Craftex CT089 Thread Cutting Problems

[SteelFred]

Hi!
I have a Craftex CT089 13x24 lathe and I've set the gears up to cut 8TPI according to the chart inside the gearbox door. These gears are M/N=28/35, A/B=35/28, C/D=30/48 where the numbers refer to number of gear teeth. The speed is 110 RPM, material 1" aluminum rod. I did a test cut and the thread is indeed 8 threads per inch. The problem I'm having is synchronizing each subsequent cut to the same thread. The chart indicates to close the halfnut when the TDI is at "2" only. My procedure is to zero the crossfeed against the workpiece and never touch it again, move the carriage and compound feed to the thread start and zero it, set the feed towards the headstock for a right hand thread and engage the half nut when the dial indicator is at "2". After the cut, the half nut is disengaged, the compound feed is pulled back, carriage rolled back to start, compound feed another 0.005", then wait for the "2" before engaging the halfnut. I've tried many times, and also using all the other thread dial indicator numbers 1-6 but with the same bad result- nothing synchronizes. I've noticed that the half nut will engage a total of 17 times even though there are 6 divisions on the thread dial indicator, so it becomes difficult to engage on an exact division. The leadscrew has 5TPI while the dial thread indicator has 17 teeth. Any ideas on what could be wrong here? I'd also be interested in any links to the math behind all this. Frustrating start to cutting my first thread. Thanks in advance!

[ninefinger]

I'd do a check on the lead screw to see if its really 5 TPI and not the metric nearly equivalent of 5mm per rev which is 0.19685" per turn versus the 5 TPI of 0.2000" - that would explain what your seeing.

Setup a dial indicator and turn the lead screw manually 5 turns -if its an inch on the nose then your 5 TPI , if its much less then its metric (0.984" or 16thou short).

Mike

[SteelFred]

Thanks for the suggestion- the chart on the lathe indicates 5 TPI for the leadscrew and it does measure 1.001" for 5 threads.
I'm wondering if the half nuts are loose somehow and not engaging where they should.

[John Bennett]

Youtube.com has multiple videos on threading. I have found "MrPete" a good source for information. Threading link for part 1 is http://www.youtube.com/watch?v=9y0MmvscBzg]THREADING on the CLAUSING LATHE south bend part 1 tubalcain - YouTube

Following your steps, you do not take any consideration into account for backlash, if you start your threading at the very start of your piece. Backup a little and engage the halfnut when it turns to 2 and disengage at end. Watch video part 1 and 2 and you'll see what is required. Do not waste the whole day watching the rest of the videos as you'll wonder where the day went.

The other part is to make sure you have a sharp tool bit.

[SteelFred]

Hi,
I finally figured out why cutting 8 TPI on the Craftex CT089 lathe was not working. Specifically, closing the half nut using the suggested thread dial indicator reading of "2" kept overcutting the previous thread. I finally decided to explore the math of the problem and here is what I found:

For 8 TPI, the chuck rotates 8 times while the carriage advances 1.000 inches. The leadscrew is 5 TPI so one turn of the chuck turns the leadscrew 5/8 turn and advances the carriage 1/8 inch.
The thread dial indicator has 17 teeth and 6 marks, so 17 turns of the leadscrew will turn the TDI one revolution and advance the carriage 3.4 inches which would cut 27.2 threads. This is the problem; it is not possible to use this TDI to measure 8 TPI because there is no integer number of threads from one revolution of the TDI. There would need to be five turns of the TDI to get an integer number of threads, i.e. 5x17 turns of the leadscrew which is 17 inches of carriage movement or 136 threads.
Putting this to practice, it worked! It's a bit of a wait for the TDI to rotate five times before engaging the half nut, but it did work.
Ideally, I'd like to change the TDI gear to 20 teeth then the half nut could be engaged on the whole turn of half marks. Not sure why 17 teeth was chosen for the TDI gear.
Hope this helps!
Cheers,

[SPIKER]

I have a CT089, when I tried my first attempt at threading, discovered that the gear chart on the door is incorrect, Found an online manual that has a different chart, Busy Bee confirmed this is the correct chart, Yet when I use the online chart I find that the "C and D' gears suggested for 20 tpi are physically too large to mate.
Am I missing something?
M=24 N=60 A=30 B=27 C=40 D=42

[SPIKER]

sorry about that, with a better light I realize that the "C" spindle is adjustable which solves my question.