Sharpening Lathe Thread Cutting Tool Bits

[Linstrum]

I have had to cut a lot of non-standard threads lately and the lathe is the only way to accomplish that. For about the last forty years I have always sharpened the tool bits by eye on a bench grinder using a fishtail gauge as the template to compare the tool bit tip with, and that's okay for occasional jobs and somebody with good eyesight.

About four years ago I bought thread cutting tool bars for doing both external and internal threads, but the carbide or ceramic inserts cost $15 each and snap off way, way, too easily when the tool bit comes to the end of the cut, that is an especially bad problem down in a hole when doing internal threads, even with the carriage travel stop set to stop the carriage before the tool point bottoms out. The tool holders I have can't be turned upside down so the lathe can be run backwards, which is the way to avoid the end-of-cut problems. I still use the tool bars when the end of the thread cut runs out into open space, but when the cut stops up against a shoulder, forget it! That is $15 guaranteed to be gone from a broken threading insert no matter how carefully the tool bit is used, they MUST run out into open space to prevent end-of-cut breakage.

I use a parting tool for thread cutting, its narrow width and vertical stability are ideal for threading and because they are narrow they are darned easy to sharpen to a 60° point. Getting that point to exactly 60° (or 55° for Whitworth threads used on Enfield, Mauser, and Mosin-Nagant rifles) every time can be a problem when the tool bit needs to be sharpened several times per job, so I made a guide for my bench grinder that works great and is extremely simple. Making the angle on the guide is the toughest part since the included angle of both right and left guide straight edges have to be at exactly 120° (110° for Whitworth) angles.

My bench grinder tool guide is made from a piece of 1/4" x 2" x 3" hot rolled flat stock with a 1/4" slot cut in it so it can adjusted in and out from the grinding wheel and is bolted to the grinder steady rest bar with a 1/4-28 grade 8 bolt. I cut the guide at the 120° included angle in my millimg machine by re-mounting my vise on the mill table at a 60° angle. I cut one side of the guide and simply turned it over in the vise to cut the other side. It works great and now I can accurrately re-sharpen a threading tool bit in seconds. When the guide is in the way of using the grinder it is easy to get off. I also drilled a hole off to one side of the threading tool guide plate so the guide can be put back on the steady rest at the 59° angle for hand sharpening drill bits.

[jhrosier]

Thanks Linstrum!

That will be a great help to me as I have recently purchased a mini lathe and need to hand sharpen my cutters.

Jack

[scrapcan]

Linstrum,

Do you have any issues with cutting large deep threads with the modified parting tool? i.e. something in 10 pitch or below on a large diameter.

[Linstrum]

No, I haven't had any flexing problems with the relatively thin parting tool bending even though I run the thread deeper using the compound feed at 60°, which does generate side thrust on the tool bit. I use a cobalt high speed steel parting tool bit, which is a bit stiffer than regular Mo-Max HSS. The tool bit has a side pressure determined by the cosine of its angle to the lathe's axis of rotation, and the cosine of 60° is 1/2, so the tool bit only sees half the pressure off to its side that is generated by feeding it inward.

About three weeks ago I cut a 6 turns per inch pitch thread without any problem even though the parting tool is less than 1/6-inch wide, which is possible because the compound feed is used to deepen the cut. The tool bit only sees the depth of cut and not the entire width of the thread face since the thread is cut using several passes. I run my depth of cut between 0.0025" and 0.005" per pass depending on the kind of material being threaded and how close I am to the finish cut, and my finsh cut is usually about 0.0005" (5 tenths). I sometimes hand-dress the tool with a diamond whet stone after sharpening it on the bench grinder to give it a hair-splitting sharp edge, which helps with lowering tool pressure besides giving a good shiny smooth finish.


rl667

[barrabruce]

For what its worth.
I 've never done gun stuff.
But I used to use a tool that I sharpened on the off side to a good clearance and used a small radius.On the leading edge the tool has to be ground for at least pitch clearane.
Some times with round stock we would turn the point to equal the pitch to lesson the thrust on the travelling steady.
Used the fish tail guage to align the tool up when the compound rest to the appropriate angle plunged the cut down one side of the thread.
As long as the cut was about 1/2 the radius of the tip a resonabley good thread could be cut.
I would then index and catch the thread and use a few small plunge cuts with a
"good sharp cutter" to finish the job.
Or just take of the side to clean it up a bit.
That way it was simple to cut many threads with one "good" tool and saved the hastles of dulling a proper toll for no benifit.

But I used to cut 48" long theads on 1" shafts of about 4mm pitch.
But normal Threads of smaller pitches on outside shafts were easy enough to do to.

I like that set up Linstrum.
If I ever decide to "tramp the boards" again I'll keep it in mind.
Cheers
Barra

[Linstrum]

Hi, barrabruce, you are quite right about the fish tail gauge, its main purpose is setting the threading tool square with the lathe axis of rotation. But it also makes a good template for grinding the tool point to 60°.

I need to cut the other end of the bench grinder tool guide plate I made to 110° so I can grind tool bits for doing 55° threads. I also need a 55° fish tail for Whitworth threads since I need one for cutting the British Standard threads used on Mauser, Enfield, and Mosin-Nagant rifles since those all use the British Standard - Whitworth System for barrel and hardware threads.

I was very surprised to find that the Germans and Russians used British Standard threads on the Mauser and Mosin-Nagant Rifles! Ricochet explained that one, back in the 1880s and 1890s the Germans and Russians bought their barrel making machinery from Britain since the British had the best barrel making machinery at the time, and once Peter Paul Mauser and the Russian arsenals started using English System threads they kept right on using that convention. I re-barreled a 6.5mm Swedish Mauser with a slower twist barrel and when I checked its threads they were indeed inch-system and 55°.

I also have a clone of a British-designed Petter diesel engine built in India and it uses both Whitworth and British Standard System pitch 55° threads for all its nuts, bolts, and pipe fittings. My first head-scratcher with it was making up a pipe fitting for its cooling system that uses 19 threads per inch! It took me some amount of fiddling with my Jet 13-60 lathe to get the right gears into the thread cutting drive to cut that pitch of thread but I was able to do it. Darned nice lathe, it does all sorts of oddball pitch threads, I haven't found one yet that I can't match in either the inch or metric systems.


rl668

[StarMetal]

Linstrum,

Good stuff here, reading it with interest. I just wanted to say about different threads on things like you mentioned on that diesel. My one trail biking friend had a two stroke trail bike by the name of AJS. The dang thing had metric, whitworth, and american nuts and bolts on it!!!! I forget the origin of the bike.

Joe

[scrapcan]

Linstrum,

Thanks for the reply. I think it is a very interesting use of tooling. I was pretty sure the pressure would be reasonable with the offset. But I was curious as that cutting edge is not very wide and if I understand cutting threads you cut on the sideyou are moving to, i.e the left edge of the thread. I was not sure if you could keep a nice clean thread if it was deeper than the side fo the threading bit.

I am a newb with the lathe so bear with me as I ask stupid questions.

[John Guedry]

That AJS was made in England.

[grumpy one]

I may be missing something, but I have not seen any reference to one of the design issues for your jig. A thread has a lead, so the cutting tool's edge must not have a vertical shape, it has to be sloped (from left to right for a right-hand thread). I have 'bought' thread-cutting tools that are made with a built-in lead to provide this slope, but of course it varies a bit with the pitch of the thread. For multiple-start threads the amount of lead becomes very large.

It looks to me as if the lead could be built-in to your sharpening jig by angling the front faces of the guide in a vertical plane. The right and left faces would need opposite angles.

[ANeat]

Grump on a cutter like that you really dont need any lead since the round part angles away from the cutter and also because the parting tool angles awy from the part. (rake?)

The area where the part/cutter meet is very small.

Like you mentioned, if a person was to cut a high pitch, multiple lead thread then you may get into a situation where the side of the cutter would rub.

[Linstrum]

Hi, grumpy one, you've got a darned good point since relief angle has to be provided somewhere so that only the cutting edge will touch the work piece. It is exactly as you say; for multiple helix threads and coarse pitch threads, the helix angle is very great and the side of the tool bit will contact the thread face before the cutting edge does when there isn't enough relief angle below the cutting edge.

What ANeat pointed is part of the solution for the necessary tool relief for external threads, the curve radius of the round piece of metal being threaded provides relief away from the cutting edge of the threading tool bit. However, there must be relief on the left side of the tool bit point for right hand threads and relief on the right hand side of the point for left hand threads. The way I set up the bench grinder's tool rest automatically provides relief on both sides of the point, which is nice since I cut both right and left hand threads. If the relief on the tool bit point is not there, then the cutting edge will not contact the work piece and all it does is gouge and gall a very shallow rough and jagged groove into the work piece. If the tool bit doesn't make a nice clean curled up shaving and there is a jagged groove when making the first pass, then your tool bit relief angle is not sufficient, and the coarser the pitch the greater the relief angle must be to make a clean accurate cut.

When I use the 60° sharpening jig I made for holding the tool bit against the grinding wheel, I adjust the steady rest so that its front is tipped up far enough above the center of the grind stone so that the tool bit gets ground away down below the cutting edges of its point to give it plenty of relief angle. I tip up the steady rest so it gives somewhere around 10° of relief angle below the cutting edge. The curvature of the grinding wheel itself is also a built-in mechanism that creates some amount relief angle below the cutting edges of the point AS LONG AS the tip of the tool bit is about 10° ABOVE CENTER on the grinding wheel. No need to cut an angle on the sides of the jig for providing relief.

Good call! The fact that you thought about this shows you know exactly what is going on about tool point geometry, a skill some machinists have trouble with!


rl692

[Linstrum]

I got around to cutting the other end of the guide plate to 110º for doing Whitworth threads used on all Mauser, Mosin-Nagant, and Enfield rifles. I have a Mauser barrel job coming up, and being able to easily sharpen my threading tools is part of making the job go well.

rl 1,163

[Frank46]

Just some info,warner tool will sell preground tool bits for threading. I have a catalog floating around here somewhere that shows preground tool bits sharpened for the 60 degree thread. And they say they can do other threads. Frank

[Linstrum]

Hi, Frank46, thanks for the info! Like Howard Hues said: "He who has the tools rules". Like I say: "There is no such thing as too many tools, especially when it comes to casting and reloading."

rl 1,164

[smokeywolf]

Linstrum,
That's a swell solution to resharpening your single point tools. One thing though, because your not sweeping your cutter back and forth across the face of your wheel, your going to have to keep dressing the groove out of your grinding wheel.
On all the bench and pedestal grinders I've serviced and adjusted, I've always set the tool rest level, but just slightly above the center line of the grinding wheel. This accomplishes two things; puts a relief below the cutting lip of the tool, and makes it so that the grinding wheel pushes the tool away rather than pushing it down or sucking it in.

I used to do a lot of single pointing of male and female threads on lens barrels for motion picture camera, projector, and printer lenses. Internal threading up to a shoulder (or even from a shoulder) is one of those things that can separate the men from the boys in the machine shop.

I've never cared for carbide or ceramic bits for single pointing; they crack and fracture too easily. If your using inserts, it's difficult to find anything else. I prefer high speed with at least 8% cobalt. Coatings are nice, but the first time you resharpen, you lose the coating on at least one side of your cutting lip.

Another example of "Necessity 'being' the mother of invention"

smokeywolf

[I'll Make Mine]

I've never cared for carbide or ceramic bits for single pointing; they crack and fracture too easily. If your using inserts, it's difficult to find anything else. I prefer high speed with at least 8% cobalt. Coatings are nice, but the first time you resharpen, you lose the coating on at least one side of your cutting lip.

There are high speed steel inserts available for most tool configurations. With any insert, you have to be sure you're getting the correct rake and relief (some folks make their own holders with the insert platform angled to help with this); most carbide inserts are built with negative rake, but that doesn't work well on smaller (read, fits in the shop) lathes or mills compared to large, powerful industrial machines, and finding them with positive rake can be a bit of a search. Best bet is to learn the "code" for the insert numbering system, and then search for the numbers that code for the cutter you actually need.

[smokeywolf]

Right you are, IMM! I use use negative rake holders (17 x 40 gap bed engine lathe) mostly for roughing, and zero rake for finishing. I use mostly CNMG 80* diamonds for turning/facing; use all 8 corners. Can't remember the code for the zero rake triangles I use for finishing. Sometimes just use a tantalum tungsten or stellite bit. Trigons in most of the boring bars. I do OK at off-hand grinding, so never had a need for HSS inserts.

smokeywolf

[Bullshop]

Ah what!!!??? I am going to need a machinist dictionary.

[bob208]

i sharpen and make new ones on the surface grinder. use a tools makers vise and angle blocks. if you don't have a surface grinder. a tool post grinder set up in the shaper will do just as good.

[Frank46]

Almost forgot, A.W.Warner does make HSS indexable cutters just like the carbide inserts. Just in case anyone's interested. Frank