Live threading question

[Chris Abaya Cues]

Hello everyone. Happy Thanksgiving!

Does anyone know if it would be possible to calculate the rpm and feed rate for cutting inside threads? For example, 3/8-10 threading on shafts.

I have a midamericapool's lathe with the CNC attachment so I would have the ability to control the feed rate. The motor is variable so I should be able to control the spindle rpm using a tachometer (probably).

I am just curious if there's a formula for cutting threads.

Thanks for your time.

Chris

 

[Kim Bye]

You would need a leadscrew of some sorts than turns at the correct speed.
Todd is here on the forum, I`m guessing he can give you a definitive answer if this is at all possible on his lathes.
I have a thread milling attachement on my Cuesmith deluxe, you can change out cogs attached to the headstock and leadscrew to get different TPI, much like a regular metal lathe.

 

[Chris Abaya Cues]

Thanks Kim. I did talk to Todd and he did say the same thing you said but out of curiousity, isn't threading just a matter of calculating the right combination of spindle rpm and feed rate? So, if i have the potential ability to control these 2 factors, wouldn't it be possble to do threads?

 

[seanjonsean]

U can do it with out leadscrew but its takes some creativity

Sent from my SM-G920P using Tapatalk

 

[tsp&b]

It would be extremely difficult without a spindle that is directly controlled by CNC. I believe that the basic formula would simply be for each revolution of the spindle the carriage would need to move 1/10 of an inch. But like I said that would require that the spindle and carriage are locked in coordination with each other either by direct gearing or by 3 or 4 axis CNC.

 

[GBCues]

It has been a while, but in the past there have been a number of different and ingenious jigs and methods of setting up live threading on cue lathes discussed here and in the Cue Machinery forum.
It is not a matter of spindle speed at all, but of advancing the cutter along the axis to match the thread pitch desired.
Try searching both of these forums for "threading" or "live threading" and see what comes up.
Hope this helps,
Gary

 

[Travis Niklich]

If you lathe is run using a G & M code for a "CNC Lathe" your feed rate would be the pitch of the thread your trying to cut.

Pitch is 1/TPI
So for 3/8-10 you use 0.100 or in lathe code F 0.100
The RPM doesn't matter on a lathe G code as the feed is in IPR (inches per revolution)

If it programs like a mill it's a little more complex

But for 10 TPI you could run

100RPM and feed at 10 IPM (inches per minute)

or

10RPM and feed at 1 IPM

or

1000RPM and feed at 100 IPM

 

[Thomas Wayne]

It would be extremely difficult without a spindle that is directly controlled by CNC. I believe that the basic formula would simply be for each revolution of the spindle the carriage would need to move 1/10 of an inch. But like I said that would require that the spindle and carriage are locked in coordination with each other either by direct gearing or by 3 or 4 axis CNC.

Actually it's not too difficult at all - assuming the configuration of your CNC machine is such that your spindle moves in both the X-axis and the Y-axis without moving the cue around. That is to say, if your cue remains stationary (except for rotary indexing) while the cutter head does the moving... then the only other thing you need is to be able to mount a shaft vertically under your machine, with the business end running up through a small hole in the machine table.

If you can to that, and mount some sort of simple jig or stationary collet to hold the shaft in place, you can cut threads around the perimeter of the shaft tenon using a single-point thread mill and helical interpolation in the Z-axis. I used to do all kinds of threading this way before I built my CNC gang-tool lathe.

TW

 

[tsp&b]

TW, what you say is absolutely correct if you are working with a 3 or 4 axis mill. However I believe the original poster (Chris) is working with a simple 2 axis lathe that is set up to be used primarily for tapering .

 

[conetip]

You need some way of linking the rotational speed of the lathe spindle to the Z movement of the carriage.You may choose say 10 rpm, but under load, if the spindle slows to 9 rpm but the carriage continues at 1 inch per minute, the pitch will be way off. Commercial lathes have a tachometer on the spindle and link the Z axis movement to feed rate per rev, in the G99 , G98 is feed per minute for lathes in ISO G code.
Neil
Did a google search and found this , http://www.editcnc.com/GandMcodes.html

I quite like Edit CNC for the code editing. But mostly now it is cad cam.

 

[Kim Bye]

If your doing live threading and only need one pass, it's possible to make som sort of contraption, but if you need multiple passes, you need some sort of way to repeat the same pattern, lining it up by eyesight alone is quite tricky. Metal lates have a counter that helps you determine when to engage the slide.

 

[Kim Bye]

Doublepost...

 

[Chris Abaya Cues]

Thanks everyone for your input. You guys made some excellent points. I appreciate your responses.