Shaping 60 Degree thread cutter

R

Reynard

Guest
Hi Guys

I have been lurking on this forum for quite some time, decided to take the plunge and ask a question.

Firstly a bit about myself, I am 46y old, married, living on a sheep farm way up in the hills in the Scottish Borders.

I finally got around to putting a concrete floor in my workshop to be, I have my Harrison M300 sat in a corner, I have used this lathe to screw-cut barrels for moderators. Up until about 4 years ago I had never used a lathe since school, I sure enjoy cutting metal.

I have been coming to this forum and absorbing all the information from you guys, that are so willing to part with knowledge that usually only gets past from father to son, In my country people are very wary of giving away their knowledge freely.

I would like to take my lathe work a step further and at some point have a go at truing an action.

I bought the John L Hinnant book which arrived by post today, It is full of so much useful information, I am going to enjoy reading it from front to back, at least twice. I printed of the tool sharpening pages, which I then laminated.

My question relates to shaping a 60 degree tool for cutting UNF threads, in Mr Hinnants book he shows 6 degree relief cut on both sides of the threading tool from top to bottom on both sides, I was wondering if this is a crucial step or can it be done without it. stupid question probably he obviously has it there for a reason.

I went out to my workshop this evening and played about with my knife grinder, finding the right angle so that the point of my 60 d threading tool would fit snugly into the little 60d threading tool guide, I was amazed how quickly I was able to achieve my task, I did cut to shape 5 tools that fit snug into the thread guide, I have not put a 5d relief on the front yet, I will complete them in the morning, then get practising on some old barrels.
I previously only threaded with inserts. I have never threaded an internal thread yet, looking forward to having a go at that.

Any other members using a M300, any tips or modifications that would be helpful, would be gratefully appreciated.

Thank you in advance for any replies

Ian
 
Hi Ian,

I have been grinding my bits for cutting threads for 40 years... I don't know if I am doing it right or not... I think the 6 degree relief is probably a minimum, if you don't have enough relief it will not cut smoothly, more will not hurt anything. I was taught to match the 60 degree tool guide while grinding on a convex stone... the relief is built in the grinding...
 
Reynard said:
Hi Guys

I have been lurking on this forum for quite some time, decided to take the plunge and ask a question.

Firstly a bit about myself, I am 46y old, married, living on a sheep farm way up in the hills in the Scottish Borders.



My question relates to shaping a 60 degree tool for cutting UNF threads, in Mr Hinnants book he shows 6 degree relief cut on both sides of the threading tool from top to bottom on both sides, I was wondering if this is a crucial step or can it be done without it. stupid question probably he obviously has it there for a reason.

would be helpful, would be gratefully appreciated.

Thank you in advance for any replies

Ian
Click to expand...
I am assuming you are talking about the side clearance on the tool?? If so, remember the tool is traveling an incline plane as it cuts the thread. The thread leade determines the plane angle. As Dennis suggests the 6 degrees on the left side is probably a minimum.

Go to http://www.americanmachinetools.com/how_to_use_a_lathe.htm then scroll down to Fig 7-7 then look at the side view. This is not a great example but notice the angle the side clearance of the tip makes with the tool bits main surfaces.
 
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=715&title=BARREL%20THREADING%20LATHE%20BITS

Sometimes for a guy that hasen't ground one before, it's nice to have a sample in hand when you stand in front of the grinder. I see as usual Brownell's has lost there minds price wise, they used to have a set of tools bits for less money than just one now a days. For the most part I buy my blanks HHS with 10% cobolt. They seem to stand up well and are cheap in the long run.

One picture ain't as good as one in hand.:)
 
Thanks guys for your input, I cant believe the price of these threading bits.

I just found out that we now have a Midway UK, which seems to charge up to double for the same items you guys can buy in the states, for instance the Hinnant book was only $35 from Midway USA, it is £41 from midway Uk, this equates to a $ price $57.36, this is a bit steep, still cheaper to bring items back from the US of A in hand luggage.

Jerry, I have viewed the link that you posted, for a beginner it is great, Thank you.

I am going to have a go at grinding the bit as close as I can get to the pictures, then give it a try.

I notice that the bits that midway and Brownells sell are not ground to a central point on the tool, rather they are ground to a point which is offset, am I doing it wrong by making the point in the centre of the bit.


Thanks
Ian
 
Ian,

Grinding to center or "right" or "left" isn't "right or wrong" ....... :D

(How's THAT for confusing rhetoric???)

A lathe bit ground to left of center let's you get just that much closer to the headstock with the bit..... it adds a little safety margin, more room to work. Using a rocker tool post and grinding the bits at an angle to the centerline of the shaft is another method of gaining clearance..... and grinding to one side of the bit can also help your chip clearance. CUTTING-wise I don't know of any difference betwixt the two.

my opinion..

al
 
Reynard said:
Thanks guys for your input, I cant believe the price of these threading bits.

I just found out that we now have a Midway UK, which seems to charge up to double for the same items you guys can buy in the states, for instance the Hinnant book was only $35 from Midway USA, it is £41 from midway Uk, this equates to a $ price $57.36, this is a bit steep, still cheaper to bring items back from the US of A in hand luggage.

Jerry, I have viewed the link that you posted, for a beginner it is great, Thank you.

I am going to have a go at grinding the bit as close as I can get to the pictures, then give it a try.

I notice that the bits that midway and Brownells sell are not ground to a central point on the tool, rather they are ground to a point which is offset, am I doing it wrong by making the point in the centre of the bit.


Thanks
Ian
Click to expand...


Yes you want the clearance on the side. If you read the text it tells you this is to be able to get as close as possible. Well? I know you want the clearance, for sure. Once you get a tool bit that works for the job at hand, it goes in my machinery tool box, it funny how as soon as you look at the tool bit, the reasons all come flooding back This becomes my master when I grind the next same as tool bit and mount and try it in the tool holder. I don't know if anyone has mentioned anything about carborundum stones for breaking edges on your tool bits before you use the tool bit. I keep the stones scattered around the shop.


I remember that the UK had a lot of home shop hobby machinist, it seems you would have lots of sources around your neck of the woods.
 
Its generally recommended that the tool point is set above center.
This, because the workpiece(barrel to be threaded) is trying to
rise against the pressure of the cutting action. How much
may depend on how ridgid your set-up is. It is also said that
this relationship allows an approach angle that shears instead
of scraping. Threads cut near a tailstock center will not be as
ridgid as those cut near the headstock.
Relief is necessary, because one side of the thread as viewed
horizontally at the centerline is leaning to one side( to the right
below the threading point , and to the left above it. Less relief
is required on the right side of the cutting tool. To much
relief and tool bits can get hungry,digging in. Tool bits
used in a rocker style tool post at positive upwards angles
may also do this. It is possible that you could have enough
relief to cut 18 TPI and not 16. Coarser threads need a bit more
and setting tool points above center can add to that requirement.
Tool points ground off center are a big plus when threading
into a relief groove near the shoulder. Wide relief grooves are
not very pretty, and re-cutting them to cover up an accidental
chamfer on a shoulder aren't either.
I have all the jigs for grinding High speed steel threading bits,
but now use Carbide inserts. All the angles are right.
 
Bought one of these to use grinding bits but haven't tried it yet. Looks like it should be easier to grind relief angles correctly.
 

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Bob Kingsbury said:
Its generally recommended that the tool point is set above center.
.
Click to expand...
Before someone inexperienced gets the wrong idea in their head about lathe tool setup, it is NOT best to set the tool above center. Tools can be ground with top clearances and lips to where they will work above center but normally not so.

If a tool tip is above center, the cutting pressure will force the tool tip down and the workpiece, in the meantime, will attempt to lift up. Just natural action of cutting forces. When the tool tip pulls down, even slightly, it will dig further into the workpiece. So, the heaver the cut load, the more the tools downward deflection and the workpiece's upward deflection. These deflected movements will cause the desired cut to cut deeper than intended.

All factory made cutting inserts and ground tooling is made to work on center. I hate to be contradictory on this, just passing along the true facts.

When grinding lathe tools by hand keep in mind several things;
1) Depth of cut I will take.
2) Do I want to end up with a square shoulder or a bevel.
3) If the metal is gummy, like 1010-1020 or such, you will need considerable top clearance (some call this back rake).
4) If the metal is generally hard, like the action lugs for example, you will need to slightly break the cutting edge with a hone. About 0.002" radius or so. A knife sharp edge will not last long in harder materials.
5) On some "grabby" metals like brass, you may even need to go to a slightly negative top rake.

Grinding lathe tools is not all that difficult if you think through what you want to accomplish and consider the depth of cut and feedrate you plan on using.

One thing about cutting metals, they will not let you abuse them. They always win if you try to overtax the machine or workpiece.
 
Reynard said:
Thanks guys for your input, I cant believe the price of these threading bits.

I just found out that we now have a Midway UK, which seems to charge up to double for the same items you guys can buy in the states, for instance the Hinnant book was only $35 from Midway USA, it is £41 from midway Uk, this equates to a $ price $57.36, this is a bit steep, still cheaper to bring items back from the US of A in hand luggage.

Jerry, I have viewed the link that you posted, for a beginner it is great, Thank you.

I am going to have a go at grinding the bit as close as I can get to the pictures, then give it a try.

I notice that the bits that midway and Brownells sell are not ground to a central point on the tool, rather they are ground to a point which is offset, am I doing it wrong by making the point in the centre of the bit.


Thanks
Ian
Click to expand...

Yes, offset it to the left, to help thread to a shoulder without collision. Or grind the left side down to keep it from damaging the shoulder..same difference.

I also like positive rake on the top of my threading tools, makes it leave a nice finish.

Take care up on the borders...

Ben
 

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JerrySharrett said:
Before someone inexperienced gets the wrong idea in their head about lathe tool setup, it is NOT best to set the tool above center. Tools can be ground with top clearances and lips to where they will work above center but normally not so.

If a tool tip is above center, the cutting pressure will force the tool tip down and the workpiece, in the meantime, will attempt to lift up. Just natural action of cutting forces. When the tool tip pulls down, even slightly, it will dig further into the workpiece. So, the heaver the cut load, the more the tools downward deflection and the workpiece's upward deflection. These deflected movements will cause the desired cut to cut deeper than intended.

All factory made cutting inserts and ground tooling is made to work on center. I hate to be contradictory on this, just passing along the true facts.

When grinding lathe tools by hand keep in mind several things;
1) Depth of cut I will take.
2) Do I want to end up with a square shoulder or a bevel.
3) If the metal is gummy, like 1010-1020 or such, you will need considerable top clearance (some call this back rake).
4) If the metal is generally hard, like the action lugs for example, you will need to slightly break the cutting edge with a hone. About 0.002" radius or so. A knife sharp edge will not last long in harder materials.
5) On some "grabby" metals like brass, you may even need to go to a slightly negative top rake.

Grinding lathe tools is not all that difficult if you think through what you want to accomplish and consider the depth of cut and feedrate you plan on using.

One thing about cutting metals, they will not let you abuse them. They always win if you try to overtax the machine or workpiece.
Click to expand...

Jerry,
You are correct , in saying inserts and factory tooling are
meant to run on center. This is of course using very ridgid production
equiptment. Repeatability in programable machines depends on it. What
I should have said is on center or above, rarely if ever below center.
Gunsmiths don't normally use large, programable turning centers.
Nearly any engine lathe tutorial will say on center or slightly above.
This thread is essentially about threading tools. Form tools, Factory
ground, upside down tooling is just right in some application, somewhere.
and all have different requirements. I recently read here that someone
thought barrels might be chambered better at very high speeds and
another suggested his reamer was actually humming from the RPM's.
Gunsmith lathes simply don't work exactly the same way
 
Bob Kingsbury said:
Jerry,
You are correct , in saying inserts and factory tooling are
meant to run on center. This is of course using very ridgid production
equiptment. Repeatability in programable machines depends on it. What
I should have said is on center or above, rarely if ever below center.
Gunsmith lathes simply don't work exactly the same way
Click to expand...

Bob, if you will scroll down the page of the link I posted above, http://www.americanmachinetools.com/how_to_use_a_lathe.htm , you will find they also recommend a tool being above center. This line of thinking goes way back, way, way back. Back to the days of Tool Steel (not HSS and HHSCo), cutting tools like Black Diamond, even as far forward as Rex 49. This was in the days that most metals being machined was cast iron, bronze and the like. Way before alloy steels and stainless steels. The thinking at the time was that the embedded abrasives would wear the tool tip down to center.

Way, way back, not all that long really though. Try as late as 1952! General Electric was just introducing their new cemented turning tool called Carboloy. Carboloy was introduced to machine a new steel called Allegheny Metal--stainless steel.

Having tools slightly above center, if they are ground for that situation, will work but I'd not recommend deliberately setting the tool above center.
 
Here is a pic I stole from someone on this board I think. It shows pretty well the offset to the left for threading up to a shoulder.

Hope you don't mind me using the pic.

Joe Hynes
 

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M.D.Spencer said:
Have you been to Border barrels?
Click to expand...

Hi

No I have not been to Border barrels, Have spoke to Geoff Kolby on the telephone, Before I moved to the farm I was a registered firearms dealer.

Only custom barrel that I have on a rifle is a shillen on my 17 Rem, I would replace it with a barrel of the same maker.

I have two Tikka M55 rifles, one in 243 and the other in 308, I would never part with them.

I have seen some groups that rifle smiths show as a result of truing an action or building a rifle on a custom action, I have on many occasions shot one hole groups with both rifles, you could sit an empty case on top of the group( not a 50 calibre).
I aint slating rifle smiths, just lucky to have two well built factory rifles. I have never seen anyone on any web sites that I have been on that have done truing work on Tikka M55 rifles. We are not talking the newer Tikka rifles that are made by Sako, I am talking the older models originally made by Tikka.

Hope this post does not offend any sensitive Rifle smiths.

Ian
 
Thanks to all for the input, much appreciated.

I was interested reading of having the tool above centre, I have been threading on centre, to be honest I would be too embarrassed to post a picture of my end result, that was with an insert, I was threading a 1/2" UNF, just for the practice, seems to have a look of galling.

My Harrison M300 is a Metric machine, so I have to keep the lead screw engaged throughout the threading process to thread imperial.

I will attempt to thread with the insert slightly above the work piece, I was threading at a speed of 50 something rpm, is this too fast.

Many thanks Ian
 
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