Broken Bolton BT-800

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aPpYe

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I was trying to make some blocks to raise the vise up closer to the spindle so I could do smaller things, namely some valve and other work on my airguns. I cut a couple of 2.5" lengths of an old trailer hitch and was happily milling the second one down when one of the gib screws got caught on the ways for the carriage (not sure about my terminology here) and caused the carriage to sort of jump a little bit. This had happened to me a few times before and I should have rubber banded the little floppy thumb screw thingies that tighten up the gibs. Kind of a dumb design. I will do something to keep these from flopping down again. Anyway, I shut off the machine to reset my work, turned it on, it spun up to speed for about 5 seconds and then quickly slowed and stopped. Removing the belts, the drive and secondary pullies spin freely. The motor spins right up. I can still turn the shaft by hand but it is tight. I can feel a tightness when turning the pulley at the top, and I can also feel a sort of chunky feel when turning the chuck. Something is gonna have to come apart I think. Might have blown a bearing? I don't know. I was running it at the full 3000 rpm for several minutes at a time, and I can see that I melted the grease in the lower bearing a little bit. I think it was that last jump that broke something though.

...Any advice?
 
aPpYe said:
I can still turn the shaft by hand but it is tight.
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sorry, i don't quite follow is that the motor shaft, an intermediate shaft or the main spindle?

disassemble and look for bad bearings and possible even a bent (spindle?) shaft, depending on how hard that lock-up and jump was.......
please post back.
-brino
 
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main spindle. The one that the chuck is connected to. I am thinking I have a blown bearing or something. Maybe something was just knocked out of place, and a broken pin or something? I will look into it tomorrow. This sucks because the mill (even the entire machine) maybe has 2 hours on it total...
 
hopefully something small.

sounds like you should also fix the root cause "floppy handles" before running it again too.

Good Luck!

-brino
 
I was looking at the Grizzly (g9729) version of that manual, this one offers a different perspective! I will grab that as well. The machines are essentially identical...

brino said:
Is the machine under warranty?

Perhaps a call to their toll-free number or email to their "contact us" link could help too.
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Lol, I doubt it! I bought the machine a few years ago and then life happened. Now that the baby is a full blown kid, I just last weekend dusted it off and starting legitimately playing with it.

... Anyway, time to stop doddling, take out the trash and look at this thing. I will post a pic of the floppy handles thing as well. They're gib screws so they shouldn't be all that tight anyway. I may just epoxy them straight.
 
If you were cutting steel at 3000 rpm that's a bit fast - I agree with your diagnosis, a bearing may have flown the coop
 
I can't help with your immediate problem as I have never had a similar machine. But I can't think why you would often need to loosen or tighten gib screws. Generally, they are adjusted to a compromise between tight enough so that there is no slop and loose enough that it doesn't take much force to move the movable part of the joint. Gib screws normally are socket head screws and have a lock nut on them.
 
Unless you are running a 1/16" diameter end mill, that is much too fast. Remember that the figure that controls is surface speed of the cutter at the OD, which varies directly with both the RPM and the diameter of the cutter. The Atlas MOLO has a table for optimum turning speeds of various types of steels plus sections on turning other materials. It also has a chart showing RPM versus SFM (Surface Feet per Minute) for various diameters of end mills, and says that the optimum cutting speeds for milling are about 2/3 the speed for turning. You haven't said what diameter milling cutter you were using but according to the charts and tables, you should probably be under 1000 RPM.
 
Huh. Here's my current theory. That "jump" pushed the bottom tapered bearing (part 559 in the diagram above) up just enough to cause it to bind everything up with the opposing bearing (563) at the top of the assembly. Once I started tapping down on the shaft (trying to figure out how to get it out), everything loosened right up. It is now TOO loose, noticeably wiggling around a bit. That being said, when I put pressure either up or down to engage either bearing, they both feel nice and smooth. I will attack the assembly this weekend with parts cleaner and check the races for damage, but it all feels pretty good honestly. If all is good, my thought is to make sure the races are seated correctly, then pull everything back together by tightening up that spanner nut (565) and then backing off once things start to bind again. I am not sure how I want this to feel. Obviously tolerances need to be tight, and I don't want the assembly to have any slop ...

wa5cab said:
You haven't said what diameter milling cutter you were using but according to the charts and tables, you should probably be under 1000 RPM.
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Yeah, it figures I was going too fast. I was totally just shooting from the hip, cowboy style! Maximum speed of course! The cutter is a 1/4" harbor freight (super ultra mega awesome quality I am sure) titanium nitride bit ... The set came with 2 and 4 flute bits, so of course I chose the 4 flute because that's what John Wayne would do. What is an Atlas MOLO?
 
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