Subject: Laser-cut live steamer frame (long)

From: [email-masked] (Greg Lewis)
Date: Tue, 6 Jun 1995 20:06:37 GMT

I decided to have the frame sides for my live steamer laser cut from 
plate. Regular readers of r.c.m. may remember my queries last fall about 
the best way to go. Here's an update:

There is a firm here that has an enormous laser-cutting machine that will 
do any profile you can program into the CNC device. It will allegedly 
cut up to 3/4-inch-thick steel. It's quite a thing to see. It will take a 
plate at least 8 feet wide and probably 20 feet long. It cuts noislessly, 
and the work is only slightly warmed at the end of the cut. You can pick 
it right up bare-handed. The quality of the cut is quite smooth. Much 
better than a torch cut; somewhat like a rather rough machine cut. 

The frame sides are somewhat like a ladder stood on edge -- two straight 
lines that are the top and bottom stringers, and the rungs, which become 
the vertical supports as well as the pedestals for the axle boxes. The 
exact profile is a little more involved than this, with a special shape 
at the front for the cylinder block, and a taper at the back where the 
cab sits. If you are familiar with the Allen Models ten-wheeler, then you 
know what I'm building.

The finished frame sides are to be 1/2-inch thick. I had seen several 
pieces of 1/2 HRS at the metal yard that had a waviness in them that 
would be hard to deal with, so I planned to use 5/8-inch hot rolled plate 
and have the 5/8 thickness blanchard ground down to the required 1/2. 
This was to avoid any thickness variation in the piece of HRS, and get 
rid of any warpage problem. I didn't want to use CRS as I was afraid of 
warpage.

Well, the *engineer* at the laser-cutting shop talked me into using 
1/2-inch CRS. He said he would cut from the middle of the sheet and flood 
it with water to keep it as cool as possible. I figured he knows what he 
is doing, so OK. 

Well, he called me after cutting the first piece so I could check it out. 
But the machine operator cut out the outside profile before cutting the 
windows (between the *rungs* of the ladder...) and the workpiece shifted 
and was way out of spec. (The laser is supposed to cut to plus-minus 
.010.) 

The engineer wasn't happy with the smoothness of the cut, either. I 
thought it was a bit rough, but acceptable. The piece had a warp that 
made it slightly dished if it were laid flat on a table. This isn't a big 
problem as I can bend it back, and the cross stringers will hold it 
straight. He tried to tell me that this was because the plate was warped. 
I didn't believe that, but....

Well, he called again in a couple of days and wanted me to look at
another piece, so I dropped by. This one was cut from a piece of 1/2
HRS, and looked good. They had put a new lens in the laser so the cut
was smoother, and the piece of plate he used was quite even. Due to
the dish-warp, which I realized was from the cutting process and not
the material itself, I asked him to flop the program in the computer
so there would be a left and a right. This way, the assembled frame
sides would pull against each other and not both pull in the same
direction.

I was actually getting four pieces, two for me and two for a friend. So 
when the engineer called again to say the job was ready, I arrived to 
find that two had been cut from HRS and two from CRS. The HRS ones ended 
up with more dish-warp than the CRS ones. I was grumbling about that, so 
the engineer said we could put them in the press brake and straighten 
them. (This shop has some *big* equipment... they can do just about 
anything with just about any thickness of plate....)

But the press brake operator said that wouldn't work... the pieces would 
just spring back. So he decided to run them through a slip roll. This 
didn't work either as it created a twist, or skew to the piece along its 
length. So, after fixing that by some careful bending in a vice, he got a 
big hammer and whacked on each frame piece in a few judicious spots and 
each one laid down just as flat as you'd please. 

So now I've got two pieces of HRS and two of CRS. My friend doesn't care 
which he gets, so I'll need to decide which to keep. Both sets are quite 
flat. The laser cutting didn't really hold to plus-minus .010; it's more 
like .020, but no matter. I left enough machining allowance to take care 
of that.  I'm leaning on keeping the CRS ones as they will machine up 
nicer, although I may have some warpage when I cut out the stringer to 
open up the bottoms of the pedestals where the axle boxes fit in. But 
that will be no problem to bend back again.

Anyway, it all ended up working out. The engineer said he wouldn't do it 
for that price again ($130 per piece including material), but hey, he 
said his degree was in business, not engineering, and it's not my fault 
he had problems with it. So far, I'm happy. Next step is to bolt the two 
sides together, take them to my friend's place and set them up on the 
Bridgeport, mill one edge as a reference point and work from there. I'll 
make one long pedestal binder to go the whole length of the bottom edge 
before I cut open the webs that are temporarily holding the pedestals 
together. With luck I'll have the frame on its drivers and suspension by 
the end of summer. (But don't hold me to that!)

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