Does anyone with a VF1,2,3,or 4 with a gearbox & cat40 spindle (1996 & newer) tap a 7/8-9 hole with a spirial flute tap in CRS? The thread depth would be approx 2 1/2" deep. What would be the best & fastest way to do this? Would peck tapping work for this or would the mill be able to tap it in 1 pass? Looking at a job with this tapped hole and trying to figure out what mill I would need to purchase. All I have are -2- mini mills & I know they won't do the job but I was hoping a VF1-4 with a gear box would be able to do it. Thanks for all the help.

I've done as large as 3/4-10 so far, on my geared VF3, without any problem.

I always use oil for tapping steel, not coolant. It seems to preserve the tips of the tap's threads. If the tap gets even slightly chipped, the torque requirement goes up very quickly. It might be good to be cautious at the outset, and peck to -1.5 first, then finish the thread.

Don't run too slow, as the motor torque may be somewhat reduced at low speeds, especially on the older machines with open loop spindle drive. 200 rpm should put your motor in a decent power range, probably near 1000 rpm....just my WAG.

On extra deep threads, you can often get by with less than 75% thread, closer to 60 or 65% should be adequate, and this will significantly reduce the torque requirement on the tap.

You could do it but a Threadmill might be better.

Here is actual program ran for M24 X 3(pretty close to 7/8-9)
My VF-2 with gear box had plenty of power
I like going slow , although I can be wrong
I stopped at 2.00 deep on the first pass because that was a blind hole.
Chips were getting too long.
My only suggestion is , try to make hole as round as possible , maybe run a reamer or sizing end mill.Improves tap life a lot.

Good luck.

T18 M06
(USE OIL)
G00 G90 G59 X0. Y0. S55 M03
G43 H18 Z2. M08
G00 X2.5 Y-1.032
G98 G84 Z-2. R0.1 F6.5
G80
G00 X-4. Y0. M09
G91 G28 Z0. M05
M00
T18 M06
G00 G90 G59 X0. Y0. S55 M03
G43 H18 Z2. M08
G00 X2.5 Y-1.032
G98 G84 Z-2.7 R0.1 F6.5
G80
G00 X-4. Y0. M09
G91 G28 Z0. M05
G28 Y0
M30

I have a Mighty Mustang belt drive 40 taper with Vickers Acramatic 2100 Control. I have tapped 1-1/4"-7 in 1018 steel at 100 rpm. Who needs a gear drive? Kollmorgen motors and drives. I'm curious. What is the biggest hole you've tapped in steel with a non geared head 40 taper guys?

Thanks for all the suggestions, it will all help (the peck tapping program, feed & speeds). I am trying to stay away from the thread milling (slower than tapping because of the depth), but would do it if I had to. I have 20000 parts per month (for 1 year) to do & just trying to figure out the fastest method to get them finished.
My mini-mill has tapped a M20 x 2.5 hole 1/2" deep in 4140 PHT (at 75 rpm). That is the largest at that depth that I would want to try it. I can tap M16x2 (2-1/4" deep) in mild steel all day long.
Thanks again for all the help.

If you want to peck tap this works for me. Once the cam spits out the code just cut and paste the G84 line as many times as you need...and adjust the -Z depth to your liking. I like to stick a G00 Z0.3 right after the G43 line to allow coolant to blow off the tap on sticky material in between pecks...

(1/4 28 TAP)
T22 M06
S500 M03
G00 G90 G120 X0.0393 Y0.3625
G43 Z1. H22 M08
G00 Z0.3

G84 G98 X0.0393 Y0.3625 Z-0.25 R0.3 F17.8571

G84 G98 X0.0393 Y0.3625 Z-0.5 R0.3 F17.8571

G84 G98 X0.0393 Y0.3625 Z-0.75 R0.3 F17.8571

G80
G00 Z1. M09

Thanks for all the help everyone. I never new about the peck tapping until I read it on here a few weeks ago.