G0709 gearbox: cutting wrong thread pitch

M

mcdanlj

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I'm trying to single-point an M4 screw thread on my G0709. It says that FARW8 is 0.7mm thread pitch. But it's actually cutting a much finer pitch, so fine that I'm having trouble gauging it accurately. Maybe 0.4 or 0.45 mm?

I have definitely confirmed, twice, that my change gears are in the F configuration. I have made sure that the ABC, RST, and WXY knobs only move to point to the three indexes, and engage at each index. I'm not sure how to confirm that the 1-8 knob is really in the 8 position since it rotates infinitely, without a stop, but it looked like it cut an even finer pitch when I moved it to 2, and FARW2 is the setting for 0.45mm pitch.

The exploded diagram for the gearbox in the manual is less than complete for demonstrating how the gearbox works. I'd appreciate suggestions for diagnosing the problem. The gear train in it is a mystery to me.

Thanks for any insight!
 
Take a picture of the thread pitch plate, your selectors and your gears so we can see what the specifications are as to the the settings. I would not be digging into the gearbox, they either work or will lock up if the gear selector forks are not operating correctly.
 
@mksj Thanks! To be clear, I've single-pointed successfully before. Most recently 24TPI. But I can't remember whether I've single-pointed a metric thread before. (I do use metric, it's just that it's all been tap/die as far as I recall.)

I wasn't going to start by taking the gearbox apart; it's just that I don't understand how it goes together to have a good mental model for what can go wrong.

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@Ulma Doctor On this lathe, the "W" configuration of the norton gearbox selects metric ratios. The change gear train has no 127-tooth gear; it gets that through a combination of smaller gears. Exactly what that combination is I don't know, but in the gearbox there are only gears with
16, 19, 20, 22, 23, 24, 26, 27, 28, 33, 35, 38, and 50 teeth, as far as I could tell from the parts list.
 
Start with the basics, you are aware that with an imperial lead screw you cannot disengage the half nut?
 
I've got a G0709, will see what I get when using FARW8. At work right now so it'll be a few hours. I've threaded metric on mine a number of times, can't recall the pitch however if any were 0.7 mm. Had no issues at all, but again, I can't recall which ones I was using.

To confirm your #8 position, maybe do an English thread that uses that same position and check it? Just use a Sharpie instead of a cutting tool, should be close enough for checking your thread if your pitch is off by a factor of 1 1/2.

Bruce
 
@BtoVin83 yes, and this shows up on a single pass; this is not multiple starts from disengaging the halfnut.

@BGHansen Yeah, I doubt it's something like wrong gears in train in the norton gearbox; that seems like an unlikely error to allow functional engagement... Good idea on trying the other letter combinations with #8 and using a sharpie, thanks!
 
Would it be possible for you to show a picture for the Change Gears from both a Face View and Edge View?
 
Combination of gears is "F" 32-76 33-61-66. There is no transposing gear (120/127) for metric, but the pitch change is achieved through a combination of change gears. A Norton gearbox is technically an open type with levers for the gear selection, you have a (semi) universal gearbox, typically no change gears for imperial similar to a Norton style. You do need change gears for metric on your lathe, and I would double check that they are in the correct position. The front panel selectors appear correct. FYI, you cannot use the threading dial for metric, you must do all the threading with the half nut engaged.
 
Another naive question - any chance you're engaging the power feed instead of the halfnuts?
 
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