How to get a good finish or proper fit?

[Jadecy]

This is a fairly broad and undefined topic so I will try to break it down into more specific questions.

1) Cast Iron (cylinders)
a) How much undersized do you make the bore to leave material for honing?
b) How much undersize for lapping?

2) Stressproof steel (crank or any part that rides in a bronze or brass bushing/bearing)
a) How much larger do you turn it to leave material for grinding or sanding to fit?
b) What method do you use for grinding or sanding?

3) Can you describe some methods/processes that you use for getting show type finishes on external parts and processes you use in the home shop to obtain proper fit?
a) Aluminum (I use mostly 6061 T6)
b) 1018 steel
c) stress proof steel
d) cast iron
======================================================================

I have common practices I use but I have been doing this for such a short time and have never had professional training that my ways don't always turn out so good.

I've used emery cloth on many steel parts. I have left somewhere around .002 on the part for finishing. The uneven pressure on the emery cloth on a long part sometimes leaves the shaft uneven in diameter. I try not to use lapping compound around my lathe for fear I will get it on my ways or some other part of my lathe and not get all of it wiped up.

I've used multple grits of sandpaper on aluminum followed by mothers aluminum polish which works pretty good for visible parts.

The only bore I've done so far that is on a completed engine was in durabar ductile cast iron and I finished the bore with a reamer. I sized the piston to fit the bore using fine sandpaper and a lot of time. The engine works well but I should have made a lap as described in the construction notes.

After seeing some of the fantastic machines posted on this site I have no doubt many of you have better techniques than I.

Everything I've learned so far about machining have either come from the school of hard nocks or from the generosity of those on the web be it youtube videos or posts on forums or sites specific to the hobby. Heck just last week I learned how to cut keyways using a broach by watching a video on youtube of a 7 year old girl cutting keyways under her fathers supervision! Up until that point I had never seen a broach used.

;D

 

[1Kenny]

For polishing I use buffing compounds on cotton wheels. The only color I dont have is the blue. On the right side of the link Caswell has a free how to guide you can down load. My 3/4 hp Baldor buffer runs at 3600 rpm so the wheels are running over 7000 feet per minute. Let the wheel and compound do the work. If you force the work into the wheel you will end up with more scratches than you started with. Have a rug under the wheel to prevent damage to the part if it comes out of your hand and hits the floor. Also, a buffer is one of the tools that you will need to wear gloves with. For little parts, have a few lined up to do as they get very hot quick and you can keep buffing while others are cooling down. With aluminum, if you push it into the wheel and hold it in one spot too long it will turn the surface a brownish color. The color can't be buffed out. I like the brown sticks for brass and aluminum but the green works good too.

Kenny