What should be the allowance for press fitting?

[ukazak]

What should be the allowance for press fitting?

Hi,

Fun shoppinn’ time to everybody,

Is there a rule of thumb or something like that which will determine the press fitting?

I mean, to a given drilled hole size, what should be the parts size could press fit? Lets say the drilled hole size is 4mm, and part to fit 4.05, 4.10 or 4.20, … what should be for proper size? What is “too much”, what is “not enough”… any tips, please..?

Unal

[PixMan]

There are so many variables to take into account that it's not easy to make blanket statements about how much to allow. Among the factors to consider are:

material type
diameters
surface finish
size tolerances

Also, press fitting isn't as firm as shrink fitting. When you can heat up a bore and have it shrink to the mating part, that is often s stronger bond than simple pressing.

[DaveH]

Unal,

This will give you some idea, this considers the Hole is dead to size and the Shaft dia. is altered.

It also assumes the Shaft is softer or equal to the hardness of the material that contains the hole.

From ¼” to 2” dia. Shaft/Hole (Sorry it is in inches)


All these numbers are in Thou’s. ie 0.5 = 0.0005”

For a Force fit the Shaft dia will be + 0.5 + (0.75 thousands of an inch per inch of dia.)

So put another way it is Half a Thou PLUS Three Quarters of a Thou for every inch in dia.


For a 1” hole the shaft will need to be .0005” + (.00075 / inch) = .00125” So the shaft needs to be (near enough) 1.0013”


For a ½” hole the shaft will need to be .0005” + (.00075 / inch) = .0005 + (.00075 x ½”) = .0005” + .000375” = .000875” So the shaft needs to be .5009 (near enough)



This gives you a “run of the mill” force fit for mild steel, brass, phosphor bronze, aluminium.

The finish is important and assumes a “reamer” type finish, to the hole and shaft.

It is also very important to take notice of what Ken has said.

Some times it is much better to “Loctite” the parts together, doesn’t stress the parts.


DaveH

[ukazak]

Thank you guys…,

I was expecting something on that nature like Pix Man said that would come. I thought of pointing out the problem would save all much time… I will do that next time.

But, I was hoping something like what Dave explained. I got the picture.

I had few times press fitted some parts on Engine projects.. each time I got lucky. Than again, working with soft materials I was.

This case is 4mm stainless shaft will go in brass crank disks. I drilled the center with 3.5mm. and thought I had somewhere around another drill bit between 3.5 to 4mm. but I cant find it. Now, I have all the other stuff fixed to the sizes I cant change. (ball bearing 4mm) so I need to enlarge the hole to a size that I can fasten to the shaft.





Someway I have to enlarge the center hole a bit more, but how?
Unal

[DaveH]

Unal,

So you want to do it “back about boot”.

One way is to drill and ream it out to 4mm dia. and loctite it in. Try the fit first on some scrap brass first.

Another way is to drill it out to 3.9 mm dia then use a hand reamer until the shaft just starts to enter the hole. Then press it in.
What is important here it to use a hand reamer not a machine reamer, a hand reamer has a long taper to assist in keeping the reamer central and perpendicular to the hole axis. So your hole will have a slight taper.

Before you do this second method it is best to have a practice on similar material and size just to get the hang of how far the hand reamer needs to go in.

DaveH

[ukazak]

Thanks, Dave,

Bummer! I dont have reamer of that size... and cant find my 3.9

but, how about taping with M4, than locktite and press. I cannot put too much of presure on the shaft, afraid of bending.

I'll take the advice, good idea to practice with scrap first.

cheers,
unal

[DaveH]

Unal,
Yes, screw it together fine, just make sure it cannot come unscrewed, especially when you don't want it to.

DaveH

[ukazak]

Dave,
not meant to screw it, just tap the hole at the crank disk and press fit the shaft through the threads, instead of straight drilling 4mm.

Unal

[DaveH]

Unal,

@#$%$^%$ ^$# *$#@ $%#!! ^^


DaveH

[DaveH]

Unal,

Here is another way

Drill the brass 4mm dia, then carefully centre punch the shaft with 12 centre punch marks (4 rows of 3) around the shaft. Then press it in, the burs may just hold it in.

Sometimes the shaft can be knurled to help the fit


DaveH