-if you are going to buy one here's some of my thoughts about the subject.
1-a Bridgeport typically has an R8 collet quill.
-I feel the R-8 shank diameter is pretty puny for swinging a large diameter flycutter.
-On normally head milling machines the typical cutter diameters are usually way larger than you'd ever use on a Bridgeport and you'll probably have to seriously compromise the diameter to get rid of chatter.
-and normal head milling machines have the cutter tilted down on the forward edge slightly so the rear of the cutter doesn't drag a pattern part way across the head as it starts to come across the back side near the end of the cutting pass.
-some people try to run a small diameter flycutter and make multiple passes and I personally feel this is nearly a futile effort.
2-I have a mill but it happened to have a #30 quill which in my estimation is substantially stronger--so I built my own cutter using a SBC cast iron flywheel.
-this gets me about a 11" cutter path----and I try to tilt the head of my mill very precisely to drop the leading edge down .001" and the trailing edge up .001" as a minimum to maybe as much a .0030" total tilt.
-even with this minimum amount of tilt this causes the cutter to machine several thousands of concavity which is actually measurable across from the sides of the head---about .002".
3-feed and speed and depth of cut---I use 6 adjustable cutter teeth---carbide tipped boring bar bits---into the underside of my flywheel based cutter.
-I wanted the cast iron rather than steel--to eliminate most of the chatter or vibration.
-I hand sharpen my carbide cutters using a diamond wheel on a 3/4hp grinder motor.
-the cutters do not all cut exactly the same in regards to the leading edges of the cutters---but they can all be adjusted to exactly the depth.
4-Cast iron heads, aluminum heads, where you are only cutting cast iron or aluminum--meaning not machining thru valve seats or other hardened rings like is common on diesel heads---means you have basically have to get the rotation speed, the feed rate, and the cutting depth figured out for each of the two metals.
-but when you have the cutters hitting hard seat or the rings you have to cut across on the diesel heads---now you have totally different issues to deal with---mainly chatter and smear.
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-I built my own because I didn't see anything on the market at the time (20 years ago or so).
-here is some story and pictures of my setup---and I can machine decent surfaces and do it all in a timely manner.
http://ryanbrownracing.com/Bill_Jones_Page_1.html
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-All I'm trying to get across here is I feel that anything you can buy for a Bridgeport is going to be a compromise---and I really don't know if it's possible to get the rigidity speed using an R8 quill.
-There should be about a hundred guys on here who are using Bridgeports---so you should be getting their advise and stories here shortly.