Posted: 3/13/2026 9:18:27 PM EDT
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I've got my mill for sale (not here) and was checking it over so I have answers when buyers ask. I think the mill is in good shape from my experience using it. I'm only selling because I don't have the power to run it anymore and I have both a smaller mill and a big ass VMC (for when I do get power) that make it redundant. When it comes to runout and backlash, how much is expected? How much is too much? The way I was checking backlash was to crank the table one way, zero my indicator, then crank it the other way .010" and compare. So if the indicator moved .005", then I have .005" of backlash. Confirm by going another 10 or 20 thou on the crank and see if the indicator was still lagging behind the same. Now that I think about it, I could have just referenced the DRO instead of digging out the indicator and mag base. I ended up seeing .005" backlash on the x axis. Didn't matter which direction I ran the test. They both agreed. I had .005"-.007" backlash on the y axis. To me, those seem like perfectly acceptable numbers in the center of travel where most of the wear will be. Now spindle runout. I'm sure I could have gotten a better setup, but I used what I had. At the nose taper, I had maybe a tenth or two. Could barely see the needle vibrate from movement. Next thing I did was to grab my drill chucks and a 12" piece of 1/2" TGP rod. All readings were taken at low rpm at the end of the rod. So about 11" from the chuck, roughly 14" from spindle nose. Chuck #1 was .023" total runout. So .0115" deviation from center. Chuck #2 was .010" total runout. So .005" deviation from center. Unknown brand, used, 1/2" collet was .007" total runout. So .0035" deviation from center. The test rod appeared to be true at the spindle/chuck. Maybe my arbors are bent, the rod is bent, or some combination thereof. Either way, considering how far out I was measuring, those numbers look fine to me. This is a 26 year old Jet bridgeport clone. So it likely wasn't even perfect when new. What do the professionals think of these numbers? |
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What are the backlash specs for when it was new? Spindle run out is most important. The user is responsible for buying collets and chucks with the concentricty necessary for their production needs. I always operate a Bridgeport type mill to compensate for potential backlash. I would rather own a tight JET mill than a sloppy Bridgeport. The Bridgeport at work has been abused and neglected, but you can still make good parts. |
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Originally Posted By Deerhurst: Drill chucks are notorious for runout. Use a collet. TGP isn't necessary straight. I've found that to be true time and time again. I did test with a collet. An old, used, unknown brand, but a collet. I guess, my feelings on the matter are that even if the TGP was dead straight and my numbers accurate, nobody is machining with a foot of tool stick out. Even if you were to run that long of a drill bit or reamer, it's going to be flexing like a wet noodle anyway. I think it's a good, tight, and accurate machine. I'm just trying to be able to prove it. If I was going to order a test bar, what should I get if TGP isn't reliable. I have a 1" bar I made for checking out my lathe chucks and setup, but need something for the mills. I have 1/2" now. Maybe a 7/8" or whatever the largest R8 collet size is would be better. Then just keep the 1/2" for my drill chucks. |
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Originally Posted By M4BlackRifle: What are the backlash specs for when it was new? Spindle run out is most important. The user is responsible for buying collets and chucks with the concentricty necessary for their production needs. I always operate a Bridgeport type mill to compensate for potential backlash. I would rather own a tight JET mill than a sloppy Bridgeport. The Bridgeport at work has been abused and neglected, but you can still make good parts. No backlash specs in the manual that I could see. Google AI says .010"-.020" is normal though. The manual did have spindle taper runout specs. Hand written, said dead nuts .0000" at nose and .00055" at 12" out. I'm going to call bullshit on that though. If somebody farts in the next room, a 12'' length of metal will move that far. |
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The backlash is fine. It's not really a critical thing anyway, though lots of people are concerned about it. The only time it is of any concern is if you're climb milling or if you're so old school that you don't use a DRO. No DRO means that you'll lose tracking of actual position if you reverse the travel direction. If you're milling to a shoulder that could be an issue. Otherwise just maintain the backlash in the same direction and it's not a problem. If runout at the spindle taper is close to zero you're probably fine, but you could check at top and bottom if you have an indicator that will get up in there. This an R8? |