Deep hole tapping

[Bmac991]

Can any one help me with this, I have to tap 2.5" 4nc in a hole 6" deep
My question is what is the minimum allowable thread engagement for this size hole. I know for most holes it is typically 75%.
I was wondering if I could go 50% because at the recommend tds from the machinery's hand book the hole is not big enough and ends up ripping threads and stalling.
Any suggestions would be very appreciated

Thanks

[underthetire]

Anything that big, we thread mill.

[Bmac991]

i have thought of that but i am kind of a novice to thread milling and was not to sure about the depth (finding a tool long enough rigidity and deflection) and time it would take to do one hole, i have about 150 holes to do.the holes are for mounting blocks for compressors.

I also have about 75 holes at 1.750" 5 unc to do as well, we have subbed some blocks out but it is getting costly so the boss wants us to do them in house.

[RICHARD ZASTROW]

Check your Machinery's Handbook again. I believe you'll find there is an adjustment for lower percentage of engagement for deeper thread lengths.

Dick Z

[underthetire]

Yer gonna need low end HP no matter what to tap that big. Can you peck tap on your machine?

[Bmac991]

i would have to double check but i believe so, would it matter that it is a cnc drill not mill?

[Shane123]

as long as he can rigid tap, he can peck tap, whether that be with a special tap pecking command, or throwing in the same coordinates and just calling the z depth larger in g84 line again. but with that many holes, one should really look into thread milling. so fast and simple.

g84 z-1. r.1 f43.722
z.1
g84 z-2. r.1 f43.722
z.1
g84 z-3. r.1 f43.722

you obviously dont need the z.1 inbetween lines, but it is just a habit i got into after running a romi lathe.

[DareBee]

Holy - big tap - BatMan
And a lot of them two.

Definately would thread mill both of those units.
Vardex has free software to make the code and they have good threadmills too.

I would be afraid of breaking my machine with that kind of torque needed. Threadmills will cost 300 - 400 but ballscrews and spindles (damaged) will cost 10G PLUS.