looking for advice on turning a custom tap

[ikoral]

The purpose of the tap I am trying to make is to cut threads into aluminum 6061 plates. I don't need a precise fit. The tool steel I have is W1 steel. The thread I need to make has about a 0.56" major diameter a 0.36" minor diameter a pitch of 6 and a flat at the bottom of roughly .050" . The particularly odd part of this thread is that it has a tooth angle of 32 degrees. I ground a HSS square bar to the 32 degree form and put a flat on the nose to just slightly narrower then the male thread. I set my top slide to slightly under 16 degrees and used a live center for support. I was using a 5C collet to hold the stock in the headstock. I started with increments of about 0.0025" depth of cut with a spring pass about every three cuts but by the time I was at about a total depth of 0.025 I noticed the stock was sliding back in the collet. So I changed to a 5C to ER32 collet adapter and added a stop to help keep the part from sliding. I continued turning and by the time I was at 0.040" depth I changed to 0.001" depth of cut. When I got to about 0.05" total depth the part rode onto my tool and sheared off.

I attempted to make the tap again but this tine I started with a 60 degree threading tool to remove material first so I made less contact when I changed to the 32 degree threading tool. But I again rode up and broke off, this time at about a 0.07" total depth and still using the 60 degree threading tool.

So at this point I am not sure what change I should make. I have no experience with turning tool steel any tips would be appreciated.

 

[gustafson]

feed too fast
crap hss
lubrication

 

[Bobw]

No relief angle is my guess.

Post a pic of your tool.

 

[implmex]

Good morning ikoral:
You have a couple of problems I can identify:
First, when the part rides up on the tool and shears off, you have it sticking too far out of the chuck so the turning forces can bend the part enough to make it want to crawl up and sit on top of the tool.

If you must have a long stickout, you must support the free end of the workpiece and also the center of the workpiece if it's very skinny.
There are two ways to do so.
The first and most common is to drill a center into the free end and put the tailstock up against it as you've done.
The second is to use an accessory called a "following steady rest" or "follower", or "traveling steady" (there are other names in use too).
This will hold the workpiece down right near the cut, so the skinny workpiece can't flex upward.
If you are unfamiliar with this accessory, Google it and you will understand how it works.

Second, your thread is terribly ambitious to try to tap, even if you can successfully make the tool.
The minor diameter is small, the threadform is very coarse and the pitch is steep.
You have an immense amount of material to remove with this skinny, fragile home-made tool and it will be difficult to get it to cut without breaking it off in the hole.

These kinds of tools are usually form-ground from high speed steel...if you make it from something W-1, (W-1 is a common, water quenching plain carbon toolsteel not suitable to tolerate heavy forces) it will likely not be tough enough, especially if you just torch harden it and quench it in a bucket of water in your home shop.
That works OK for robust, finer pitch taps but not likely for something like this.

So I believe you will struggle with this project...it's ridiculously ambitious for a guy just starting out. (that's my impression from the nature of your inquiry).

A thread of this kind is difficult to make no matter what tools you have; the small diameter, coarse pitch and deep threadform make it impossible to threadmill on anything other than a 5 axis mill.
It is hard to single point turn because the tool gets so skinny if it has to fit in a 0.360" diameter hole.
It is a nightmare to try to tap for the same reason, unless you can use graduated taps. (the first one small in OD, the next one a bit bigger, the next one bigger yet etc etc).
A taper tap to start is also useful, and small diameter, coarse pitch ACME profile threads are manually cut routinely using these tactics.
(Look up taper taps on Google and you'll see what I mean.)

A good way forward if you MUST have this thread is to sinker EDM machine it with a rotating electrode that the machine advances into the block just like a tap.
The benefit of sinkering it is that there are no forces on the electrode, so nothing to snap off in the hole.
You may be able to get away with an orbiting electrode (easier to implement on a sinker)...it depends on whether the trailing flank forms an undercut...in which case you can't.
Obviously if you can't find anyone with a CNC sinker EDM to take this on you cannot invoke this method...I mention it only for completeness' sake

Another way is to have a block metal 3D printed and accept that it will be a crappy thread.
Yet another way is to cast the thread in something like Moglice and accept that it will be relatively weak.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

 

[gustafson]

Good morning ikoral:
You have a couple of problems I can identify:
First, when the part rides up on the tool and shears off, you have it sticking too far out of the chuck so the turning forces can bend the part enough to make it want to crawl up and sit on top of the tool.

If you must have a long stickout, you must support the free end of the workpiece and also the center of the workpiece if it's very skinny.
There are two ways to do so.
The first and most common is to drill a center into the free end and put the tailstock up against it as you've done.
The second is to use an accessory called a "following steady rest" or "follower", or "traveling steady" (there are other names in use too).
This will hold the workpiece down right near the cut, so the skinny workpiece can't flex upward.
If you are unfamiliar with this accessory, Google it and you will understand how it works.

Second, your thread is terribly ambitious to try to tap, even if you can successfully make the tool.
The minor diameter is small, the threadform is very coarse and the pitch is steep.
You have an immense amount of material to remove with this skinny, fragile home-made tool and it will be difficult to get it to cut without breaking it off in the hole.

These kinds of tools are usually form-ground from high speed steel...if you make it from something W-1, (W-1 is a common, water quenching plain carbon toolsteel not suitable to tolerate heavy forces) it will likely not be tough enough, especially if you just torch harden it and quench it in a bucket of water in your home shop.
That works OK for robust, finer pitch taps but not likely for something like this.

So I believe you will struggle with this project...it's ridiculously ambitious for a guy just starting out. (that's my impression from the nature of your inquiry).

A thread of this kind is difficult to make no matter what tools you have; the small diameter, coarse pitch and deep threadform make it impossible to threadmill on anything other than a 5 axis mill.
It is hard to single point turn because the tool gets so skinny if it has to fit in a 0.360" diameter hole.
It is a nightmare to try to tap for the same reason, unless you can use graduated taps. (the first one small in OD, the next one a bit bigger, the next one bigger yet etc etc).
A taper tap to start is also useful, and small diameter, coarse pitch ACME profile threads are manually cut routinely using these tactics.
(Look up taper taps on Google and you'll see what I mean.)

A good way forward if you MUST have this thread is to sinker EDM machine it with a rotating electrode that the machine advances into the block just like a tap.
The benefit of sinkering it is that there are no forces on the electrode, so nothing to snap off in the hole.
You may be able to get away with an orbiting electrode (easier to implement on a sinker)...it depends on whether the trailing flank forms an undercut...in which case you can't.
Obviously if you can't find anyone with a CNC sinker EDM to take this on you cannot invoke this method...I mention it only for completeness' sake

Another way is to have a block metal 3D printed and accept that it will be a crappy thread.
Yet another way is to cast the thread in something like Moglice and accept that it will be relatively weak.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

Oh just show off why dontcha

 

[wesg]

Can you make a simple turning cut at that depth with this tool?

Do you have enough relief on it for the thread pitch?

And how long is the thread? As short as you can stand would be your best bet, like maybe 1 diameter plus a lead on the nose. Which is probably going to have to be pretty long.

W-1 sucks to cut, IMO. And at the low speed you're likely threading at, it's gonna be worse. You might try wiggle-wagging it in ;-)

Something else to try:

Set compound and cross slide to zero at finished depth. Cut a bit with the compound, and then back out, set the compound to zero, and deepen the cut with the cross slide. Back and forth on them until you get to zero on both. You'll need a decent cutting edge on the back side though. Typically a hand ground tool is going to be negative rake there because of the positive rake on the leading edge. But it might cut half decent with light passes.

Sort of a manual hack on a CNC threading option for coarse threads.

 

[john.k]

havent read the blurb,no doubt someone has mentioned the importance of form relief in a tap....if you cant include form relief (likely),then the alternative is to back off the flutes to near points so the tap doesnt drag and break.........You quickly discover all this making taps for half nuts and tailstock nuts in small lathes.

 

[implmex]

Hi john.k:
You wrote:
"no doubt someone has mentioned the importance of form relief in a tap"

Nope, nobody's mentioned it yet, so you can grab the credit.

You're right of course
It is an important consideration, and as you point out, it's not easy to achieve.
However, the OP is still struggling with just getting a reasonable threadform on his blank.
Maybe we should just stick with one thing at a time.

On the other hand, as you correctly point out, it is an important part of the puzzle and he's going to have to confront it sooner or later.
I like the solution you proposed...I hadn't considered doing it that way.
Of course the skinny cutting edges will be terribly fragile, but if you can get one good thread out of it you have a winner.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

 

[john.k]

Well ,without form relief, no long fragile coarse thread tap is ever going to survive in a machine drive .........with the 1/2"x 10(or 8) Acme taps ,they need to be very long ,very slow taper ,transition from sharp to acme ,and be used very carefully....by hand ....backing out every time they load up.....another tip from the old books ....cut the centre divot off the tap before hardening.....the tap will crack from the centre up one flute when quenched.

 

[ikoral]

Thank you all for the advice! I did check the relief angle in relation to the helix angle on the 32 degree form tool I ground but I did not think to double check what the relief angle was on the 60 degree form tool.

I was leaving myself enough stickout to hopefully give enough of a taper for the tap to work. I will try shortening the stickout ( I do not have access to a following steady rest) and making graduating taps.

Is there an alloy of tool steel that would be better suited to this particular job?

 

[EPAIII]

The fact that your tool jumps over the tap being cut when the depth of the cut is increased seems to point to insufficient clearance on the leading edge of the tool.

When you checked the relief angle on your tools, both the 60 degree and the form tool, what diameter helix did you use?

You need to use the worst case helix which is at the minor diameter of about 0.36", not the OD or the pitch diameter. The relief angle will be greater as the diameter of the helix becomes smaller. For 6 TPI I get:

At the OD (0.56"): 5.41°

At the PD (0.46"): 6.58°

And at the minor diameter (0.36"), which is the one to use: 8.38°

This suggests that a 9° or 10° relief angle is needed on the leading edge of the tool. 8° or less will rub and probably cause your jumping.

I would welcome someone checking my numbers here.

 

[EPAIII]

As for the difficulty of using the tap after it is made, I would suggest making several taps, perhaps three or four. All of them would have the same minor diameter and the same form from the minor diameter outwards. But they would be made with steps in the OD with the first one at about 0.41". Then the second one at about 0.46", the third at 0.51". And the final one at the final 0.56" diameter.

That way the first one only cuts about 25% of the thread. The second one then cuts the second 25%, etc. until the last one completes it. You still need the tapered section at the tip of each tap but contrary to what others may say, there is no real need for any relief on those, cutting teeth.

Do use a good tapping fluid when using them.

 

[ikoral]

The fact that your tool jumps over the tap being cut when the depth of the cut is increased seems to point to insufficient clearance on the leading edge of the tool.

When you checked the relief angle on your tools, both the 60 degree and the form tool, what diameter helix did you use?

You need to use the worst case helix which is at the minor diameter of about 0.36", not the OD or the pitch diameter. The relief angle will be greater as the diameter of the helix becomes smaller. For 6 TPI I get:

At the OD (0.56"): 5.41°

At the PD (0.46"): 6.58°

And at the minor diameter (0.36"), which is the one to use: 8.38°

This suggests that a 9° or 10° relief angle is needed on the leading edge of the tool. 8° or less will rub and probably cause your jumping.

I would welcome someone checking my numbers here.

Thanks for the tip! I just double checked using an online calculator as well as machinery's handbook and got the same angles. I will make sure I ground at least that much relief

 

[winger]

When I'm pushing hard during roughing or the I need to clamp a little lighter, I turn the end down a bit so there is a shoulder to but against the jaws.

Dave

 

[implmex]

Hi ikoral:
I have a soft spot in my heart for A-2 and D-2 for making specialty one time tools that I will have to heat treat myself.
Both are air hardening steels that get really hard...63 RC is achievable with both.
A-2 is a bit more stable, and D-2 is quite a bit tougher.(it's used for punch dies)
D-2 is a cast iron bitch to surface or cylindrical grind once it's hardened.

The problem with both of these steels is that you cannot harden them with a torch...you need a furnace in which you can control the temperature accurately over time.
That may or may not be a show stopper for you.

If you have to use a tool steel you can harden in a home shop environment I'd pick O-1 over any of the water hardening steels.
O-1 can be flame hardened easily but it's quenched in oil not water, and just about any oil will do just fine although a proper heat treat oil will do a slightly better job than the contents of your automotive transmission will.

The water hardening steels all suffer from the bad effects of the super rapid quench, and the worst of these effects for your purposes are warping, crack propagation and embrittlement.

O-1 also is decent to machine and gets very hard too...under controlled conditions it will go to 63 RC too and has decent toughness but pretty crappy abrasion resistance compared to something like D-2.

I concur with EPAIII's comments on making staged taps and digging out the material in the tapped hole 25% at a time.
You'll still have difficulties with the coarse pitch, and with the absence of form relief.

The form relief issue can be solved with the application of patience, a microscope, a steady hand and a pencil grinder...the result doesn't have to be pretty to be effective; you just need to get the material behind the cutting edges to go away without removing so much that you weaken the teeth beyond usefulness.

For that purpose, making the tap shorter is beneficial...fewer teeth to relieve.
You may also consider starting the tap in the lathe so you can gear it to the rotating workpiece and drive it forward at the correct pitch.
I'm NOT recommending that you drive it under power...but if you can get a good start at the proper pitch by mounting the tap on the carriage, engaging the leadscrew and turning the chuck over by hand, you can get it started straight, and cutting the proper spiral rather than just chewing a bellmouth at the start of the hole.
Of course, this depends on whether you can actually swing the workpiece in the lathe.

You can also make a threaded guide at the correct pitch to drive the tap.
Attached are photos of a 3 mm, 2 start coarse pitch ACME tap I made by grinding a HSS blank that has only one tooth per thread start.
I drove it at the correct pitch by making a threaded male collar on the tap and a threaded female socket positioned in front of the workpiece.
To get the proper threadform, I ran the tap through, then repositioned the guide a bit further forward, ran the tap through again and repeated the process until I had the proper width of the thread flanks.
If you compare the tap threadform with that of the brass screw in the second photo you can see that I had to incrementally move the tap guide forward quite a bit to nibble out the thread in stages.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

 

[EmGo]

Attached are photos of a 3 mm, 2 start coarse pitch ACME tap I made

Do they normally use a hammer drill to install these, Impl ? I swear my dentist uses you as a supplier ...

To get the proper threadform, I ran the tap through, then repositioned the guide a bit further forward, ran the tap through again and repeated the process until I had the proper width of the thread flanks.
If you compare the tap threadform with that of the brass screw in the second photo you can see that I had to incrementally move the tap guide forward quite a bit to nibble out the thread in stages.

Same instuction manual, too It was #28 or 29, I swear you could swing a two-lane suspension bridge off that thing. First time I've ever passed out in the doctor's office, never again. Gimme Goerge Washingon's wooden ones, I don't care ....

p.s. This is probably a case where having someone with an edm do the job makes more sense. OP's thread, I mean. Although maybe on the next implant, too. Couldn't hurt any more.

 

[implmex]

Hi Emanuel:
That is not supposed to happen when you get dental implants...there's no reason for it to hurt like you're describing.
I put in a lot of implants when I was still a dentist (in a past life now happily almost forgotten).
I never hurt anyone, and I've never heard of anyone being tortured in the dental office getting them put in by anyone else either, so it's not the common experience.

Everyone got frozen real good, and they all got a shot of dexamethasone afterward too, to keep the postoperative pain and swelling down to a reasonable level.
I can't recall a single incident where anyone was beat up by the experience...I hurt people a lot worse taking out their impacted wisdom teeth (postoperatively, not during the procedure).
So it sounds like you were in the hands of a "somewhat brisk" and "not excessively compassionate" practitioner.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

 

[dian]

what might be happening is that the material hardens on you when turning (too lazy to look up what W1 really is). concerning taps made out of tool steel i dont see a reason why they should break easily. on the contrary, they will not be as hard as hss and will not tolerate heat but should be more flexible. i have a set of very old taps that are designated as "tungsten steel" and i (sparingly) use them when there is a risk of breaking them.

 

[john.k]

I suggest you forget the tap,and start on something interesting ,like a model steam locomotive.

 

[EmGo]

So it sounds like you were in the hands of a "somewhat brisk" and "not excessively compassionate" practitioner.

That's the weird thing ... she's really good. REALLY good. And she shot me up about ten thousand times and it's not even the old tomato-cheek stuff.

The scraping on the bone part made me cry, but at least thought that was the worst, until it was installation time and out came the big hammerdrill.

Maybe I have excessive nerves in the jaw ? agree with you, no one else I have talked to had this experience but ... god was punishing me ?

Back to topic, edm really would be smarter but someone brought up a good point - if I wuz to try to make a tap without any relieving equipment and not much experience, and the tap was going into aluminum, I'd choose more of a ductile steel than a hard one. Especially for this thread ... maybe just 4140 prehard or something similar ?