Screw or nail file?

[Peter WParticipant @peterw]

Well I completed my first screw cutting project on the ML7, and the screw will do the job for which it was intended …. but if I decided not to use it as a screw, it would double up nicely as a nail file! The finish is rough to say the least.

It's 303 stainless which I cut with a tungsten carbide 60 deg thread cutting tool. For the first attempt I set the top slide over at 30 deg and fed the tool with the top slide. I'd read this was a good thing to do. For the second attempt I just came straight in with the cross slide to see if that would give a better finish, but it didn't.

Any advice on how to get a better finish would be very welcome!

Thanks, Peter

[Peter WParticipant @peterw]

[Anonymous]

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

[BazyleParticipant @bazyle]

With the set over topslide method the idea is that the tool can have good top rake for the left hand flank as you view it so can actually cut rather than scrape. The right hand flank is just the side of the 'groove' being cut so doesn't get cut as such so the bad geometry that side doesn't matter. It would be possible to use a tool less than 60 degrees provided the left edge is presented at the correct angle and then the toplide angle ensures the other flank is correct. In practice the tool is often set a tad off 60 degrees so the right flank does get a fine scrape to avoid ridges from each pass. Ironically it is mostly that right flank of the thread that ends up in contact with the nut

Try some plain turning on your stainless stock and find a tool geometry that works on it. Then translate that into a home ground HSS screwcutting tool,. It may be you can't get a good finish in staright turning if it is crappo steel and then you have no hope of improving that when scdrewcutting.

[Russell EberhardtParticipant @russelleberhardt48058]

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

[Peter WParticipant @peterw]

Thanks for all the helpful and encouraging comments.

Answering some of the questions:

Here's a picture of the tool. I was running at 210 rpm. My lathe does have a clutch, but if I were to go much faster I think I'd have to make the groove under the shoulder a bit wider so I could stop the lathe before the tool hit the shoulder.

Andrew it is M8 x 0.75. I seemed to get the roughness even with cuts of 2 thou. There didn't seem to be much push on the work.

Bogstandard2, regarding the flat crest. Good point. Since I was starting from hex bar, I just turned the corners off and stopped when it looked like that was complete. My learning there is that I should take care that the major diameter is correct before I start the screw cutting, and only go in the right depth so I leave that crest flat. Otherwise I need to file down to a flat crest as you mentioned.

Bazyle, Russell, as you can see there's no top rake on this tool. Is it simply that the finish I'm getting is just what you get with this kind of tool? When you talk about HSS tools, is that going to mean I'm going to have to learn how to grind tools? No problem, but I was hoping that was a skill I could work on later! Maybe the tool I'm using is blunt?

As I mentioned before, the screw will work (both as a screw and a nail file), but it's also about understanding what I'm doing so I learn and get better. Thanks for all your comments.

Peter

 

Edited By Peter W on 25/08/2013 11:30:30

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

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

[Russell EberhardtParticipant @russelleberhardt48058]

Yes, a flat top is usual for screw cutting. If there is any back rake it will change the thread angle.

Is that tool exactly at centre height? That's important especially for small diameters.

That tool seems to be made with a slight helix angle and very little side clearance. Could the tool be rubbing?

There is a very good chapter on screw cutting in G. H. Thomas's book, "The model engineers workshop manual. Well worth a read if you can get hold of a copy.

Russell.

[BazyleParticipant @bazyle]

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

[KWILParticipant @kwil]

Screwcutting is not difficult on S/S if the tip is of the correct form. I have cut M2.6 (yes) for replacement screws for Japanese tooling using 303. Correct tip, cutting depth and speed are the essentials.

[HopperParticipant @hopper]

Posted by Peter W on 24/08/2013 21:14:04:

Well I completed my first screw cutting project on the ML7, and the screw will do the job for which it was intended …. but if I decided not to use it as a screw, it would double up nicely as a nail file! The finish is rough to say the least.

It's 303 stainless which I cut with a tungsten carbide 60 deg thread cutting tool.

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.

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