Trailing truck advice.

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Jim_Nolan
Posts: 501
Joined: Sat Jan 04, 2003 11:03 am
Location: UK, North of the wall
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Trailing truck advice.

Post by Jim_Nolan »

Gents I might be over thinking this so I am interested in your thoughts. I am just about to machine the trailing truck on the 25NC. I am not sure if my thought process on setting the truck up is correct though.

Setting up front to rear was a simple DTI job but there were all sorts of ways to set it up side to side. I decided that getting the back cross beam square would probably best in mind of the subsequent machining needed. I could just get a vernier height gauge under the back cross beam. So I was able to set the little pads up that the rocker plates fit to square. Both pads will still need machining when the truck is turned over.

Image



The thing is though; this meant I had to shim the left hand corner nearest the camera up about 40 thou. The knock on of that is where the frame sweeps up to the machined tabs that the binder fits over this is only about 30thou below the machined face.

Image


So in the current configuration I will be machining into the frame a tadge.

So the conundrum is this, is it better to get the frame square and worry about the pads when I turn it over. Or do I keep it as it is and perhaps just machine the tabs for the binder a might shallow on that corner or is there a third way?

Things area at a stop at the moment as the grub screw key in the spindle has given up the ghost and the R8 arbour is slipping. I won’t get any more grub screws to the middle of next week but it’s a four day weekend in the UK next week so I am thinking I could box the whole thing off next week if you can make my mind up which way to go

Jim
www.northernsteam.com
AMatthews
Posts: 90
Joined: Mon Jul 27, 2009 8:46 am

Re: Trailing truck advice.

Post by AMatthews »

Hi Jim,

You are kind of between a rock and a hard place, my suggestion is to split the difference and machine them all to the same depth and make all of the tabs a little shallower that way a 0.015" difference in relation to the casting is negligible, the important thing is to ensure all your machined surfaces are square and true to each other, machine all the horn areas to the same measurement all the way round then turn over and use those machined surfaces as a datum to do your pads and pivot area. make sure you tram your diagonals from your pivot point to the horns so it rides square as well this should work out if you get it dead square on the mil, but it's a handy cross reference check. Will get back to you on your email tonight after work.

Cheers,

Andrew
JKreider
Posts: 311
Joined: Fri Oct 07, 2005 8:38 pm
Location: Redlands, CA

Re: Trailing truck advice.

Post by JKreider »

Jim:

I strive to achieve what Andrew is saying.
The way I lay these out for initial machining is to position it as you have shown and then shim it on the four surfaces so that I am machining all four bottom pedestal toe surfaces about the same amount or as close to a compromise as I can get. Now you have your vertical machining datum points. For squaring the casting side to side, I position the casting such that I am machining about the same amount on the pedestal jaw outside surface on say the upper left hand corner and then position the casting side to side such that I am machining the same amount on the lower right outside pedestal jaw surface. Now you have the side datums.
You may have to machine or file the four resting surfaces on the casting a very small amount to get the casting to lay as stable as possible without wobbling before it is clamped down. I try to clamp the casting such that the casting moves very little when clamped so that it does not distort very much when I remove it from the machine.
Machining the datum points on a sand casting like this very often involves making some compromises in regard to material removed and the final position of the datum points being in the same position relative to other casting detail.
Everything else is then machined from these datum points.
Sometimes function of casting details have to be brought into consideration. For instance in machining the Buckeye tender truck side frames for the Berkshire it is imperative the journal box pedestal jaws, transverse bolster slots and equalizer pivot points be machined accurately to the relative positions shown on the drawing but if a boss or detail for say, holding the brake hangers comes off some from the drawing, I’ll move the hanger pin hole to the center of the boss since I can tolerate some latitude in regard to the location of the brake hanger.
Complicated castings can be fun if that’s the right word.

Jim Kreider
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