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Form Tapping 4-40 in 303 Issues

smb44

Plastic
Joined
Nov 26, 2019
Long story short, I can't get a single tapped hole without the tap breaking in 303. It doesn't even make it .125 deep before snapping. This is my first experience with stainless, so I'm sure there's a myriad of things I'm screwing up that I'm not aware of, despite my best efforts at utilizing Google.

Just to verify I wasn't getting slippage or encoder problems, I tapped a bunch of 1/4-20 in aluminum with no issues, so the machine seems fine (Haas 2005 Mini Mill w/ rigid tapping). I was also able to use a straight flute plug tap from a cheap tapping set from amazon to tap about .2 deep (simply a 'screw it, let's try this moment'). It snaps if I go the full required depth to get full threads, unsurprisingly. It got further than the form taps have though.

Anyways, details....

Material: 303
Tap: Form Balax 4-40 2B TiN
Tap Drill: #39

I'm using 20 SFM. Flood coolant (Kool Mist)

When drilling, I'm taking a .002 chip load at 50 SFM and .045 pecks.

I'm using an ER16 collet for holding. I don't appear to be getting slippage when marking with a sharpie.

I raised the feed height to ensure the spindle is up to speed and synced with the Z axis.... just trying to cover my bases.

I'm considering going to a #38, or even a #37 (#39 is recommended by Balax), but figured I'd see if anyone had some input here before continuing my campaign of death to 4-40 taps and my bank account.
 
You say Kool Mist, I don't believe that is flood coolant? You need a lot of lube when form tapping. Maybe program the machine to dip your tap into a bottle of real tapping fluid before it enters the hole to be tapped.
 
The KoolMist is a red flag to me. That stuff is super light duty coolant, with very little lubricity. You need real lubricity, especially form tapping stainless, and especially junk stainless like 303. Before the machine taps have it program a stop and open the door, put tapping fluid in the hole and then run it. See how that does.
 
I'd probably try a #38 (.1015") Drill. The #39 (.0984") is about .001" smaller than the theoretically ideal start hole size (.0993"). 303 SS is squishy enough. Either way, get a set-up piece, and try a few holes. Roll tapping small holes always takes me a try or 4.

R
 
Have you tried starting the thread with the machine and then finishing by hand? If the tap doesn't break off then my guess would be a machine problem. I understand you tapped 1/4-20 in aluminum but these are a lot finer threads in a lot less forgiving material. Do you have a floating tap head by chance?
 
Sorry, but IMO using a floating tap holder, when your Machine is equipped with Rigid tap, is only compounding an issue.

R
 
I'm with Rob, I don't think you can form tap with a tension/compression holder. Form tapping you need to push HARD, there's no cutting edge to progressively make a groove and pull the tap in. I suspect you're dancing around the top until you run out of spring in the holder, and by then the hole's too boogered up to do anything with.

303 cuts nicely because of the sulfur, but it doesn't bend or form worth anything, for the same reason.
 
You say Kool Mist, I don't believe that is flood coolant? You need a lot of lube when form tapping. Maybe program the machine to dip your tap into a bottle of real tapping fluid before it enters the hole to be tapped.

I've been using Kool Mist #77 for a while as a flood coolant, and they say it can be used that way, but I'll look into doing what you mentioned with tapping fluid. Thanks!

I'll also give a #38 a try as well, thanks R.

Thanks for the input all, I'll get back to it after the holiday and report back.... fingers are crossed!
 
.1015 dia drill and tap form fluid(i like hangsterfers red stuff personally)

I Roll Tap hundreds of 4-40 holes in Ti cp2 every month and almost(knock knock)never break one, BUT i dont tap to depth in the machine, i only go .100 deep then bring it to .220 deep by hand
 
You give s/f details for drilling but no info at all for the s/f of the tap.

Lose the spray mist. Use a brush and apply high sulfur cutting oil (available at Lowe's, Home Depot, etc.) to the tap. Less mess than mist and works better.
 
You give s/f details for drilling but no info at all for the s/f of the tap.

Lose the spray mist. Use a brush and apply high sulfur cutting oil (available at Lowe's, Home Depot, etc.) to the tap. Less mess than mist and works better.

smb44 said:
I'm using 20 SFM

And if it's a 4-40, I'd hope like heck he's using a feedrate of .025/Rev
 
Here are the parameters we use:
flood coolant in Haas vf3 (we use cool rite)
spot drill first
#38 drill running 2900rpm 4.3 feed, peck full out .1 increments to .43" deep
ridged tap in er 16 collet 250 rpm to .312 deep.
never had a problem breaking taps. maybe have 5 or 6 hundred holes on the tap now.
 
Here are the parameters we use:
flood coolant in Haas vf3 (we use cool rite)
spot drill first
#38 drill running 2900rpm 4.3 feed, peck full out .1 increments to .43" deep
ridged tap in er 16 collet 250 rpm to .312 deep.
never had a problem breaking taps. maybe have 5 or 6 hundred holes on the tap now.
That's aweful fast for inconel!

Sent from my Pixel 3 using Tapatalk
 
i also use a premium substrate for the tap HSS with cobalt and a coating like altin plug tap if you have the room instead of bottom taps
the drill size does make a difference try the #38 it should give you 61.75% full thread the #37 will probably be to large giving about 47% thread causing the minor dia to be to large
Also have found when tapping 303ss material you can buy 303 or a premium grade 303 that is made to be easier cutting
 
As others have said, get some decent tap lube in there. Kool Mist is only good for cut tapping small holes in 6061, and even then would not be my first choice. It's about 2 clicks above water if sprayed. High sulpher cutting oil would be good. I've used STP in a pinch when nothing else was around or open and I had to get the job going. There's also some sort of grease type stuff in the yellow and green tube laying around most shops that I use for center lube for a dead center (can't remember the name right now). That would tap far better than Kool Mist in your situation.
I've had to ream holes at times when form tapping in some materials to get hole size after tapping to be in tolerance. Might be something to think about since you have a problem at this point.
Good luck!
 








 
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