Press Fit Help Request

[Brian Rupnow]

The engine I am currently building is going to use 3/8" cold rolled shafting for the crankshaft. This stuff measures 0.374" with a micrometer. I want to buy an undersize reamer to ream for a press fit of the shafts into the 3/4" thick brass flywheels. The question is, how much undersize do I want? there seem to be an endless supply of different undersize reamers, but at $25 each, I don't want to guess wrong and buy the wrong undersize reamer. The piece of crankshaft that gets pressed into each flywheel is only about 3" long, so shaft deflection shouldn't be a problem. Any knowledgeable help would be much appreciated.---Brian

 

[enfieldbullet]

according to the machinery's handbook:

force fit FN 6(permanent) for 0.20' - 0.40' - about 0,5 - 2 thou interference.

 

[canadianhorsepower]

Hey Brian
this is the set I was talking about yesterday

http://www.micromark.com/overandunder-chucking-reamer-set,9757.html

As for pressing the crank together I will drill and ream a 1" piece of steel
to guide everything

 

[Cogsy]

Using my bearing books, the tightest interference fit for a shaft that size is 1.5 thou. I realise it's not a bearing fit we're talking here, but knowing how tight that fit would be, I'm sure it would be classified as 'permanent'.

P.S. I was out the back having a smoke and I worked out you weren't intending to press the whole assembly together, came back inside and you'd responded - sorry - just made an assumption and jumped to a conclusion...

 

[Woodster]

Any use to you?

 

[kvom]

If it were me, I'd just use green Loctite (e.g., 620) with a sliding fit. Very solid hold, and can be removed after application of heat.

Alternative would be a shrink fit - heat hole, freeze shaft, slide in.

 

[Brian Rupnow]

Okay--Faint heart never won fair lady!!! After reading all the responses on this forum and the other forum I hang out on, the general consensus is about .0005 to .0015 interference for a press fit on a shaft this size. Since the shaft I want to use is 0.374", I have ordered a .3735" chucking reamer, which is .0005 undersize. That should get me close to where I want to be. I will leave enough material on the flywheel for a "skim cut", and true it up after pressing by holding the shaft in the chuck jaws of my lathe and taking a very light cut on the flywheel.----Brian

 

[Dave Sohlstrom]

Why not just drill undesize and bore to the fit you need. Then you know the hole will be concentric with the OD of the fly wheel.
You run a real chance that you drilled hole will not be concentric and that the ream will ream over sized hole.

Dave

 

[OrangeAlpine]

I generally ream .001" under. The resulting fit is definitely a force fit, but not real strong. I think you will be disappointed with the strength of the half thou fit. I would not attempt to turn anything with such a fit.

My crankshaft has 9/16" thick discs, 3/8" rod journal and the one under reamed hole was not sufficient. The crank came out of register while I was working on it. But then I'm pretty clumsy, can't be sure I've really picked up something until I've dropped it at least twice. Had to drill and tap so I could secure things with set screws. The mainshaft seems to be holding nicely with the interference fit, but then it has no real torque on it. At least not while I'm working on it.

Bill

 

[Brian Rupnow]

Yes, and the world might end tomorrow, and the Russians might drop the bomb!!! The whole reason I am going to a reamer is to AVOID the necessity to bore to within a .0005 fit. Its okay guys. I'm learning here. Worst case scenario is that Brian has wasted $20 on the wrong size reamer. If so, then a lesson will have been learned.

 

[mu38&Bg#]

Brian is that .374 a machined surface (round and decent surface quality)? I don't think I would count on a .0005" press fit on cold rolled stock. Press fits generally require a machined surface.

I agree that 3/8" is not too small to bore, but you'll want to make gauge pins or pull out that .0001" bore gauge to measure it accurately.

Greg

 

[Swifty]

Brian, when working as a toolmaker and having to bore a mild steel dieset to accept a pressed in case hardened guide pin of 1 1/4" dia, I allowed an interference of .0005". If the hole surface was a bit rough, a bit more was left on as the high points would push away as the pin was pressed in.

You also have to bear in mind that a size for size pin and hole will not go together, ie a .3750" pin will not go in a .3750" hole without a bit of pressure. Also watch out that the pin does not seize in the hole because of being similar materials. Also if too tight you run the risk of mushrooming the end of the pin.

When I was grinding cylindrical hardened parts to various fits, I used the same tables that Woodster has posted.

I don't want to be negative on this, but it may pay to do a test on a piece of scrap first. Have the pin and hole finish as smooth as possible and use a bit of oil when pushing together, or maybe loctite.

Paul.

 

[Brian Rupnow]

Its a cold rolled steel shaft going into a brass flywheel. I will smear a bit of green Loctite on to act as a pressing lubricant. I will start out with my manual arbor press. If that fails I move to successively larger hammers----

 

[Swifty]

If that fails I move to successively larger hammers----

Brought back memories of a press shop that I did a lot of work for, they had about 6 hammers next to a bench ranging from about 2lb up to the heaviest sledge hammers, each one had a longer handle as they got heavier.

Paul.

 

[kvom]

If you're going to use green loctite you don't need a press fit.

 

[Goldflash]

Why make it a press fit. Make a shrink fit.
Make it an Interference fit as per the tables supplied in preceding posts . IE about .001" interference.
But heat the Flywheels in the oven to about 200 deg C / 450 F to expand the bore that the shaft will fit in . ( shouldn't take more than 1 hour to reach a saturated heat level )
Put the shaft in the freezer overnight to shrink the shaft.
The shaft should just drop in the flywheel.
I would make up a bush / Guide that would sit on top of the flywheel and allows for the shaft to slide down into the flywheel and will insure that the shaft goes in Square.
In the past I have used liquid nitrogen or dry ice mixed with methylated spirits to shrink shafts / valve seats / valve guides etc but using mums fridge freezer worked as well.
The problem you have with a press fit of a hard material like steel into a soft material like brass is that sometimes when pressing the shaft in it actually removes some material from the bore and the result is that you end up with a loose fit.
Requires a good set of leather gloves ie welders gloves or fingers might get burnt or frozen.
Food for thought

 

[Brian Rupnow]

Goldflash---That is very interesting. I have seen this suggested numerous times on different forums, but have never tried it. I know that many times over the years I have heated a "frozen" nut or bushing with oxy acetylene to expand it enough to start it turning with a wrench or press. I am a total "Babe in the woods" when it comes to press or shrink fits. My flywheels are 3/4" thick. I plan on "tapering' the first 1/8" of shaft with some emery cloth, in the lathe to get the shaft started. The flywheels will be polished brass, and I'm afraid any kind of temperature sufficient to expand the material would also discolor it.---Brian

 

[Brian Rupnow]

If you're going to use green loctite you don't need a press fit.

Kvom--I have a bottle of green Loctite #648 which specifies "For Press Fit"---Even if it doesn't have any real bonding effect, it will act as a pressing lubricant.

 

[Goldflash]

Brian, Shrink fits are quite easy to do and there is less risk of things jamming or misaligning when doing the assembly.
I wouldn't worry about the brass discoloration as once the flywheel is on the shaft you can always spin it in a lathe chuck and clean it up with a bit of metal polish.
Long time ago I did my time as an Aircraft Fitter Turner / Toolmaker. Shrink Fit was standard practice for replacing bushes, shafts etc in aluminium forgings on aircraft parts.
Shrink Fit was also standard practice in the old days of built up crankshafts on Steam Engines, Motorcycles etc. ( Mainly on the Axles but sometimes on the crank pins )
Ralph

 

[kvom]

The steel "tires" for steam locomotives were installed on the wheels via shrink fitting. The coefficient of expansion of brass is greater than that of steel, so reheating should allow separation later if necessary. You can always practice with some smaller bits of scrap before commiting.

OTOH, I still recommend Loctite 620 and a sliding fit for simplicity.