bar, but it only gets taken out for special occasions.

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Fri Dec 28, 2018 11:18 pm
by Glenn Brooks
In similar situations, I’ve had good success drilling my initial hole over the top of an existing hole using an end mill, rather than a drill bit. The end mill doesn’t deflect and cuts where placed, unlike a drill bit. Afterwards, you can enlarge as necessary, without difficultly.

Glenn

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Sat Dec 29, 2018 10:43 am
by earlgo
If I am not mistaken the torque from the drive train or brake disks is transmitted to the wheel thru the lug bolts. By adding the holes between the holes, you have reduced the wall thickness to about 1/4 to 1/3 what it was. The wall crush strength may give you problems in extreme torque transmitting situations. You might be able to alleviate that somewhat by tightly plugging the old holes and counterbores. This looks sketchy at best.
JMHO
--earlgo

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Sat Dec 29, 2018 2:36 pm
by BadDog
Glenn Brooks wrote:
Fri Dec 28, 2018 11:18 pm
In similar situations, I’ve had good success drilling my initial hole over the top of an existing hole using an end mill, rather than a drill bit. The end mill doesn’t deflect and cuts where placed, unlike a drill bit. Afterwards, you can enlarge as necessary, without difficultly.

Glenn
I did this exact thing on Thursday. But in my case it was a steel bracket for a custom engine mount crossmember that needed both larger hardware and moving over slightly. End cutting end mills are just the thing.

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Sat Dec 29, 2018 2:45 pm
by BadDog
earlgo wrote:
Sat Dec 29, 2018 10:43 am
If I am not mistaken the torque from the drive train or brake disks is transmitted to the wheel thru the lug bolts.
It is my understanding that if the bolt shear is restraining brake or accel induced torque, then the system is flawed and already doomed in much the same way as restraining a taper drill with the tang. Even if all else was more than up to the task based on an FEA, there is always clearance, which means movement and hammering, which means eventual failure.

Again, as I think I understand it, the real key here is clamping with resulting high static friction to transfer torque loads. Which is another reason for the importance of properly torqued wheels, more so on higher performance applications.

However, some wheels are hub centric, and others are lug centric, the latter typically having cone seats to address accurate location with the required clearance. And I've heard folks claim the cone seats transfer for torque, but also that they are wrong in that claim.

At least that's my understanding. I'm no engineer, so I'm fully prepared to be wrong...

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Sat Dec 29, 2018 3:04 pm
by warmstrong1955
You are 100% correct Russ.
Clamp load is what transfers the torque, not bolt shear. If it comes down to bolt shear, it's because your wheel was loose...and yeah....wheel fastener torque is very important, with both designs. Bolt shear would require 'body-fit' studs & wheels....not an inexpensive endeavor.
Heavy equipment moved away from both conical & spherical wheel centering to center pilot (center hub) centering years ago. That's all the taper/cone/spherical design is for....centering. Conical and spherical (Budd) nuts are pretty much a thing of the past, and have been replaced by hex-nuts with hardened (SAE) flat-washers.
I am much more familiar with off road equipment design, but I know many automotive designs are taking the same path.

So Russ....you be right!!
Engineer? I think it's spelled injunear.....
:)
Bill

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Sat Dec 29, 2018 9:13 pm
by Bentworker
Thanks guys,
The endmills worked well, but I am always curious what others are doing to accomplish the same thing.

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Sun Jan 13, 2019 1:07 pm
by Orrin
I visited a shop this last week that featured a rack with a complete set of annular milling cutters in sixteenths. At first, I wondered why; but, the more I thought about it, it seemed like a brilliant idea. An annular cutter might have been the perfect solution to this problem.

Re: Holes with interrupted cuts / offset holes, how to cut?

Posted: Mon Jan 14, 2019 6:20 pm
by duckman903
If these are for you all well and good, if for another person don't charge them and if asked you never saw them.