A few lathe questions.

I recently purchased a 15 x 30 Colchester Triumph lathe which needed a half nut. Being a cheap hobbiest and not wanting to spend ~$700.00 for a new one, I decided to make my own which I must say turned out very well. Since I could use taps and dies to do most of my threading, I don't really cut many threads on the lathe. I decided to try out my new half nut last night and set up to cut a 1/2 -13 thread on some brass round stock that I had laying around. It was a disater of a thread when I was finished. Nothing seemed to work as it should....except my new half nut.
Cutting an odd size thread, the thread chase dial should read any whole number...correct? I had to land on the same number (4) to have the thread chase correctly.
I found a thread chase dial in the drawer of the lathe and this is what I used and the attachment holes dont line up with the dial.
The top pivot hole seems to be correct for the allignment, but the bottom hole doesnt. I attached it with one bolt.
1) Where should the gear of the thread chase dial intersect the leadscrew?
2) How many teeth are on the gear of the thread chase dial when the lead screw is 1 1/4 x 4TPI
3) Why doesn't the Colchester Triumph have the compound marked in degrees? Real PITA
4) What caused my threads to look like a saw tooth? Compound angle off slightly or incorrect?

A few more pics of the thread dial mounted, of the dial gear engaged with the lead screw, etc. Regards, mike

I will try to take some pictures tonight. Great idea with the marking pen. Also I read that the Colchester compound should be set at 60.5 degrees for thread cutting. Is this perpendicular to the work piece or cross feed? I think that once I get this correct, I will scribe the compound.
I used a halfnut from a smaller lathe for a pattern. I had to enlarge the dimensions of everything for my 15" lathe.

Nicely said, Bill - to simplify - you can cut a thread by feeding in with the cross slide with the compound at any angle you want it at, as long as your threading tool is square to the workpiece. But the tool will cut on both sides, doubling the already heavy cutting force. When you're cutting a 60 degree thread, turn the compound 29 1/2 degrees from the face of the part (to the left) and feed in only with the compound, returning your cross slide to zero at each pass, causing the tool to only cut with the left side. If your compound was facing straight along the x axis, 29 1/2 degrees to the left is how you wanna turn it. Make sure your tool is on center and line it up with a fishtail (thread gauge) to the diameter of the part

lathe is mainly for machining of relatively hard materials. It includes some intermediate gears that allow the correct ratio and direction to be set for cutting threads and worm gears. In order to accurately convert from one thread form to the other requires more than 120 tooth gear.

Tarawa, I made the half nut for my 15x48 Clausing, also. It works like a charm. The bushings (bronze I think) that the lead screw turns in the apron were worn out-egg shaped-, also.

Back then, I had the purchasing agent at the plant I worked at order me some sort of nylon bushings that same dimension. I installed them at the same time as the new halfnut and so far so good- that was in 1996! When he ordered he got me a whole bunch of them. I only used 3 or 4, I can't recall exactly how many now, but I have a bunch left. If our acme lead screws are the same diameter, and you want to replace yours, you are welcome a set for your machine.

No help from me, though, on your threading problem.

Edit: I just went looking for those bushings and evidently they've been relocated in a cleaning frenzy. I withdraw my offer til I re-locate them. I hate that....I moved them to clean the place up and now I can't remember where I put them. Sure I didn't toss them....

JoeE
thanks for the offer. If you find them , then I accept!

To All
I am thinking that my threading problem is operator error for the most part. I am having difficulty with determining the angle of the compound in addition to everything else. I tried the reverse method of cutting a thread and the thread still looks like a sawtooth.
why a lathe company would not have a marked compound is beyond me. After all these Colchesters were used in a lot of learning institutions.
I will probably need a picture to show me a correctly positioned crosslide.

tarawa said:

JoeE
thanks for the offer. If you find them , then I accept!

To All
I am thinking that my threading problem is operator error for the most part. I am having difficulty with determining the angle of the compound in addition to everything else. I tried the reverse method of cutting a thread and the thread still looks like a sawtooth.
why a lathe company would not have a marked compound is beyond me. After all these Colchesters were used in a lot of learning institutions.
I will probably need a picture to show me a correctly positioned crosslide.

Click to expand...

Don't have a pic handy, but it is pretty simple. If your compound is at 90 degrees(perpendicular) to your workpiece and you want to thread toward the chuck, rotate the tail of your compound 29 1/2 degrees to your right. Then set your tool to the work using your fishtail gauge. Feed with the compound, and come back to zero with the cross after each pass. Make your last cleanup cut with a slight infeed of the cross slide to cleanup the back side of the thread.
James

Other advantage of threading with the compound is that is positively loads the leadscrew in one direction. This keeps the tool from wandering in the cut as it is pulled forward by the leadscrew, pulled by the previous pass and generally meandering all over the place. If you weren't at least keeping a light drag on the carriage handwheel to keep the half nut and leadscrew flanks solidly engaged, that could very well explain your results.

J. Randall said:

Don't have a pic handy, but it is pretty simple. If your compound is at 90 degrees(perpendicular) to your workpiece and you want to thread toward the chuck, rotate the tail of your compound 29 1/2 degrees to your right. Then set your tool to the work using your fishtail gauge. Feed with the compound, and come back to zero with the cross after each pass. Make your last cleanup cut with a slight infeed of the cross slide to cleanup the back side of the thread.
James

Click to expand...

James
I have it set up and I will try it tomorrow. I wish that this lathe was marked, but it should be pretty easy to put a scale on the cross slide.

one other advantage of having the compound set at 29.5 degrees is that the sin of 30 is 1/2. makes the trig easier.
Bill D.

Why is the compound set and 29.5° and not 30°?

Hello, I believe that the half a degree is to give a slight shaving cut to the trailing flank as well. Set to 30 degrees, ie, half the angle, all the cut is on the leading flank only. I think that's right !!. Cheers.

Today I chucked up some junk tubing and set a marking pen in the holder. The half nut and thread dial are working perfectly. I removed the compound and marked the cross feed reference lines. Set the compound for 29.5 and squared the tool. I cut a perfect 1/2 - 13 thread on some scrap delrin.
I guess I was habing a senior moment when I set it up the other day.
I want to tank everyone for the great advice......especially using the marking pen.

taylor21 said:

lathe is mainly for machining of relatively hard materials. It includes some intermediate gears that allow the correct ratio and direction to be set for cutting threads and worm gears. In order to accurately convert from one thread form to the other requires more than 120 tooth gear.

Click to expand...

Lathe is very useful if treated well.





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Pictures of the thread dial engagement.