I've very carefully avoided learning too much about Bridgeports, because I didn't expect to ever be able to get one - but my wife has just given me the green light...so now I need to do a little learning!
Is there a Bridgeport FAQ? I'm looking used, in the $2k - $3k range, what should I expect? Can you get a 220 volt single phase version? In a non-production environment, is variable speed essential or will an 8 speed belt change work as well, if harder to change speeds? What is essential to use it to do valve seat work on automotive heads? What are good alternatives to Bridgies, beyond the off-shore stuff?
Thanks, Guys, I want to get the right machine the first time!
I like the step pulley belt models with a VFD for speed control. Its better than the mechanical vari-speeds since you get "instant" braking and reversing. I hardly ever change the belts on mine anymore, just sometimes put in the back gears. If you're tight on $ you could start with a step pulley machine first and add the VFD later.
Also if you only have single phase power available with a VFD you get 3 phases to the motor and full power without an additional phase converter.
A lot of the used machines you will come across will have 3 phase motors. If you don't want to get a VFD but find a good machine that needs 3 phase power, the "static" phase converters work ok for most peoples usage. You only get 2/3's of the full motor power, but thats plenty for most machining operations. Before I got my VFD I used a static converter for many years and never had a lack of power, even taking pretty heavy cuts on steel. A static converter (which is really just a capacitor and relay in a box) costs about $100.
There are alternatives to a real Bridgeport that are pretty good- Lagun, Sharpe, Gorton, and others. Poke around on the web groups for some opinions on these. In my opinion all the chinese machines are a big step down, although some of the Taiwanese machines aren't too bad, especially the earlier ones.
One thing thats important is to be able to tell the difference between a machine that's been used hard (ie production) versus one thats been in a prototype or tooling shop and doesn't have a lot of miles on it.
Backlash, condition of the ways, spindle runout and general appearance of the table and the spindle can usually tell you how the machine has been used.
Take your time and look at a bunch of machines before you buy and you'll get a feel for a machine that's worn out versus one thats in good shape.
Probably the best deal today is to get the 8-speed (1J) model with the
3-phase motor, and use a VFD to get variable speed AND the single to 3-phase conversion. The variable speed feature adds height to the machine, and also some expensive and hard to fix parts that tend to go bad about the time the machines are retired. This is what I have, and I have never really missed the varispeed option (that is the 2J head). I can't, in fact, use a 2J, as the top of the motor would be within 1" of the ceiling, and cause a lot of problems.
For $3 K you should be able to get a very good machine. The klunkers usually sell in the $1750 range, and can be anywhere from good but rough looking, to very badly worn. Get the chromed ways if you can, they are far more robust. Examine the ways for signs of wear. Also, at the center of travel on an axis, measure the backlash in the screws, by turning the crank until the table moves, then turn the other way until the slack is taken up. Then, repeat the measurement at the end of travel. If the amount of slack differs, that is the differential wear in the screw from middle to end. It also becomes a pitch error in the leadscrew, over roughly half the length of travel, and therefore will limit the accuracy of any movement calculted off the dials next to the handles. If it is much more than one dial division, you will likely want a DRO, so you might as well find a machine with a DRO already on it.
Changing speeds on the 8-speed 1J head is trivial, it only takes me 15 seconds or so.
Valve seat work? I think much of that can be done with a good drill press. But, certainly a Bridgeport can do anything a drill press can do, only better. (With the exception of drilling a hole in the end of a 5' bar or pipe. There are ways to do that, too, by swiveling the turret.)
The advantage of a REAL Bridgeport is parts are available anywhere in the world for them, new or used. Some knockoffs have a number of interchangeable parts, some DON'T! And, you don't want one of those, in the event you have something major, like a casting, break on you. Also, the knockoffs may or may not allow various accessories to bolt directly on without modification.
Specifically for valve seat work, I guess a fly cutter would be used to cut out the old seat and prepare the head for a new insert. Then it would be used to cut the new seat to the proper angle. This could be done with carbide lathe tools in the flycutter. There are probably tools made specifically for this purpose, if you'd be doing a lot of that sort of work.
Once you have the machine, you will find a huge number of other kinds of work you can do on it. You might have a need for special fixtures, tools and holders if you do a lot of engine work.
Thanks for the great info. I know that BP's get discussed a lot, but this is such basic stuff that I was embarassed to ask, honestly. I have a Harrison horizontal mill, but the BP is quite different. I have a friend who uses them quite a bit, and he should be able to help me out here.
I'll look for an 8 speed as my preference, I don't like the idea of old worn out vari-speed pulleys.
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