Question for the REAL machinists out there...

[crazy steve]

...and being entirely self-taught, I lack the book knowledge and experience that somebody that is or has done it for living would know. So, what exactly constitutes an 'interference fit'? I want to press an aluminum 'plug' into a hole so I can re-drill it offset a bit. The part I'm pressing into is also aluminum and a casting, so I don't want to break it. Or would I be better off with a 'light' fit and 'pinning' it for retention and/or to prevent rotation?

TIA!

'78E original owner

[randy]

The way I understand an interference fit is similar to the way a bearing is sized to be pressed onto a shaft. As in our lower steering stem bearing. The size of the bearing is ever so slightly ( i think that's the actual engineering term ) smaller than the shaft so it must be pressed on.

[red bandit]

Yep, an interference fit is where the part is slightly bigger than the hole it is going into, a press fit. Hard to say how much press you could get away with in an aluminum casting. I wouldn't go more than .0005 without seeing it. If you are worried about the inner piece turning or moveing you can drill and tap for a small set screw (#6 or 8) right on the line between the inner piece and the outer piece. So that half of the set screw is in the inner piece and half in the outer piece. Don't drill and tap all the way through. Run the set screw in until it bottoms out on the threads and use lock tight. No way can it move.

[oseaghdha]

Depends on size, material and surface finish.
The smaller the diameter, the less press you try for.
I've done everything from .0003" to .030"
Hot and cold are definately your friend.

On a bad hole in aluminum, I would try to find an aluminum bolt. Drill, tap and loctite. Saw/grind flush.
Redrill on position.
Easy squeezy.

[Ivan]

That really depends a lot on application. Size factors in too.

I have a "Machinist Handbook" I can look into and get you a general idea. Of course the more specifics the better the chance at finding what you are looking for.

[HobbyMan]

Granted, I am no Machinist, but I can tell you what I have done. Oversize the shaft by .0005. If you are still worried about it spinning, take a punch to the shaft and giver her a few dimples. This is usually what gets done when there is no round stock available to make a new pump shaft on a ship.

[Ivan]

Well, I looked at that book and there's only about 38 pages on interferance shaft fit. That doesn't include running fit, sliding fit, locational fit or force fit. Most of my info is for steel shaft and steel hub. I imagine if I read closer, I could find the different expansion coefficient for aluminum.

So, it varies by material, fit, size, and force available to push them together.

What are you trying to accomplish?

[crazy steve]

What I want to do is fill an axle hole, then redrill to a smaller size and offset it a bit. This won't turn, it's strictly a locating hole. I'm installing HD 4-piston calipers on the FJ front end I'm currently thrashing to install, and have decided to go whole-hog and use a hd 4-piston rear too. The caliper looks like this:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=160425492340

The existing axle hole is .750" and I need to reduce it to 17mm; the offset is to correct the slight difference in rotor diameter (about 1/4").

I like the hot/cold idea; maybe I'll put the caliper (when I get one) in my oven and see how much expansion I get....

'78E original owner

[Ivan]

I think I see what you are trying to do. If you need to move half the .250 for the rotor diameter, then the difference between .750 and 17 mm is only about .08, which is less than the .125 you theoretically need to move the hole, so the new hole would not fully fit in the old. If that's the case, no matter how much interferance there is, it will fall out at some point. Maybe someone could TiG the hole, then you could rebore it where you need it?

If I am missing something, forgive me.

If the hole will indeed fit, I would go with about .001 oversize and get some green loctite and primer. That should hold things in place nicely, since your load is radial as opposed to torsional.

Just my .02$

[tarzan]

I agree with Ivan in that if the hole won't fit well within the plug you press in,
it will probably fail.If that is the case then I would make the hole larger with a drill and then ream it size.The reason to ream it would to make a good uniform sealing surface for your plug.I would then make a plug that is .001 to
.002 bigger than the hole.That would be an ideal interference fit.

Another thing you could do is chamfer the hole (big chamfer) and press a
plug into the hole and weld the plug in.Then drill your new hole.That would
work well.

Either way you go you need to make sure that your plug is the same material as the material the plug is in.

[fredintoon]

...and being entirely self-taught, I lack the book knowledge and experience that somebody that is or has done it for living would know. So, what exactly constitutes an 'interference fit'? I want to press an aluminum 'plug' into a hole so I can re-drill it offset a bit. The part I'm pressing into is also aluminum and a casting, so I don't want to break it. Or would I be better off with a 'light' fit and 'pinning' it for retention and/or to prevent rotation?

TIA!

'78E original owner

Hi Steve,
what also works for what you are doing is to make the plug a slip- fit (so it slides in nice but don't rattle) and fix it in place with Loctite stud retainer.
Lest this be thought a shade tree mechanic's trick, it's often used for installing bearings in aircraft landing gear, the hole need not be held to such a tight tolerance and the cushioning effect of a 0.001" layer of Loctite increases the bearing's resistance to shock loads.

[oseaghdha]

+1

I would have it welded up.
Otherwise, like Ivan said you need to go twice oversize from what you want to move it, plus some for wall thickness.

[crazy steve]

I think I see what you are trying to do. If you need to move half the .250 for the rotor diameter, then the difference between .750 and 17 mm is only about .08, which is less than the .125 you theoretically need to move the hole, so the new hole would not fully fit in the old. If that's the case, no matter how much interferance there is, it will fall out at some point. Maybe someone could TiG the hole, then you could rebore it where you need it?...

This is why I asked..... Hmm, so you don't think the plug would hold with the hole offset that much. I'm only going to be into the original hole about .050". How about if I pinned the plug at two points 90 degrees to the 'break-through'? To get any TIG done means a long drive and money out of pocket, so I'm trying to avoid that, cheap-a$$ that I am.

There is an alternative; Harley went to 1" axles on the later bikes ('02-up touring) so with a bigger hole maybe one of those would be a better choice. But they seem to want more money for those...

'78E original owner

[jmnjrpa]

Damm

A slip fit allows two mating parts to "slip". A interference fit is when the two have the same OD/ID size. A press fit is when the inner part is larger than the hole you want to "press" it into. Typically allow .0005 press fit per inch diameter on steel and twice that on aluminum. A press fit with large chamfers on both ends then weld up is the most acceptable method for a job shop. Grind flush and then drill to fit. I have been plant manager, foreman, machinist, welder, etc for 40 years. I have done flight critical aircraft work to oilfield pressure systems.

[jmnjrpa]

Sorry,

But that is not the place to get cheap on. Just imagine the caliper breaking off at a hard stop from 70 MPH.