Problems with thread taps

[trapper]

Earlier today I made a section in stainless steel and threaded the collar (the insert to the main body) I then turned up the main body drilled the bore and proceeded to cut the thread. I did tis by hand on my metal lathe ie lathe not under power. The body of the pen became hot and the tap acted as a reamer and indeed stripped the cutters from the tap. I had drilled at 7mm for an 8mm x 1 tap.
Has anyone got any ideas as to what went wrong ? I have used this tap many times without problems

 

[Curly]

Some SS work hardens as you drill it etc., getting very hard. That might be a reason. What did you use as a tap lube? What alloy of SS is it?

The other is that even though you used the theoretical correct size of tap drill of 7mm it might have been a tight hole and it was too much for the tap. Going to 7.1 or 7.2 might work better.

Sent from my iPhone using Forum Runner because I can't reach my desktop!

 

[frank123]

Have you tapped stainless with that tap before?

Plastic and aluminum, even brass, taps easily even with a dull or poorly made tap, harder metals do not.

If the tap is in good condition, lube designed for metal and possible a slightly larger drill size may solve the problem.

And as mentioned earlier, the type of stainless is a possible cause, some of it -usually the softer stuff- can be pretty gummy and gall up and tear out easily. Try advancing the tap a third of the turn and backing it out a quarter then advancing a third again to keep the chips broken free and out of the cutting edge.

 

[KenV]

Work hardened stainless will eat a premium high speed steel drill bit. Been there, done that. Slow speeds help avoid it, but once work hardened, tis grinding time.

80% threads are probably much better plan until you get a feel for working with the diifferent stainless alloys.

 

[Donovan]

Welcome to CNCCookbook! « CNCCookbook CNC Blog CNCCookbook CNC Blog Go to this site and search for "tap and drill sizes" it makes for interesting reading. This is a short piece."Imagine the fully formed internal thread. Each thread rises from valley to peak. Now suppose you ran a twist drill down the hole and shaved off some of the peaks. They're pretty delicate anyway and will wear off quickly. In fact, they contribute surprisingly little strength. Kennametal says a 100% thread is only 5% stronger than 75% thread. But here is the real kicker:

That 100% thread requires 3 times the power to tap!

 

[trapper]

Thanks Guys.
I think Curly and Frank123 are probably right, Even though i use cutting fluid while drilling the workpiece did get very hot. I drilled another piece yesterday evening and it appears to be almost case hardened possibly the penalty of recycling old metal rather than buying fresh!!! Originally it was a deck rail on a salvaged yacht. I think tungsten carbide taps is an answer i'm fairly sure the one i used was not the best quality

 

[Rich L]

You might try some different stock. If what you have is marine stainless it's likely the 316 alloy which is one of the difficult to machine austenitic alloys. If you've already work hardened it with a dull tap or cutter you've got a difficult problem rescuing it.

 

[Tom_W]

Stainless comes in lots of different grades. What grade did you use?

Some stainless (like 440C) can be harden and tempered and is used for knives. If you use this type of stainless then in the act of drilling, especially because of the thin tube wall, you probably hardened and air cooled it (rapidly) hence you could produced a tube that is harder than your tap - this is the most likely outcome if you haven't used flood coolant and cutting drill oil.

Other stainless (like 304) does not harden but like all stainless it does not like heat. A product of the heat is that the stainless expands and clamps your tap - which restricts the hole, which clamps tighter on the tap, which creates more heat as you force the issue..... until either the metal and tap binds or one simply degrades to the point of useless. (alternatively if you get away with it you get an odd dimension hole)

If you are tapping anything in stainless, or most metal, then you must use a decent EP tapping fluid (Extreme Pressure). There are ones specifically made for stainless (although any is better than none) as its benefits to your process cannot be understated.

However that alone would not have solved your problem as I think this probably occurred in drilling; if you are deep hole drilling (x5 drill diameter), use a cutting fluid and coolant and slow the speed right down - peck drilling if required.

Additionally give it a bit more clearance and use a 7.1mm drill as the min and max core diameters for a M8x1 are 6.917 and 7.153 respectively so 7.1 ain't going to hurt and will give you approx 70% full thread.

Old Machinists Lore - Don't use tools you have used on Brass on Stainless or your stainless will be worth less brass.

T

 

[trapper]

Thanks Tom........ this is first set back in a magnetic cap idea on a kitless rollerball lazy mans burden really I should have made it in alloy as the nd magnets Ihave will be recessed to give a flush fit cap to body without tripple start threads. New plan is alloy annodised section acrylic cap and body but slender at 9mm outside diameter just waiting on some alloy rod being delivered

 

[Tom_W]

Ah, that will be (more than likely) martensitic stainless then. It will heat treat well ;-)

T

 

[Rich L]

Stainless comes in lots of different grades. What grade did you use?

Some stainless (like 440C) can be harden and tempered and is used for knives. If you use this type of stainless then in the act of drilling, especially because of the thin tube wall, you probably hardened and air cooled it (rapidly) hence you could produced a tube that is harder than your tap - this is the most likely outcome if you haven't used flood coolant and cutting drill oil.

Other stainless (like 304) does not harden but like all stainless it does not like heat. A product of the heat is that the stainless expands and clamps your tap - which restricts the hole, which clamps tighter on the tap, which creates more heat as you force the issue..... until either the metal and tap binds or one simply degrades to the point of useless. (alternatively if you get away with it you get an odd dimension hole)

...
T

Two ways to get hardness - work hardening and heat treating - totally different.

304 (300 series stainless) cannot be heat treated to get hardness, as Tom says, but it will work harden (or strain harden) enough to give you significant trouble machining or tapping if you don't do it right. 400 series are martensitic and can be heat treated to significant hardness hence their use in knife blades, etc, as Tom says, and they are easier to machine. If you search the professional machining forums you will see overwhelming complaints about machining, tapping, 304, 316, etc because of the work hardening characteristics - broken tools, overheating, dulling tools. One guy even said 304 was a "design error." (I liked that) It's generally all because of technique and close attention to "making the cut right." I'd stay away from 300 series for pen making if I could unless that's all the material I had and I wanted to gain more experience cutting it. IMHO I can't think of any real design reason to make a pen out of 300 series stainless unless you want to make one out of some of the really pretty Damascus steels such as what is made by Damasteel.

Unlikely you will be able to rethread with a carbide tap but maybe you can get it to work. You can, however, anneal the stuff:

if 300 series - heat red hot and let cool any way you want
if 400 series - heat red hot and let cool very, very slowly (like a few hours or more)
if you don't know - heat red hot and let cool very, very slowly (like a few hours or more)

You will likely have your work cut out for you re-finishing your surfaces after that.

Good luck - I hope you can fix it!

 

[trapper]

thanks for the info guys all now written up in my book of foul ups and how to avoid them lol . I'm restarting the project and will do the first one in alloy and acrylic then have a fresh attempt with stainless BUT THE RIGHT STUFF!!!