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Machining an 8" Featherweight SE/Prism/Parallel/Level Casting


Jun 14, 2008
Bellingham, WA
I was contacted today by a PMer inquiring as to whether I have a write-up for my preferred way to machine my 8" Featherweight casting. I looked back in PM and could not see where I have detailed the methods that work well for me. So, I will start by briefly describing how I do it and then, the next time I do a batch of 8's I'll take pics. and add them to this thread. The 8 is my best seller with by far most of them shipping as raw castings, but I have machined about 30 of them on my Bridgeport mill for folks that did have the time or means to do so. I have machined in the vial pockets on only a handful.

So let's start at the end. ;-) Here are some pics of a finished 8 that includes a level vial and a cross vial.

Machining notes:

Firstly, one nice feature of the 8 is that it is short enough that it fits into my Kurt vise---if I remove the removable portion of the jaws. So, do I grip them on the steel surface of the permanent jaws? No, I use 1/4" thick plywood (very) soft jaws as padding. That plywood padding grips the jaw extremely securely as the wood/steel interface has much greater friction than a cast iron/steel interface. I first rotate the vise so its long axis is parallel to the x of my table. I secure it with hold-downs and indicate it parallel to the x axis though you really do not have to do so. WIth the hold-downs securing the vise long-ways on the table it is not convenient to use any of the standard Kurt vise handles to close the vise, but a 3/4" socket on a 6" exension works great with a ratchet handle to open and close the vise jaws.

Once the vise is secure I usually start milling the back of the prism. (The back will end up vertical in ordinary prism use.) I place the casting in the vise with the jaws just snug enough to prevent the vise from falling out of the vise and insert a 3 or 4 inch face mill in the mill. I locate the prism under the face mill and lower the face mill so that it touches the back. I wiggle the casting a bit until it the back seems to be tangent to the face of the cutter. At that point I snug up the padded jaws. Then I touch off the cutter on one end of the prism and confirm that it is more or less making a flat cut across most of the back. (Normal casting irregularity makes this an approximate process.) I zero the z axis and move to the other end of the casting and see if I make similar contact on that end and see how much difference there is in z elevation on that end vs the first. I figure 10 thou or so variation is close enough. If needed one end can be pushed up or down a bit. Now it is time to cinch down the vise and partially crush the wood pads.

My mill will easily take 30 to 50 thou passes moving at 10 to 15 IPM and running full speed in back gear. I can usually clean up the back in 3 or four passes. If your cutters are sharp, very little heating will occur in the casting---just slightly to mildly warm to touch.

At this point, I slow my feed to about 4 IPM and reduce the cut to about 4 to 5 thou and make sure I am trammed so that a good cross-hatch pattern occurs. That pattern will usually only fully develop on left or right travel but not both directions due to some normal deflection of spindle bearings. Cross hatching will be easy to achieve with heavy fast cuts, but a little tweaking may be needed for light slow cuts. One other consideration is swarf control. I like to use a piece of cardboard strategically magnet-located on the vise to keep from having cast iron bits 5 feet from the mill, down your neck, etc.

Next up, I usually do the sole. I like to indicate in the just-machined back so that it is within a few tenths of vertical. It really does not have to be exactly parallel to the x axis. Just good and vertical is good enough. I mill the sole much like the back.

Next, I do the 45 degree (at least close to 45) face. For that face I use a simple mechanical drawing plastic triangle to check 45. (You could be more exacting using a little trig and your DRO. But that 45 angle is a clearance angle and not a reference angle) I am careful to make sure the intersection of the sole with the face is quite parallel to the intersection of the back and the top rail and the finger relief groove. Otherwise, the intersection of the face and sole can be wonky. The function of the prism will not be impaired. But it looks like garbage. Follow the previously outlined cut strategy. The whole while it is worthwhile to make sure that not much heating of the casting is occuring. Mild uniform heating seems not to result in any measurable twist, bend, bow etc. But I would be wary of final machining a hot casting.

Next comes the top rail. I check the bed of my vise with an indicator to be sure it is parallel to the x travel. Then I stone the sole and place the sole on the bed of the vise long-ways and tap it down while snugging up the wood-padded jaws. The cuts on the top rail are similar to the faces. After making a cool final cut, I mic the top rail to the sole on each end and make sure the difference is less than a thou. It may be necessary to use a shim of mylar (.001") under one end of the casting and repeat the rail cut until it is within the tolerance you want.

Finally, I clean up the ends. I did make a fixture that holds the milled casting very accurately vertically, nesting the casting on both the sole and back surfaces. For practical use, just a nice neat cut would be good enough.

Once that is all done, I take the casting to my 2X72 belt grinder to just take off the sharp edges. Running the vertices parallel to the belt travel results in a nice smooth neat edge treatment. All corners are just lightly touched. If needed, I'll use a die grinder on the finger relief groove.

I do then check the milled surfaces for flatness on my surface plate. I am amazed at how well the Bridgeport maintains flatness on the 8 and 18 inch castings.

At that point I feel it is a matter of how pretty someone wants to make it and I don't want to spoil all their fun.

I hope the above makes sense. And thanks to the PMer who inquired.


PS: I hear lots of talk of people machining various iron castings and running into hard spots, foreign material, voids etc. So far, I have yet to encounter such in a Featherweight casting. And no one has ever reported such. I hope the record holds for a long time. I do my absolute best to keep the casting quality top notch.


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