Thread Cutting 3mm Pitch Thread on HLV-H...

[cinematechnic]

Last evening I was setting up to cut a M24 x 3.0 thread (in soft aluminum) for a one off project. Then I looked at the threading chart and had to stop. According to the Hardinge metric threading chart, a 55T idler gear pair is used to cut 3mm pitch. I only have one 55T gear.

On (mercifully in L.A.) short drive home, I got to thinking: Why was I convinced that I could do this? Then I realized - for 0.5, 1 and 2mm pitches, the gear combination is the same and you switch the threading knob position 3 for 0.5mm, 2 for 1.0mm and 1 for 2mm.

I had successfully cut 0.75mm thread many times (I mostly do fine pitch threads). The gear combo (assuming 127T gear on shaft with 66T gear) for 0.75 / 1.5mm pitch is:

50 fist gear
66 2nd gear on stud
30 idler gear
22 screw gear

So why does that combination NOT work for 3.0mm pitch with the threading knob on "1"?

I studied the threading chart and in MOST cases, there is a 1:2:4 relationship to pitches that are cut with the threading knob in positions 1-2-3. But this logic does break down for a few pitch combinations. The ones I could find in a quick search are 2.5 and 1.25mm, and 0.4 and 0.8 mm where the gear combos are not the same.

Update: I did some further research and it seems that the idler gear does not affect the overall gear ratio. But I'm still confused why a gear setup that -should- cut 3mm pitch doesn't work. What was the reason for changing to the 55T idler?

BTW, I have a M24 x 3 die so as long as I can get some starter threads on that piece of Al I can finish the job with the die.

 

[L Vanice]

I do not own an HLV-H, but I do have three of the external gear sets for cutting 10 TPI threads. Each set includes a 22, 55 and 60 tooth gear and the bracket and stud assemblies.

So, I could sell you one of these sets and you would have extra 22, 55 and 60 tooth gears, which can be used for other pitches. You could resell the set after using the 55 T if you don't think you will need it again. Or you could look on eBay and see this 55 T gear for $67.82.
Hardinge HLV-H 55 tooth change gear $1 per tooth | eBay He has similar prices for other size gears.

As I read the manual, the two 55 T gears are on one stud and have no effect on the overall ratio of the gear train. That means, you could use two identical gears of any size that can physically fit in the train, like two 50 T or two 60 T.

Larry

 

[rons]

One day I might go for it...

 

[cinematechnic]

So I've confirmed that the idler gears do not affect the overall gear ratio. So why did Hardinge suggest using a 55T idler pair ONLY for 3.0mm pitch?

It is not because of fitting issues, since the gear combination is the same as for 0.75 and 1.5mm pitches.

Anyone know or care to take a SWAG (Scientific Wild Ass Guess)?

 

[L Vanice]

Every HLV-H lathe was shipped with a set of 22, 55 and 66 T gears to allow cutting 10 TPI. Maybe the 3 mm gear set was selected to use two 55 T idlers because you already have a 55 T gear, so you only have to buy one more gear. They have no way of knowing what extra non-standard gears a shop might already have, so maybe they went with a gear combination that would be least costly, rather than asking you to buy two new gears of, for instance, 50 or 60 T.

I will also mention that 3 mm is a coarser pitch than 10 TPI, the coarsest inch pitch that Hardinge includes in their TPI gear table. Such a coarse pitch may overload the half nuts and gear train/shear pin if you try to take too big a cut in steel.

Larry

 

[cinematechnic]

...I will also mention that 3 mm is a coarser pitch than 10 TPI, the coarsest inch pitch that Hardinge includes in their TPI gear table. Such a coarse pitch may overload the half nuts and gear train/shear pin if you try to take too big a cut in steel.

Larry

Thanks for the info.

I realize this is a very coarse pitch, equal to about 8.5 TPI. I will be cutting soft (6061) aluminum with a sharp HHS tool. The plan is to take a series of light cuts, 0.15mm / 0.006" per pass. I plan to use the "advancing the compound" technique for the first time (up to now all threads were fine pitch).

I was thinking that one possible explanation for the 55T idlers might have to do with the shocks when reversing the threading control. I've had the change gears shift on me because of that (and because I didn't tighten enough).

The HLV has a VFD and it soft-starts. So one way around that (the torque reversals) would be to shift the threading control with the motor off, then start the motor.