Hello all, I've been a hobbiest for several years now. I have a Bridgeport mill series II and a Taiwan Enco lathe model from .
Lately I've decided to try my hand at single point threading. I've watched countless videos and read many posts on how to do it but of course my threads don't come out correctly.
So here is my set up to start with. I decided to pick 14 tpi as a random thread, but it doesn't matter what I pick as I've tried several different ones. I'm not interested in the diameter or even using a test nut, just trying to make consistent passes to cut the threads repetitivly and then check with a thread gauge (like the kind that comes in a standard tap and die set) to see if I'm getting the timing of the half nut and the threads right. My problem is I'm off, or the machine is but I'm guessing it is me.
The machine is set on B8 per the chart on the lathe for 14 tpi. I've set the speed rather lowish to keep things simple 240 rpm. Compound is set at 29.5*. Using a carbide insert 60* thread tool. I catch the thread chasing dial on any number since it is an even thread. So I start with a scratch pass, terminate in an undercut, disengage the half nut, back out the cross slide, move the carriage back to the start, zero the cross slide, increase the compound a few thousand, wait for the timing of the thread dial to catch a number and engage. Repeat as necessary and then check with the thread gauge and I'm off by one tpi. When I set it for a 14 tpi, I end up getting 13 tpi on the gauge.
This happens consistently and with different tpi and settings according to the chart.I have neve had one match up correctly. I've used different thread gauges in the thought that maybe I had a bad one and that was really the problem. Not so as I've also check with nuts after turning the diameter to what is needed first. Nut doesn't want to thread on correctly. I've checked the threads with the dreaded three wire (Pee Dee?) gauge and the size is correct but the tpi is off.
I'm starting to wonder if maybe this lathe is set up to cut metric threads? My next step is to get a metric thread gauge and check the tpi and see.
What is the chart depicting in the third pic? I don't understand this one.
What am I doing wrong?
Thoughts?
Thanks.
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First off 24O RPM is way too fast for a guy new to threading. 60 to 100 for a new guy.
Start the travel a bit before the part to see all is tightened up on the feed.
Guess with set for 14 thread I would drop on the very same line for a number of passes to see a perfect match/or not to a number of cuts at the same in-feed perhaps .005 just to prove nothing is slipping.and to see if you get a 14 thread or what.
Next set the gears to a metric thread ..doing the same with dropping and not releasing the thread dial. to see if you track-same and make the chosen metric thread.
This may tell if you have a metric change gear in place..
oh! see that you have the cross dial set to a zero and coming back to that same zero every time.
The compound set to a zero and the in-feed coming with in-feed of the compound.
Be sure the thread dial is well fixed and close mesh with the screw when dropped..
See the bit and tool holder are tight.
Put a strip of masking tape on the drive shoulder and on the lead screw marked with a sharpy to see they do not chang/slip as if a broken pin.
Timm,
My hunch is that you are not actually getting 13 tpi, but 13.125 tpi. The easiest way to confirm or infirm that is to engage the half nut when the carriage is away form the headstock, feed it under power for a few turns/seconds, then shut it off without disengaging the half nut.
Then, set up an indicator on the ways between the headstock and the carriage and measure the travel of the carriage over say 10 turns of the spindle. Put a mark on the spindle or chuck and on something fixed so that you can rotate the spindle exactly 10 turns. See what the travel is. If you are dealing with 13tpi, the travel should be 0.769. If it is 13.125 tpi as I suspect, the travel should be 0.762. The difference is not huge, but it should be clear which one you get. If you have a 2 inch travel indicator, you can use 20 turns perhaps.
To me, there is something inconsistent between images # 2 and #3. According to image 2, B1 is 8tpi. With the bottom inch configuration in image 3, B1 is 7 1/2 tpi.
The advice of looking at the gears setup is perfectly accurate. I think that the setup you have is the one in the bottom inch configuration corresponding to 7 1/2 tpi for B1. If I am correct, it would mean that B8 would give you 13 1/8 tpi.
If you can confirm that, it means that now, you can predict the other resulting pitches. That's a good thing. Evidently, the chart in image 2 corresponds to a gear setup that is different than the one you use and it can be figured out also.
Hope this helps.
Jacques
Not to put too fine a point on it, but the way that you "confirm" the gear set-up is to first have a picture/diagram of what the correct set-up should be. For that you normally need the manual for that machine.
Your machine would certainly have a "normal" gear train for use with the thread table you show. You need to know what that gear train should be. GET THE MANUAL.
As for getting the correct gears, that can be done several ways. If they are no longer available, you could measure the gears that you have to determine the DP and the bore diameter and keyway (if any), and the thickness. With those factors you could search places like E-bay for suitable substitutes. A gear shop (or you) could make them, but that can be expensive.
It seems that you purchased this lathe second hand. You could try contacting the seller and asking about any missing gears. And the manual. Yea, yea, I know. But it is worth a shot.
I have never been able to decipher those charts on the import mills either!
Don’t overthink the treading too much, when you first start you will make mistakes! Get some stock and do lots of practice cuts, get familiar with how your machine acts and keep at it for awhile before you actually cut threads on a part you spent hours machining and ruin it!
I’ve been cutting threads for several years and I still screw it up sometimes! I finally bought a set of thread wires recently, but still haven’t used them.
When practicing just try to get the threads to fit in a nut without too much slop at first also remember to take small depth cuts with each pass.
Perfection comes with experience!
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The photos of the lathe showing the charts on it appear to only show feed per revolution in both imperial and metric. There should be additional charts that show threading combinations. The layout of the gear train appears to show that the lathe is capable of changing gears to achieve different rates of advance of the lead screw per revolution. What make and model lathe is it? Did you get the lathe new? Did it come with any additional change gears? Do you have a manual for use of the lathe?
All the information you need to change thread pitch is in those charts.
In image # the lower part of the chart (white) is for threads per inch.
The upper part of the chart (blue) shows the gears and locations for them to produce the pitches listed below .
Image # shows a simplified map of your lathe's gear train. Your lathe will also have a means of adjusting the gear train to accommodate different diameters of gears.
It's a dirty job, bring an nice rag out.
In school machine shop, the teacher showed us how to grind a tool to fit the fishtail gage, how to set it up in the lathe, using the fishtail gage and how to set the tool on center using the point of the tailstock center; we learned how to set up the QC for the thread we wanted to cut, and just went at it, using only a appropriate nut to gage the fit. I hardly ever use thread wires, they are a pain in the butt! If a thread is fussy for fit, I use a thread micrometer, they are much easier to use than wires, and can be bought used for not a terrible amount of money. Do not waste your cash on insert threading tools, they break way too easily, I use HSS tools made by Aloris, they are sharpened only on top and last a very long time.