This will not be so much a straight how to as a rundown of my own methods for engine turning a firewall for my roadster. When someone asks an engine turning (or jeweling) question the answers range from broomsticks and scotchbrite pads to buying pre-turned sheets or vinyl to simulate the look. There tends to be a lot of good info too, I'm just suprised at the number of people willing to accept some of these alternates in leu of the real thing. I realize that the pre-turned sheets are really turned - but it loses something when you just cut the shape you need from the pre-fab stuff. I think the Scotch Brite method - while resourceful - doesn't have a nice crisp edge to each swirl and the vinyl idea is just goofy - looks more like 70's prism tape than engine turning. If you didn't see the post last week I had started the process with a hardwood dowell and some parts store valve compound marked "fine". Check the photo. Some experimenting and I ended up with some decent results. You can see the RPM I used marked on the dowel. The problem was having to constanty load the dowel with compound - about every half row. Then it would get too much(?) and gall the material, plugging the dowel and leaving a ring on the next cut. I think you can see an example in this photo. (It shows as a dark ring in the swirl) The slot I cut in the dowel helped, but the problem was still there.
So I decided to use a stick of Cratex. I bought this from the local machine tool supply house. Good stuff. It is 7/8" and "fine" grit. The next time I order I will probably step up to the Medium grit because the fine doesn't cut quite enough to eliminate the previous swirl. Meaning there is some ghosting - but I'm jumping ahead.
From there you pick your pattern and go. I did a sort of staggered pattern with each swirl overlapping the previous one by roughy 1/4 to 1/3 the size of the swirl itself. I didn't really do the math - I just eyed it, noted the digital scale and took advantage of it. I chose to do a staggered pattern with each consecutive row offset exactly half a swirl width from the previous row - and overlapping the aforementioned 1/4 to 1/3. Here is the first row being layed down. You can pick up a little of the ghosting I mentioned earlier. I think I was running close to 700 RPM with this.
Cool thread. There's a fine line between pre-fabbed perfection (those turned blanks) and a piece that has a SLIGHTLY less perfect, home shop appearance. Just seems more like the real-deal. Sucks, the fire department's rides use the cheezeball vinyl engine turned stuff, not to mention the vinyl cut characters. YUCK
Here's the digital scale I was sort of using. The only thing it was really good for was figuring out where I needed to be to pick up another row and be in the right spot. For instance, the top of the firewall is curved so I didn't cut the whole square - the top row was short. The scale gave me the right place to start and pick up that previous row dropping a swirl right between the two above. Like I said I really didn't do the math, but the eye worked pretty good. Does .6424 fall anywhere within the 1/4 to 1/3 range with a 7/8" circle? Wonder how well those numbers plug into the Fibonacci system?
Here's the pattern really starting to take shape. You can see the offset. I talked to Bruce L. about making the swirls perfect, or purposefully making a few of them "off" to make the piece seem more real, for lack of a better word. What I came up with after talking to him was not to purposely miss now and then, (that would be kind of lame) and not to use the readouts so much that everything was cold and machine like. But to use the accuracy of the tool and my eye to get it as perfect as possible. The staggered rows help this greatly. I think the key is to get the top to bottom spacing of the rows dead on - even using the digital bits if you have to - and eye the horizontal.
.64225 is 26.6% of .875 (7/8")...yes it does fall within that range at the very bottom, but in the range nonetheless...Cratex works great for engine turning, until it starts chipping off...it sucks trying to get it flattened out again...might as well pick up some medium and coarse while you're at it, you might need the coarse for stainless or other types of steel...and there's a bunch of different sizes of it too...a trick I liked for using the wooden dowel was to spread the lapping compound out on the sheet using a plastic body filler spreader...get it even and then you don't have to reload the dowel with compound, and the swirls in the compound show up as your guide for the next swirl to go...
And again, here is a shot of the shop I had the good fortune to use. That's Phil Wasson, owner/operator of SRMW behind the computer tweaking the code on the sprockets you see in the foreground. He's a true hero.
So lets see the finished product. How long did it take? I'd like to see some more shots of that tool with the digital x,y readouts. I'm with Seymour, I never thought I'd hear talk about the Fibonacci Sequence on here either!
For turning small parts on my drill press, like aircleaners and valve covers, I made a little X-Y axis table out of some wood I had laying around. 1x10's I think. I covered each face with masonite so things would slide easily and then had a fence for each direction with 1/4" marks that I layed out with a tape measure. It's like you said about it not being perfect... the marks were layed out by hand (with a sharpie) and when each mark is indexed during the process there is some error. Things look very nice and have less of a CNC look and more of a hand made look. I'll try to get some pictures of it for you guys tonight.
What you see in the photos is what I had after roughly three hours of work. I took plenty of breaks to clear my head and just dork around. (There is a BMX shop, some ramps, and a few lowriders being built in the same space) Also the firewall is about three inches bigger than the table will travel - so there was a lot of clamping, unclamping and moving to finish off each row. I could try to get a shot of the whole machine next time I'm down there working, but I'll bet if you snoop around the Super Rat site (link above) you'd find a shot of it somewhere. It's definitely labor intensive work to do a piece this big if you're wanting really nice results. Don't know if you can make out the red Sharpie line in the photos, but that is the outline of the firewall. The heavy black line was the outline from my last attempt with valve compound - I flipped the material and started over. I'm pretty amped - after the turning is finished, I'll clean the piece, retrace the outline with a sharpie, chop it out, dress the edges with a flap wheel and bolt it in place. From there I can start working on my new floorboards and transmission tunnel. These are probably stop-gap measures until I can get a bellypan and some sort of foot-wells made.
I've given it some thought. Maybe a limited run? Right now I'm researching engine turning round objects and things with compound curves. Django - What I'm calling a digital scale is just a glorified calculator that's hooked into the mill.
The Jalopy Journal
