Press Fit

[Cory]

Originally Posted by Peck

he isn't talking a generic drill set, he is talking a real machine shop's drill set where they have the drills accurate to tenths of a thou. An E drill is a generic and it is typically .250, accurate to the thou. If you are pressure fitting with generics... be really careful and take plenty of measurements.

There is no twist drill that can create a hole that is accurate within a tenth.

The diameter of the drill itself may be ground more precisely than on a cheapo drill, but the geometry of the tool inherently prevents it from making extremely round/accurate sized holes.

[Brandon Holley]

Originally Posted by Cory

The diameter of the drill itself may be ground more precisely than on a cheapo drill, but the geometry of the tool inherently prevents it from making extremely round/accurate sized holes.

In addition to this, any sort of runout on the drill/lathe/mill you are using will compound these issues.

Reamer is the way to go if you want to have repeatable results from plate to plate. Drilling holes, milling them, laser cutting them or waterjetting them will all result in a different sized hole for the same "as designed" size.

The best way to deal with it is to ream.


One understated method of holding bearings in is green loctite (gap filling). Having the bearing slip fit inside a hole, and using appropriate green loctite does a good enough job of holding bearings in.

Also- the ball bearing/ball peen trick is a nice one if youre in a bind. We have a nice 1.5" diameter steel ball bearing. If a hole is a little bit too large, we can just stuff that bearing into the hole and whack it with a hammer. It creates a little curl of material just big enough to bite the bearing and hold it in place.



-Brando

[AdamHeard]

Originally Posted by Brandon Holley

Also- the ball bearing/ball peen trick is a nice one if youre in a bind. We have a nice 1.5" diameter steel ball bearing. If a hole is a little bit too large, we can just stuff that bearing into the hole and whack it with a hammer. It creates a little curl of material just big enough to bite the bearing and hold it in place.
-Brando

On Emperor Swerve we had a 37mm bearing bore that we attempted to waterjet to exact size cutting super slow (like .7 in/min). The walls ended up being nontapered, but the kerf was larger than normal so the hole was easily .002 oversize.

We used some punches and ball peen hammer to rough up both the lip, and the inner diameter of the bore quite a bit in an even fashion around the diameter, and finished the assembly with bearing loctite.

We were able to "fill" that .002" gap in a really high load application with these methods and the bearings still have not loosened.

[Gray Adams]

A slightly undersized reamer is definitely the way to go. We've done all kinds of things in the past, but if you want a hole precise to .0001", use a good reamer.

We machined bearing blocks this year on our CNC mill, and some turned out as slip fits, some were press fits. Keep in mind this is the same end mill, same program, same bar of stock. You would think they all come out the same, but there is never a guarantee. Between a bit of high grit (400 or so) sand paper and snap rings, things were easily "good enough" for FRC. But...

Ideally, I would drill it undersize for the reamer and then ream it slightly undersize. Picking the size reamer you want is the hard part, but I would think .0005" under or so would be good enough. Other methods may work, but a reamer is going to give you the closest thing to a true cylinder that you can get.