Were you using carbide threading inserts? If so, were they partial or full profile? To get a good finish I find that threading inserts need to run fairly fast, at least several hundred rpm. A picture of the tool would be helpful.

The screw looks fairly small? I've also found that small diameter stainless can 'push' away from the threading tool and rub rather than cut, which can give a poor finish.

Regards,

Andrew

That looks better than my first attempt Peter!

I now set the topslide at 29 degrees so that a fine scrape is taken off the right hand side each time. I also reduce the cutting depth on each pass untill the final one is just a scrape. I use relly sharp HSS tools for threading but others prefer carbide tips.

You can always tidy it up using a chaser or a thread restoring file.

Russell

Edited By Russell Eberhardt on 25/08/2013 09:34:34

Some tipped tools can be very sensitive to setting height .
A bit below centre doesn’t matter too much but even a small amount above and they almost don’t cut at all – just rub and scrape .

This effect will be particularly noticable on smaller diameter work .

It’s all down to the cutting edge geometry .

Regards ,

MikeW

Ha ha. I'm laughing at myself. After reading Andrews comment about inserts I wrote but deleted the following from my post " Probably using the tool from a set painted blue or red. Those tools are designed for scraping cement out of bricks.".

There are a lot of threads on here about those sets of tools. Like cheap sets of drills they are mostly not as good value as they seem, but sooo tempting.

If you look under a jewlers eyeglass (you need one in your kit) you will see the edge is quite rounded so although it sort of works for a deep cut it is no good for a 1 thou finishing cut.

A mandrel handle is also good for threading. 'it's what your left arm's for' to paraphrase an advert.

One problem with the set over technique is for big threads, eg chuck mounts, you can end up with a 1/8 inch cutting front. Not good news on a little lathe. That's when you switch to the squared up topslide method so that you can keep going back to runs of lower depth (on crosslside) with horizontal offset (topslide) to nibble off bits of the flank . This also results in the bottom of the thread not being a V unless you take a final plunge. However careful study of the thread profile shows how you can make this work for you since the true form is rounded or flat bottomed.

Get grinding. It is not actually difficult and you don't need a fancy rest to start with. The only aid I suggest to start with is a bit of card cut to the angles you are aiming for to train your eye. The human brain/eye is not designed to understand 'ten degrees' but it is designed to compare A to B with amazing accuracy.

Edited By Bazyle on 25/08/2013 13:40:08

I would suggest not using stainless for your first threadcutting experiences — it is notoriously tricky to machine for the beginner (and even plenty of old hands). Do a few test runs on some nice free-machining mild steel until you get the hang of screw cutting. Then graduate to stainless.

And remember that even the pro's will often use a three-sided file or die to final finish a thread. It's just quicker and easier. (Purists in the back row stop howling please, that is the way it is when t'foreman wants it done by yesterday)

And +1 of be suspicious of cheap pre-ground carbide tools. Use good quality carbide inserts or learn to grind your own HSS tools and finish them on an oilstone for a good sharp tool that will cut well.

Another thing that helps screwcutting is plenty of ordinary mineral oil (engine oil etc) on the tool and material as it is being cut. Seems to work better than "suds" due to better lubricating qualities.