Question about machining sizes for press fits

[AndyStobbs]

Just wanted to pick the brains of those members who have experience of machining parts to achieve different fits.
I have been making a repair part for a machine that formerly had a taper fit with key between the shaft of a hydraulic motor and a wheel for driving the machine.
I have turned the (damaged) shaft down to a parallel size and machined a top hat shaped sleeve to fit the shaft.
The wheel in turn is bored out to a parallel size so as that the sleeve OD is a sliding fit inside it. The wheel being secured to the top hat with bolts - much like a car.

Question relates to the fit between the motor shaft and the new sleeve. The shaft ended up at 1.027" so based on previous experience (I'm not a machinist by trade and have had no formal tuition in such) I fancied a 0.002" squeeze as it were. So bored the sleeve to 1.025", with a gentle lead in. The contact area between the two is 1.5" long. I was quite surprised to find it took 25 Ton to mate the two in the press.

Is it that my measuring equipment is off or is that pretty much the norm?

FWIW I have come to feel that two parts made to the same size give a press fit that can be easily achieved in the vice or with smallish threaded bar ie. 8mm.
0.002" over size seems to make for a nice sliding fit, such as found between a pulley to be grub screwed to a shaft.

Are my measurements incorrect?

 

[MattH]

I'd off also gone for 0.002 under size but would of heated it before pressing.
To really work out the interference sizes the material and wall thickness of the bush need to be taken into consideration.

 

[daleyd]

Have you got a set of Zeus tables, they give the shaft sizes for interference fits/clearance fits etc? I think you can get a download online, can't remember where though..

 

[AndyStobbs]

I did heat it, and naively expected it would drop on haha. Couldn't heat it much after the fact due to seals and such in the hydraulic motor.
Material EN1A, 0.900" (length) at 0.320" nominal wall thickness, remaining length at 1.5" nominal wall - due to top hat shape.

Sorry that should be EN3B material.

 

[mike 109444]

How are you measuring the two items ? outside mic for shaft ? and DRO calipers for inside ? There will be difference between even the inside measuring with the calipers compared to the same calipers used on the outside diameter. Remember the accuracy of your measuring equipment when dealing with these sorts of tolerances plays a BIG part in correct sizing. Also how ROUND was the finished item !, it may mic up ok on some parts but vary over length hence why stuff like that is normally rough turned (+ say <0.010" )then ground to finish.

 

[AndyStobbs]

Digital calipers on both. The shaft I did emery cloth as a final step. The inside bore I didn't polish etc.
I must get set up with a bore gauge and additional micrometers.
I agree that the surface consistency and finish are not going to be exceptional. That said the question still stands in respect of what translates from finished size to fit up.

 

[gordon stephenson]

0.002" should be ok, Did you find that the o/d of the sleeve had increased after pressing onto the shaft?

 

[AndyStobbs]

0.002" should be ok, Did you find that the o/d of the sleeve had increased after pressing onto the shaft?

Thing looks like a mushroom now. Got some filing to do tomorrow.

 

[MattH]

You'll be amazed how much a set of bore gauges and a decent mic show up how loose a caliper is at getting good measurements.

 

[brightspark]

You'll be amazed how much a set of bore gauges and a decent mic show up how loose a caliper is at getting good measurements.

a caliper will do 99 percent of jobs the extra few percent use a mic for accuracy

 

[Simon-]

Telescopic gauges and a nice Mic work well for me.

A Zeus chart is a great investment for just a few bucks.

http://us.misumi-ec.com/pdf/press/us_12e_pr1263.pdf

Something like a u7 force fit tolerance is probably what you have.

Sometimes I heat the bore in an oven if possible and freeze the shaft in dry ice from a c02 cylinder before pressing parts together.

 

[CKJDJK]

A download pdf of Zeus is available here http://www.green-oval.com/data/zeus.pdf