Threading problem...

[Th232]

Short problem I'm having with cutting threads, they're just not coming out the right size. For reference, I'm on a 9x20 and I'm making an M12 drawbar for my mill, thread should be M12 x 1.75.

I've followed the charts in the manual, same as the one shown here:
http://www.finelinehair.com/home/9x20_lathe.htm#Gear_Ratios

Did a light pass to check whether my ratios were right, and the result is something with a pitch of ~0.5 mm, although I really can't say for sure, I think I need to get pointier calipers.

My troubleshooting so far has resulted in no hits:
* Change gears are the right ones (42 and 36), I've checked them at least 4 times now.
* Gearbox is set to the right position
* Gear "b" is engaging the 120 t gear (not the 127 t)

Before I go off and manually count the teeth in each gear, does anyone know what I'm missing?!?

 

[Doh!Nut]

Going by the chart, if the 42 and the 36 were swapped over, you would get a pitch of around 0.5 :thinking:

Other than that, I would get a test piece and try to get another thread size, perhaps one that requires the lever in a different position, that might eliminate some source of error.

 

[Th232]

Swapped gears was one of the first things I thought about, definitely in the right positions though.

Thanks for the suggestion about a different size, will try a different pitch tomorrow (it's 6 pm here and I've already cleaned up), see how that goes. 2 mm should do it, will require changing both gears and the lever position.

 

[precisionworks]

If all else fails, McMaster carries all thread rod made from 4140 chrome moly steel - probably better than the drawbar that came with the mill. $20 USD plus shipping for a 1m length.

Part Number: 93325A200

http://www.mcmaster.com/#threaded-rods-and-studs/=6k1czf

 

[gadget_lover]

Going by the chart, if the 42 and the 36 were swapped over, you would get a pitch of around 0.5 :thinking:

Other than that, I would get a test piece and try to get another thread size, perhaps one that requires the lever in a different position, that might eliminate some source of error.

When trouble shooting it's generally a good practice to follow up on an explanation that fits the observed behavior. If the transposing of the two gears will give the .5 pitch, then reversing them from the current position should give you the 1.75 that you are looking for. If that does not work as expected, then there are other problems. If it does work, then you need to check your interpretation of the charts.

Daniel

 

[Th232]

Picture of the gearbox when I was attempting a 1.75 mm pitch thread (yes, the lathe was on the ground. Long story):

That was yesterday. Summary of the last 3 hours today:

* Swapped gears, thread pitch only got smaller.
* Confirmed that the gear sequence was correct (again).
* Tried 2 mm thread pitch (a = 60, b = 30, lever = 7). Pitch was (again) too small.
* Moved lever back to position 1 (that should now give me a 3 mm pitch), tried again. Pitch was too small, turned out to be a bit over 0.9 mm. I don't have a set of imperial thread pitch gauges. The closest imperial pitch I can see is 28 tpi, but if it is indeed 28 tpi, that means:

a) Change gears are both meshing with the same gear, which they aren't.
AND
b) Gear a = b = 30 t, and they most definitely aren't the same size.
AND
c) Lever is in position 9 instead of position 1. I checked the gearbox, and the gears in there are definitely in the right locations.

120/127 = 0.9449, might mean something?

Very confused.:sigh:

 

[Th232]

Well, when in doubt, read the manual. Then read the parts you only skimmed over.

I was using the feed lever, not the half-nut lever.

Had to wiggle the feed lever to disengage the safety. All working now though.

Sorry about wasting your time guys, hopefully this thread will serve as a good reminder to RTFM!

 

[darkzero]

yes, the lathe was on the ground

Wait, so you had the lathe on the floor during your threading operations? Oh man a new type of machinist, "the lazy machinist", machining while laying down. Good idea!

Anyways, good to hear you got it figured out.

 

[Th232]

I wish that was just laziness, neither the lathe nor the lathe stand had the bolts to attach the lathe to the stand, and the parts lists didn't specify the size of two of them! The blocks the lathe sits on were off with a friend who's digging up the bolts.

That and I was a bit impatient, I wanted to make that drawbar.

Lying on the ground working on the change gears is not fun!

 

[gadget_lover]

Now that we have established that you can cut thtreads, you can make your own bolts.

Nice looking lathe.

Daniel

 

[Torque1st]

Study the use of the half nut and the lead screw.:thumbsup: