allowable taper when turning a shaft

savarin said:
Thanks Bob, I plead a seniors moment that I didnt think of searching for that. Doh! Looks like I did get it correct after all but need way more practice holding it square.
Thanks RJ, I've only used a follower once before and it was not a happy experience.
I will be turning the shaft down to 10mm using the tail stock and a travelling steady to get as accurate as possible but cutting a thread with the steady will be my main challenge.
The first cuts to get to size yesterday without the steady took 4 spring passes to get to size.
I think I will set up the steady with the fingers on the left hand side of the work and the tool bit exactly opposite the fingers and have more shaft spare on the right hand side that can be removed once the length of thread required is cut to size.
As its a left hand thread I think this will work.
My biggest worry is the roughness of the cuts wearing the brass fingers. I've read somewhere that running a file after every pass mitigates this.
But, as its a metric thread being cut with an imperial lead screw on the lathe so hopefully there will be sufficient space and time to do this on the return without disconnecting the half nuts.
so much to think about.
Click to expand...
Your lead screw is a left hand thread so you will be threading towards the tailstock. I would thread a longer length than necessary to give you additional time to clean up the thread burrs. If necessary, you can run back and forth with the half nuts engaged but the threading tool backed out to clean up those burrs.
I would encourage you to watch the video by Tom Lipton (OxTools) on metric threading with an Imperial lead screw.
By follower, I meant the rest that mounts on the cross feed ad travels with the cut. The steady rest mounts on the bed and is stationary. It seems that you have the two interchanged.
 
An interesting tale about measurement instruments.
In a nearby farm town a machinist friend decided to open a job shop but was struggling to make ends meet due to lack of business. To augment his walk-in business, he decided to design and build a mortising machine for sale to woodworking shops. The machine was awesome, using hydraulic motors for cutting the mortise and tenons on aqll joints in a single setup and would cut the mortises and tenons to .001" accuracy.
He took the machine to a woodworking convention and was demonstrating it to his potential customers. As he was touting the accuracy of the joints, one of the owners of a furniture manufacturing company picked up one of the samples and proceeded to measure the joints with a wooden yardstick. My machinist friend decided right then and there that the industry would never appreciate the accuracy that he had built into the machine. True story
 
Savarin -
I just thought of something. Have you added a "reverse tumbler" to the gear train in the headstock of your lathe? Most 9x20s (mine included) don't have any way to cut left hand threads! I added a reverse tumbler to a previous 9x20 that I owned, but later sold. My current one doesn't yet have one.

////OOPS//// Never mind! I just went searching on the forum, and the first hit was your 2012 thread. Good for you!
 
I must admit John I find it difficult to believe I've only been doing this machining stuff for 6 years (I think I joined when I purchased the machine)
 
I find it difficult to believe also Charles. You have come up with so many new, different ways of doing things, all because you had no preconceived ideas about how it should be done, so you just did what you thought might work. And look what you have achieved. A Master machinist by any book.
 
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No need to get a swelled head over it Charles,, it's just fair comment where it's due, and I'm sure every one here who follows you will agree.
 
You are making this part for personal use?
If so the allowable taper is what You Require, there is no standard. If you are producing this from a Customer drawing that requires a lead accuracy that has a value, say .001 +- .001" per 12" of length then you know where you need to be with it.

If you try to produce a leadscrew on a lathe to very high accuracy. sub .001" taper and lead you will quickly go mad.
This is what cylindrical grinders and thread grinders are built for.
 
Ha Ha, too late, I'm probably mad already looking at what I'm working on.
 
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