This comes up from time to time so thought I’d start a thread to refer to. I was able to make this pattern in 75 minutes because I already had pin router profiles from another project. This pattern was the body for the bearing carrier in an automotive upright project but it looked like an air cooled engine cylinder without the fins to me, so I mounted it with some foam and in about 20 minutes at my pin router had these blanks. Then sliced them like salami on my hotwire Then glued them together and had this….. It will fit in a 5.5” cube. The bore is 3.25”, the fin thickness and spacing is .090”,and the fins are .875” deep. If it was a two stroke, you could add the ports. We’re going to call it a double overhead cam cylinder. It’s not bad for a quicky but if I was making an actual part I’d probably have to machine the fin features out of a block because the cumulative error from the variation in part thickness and glue joint thickness makes one side about 1/8” taller than the other. I kept all the pieces clocked with respect to how they were sliced for assembly but in hindsight if I clocked every other one 180 out from each other the error may have been less. To machine fins this size I’d need to buy or make a slot or winged cutter because I don’t have one able to cut these depths and wasn’t willing to put the effort into it for a sample/demo part. I’ll have a go at casting it next time I have the furnace fired up. Best, Kelly
Double dipped it in refractory coating. Cast it today. I was set up to video the pour but the battery on my phone gave up in the cold……it was only 10F in Eastern Iowa this morning. Casting is ok. It’s passable I suppose, but not totally satisfied with it. Between cylinder heads and heat sinks the need for finned castings comes up often enough for me that I’m going to have give a little thought to the fin thickness, spacing, and cutter options. I have some inexpensive 3” diameter carbide tipped saw blades I use on my die grinder for degating castings that might work well in a stack…. Best, Kelly
I think these little saw blades might make a good fin cutter. I had a 1/2" mandrel for my router for various winged cutters. The blades have a metric arbor diameter so I turned a bushing and some spacers from washers on the lathe. The blades say 7000rpm max. The lowest I can dial my router is 10krpm......hmmm....gonna be some butt pucker first time I flip the switch. With a dozen blades it will cut ~1.5" of height if I've got the nerve to load that many.....less with larger fin thickness and spacing. Best, Kelly
I have those same blades and was going to try a similar stack for making finger joints for boxes to organize the workshop storage. It would be good to know if super thin fins tightly packed actually work any better than thicker, more conductive cast fins considering cast aluminium is less dense (10% I think?) and less conductive than extruded. Tightly packed dense fins may impede airflow, although those radial aircraft motors would have shrouds to force airflow into their thin cylinder fins.
I'd be even more nervous in hard materials. It's a lot of teeth in contact. Foam is like cutting air. I'm sure you're right about there being a limit but I don't think I'm anywhere near it. There was always a push for finer fins because conduction is very fast and the additional surface area was needed for convection. Even though aluminum is not the best conductor, it's still a very good conductor, even cast. I think a tapered fin is actual preferred for the conduction path but I wasn't going to go to the trouble of trying to grind tapered blades. I've got stacked bits both straight and tapered. They are only 3/8" depth of cut but work very well... http://forums.thehomefoundry.org/index.php?threads/machining-xps-foam-patterns.14/#post-61 Finger joints can be very effectively cut on a table saw with a simple fixture. Best, Kelly
A multi blade tablesaw for wood is going to increase the risk of kickback, especially as the teeth get blunt and need more force to cut. I guess you're only cutting the ends and the teeth could be arranged to cut down towards the table but the result is still looking like a multi-blade, powered meat axe . One thing about aero engines, in a lot of cases the super fine fins are cut into the iron? cylinder barrels where the much lower conductivity of the material is a problem compared to aluminium fins. Somebody has crunched the numbers and thin fins with high surface area was the answer, much to the annoyance of foundry workers everywhere. I saw a photo online somewhere of sand casting cylinder heads and they were using lots of wire nails to keep the sand mould fins from breaking off. Edit: Compare the (much easier and saner) lost foam with this method: https://www.practicalmachinist.com/...se-elegant-pratt-whitney-radial-heads-307618/
You don't need a spaced multiple blade stack. Screw/Clamp a board to your miter gauge Cut a small groove in the bottom of the board and install a tooth the size of blade kerf. Then reattach the board to your miter gauge with tooth spaced the kerf width from your blade. If you want a wider kerf stack two blades adjacent or use dado. Set the depth of cut to your stock thickness. Now stand your drawer/box sides on end clamped to the jig attached to your miter gauge jig against the tooth for the first cut. For the next cut place the previous cut on the tooth and index your way across the board in successive cuts. The jig prevents chip out of the backside. If you make the tooth a light press fit in the kerf, cumulative error wont be a problem unless you are doing very wide joints or very fine fingers. It takes about ten minutes to build the jig and you can build it to suit any size joint you want I have fancy dovetail jigs but have used this method for years. There's probably a YouTube video out there if the above doesn't make sense. Best, Kelly
Thanks Mark. That is what I was trying to recall in the other thread. I thought there was video of same, but maybe just pictures......and just think how many they made that way...amazing!! If you're not a member of Practical Machinist I dont think you will be able to see the pictures. Here's the source thread for viewing. https://buickcity.blogspot.com/2010/08/liberator-engine-work-at-buick.html https://buickcity.blogspot.com/2012/02/r-1830-twin-wasp-radial-engine.html Best, Kelly
Not entirely relevant but I made a lighting fixture pattern that wouldn't pull because of the fin (heatsink fins) on its rear (green sand). The solution was to cut the fins off and dowel them back on. I could pull the main body of the pattern without pulling the fins then tease the fins one by one. Yes, that was a painfull casting exercise but it produced quite a few good castings when my back was up against the wall!
Here's the video of Curtis Wright aero engine cylinder heads moulds being rammed up for casting at the 3:59 minute mark.
Thanks again Mark. I knew I had seen it before so maybe my memory isn't completely shot, but had to watch that one start to finish. That was great. The numbers and man hours encapsulated in those production facilities let alone the level of technical accomplishment for 80 years ago is truly humbling. The cylinder head castings are a work of art but on the subject of finned cylinders, they had 43 fins cut at "seven times the thickness of a human hair" (about .021" thickness I would think), in Nitraloy (had to look that one up) that starts as 60lbs and finishes at 16lbs. That's like 43 parting tools cutting at once. Parting is my least favorite lathe operation. On machining deep thin fins on foam patterns, I've found machining them all at once and single depth of cut to be best because the foam is supported. Trying to cut them separately causes the unsupported side to deflect causing irregular thickness and often get torn off or damaged. Best, Kelly
I also watched start to finish and some parts over again. Lathe parting has been and still is a painful process, one thing I found with my 1967 small lathe and then the larger 1950's 14" swing lathe was that the base of the compound and mating top slide surface on both lathes were worn and allowed rocking under heavy cuts as evidenced by oil moving out from under the compound base. I was able to get the top slides surface ground and machine the compound bases slightly concave and all of a sudden parting off became a lot easier. There's a couple of Gypsy Major crankcases with conrods and crankshaft my father salvaged under the house, He had new cylinders and pistons still in the oiled paper from Australian WW2 production but gave the cylinders away to a friend who needed them in the 1980's. Again the fine pitch on the fins machined in the 1940's by GM Australia for the war effort was impressive. These are a 6100 CC four cylinder motor designed in the 1930's. https://collections.museumsvictoria.com.au/items/405154
Figured I’d do a little experimenting while waiting for some more cutting blades to arrive. The top speed of my drill press is 4600rpm which seemed ok for 24 teeth on a 3 3/8" diameter cutter. It cut just fine at that speed. I made three different thickness sets of spacers and between combinations of spacers and placing blades adjacent to one another can produce a range of fin and slot/spacing combinations with depth of cut up to 1.25” and used them to cut these samples, all at 1.06” fin depth. Since I have an empty side on the sprue, I might need to add a .110" Slot and .075" fin for grins. I don’t think they stand a chance of filling (the well-known lost foam prayer). Sprued and tree’d. Fins up and fins down for each size. Best, Kelly
I added a fourth test sample to the sprue and dip coated it. The thinnest fins deflected and stuck together in pairs due to the slurry surface tension. I separated them with a razor blade and they drained and dried fine. Now there is the matter of that foot of snow, more on the way, and my icy casting area……. So the additional saw blades arrived and I promptly conscripted them into service. I ran the stack at 10krpm on my pin router and it seemed fine. When I cut the first pattern I noticed the fins cut unevenly where the cutter was extracted from the pattern. So I mounted another blank figuring I’d just leave the pattern parked then extract it from the cutter after I shut the machine down. Traveled around three sides and then blew the fins off the final side……At least the fins are the first cutting operation on the blank so not much lost. So then I decided to try climb cutting………and that was the ticket. First climb cut pattern: So I cut some more…... These are very nice patterns compared to my initial pieced together approach. So I assembled a complete pattern. I think what I’ve learned is if I try to cut fin depths more than 1” and less than .1” fin thickness the fins deflect and can flap around causing uneven fin thickness and spacing, but climb cutting seems to help. Slightly higher density foam might help too. The test parts seem to cut the gaps .015” oversize and fins that much under size. I didn’t climb cut those test patterns. The thinnest fins are only .050” - .060”. I don’t expect those to fill to full depth but will be interesting to see how deep they cast. The cutter worked well enough I decide it deserved a box. I repurposed one side of box that came with a set of bits and attached a flat top to it. When disassembled it all goes in there. Best, Kelly
I was able to get in a quick casting session this morning and cast the fin samples. The first sample cylinder cast in this post gave its life and donated its body to science. Here’s how it came out of the sand. Fins pointing down are the lowest on the sprue. I used an ultra-fine point sharpie to part mark them and their orientation in the flask. Here they are degated and cleaned up. ……and labeled The larger two thickness and spacing samples cast fine irrespective of the fin orientation. For the two thinner finned and spaced samples, the fins down seem to fair better, but they were also the lowest on the sprue so had another couple inches of sprue pressure even though they might have received slightly cooler metal. But, that .075” Slot by .075” Fin almost filled completely. I think 5" or 6” more sprue height/head pressure and maybe a little heat and it might completely fill. Inetrestingly, the wider spaced of the two didn’t seem to do as well, maybe the thinner spacing concentrated heat and delayed freezing slightly…? One other note, the fins actually cut under size and measured .050”-.060” thick. They are all a little over an inch deep. The new cylinder pattern I made in the previous post is the widest spacing and fin thickness shown in the samples. It just looked right to me for the part. This test makes me feel pretty good about my prospects for a successful pour with the new pattern. Best, Kelly
Only managed a little shop time today. The pattern is dipped and dried. Looks good. I made another cylinder pattern. It’s a little smaller than the one above. Starting back 2 ½ years ago, I had been cutting fins with a stacked, winged, finger joint cutter. http://forums.thehomefoundry.org/index.php?threads/high-aspect-ratio-features-and-deep-draws.13/ The new saw blade stacked cutter allows twice the fin density which should be a significant improvement in cooling capability. Best, Kelly
Messed around with some simple shapes and added an Inline 4 cylinder pattern to the mix. Got more snow……ughh. Maybe an indoor melt but outdoor pour….no lost foam pours in my shop. Best, Kelly
