Boss or no boss?

[Runner]

Hi all,

I am just about to start machining the driver and driven wheels for a 3.5" gauge steam locomotive. They are 4.5" in dia machined. Unlike wheels I have machined before, these come with a boss at the centre of the reverse side. I don't know if this boss is an aid to machining or just an artefact from the casting process. I assumed at first that it is an aid to machining and tried to use the boss to mount the wheel in a 4 jaw chuck. I was able to adjust the positioning so that the wheel was reasonably concentric to the lathe axis, but there was a significant wobble and the wheel could not be made to run square to the lathe axis. This is because the chuck jaws are stepped and the centre of the chuck the area of the jaws in contact with the wheel's back face is small. Should I cut off the boss and machine the wheels using a backplate slightly smaller than the wheel dia? If anyone has experience in machining wheels with the boss I would like to know the steps involved.

Thanks in advance

Brian

 

[Herbiev]

Just my suggestion. Mount the wheel centerally on a back plate then machine the boss round. You can then mount the boss in the chuck and proceed from there.

 

[GWRdriver]

Hello Brian,
Yes this is in all probability a chucking boss rather than the remnants of a casting spue. You can tell which one it is, a casting sprue will have an uneven and rough finish from an abrasive grinder saw. My recommendation would be to cut or turn this boss away and turn the wheel by conventional means. The reason for this is even if you have everything aligned the boss is relatively small in diameter for that wheel OD and won't provide enough stiffness during machining to avoid chatter at the rim. Chatter is recurring problem with castings of thin section, such as driver castings, which aren't well supported at all points because the wheel and tool want to "run away" from each other. The other consideration is alignment. Most good builders I know set up a wheel for machining by setting the inside edge of the wheel rim, not the tread, but the rim, where the spokes terminate, true and concentric and work from that point. This gives a wheel rim which doesn't wobble as it rolls along. When there is a machining boss, you don't know if it's concentric to the wheel face or not, so to insure concentricity you must begin at the wheel front rim and work toward the back, making rough facing and OD cuts, from which you can finally indicate and turn the boss. That's a lot of steps and a lot of bother and you will still have to contend with chatter. My own approach would be to use a 4-jaw chuck to set the wheel BACK to run true with front rim and carry out the rest of the machining with the wheel mounted on the faceplate.

What locomotive are you building?

 

[Runner]

Thanks Herbie that was something I was considering.

GWRDriver, I think that you have identified it correctly as a casting sprue since the face shows signs of a being cut with a grinder. Thanks for the tip of using the inside rim as the reference, its something I shall use. The locomotive is based on LMS Black 5.

Regards,

Brian

 

[chipenter]

If you mount the wheels by the boss , the chances of chatter when the treads are machined especially the radius are really good , machine them all the same size and use that to locate on a face plate or fixture .

 

[GWRdriver]

The locomotive is based on LMS Black 5.

I knew it was a BR standard something from the counterweights.

 

[Runner]

Yesterday was a bad day in the shed. Apart from Adelaide reaching 42 deg C the wheel castings have hard spots at the rims and is destroying the HSS cutting tools. I was attempting to machine the back face. The centre bosses are OK. Does anybody have any ideas on what I can do to overcome this problem?

Thanks in advance

Brian

 

[GWRdriver]

Brian,
Two options, first go to brazed carbide bits to see if you can get under the chilled spots. That may solve the problem, but if the chilled spots are too hard and/or too deep even carbide won't do it and you will want to look for someplace which can heat-treat (anneal) them for you. Sorry to hear this. There is nothing as nice to machine as a good cast iron (IMHO) but nothing as bothersome or aggravating as chilled iron.

 

[Runner]

Thanks Harry,

I was thinking that maybe an option. I have managed using a coarse file to break the skin on the affected area, so does this indicate that a carbide tip cutting tool can do the job? It was much harder than filing steel but it does indicate that progress may be achieved.

Brian