single point tool grind profile

Hi
As a challenge to myself my Harrison M300 can cut a 14mm or 2tpi thread. What i need to know first is to what profile i need to grind a tool. My machinist handbook states all kinds of info but sadly not being a qualified engineer goes over my head a bit. I kind of get it but it seems very complex. I will try to cut a whitworth 55 degree thread into a 60mm plastic rod to start with as i can use the half nut release technique on my imperial machine. It would be nice to buy a carbide tip correct to profile that i could braise onto a square bar but there does not seem to be any large enough to buy. I also like to design things in my cad program before i attempt them. I sometimes 3d print them too first for good measure. Is there something more simple as to explain the width, depth, allowance, profile tip of the thread both internal and external.

Thanks
John

My first question is WHY ??????
Your question indicates to you being a dead beginning novice at machine work...
Why would you start with cutting a 2 TPI thread.....
As threads get more coarse the depth of the cut becomes deeper, more depth means more tool width in the cut,means more chance of chatter.
Coarse pitch means that the lead screw must turn faster relative to the spindle RPM..means that engaging the half nut happens much faster...means that the tool will be advancing much faster toward
the chuck etc.....

The tool is ground the the included angle of the thread your are cutting...Don't know why you would want to purchase a hunk of carbide and braze it to a piece of steel...They make these already done.

Learn to do the simple stuff first,,,Buy a copy of the South Bend "how to run a lathe" book and read it cover to cover three times.....

Cheers Ross

AlfaGTA said:

My first question is WHY ??????
Your question indicates to you being a dead beginning novice at machine work...
Why would you start with cutting a 2 TPI thread.....
As threads get more coarse the depth of the cut becomes deeper, more depth means more tool width in the cut,means more chance of chatter.
Coarse pitch means that the lead screw must turn faster relative to the spindle RPM..means that engaging the half nut happens much faster...means that the tool will be advancing much faster toward
the chuck etc.....

The tool is ground the the included angle of the thread your are cutting...Don't know why you would want to purchase a hunk of carbide and braze it to a piece of steel...They make these already done.

Learn to do the simple stuff first,,,Buy a copy of the South Bend "how to run a lathe" book and read it cover to cover three times.....

Cheers Ross

Click to expand...

Yes.

I will add that the South Bend book is available online for free, so you do not have to buy a paper copy if you do not want to. In either case, look at it online and see if it helps before paying for a copy.

South Bend Lathe Works - Publication Reprints - How To Run A Lathe 55th Edition | VintageMachinery.org The whole book from 1958 (first 61 pages)
http://vintagemachinery.org/pubs/1617/17727.pdf (Pages 62 to 128)

South Bend Lathe Works - Publication Reprints - How to Cut Screw Threads in the Lathe - SB Bulletin 36-A | VintageMachinery.org Just screw cutting

Larry

AlfaGTA said:

My first question is WHY ??????
Your question indicates to you being a dead beginning novice at machine work...
Why would you start with cutting a 2 TPI thread.....

Click to expand...

I think he meant a 14mm x 2.0 thread but stated a 2tpi thread.
I could be wrong though.

Thanks Larry. I will say no more here.

Mtndew said:

I think he meant a 14mm x 2.0 thread but stated a 2tpi thread.
I could be wrong though.

Click to expand...

The Harrison M300 is a fine machine that I would be happy to own. To quote from Tony's description, "[FONT=&quot]Controlled by three levers and an 8-position rotary dial the box will generate both English and metric threads: 35 Inch pitches from 2 to 56 t.p.i and 39 metric pitches from 0.2 to 14 mm. [/FONT]" So post #1 was quite clear, in that it stated the coarsest pitches that the lathe will cut and the owner's ambitious desire to try to cut a challenging 2 TPI thread.

Here is Tony's page on the M300. Harrison M300 & T300 lathes

Larry

Gotta ask - how would one go about cutting a 2 tpi thread on a relatively small lathe like the M300? Hard to imagine going about it like a smaller thread.

Turning a thread that course, I would thread away from the chuck. There a a few videos on youtube about this.
here is one and he has a lot of other good videos.
Threading on a manual lathe BEST TECHNIQUE EVER !!!! - YouTube

Dave

It is surprising that a 55-degree tool bit is not included in the bargain carbide sets.

You can get a 55* HSS tool bit at Brownells gunsmith store/internet but likely 30 USD for one tool bit.
You can buy a fish gauge and grind your own, might find a local guy or lathe shop that would hand grind up a tool bit for you. Yes, take a bit to the shop.

Good to practice turning and chasing threads on mild steel because plastic will turn differently,

Good to get the book mentioned and turn/chase threads exactly as the book recommends.
For a 55* thread, the compound in feed is at 27 degrees, Not 30* or 29 1/2* as would 60* thread
Good to get a fish/center gauge set, a pack of thread wites, and a thread pitch folding set.
I forgot the infeed for a 55* but when it almost looks like a thread you start to measure with a new high-grade nut or with your 2 wires. For a 14mm you should stick out 2 1/2" or less and take light in-feeds (least stick-out is best). Thread at a low RPM 50-70 is Ok for a guy not having much threading experience.


Note that the book states certain top rake angles for steel, which can help for threading also.
The tool bit photos show a back rake on the threading bit, I add some side cutting edge also and never/rarely have any chatter.

Glarks 5Pcs Stainless Steel Screw Thread Pitch Cutting Gauge Tool Set, 51Pcs America SAE Type + 52Pcs Metric & America SAE Type & British Whitworth + 3Pcs Center Gage, Packed in Plastic Box: Amazon.com: Industrial & Scientific

Tool bits grinding
https://littlemachineshop.com/images/gallery/instructions/grindingtoolbits.pdf

Be very aware that back rake (we always called it top rake) will alter the thread angle. Not too important on shallow threads but on deep threads you will struggle trying to get the mating part to fit.

camscan said:

Be very aware that back rake (we always called it top rake) will alter the thread angle. Not too important on shallow threads but on deep threads you will struggle trying to get the mating part to fit.

Click to expand...

That is part of the reason I included side cutting edge positive rake on my mild steel threading bits, so I can have the free cutting of a positive edge, with not changing the angle of the left edge, and not having excessive top rake.

Also why I advise learning the by-the-book threading method so one can understand how rake angle affect cutting various materials.

Very likely new guys going to away-from-the-chuck, and upside-down bit don't think about tool bit geometry.
likely that is a good part of all the chatter problems mentioned here.

Flat tops tend to increase cutting forces so to push the part away, and tend to compact steel into is self rather than free cutting flow it across the bit face.

A flat-top can be like trying to whittle a stick with your knife blade 90* (right angle) to the stick.

You are telling a rank beginner to thread away from the chuck. And that chuck is probably attached to the spindle with a thread.

So are you setting him up for a chuck in his lap?

winger said:

Turning a thread that course, I would thread away from the chuck. There a a few videos on youtube about this.
here is one and he has a lot of other good videos.
Threading on a manual lathe BEST TECHNIQUE EVER !!!! - YouTube

Dave

Click to expand...

camscan said:

Be very aware that back rake (we always called it top rake) will alter the thread angle. Not too important on shallow threads but on deep threads you will struggle trying to get the mating part to fit.

Click to expand...

Absolutely. I remember many years ago I cut a 3/4 tpi thread on a Mori SL35 in Ampco Bronze, man that was fun.

There's a formula for how the thread angle changes with top rake I can probably find if anybody wants it. My big question is why in the world would anybody cut a Whitworth thread unless they absolutely had to have it? Whitworth needs a specific radius on both the crest and root, so you can't single point it with a simple tool. Learn to cut a standard ISO 60 degree thread first. In the normal direction so the chuck stays attached. At some reasonable TPI or pitch so you don't break anything. With an HSS tool that you ground yourself so you can understand the cutting angles. With safety glasses.

EPAIII said:

You are telling a rank beginner to thread away from the chuck. And that chuck is probably attached to the spindle with a thread.

So are you setting him up for a chuck in his lap?

Click to expand...

Fact check, the M300 uses a camlock system not a screw on chuck!
Tony