Thread cutting tool grind

[pgk pgkParticipant @pgkpgk17461]

I understand the concepts here and those of grinding a helix angle ut I notice that preground HSS tools sets appear to include a thread cutting tool.

I can also see that a carbide thread cuttign tool has a thin section whereby helix angle becomes academic.

So my question is if grinding my own thread cutting tool is there a compromise design for a subset of thread diameters/pitches where either one can avoid the need for a helix angle or grind a compromise angle?

Or to rephrase if I make a much larger clearance angle under the cutting edges or thin down the tip of the tool with appropriately shallower cuts when weakened will that work rather than regrinding for every thread pitch fo it's helix angles.

I'm just starting to make swarf and I fancy having a go at say an M10 or M12 external.

Second question.. what is the practical small size to get to on internal threads? Yeah, I'm sure some watchmaker can get to invisible.. but I'm guessing there are sensible limits ….

[pgk pgkParticipant @pgkpgk17461]

[Neil WyattModerator @neilwyatt]

Interesting thoughts. I found it took a fair bit to get decent clearance for cutting internal 1/2" by 20tpi threads, which isn't very small.

I need to cut some external 0.5mm pitch threads soon which will be interesting (s-mount thread for a webcam to telescope adaptor). If I was doing them internally, I'd probably make a tap, or at least a cutting tool made like a tap, but with only one cutting edge.

Neil

[Russell EberhardtParticipant @russelleberhardt48058]

I use homemade single point tools ground from round HSS and mount them in a boring tool holder. That allows them to be rotated to match the helix angle as suggested by the late G. H. Thomas.

Russell.

[pgk pgkParticipant @pgkpgk17461]

Posted by Russell Eberhardt on 02/01/2015 21:43:24:

I use homemade single point tools ground from round HSS and mount them in a boring tool holder. That allows them to be rotated to match the helix angle as suggested by the late G. H. Thomas.

Russell.

Elegant!

[Gordon WParticipant @gordonw]

I only do rough stuff threading, and not to fit any standard thread. So long as outer and inner fit ok then I am happy. I also use a hand ground single point tool, round HSS in a holder, or boring bar for inside threads. It can be a bit of a faff getting the tool set up but works and only needs doing once or twice a year.

[pgk pgkParticipant @pgkpgk17461]

I'm guessing even a theading insert compromises over the rounded base of the thread unless specified for an exact thread size?

[Clive HartlandParticipant @clivehartland94829]

Chasers of the correct TPI are suitable for threading, on small threads I rotate the chuck by hand and you can observe the thread minutely as it cuts.

Chasers will clean threads that are damaged and also make fine adjustment of thread fit as they have the correct thread profile and crest form.

Clive

[VicParticipant @vic]

I'd be interested in an answer to the OP's question as well. I never noticed the helix angle on carbide tools or even if it had one! One of my next jobs is to make a simple slotted jig for thread tool grinding so I'd like to allow for this if I can.

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Clive Hartland on 03/01/2015 09:49:59:

Chasers will clean threads that are damaged and also make fine adjustment of thread fit as they have the correct thread profile and crest form.

I have found that these are more convenient than chasers for cleaning up threads. It had a lot of use when I was restoring vintage cars.

Russell.

P.S. can't vouch for the Chronos version, mine are Presto but I haven't seen them for sale for ages.

Edited By Russell Eberhardt on 03/01/2015 10:52:20

[ChrisHParticipant @chrish]

Tom's Techniques website (http://tomstechniques.com/) feature several short videos on external and internal thread cutting which are well worth a look, as are his other instructional videos on lathe and milling work.

Chris

[BazyleParticipant @bazyle]

Russell, did you mean to link to some thread files at Chronos? I saw them in their warehouse the other day but a site searh isn't coming up with them. I might have got the wrong term.

I'm a little confused by the use of 'helix angle' when I think of it just as front clearance on the tool or am I missing something. It turns into helix angle in my mind only when thinking of a tool for an acme thread where the shape and size makes the helix more obvious. For regular screw cutting it translates in making front clearance 10-20 instead of just 5-10 degrees.

[Russell EberhardtParticipant @russelleberhardt48058]

Sorry Bazyle, finger (or brain) trouble. This is the link.

Turning the tool to match the helix angle allows you to use a smaller clearance angle and make the tool stronger. It also maintains the correct thread angle. G. H. Thomas gives an excellent treatise on screw cutting in his book "The Model Engineer Workshop Manual". Well worth a read. He also advocates setting the topslide to 25° rather than 27.5° for W.W. style threads so that the trailing edge of the tool always gives a very fine cut, resulting in a better finish on the flank that does the work.

Russell.

[VicParticipant @vic]

Now you've confused me (easily done!) but I thought the preferred angle was 29°?

[pgk pgkParticipant @pgkpgk17461]

Posted by Vic on 03/01/2015 11:58:55:

Now you've confused me (easily done!) but I thought the preferred angle was 29°?

I'm guessing that depends on whether you cut a 60 deg or 55 deg profile thread?

Again refreing back to tom's techniques.. he makes the point that this is a form tool and the final cuts will be plunged so just sets his angle to 30deg instead of worrying about 29 or 29.5 degs since the final plunge tidies it up anyway. (or that's my interpretation of his video)

[VicParticipant @vic]

Ah, I'm metric so 60°.

[BazyleParticipant @bazyle]

That thread restoring file looks quite good value as I could use it ahead of thread chaser to reduce wear on them (which I like to think are reasonably precision tools). I wonder why they don't do a metric one.

The 'not quite 30etc' angle looks well intentioned but means neither flank is true to the nut which must result in extra stress when tightened hard.

[JasonBModerator @jasonb]

Posted by Bazyle on 03/01/2015 12:36:01:

 

The 'not quite 30etc' angle looks well intentioned but means neither flank is true to the nut which must result in extra stress when tightened hard.

 

The tool is still at 90degrees to the lathe axis so if equally ground the two cutting edges are correct, its only the direction the cut is put on that the top slide effects.

As for cutting small internal threads, this was posted the other day. 10-32 is 3/16" major dia.

 

Edited By JasonB on 03/01/2015 13:39:34

[Neil WyattModerator @neilwyatt]

> As for cutting small internal threads, this was posted the other day.

That's essentially a description of the GHT tool.

Neil

[Gary WoodingParticipant @garywooding25363]

Posted by JasonB on 03/01/2015 13:34:11:

As for cutting small internal threads, this was posted the other day. 10-32 is 3/16" major dia.

Edited By JasonB on 03/01/2015 13:39:34

I took a look but couldn't figure out how to view the photos. What am I missing?

[JasonBModerator @jasonb]

Sorry, they were attachments so you will need to join MEM forum to view. Basically a very small tool made to cut internal threads

[Ian S CParticipant @iansc]

You only really need a chaser with Whitworth threads to give the rounded top and bottom to the profile.

Unless you are cutting larger coarse threads, it's just as easy to plunge straight in. On the fine threads that I cut, I find that the front clearance is enough without adding helix angle. I use HSS, and leave the top of the tool flat.

Ian S C

[pgk pgkParticipant @pgkpgk17461]

I did find an article from sandvik which stated that gave threading insert codes for different thread ranges.. so not a one size fits all but probbaly one size does a hobbyist for all he's likely to cut.

[VicParticipant @vic]

Seems to be done with shims, page 46.

http://www.sandvik.coromant.com/sitecollectiondocuments/downloads/global/catalogues/en-gb/turning/turn_c.pdf

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Bazyle on 03/01/2015 12:36:01:

That thread restoring file looks quite good value as I could use it ahead of thread chaser to reduce wear on them (which I like to think are reasonably precision tools). I wonder why they don't do a metric one.

Try Amazon

Russell.

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