Union Pacific Big Boy in 3/4 Inch Scale

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RET
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Joined: Wed Jun 07, 2006 8:36 am
Location: Toronto, Canada

Re: Union Pacific Big Boy in 3/4 Inch Scale

Post by RET » Sat Dec 29, 2018 2:34 pm

Hi,

A little bit about the mandrel. It is made in two parts, the right hand (long) piece fits inside the 9/16" bore and extends down to the gland packing for the end of the cylinder. The other side bolts through the rod hole and has an anti-rotation pin that withstands the high cutting forces necessary for machining the outside profile. I use a cap screw so I can torque it up tight. If I don't, the cutting vibration will loosen it (don't ask me how I know that). The following pictures show the progress of sinking the profile in the blank.
Power Reverse Crosshead Guide-3.jpg
Start of the sinking process.
Power Reverse Crosshead Guide-4.jpg
Next step.
Power Reverse Crosshead Guide-6.jpg
Further along.
As you can see, the cutter is advanced in depth (10 thou per cut) and it makes multiple passes in the course of a cut to traverse the entire surface. When this process is completed, the wedge shaped pieces on each side will be cut away completely leaving the two half moon extensions and the ribs that support them.

RET
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Location: Toronto, Canada

Re: Union Pacific Big Boy in 3/4 Inch Scale

Post by RET » Sat Dec 29, 2018 2:51 pm

Hi,
Power Reverse Crosshead Guide-7.jpg
Where I am at the moment.
As you can see, its just a case of carrying on until the deep cuts reach the shaft of the right hand mandrel. At that point the "triangular" pieces will fall away and I'll use a combination of regular and riffler files to finish the part. Since I'm making two power reverse units, I'll have to repeat this process with a second blank.

Once the code is written and debugged (not hard) the whole thing is quite boring. You will find that once you get the bugs worked out, this is true for quite a few things.

It takes an hour and a half to do one pass and you are looking at 16 passes or more. Yes, I know I could have alternately used a built up process instead, but I wanted to see what I could do this way and I think the end result will look better, like the two power reverse cylinders I made previously. While they took a lot of steps to make, they look like castings.

I'm currently also working on the 4 poppet throttle and the top feed check valve assembly They are also "finiky" but they'll get done eventually. Big Boy is still a work in progress, but I get great satisfaction when each piece is completed and works the way it is supposed to.

Richard Trounce.

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Carrdo
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Re: Union Pacific Big Boy in 3/4 Inch Scale

Post by Carrdo » Sat Dec 29, 2018 10:06 pm

Ladies and gentlemen,

It is a lot more tricky than what Richard is telling you.

For example, as one cuts deeper into the part as the 4th axis rotates the part, the ribs will come out tapered (because the rib edges are rotating radially in this case) unless one does some program trickery to reconfigure the cutter (or is it the part?) midstream to have the cutter cut them parallel.

Richard explained to me twice how this was done and I still didn't get it even though I was there seeing it happen. Maybe it is just me as I am a CNC luddite.

RET
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Re: Union Pacific Big Boy in 3/4 Inch Scale

Post by RET » Sun Dec 30, 2018 10:48 am

Hi,

To explain what Don is talking about, when you make this part, all the CNC process is really doing is going through a similar series of the steps you would have to do if you made it using traditional methods. The big difference is CNC doesn't make mistakes, but it will do EXACTLY what you tell it to do even if that isn't what you want.

"Home" position is where the cutter is shown in the first picture after the drawing. In "Home" position, the cutter and part axes intersect, with the cutter axis (as you can see in the picture) at the big end of the part. I always choose "home" to be an easily recognizable position which I can get back to if I manage to make a complete mess of things and have to start over.

From the home position, to cut the ribs you have to offset the cutter either to the left or right (forward or back in the pictures) half the diameter of the cutter plus half the thickness of the rib, so you will be cutting with the side of the cutter on the side of the rib. I always use climb milling wherever possible, so the first pass will be on the front side of the top rib in the picture. The cutter is a 1/8" dia. ball end end mill and the ball end is there to form the radius that would be there on a casting. I used the same cutter for the cylinder body for the same reason.

Once you get half way around on that first outside cut, you must step back the diameter of the cutter plus the thickness of the rib so when you get to what is the other side of the bottom rib, you will be cutting the proper side of that rib (sounds complicated, but its simple when wrap your head around it the right way).

Once the outside pass is complete, the intermediate fill in passes (until you get to the inside trapezoidal block which will be cut away) don't really need to be done using the offset steps, but if you don't, you have to figure out what the cutter depth would be to get the finished depth right. Its easier just to continue the way you started.

Sometimes the logic of why things are done a certain way isn't immediately apparent and someone else might approach it differently. This is just the way I see it. Hope this helps. Maybe in a day or two, the cutting passes will be finished.

Richard Trounce.

RET
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Re: Union Pacific Big Boy in 3/4 Inch Scale

Post by RET » Mon Dec 31, 2018 4:33 pm

Hi,

I've just finished with all the passes and it worked just the way I hoped it would. The cutter actually cut into the mandrel a little bit as I expected and since one side of the cutter went a little deeper, that side broke free first.
IMGA0961.JPG
Almost at target depth
Originally, I thought the cutter would simply cut through and free the unwanted portion, but I thought it would be safer if I simply used pliers and broke off the excess bit. That way I didn't have to worry about the part possibly jamming the cutter.
IMGA0963.JPG
One side removed
Power Reverse Crosshead Guide-12.jpg
Both sides removed. You can see the surplus pieces on top of the rotary headstock.
Now the next step is to use files to finish off the part. This and the cylinder are the major parts for the power reverse; making the cylinder cover, piston, piston rod and crosshead guide are relatively easy.

Since I'm making two, I'll have to make another blank and run the mill again. As you can see, its a slow process, but it works.

Richard Trounce.

RET
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Joined: Wed Jun 07, 2006 8:36 am
Location: Toronto, Canada

Re: Union Pacific Big Boy in 3/4 Inch Scale

Post by RET » Mon Dec 31, 2018 4:51 pm

Hi,

Part way through this process I overloaded the Sherline spindle motor and it burned out. I'd previously bought a two HP permanent magnet DC motor from Princess Auto (cheap) and I decided to use this as the replacement for the Sherline fractional horsepower one. It took a while to install it and I had to buy a control for it, but the end result works just fine.
Power Reverse Crosshead Guide-8.jpg
Replacement spindle motor.
The motor can accept up to 130 volts DC. which is oddball and the control puts out 90 VDC if you feed it 120 VAC so they aren't really a match, but with that combination I'll still get one horsepower, which is more than enough and a lot more than I had before. If you feed the motor 130 volts, it will run at 3200 rpm, so at 90V, it should do 2400 rpm. With a 2 1/2 to one pulley speed increase, that works out to 6,000 rpm max. spindle speed. Much better than I had before. Its great to have all the power I need.

I ran the cutter at 3600 rpm and it didn't show any signs of getting dull, so that's another bonus.

Richard Trounce.

RET
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Joined: Wed Jun 07, 2006 8:36 am
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Re: Union Pacific Big Boy in 3/4 Inch Scale

Post by RET » Sat Jan 26, 2019 12:23 pm

Hi,

I have used riffler and ordinary files to finish one side of the crosshead guide and the machining grooves disappear quite easily but in this last while, I have been working on the top feed assembly. What you see isn't quite finished, but the finishing touches are relatively minor. The 4 bolt holes have to be countersunk for the capscrews that hold the part onto the boiler. The boiler connection will be sealed with an "O" ring.
Top Feed -5a.jpg
This picture shows one view of the part.
The silver soldered fittings are angled to fit the curvature of the boiler and will have an "O" ring to seal both the screwed in fitting and the stainless ball that forms part of the check valve assembly. Where possible, I like to have the same part perform multiple functions, just good engineering.
Top Feed -6a.jpg
A view of the top of the part showing the ball check position.
The caps still have to be made that cover the oval holes in the part. These caps will be bolted in place and will seal with an "O" ring in a groove in the underside of each cap.
Top Feed -7a.jpg
Final assembly.
This shows how the final assembly of the piping bits and pieces would look including the size of the stainless ball that will be used on each side to make the check valve.

It takes a while to plan all this out; using CAD to get the sizes and angles right and then playing around with the Bridgeport to make the part. Because the groove isn't round, I will have to use the CNC mill to make the "O" ring groove in the underside of the cover plate.

This is where I am at the moment. More to come in a bit.

Richard Trounce.

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