[Rleap619]

alright,

having a little bit of trouble hope you guys can help.

Im trying to hand tap a 1/4-28 .5" deep blind hole into heat treated 17-4 SS (Cert says it was heat treated to condition H900)
i am using a TiCN Coated HSS plug tap with tap magic extra thick tapping fluid.

I have 12 parts to do with 3 holes each. so far i've done 1 part and the tap barely got through the last hole.

so my question is should i be using a different tap material Cobalt? Carbide?
Should i increase the hole size? (there will not be much strain put on the treads). any help would be appreciated.


thanks
Ryan

[txcncman]

That should be somewhere near a Rockwell hardness of 41. Not impossible, but difficult to tap. Since this is CNCZone, thread milling is the preferred method. But I am sure you have excuses for not thread milling. You have to determine what the required thread specifications are. Then you will know what the minimum required thread is for your application. Then you can find the largest allowable minor diameter and drill for that size. If you were machine tapping, I would recommend powdered metal taps. For hand tapping, use all three tap types: starting (taper), plug, and finishing (bottoming). A heavier lubricant will help more that typical tapping fluid.

[sld4121]

Since you are hand tapping, you have limited yourself to the type of tap you can use.
i.e. carbide is out.

Open up your hole as txcncman suggests and VERIFY you have actually cut that size hole. Do not assume the drill size is .213 just because you used a #3 drill per the chart.

Also, how clean is your hole - I presume that since you are not rigid tapping, that you probably did not drill them in a CNC either. That could mean the material is work hardened as well.

Since SS tends to be gummy, you may have to upgrade to a better taps. I use Emuge in the worst stainless applications. They are spendy, but they will work. One of my customers used to buy his taps at the local hardware store and wondered why they had issues.

Lastly, because it is gummy - I prefer machine oil for its lubricity.

[JimBoyce]

The handbook has a 2B ID as 0.211 to 0.220, 3B 0.211 to 0.219, so get the hole up to the upper limit. Castrol Moly-Dee tapping fluid is probably the best, also the most expensive, but worth it. Good Luck.

[Rleap619]

Thanks for the help guys ended up drilling it out to .220 and got some Emuge taps cut through it like butter.

[Orange Vise]

Thanks for the help guys ended up drilling it out to .220 and got some Emuge taps cut through it like butter.

Glad you got it done.

.220 is pretty close to a #2 drill which would give you a 60% thread. Plenty strong, yet requires much less torque to drive a tap than a 75% thread.

Tap Drill Chart - 75% and 60% Threads

[basalt]

Please help me out here, according to my calculations, for a 1/4-28-UNF-2B (Internal Minor Diameter) a tap drill size of .221" would give a 75% UN thread which is the equivalent of a 62% AN thread. Also, a tap drill size of .213" would give a 96% UN thread which is the equivalent of a 80% AN thread. My point being that there is a difference between the old American National Thread and the Unified National Thread based on the formulas from Machinery's Handbook in the attachment. My question is this--if this is correct couldn't we go to 79% UN as recommended (see attachment) which would give a tap drill size of .219" based on the formula TDS = D - (1.0825 X %)/TPI which is .250 -(1.0825 X 0.79)/28. Does anyone agree and do you think this is worthwhile information?

[txcncman]

Sure. Why not? What is your point?

Shouldn't you use UN or AN as called out by the thread specification on the part print? Most prints I have seen only call out UN for specification.

[basalt]

Hi txcncman, thank you for your reply. My point is that the tap drill charts all give % thread based on AN and that by using UN the tap drill size can be bigger thus alleviating a lot of problems.

[tony978]

all you need is castrol moly dee tapping fluid. I use it all the time to tap 1/2-13 one inch deep in 17-4 ph by hand and power taping .

[neilw20]

intermediate gun tap.
Use machine for perfect alignment of tap and do it by hand with an arbor guiding the tap. You only have a few to do.
Tap magic is good.
Gun taps actually cut very nicely with spiral chips coming forward of the tap.