What should be the allowance for press fitting?

[ukazak]

Dave,

I knew, you'll like the idea...

unal

[ukazak]

Dave,

That will work. since it is small engine and crank will not have a lot of weight to transmit, it may stay. I did the ball bearing fit taht way (had no other choice)

only thing is, crank disk 3.5mm thick.

first thing in the morning, I will fit them right after making the rots.

unal

[DaveH]

Unal,

Don't forget the pics please

DaveH

[ScrapMetal]

(05-29-2012, 12:37 PM)ukazak Wrote: Someway I have to enlarge the center hole a bit more, but how?
Unal

Why not get, or make, a very small boring bar and chuck them up in the lathe? Could be made with a chunk of carbide saw blade, grinding down an old piece of HSS tool bit, etc. Keep the length short as possible and it should be rigid enough to cut brass.

These center holes are slightly larger (about 6mm) but that's how I enlarged them after using too small of drill to make them:



Pain in the rear but it worked.

-Ron

[EdK]

Ed

[Tony Wells]

One approach I have used with good success is, lacking a proper reamer, use a standard sized single bearing ball. They are hardened, and smooth. If you drill a little undersized, the finish will allow the ball to burnish the hole to a precise size when pressed through. Then the mating part (shaft in this case) can me made to have the appropriate interference fit. Finish is very important, because in the case of an interference fit, assuming the shaft is solid and the hole is in a massive piece, there is no where for the displaced material to go, except for into the microvoids left in the surface finish. The ball method leaves a very smooth hole, and if the mating part is also turned or polished smooth, the amount of interference should be minimal.

Also to be considered is length of engagement. Naturally, the longer the engagement, the more pressure it takes to assemble, and you must consider the compressive strength of the shaft when calculating the fit. Some people actually use a little taper to make the fit a bit tighter at the makeup point, thus reducing the stress on the pressed part.

I believe that Machinery's Handbook has a bit of data on this subject as well.

[DaveH]

Tony,
I used that method once making some phosphor bronze bearings 6mm dia ball bearing for the holes, I have never tried it on steel, do you think it will work on steel?

DaveH

[Tony Wells]

Dave, it does work on nearly any material. Unless the hardness approaches that of the ball, or the finish is already too smooth in the hole, it works. It is one method used to lock in and resize valve guides on OHV engines. I used to run a "head shop" and did a few rebuilds of heads that required sleeved guides. Those liners were bronze, but sine it worked on them, I tried it on steel parts other places. I like it because you can get BB's of most sizes pretty cheap. In the head work, I used an air hammer to drive them through. As long as the hole was straight to begin with, the liners came out straight even driving a ball through. For other work, I prefer to press it through rather than drive it through though.

[DaveH]

Thanks Tony

DaveH

[ukazak]

Thanks to you all

, I improvised, combined, little bit of every ones suggestions.

Dave, you asked for pictures, here they are, sorry for the fuzzy's

Well, I made a micro boring tool from an old and dull 4mm drill bit.



Fixed to QC holder,



Carefully bored (regularly stop and check), there was only 0.40mm to go.




So, stopped when I reached that 3.90mm dia.



Than pressed using my mills vice.



Made sure it is straight and perpendicular,



Checked again



Than did the others



All went fine. As it looks from the assembly,





If it works, it is worth the trouble, at least, learned that; if you want to keep out of a trouble, start right at the beginning.

Cheers,

Unal