crepping up on it

[omegaghost]

i was wanting to know how or what is the best methode is for making a press fit. i know this is possibley a can O worms subject but i could use some advice as i have blowen past a press fit twice now and it is getting me anoyed. first time i goofed second time i did not take that the part i was turning down was pretty warm and upon cooling it contracted blew past it agine. method and technich would be helpful as i am learning as i go only had the lathe for about a month .

[sdopp]

You said it right in the title of your post!
Creap up on it, let it cool or maybe even use
a damp cloth.
Skip

[handlewanker]

Hi 'Mega, as an oldie from way back, we always used the "rule of thumb" guide of .001" per inch of diameter for a reasonable press fit on shafts.
We used to machine loco wheel tyres that were 36" diam and had an interference fit of about +.036". They were shrink fitted with heat.
It took 80tons of pressure to press them off when the tyres needed renewing.
For a light press fit we always used the "size for size" formular whereby a 1.000" diam shaft would be pressed into a 1.000" diam hole.
The hardest part is to measure the bore size as it takes a bit of experiance to "feel" the inside mike or telescopic gauge to get the size right. Just takes practice.
That is one of the problems with getting fits right.
If you wanted to get a 1.000" diam hole to be a slideing fit for a 1.000" diam shaft, you would have to bore the hole to at least .001 oversize to get the shaft to enter.
It can't be done with a reamer as the standard reamer size would give you a 1.000" diam hole and you want a 1.001" size.
If you're making to a customer's requirements, then the hole or shaft size will be stated, on the drawing, with a tolerance, and not merely indicated as a "press fit".
Then you will be using plug or snap gauges if the volume is big enough, or just a micrometer, inside 'mike or telescopic gauges as required.
Using the rule of thumb for press fits of .001" per inch of diam will cover almost all of the average press fits, and gives you "muchee plenty squeezy and no plenty bustee", a phrase coined by LBSC in the '40s.
At the same time, it must be realised for instance that metal displaced goes somewhere.
If you were to press a bronze bush into a hole then the resulting squeeze will reduce the bore size of the bush by the amount of the press fit, and this must be added to the running clearance for the bore diam, otherwise you'll be scraping your heart out.
Ian.