Thread milling cutterdia / hole ratio

[HuFlungDung]

Hi Guys,

I've been considering the purchase of some thread mills, but find myself wondering whether the milled thread geometry is correct, when the cutter has a diameter, for example, of 1/2" and the hole to be threaded is maybe 11/16 (for 3/4" NF). When the cutter diameter is an appreciable fraction of the hole size, there would be quite a bit of cutter sweep across the thread helix, which would tend to change (increase) the included angle of the internal threads somewhat.

This effect is plainly seen when milling helical gear teeth, wherein the gear cutter cuts a groove wider than the cutter on account of the entry/exit ramp of the cutter tooth. For this reason, a machinist actually uses a lower numbered cutter for milling helical gear teeth, than would normally be used for an equivalent spur gear.

Is this ever taken into account for thread mills, do you know?

[brtlatjgt]

Hu,

Don't exactly know the techinal info you want but I do know this---Threadmilling is the way to go. We recently bought an Okuma mill and have been doing quite abit of threadmilling on it. We've been cutting 1/4"npt female threads in 316 s.s. . I've been using an advent solid carbide threadmill with coating. At 3000 rpm I've cut over 800 holes with the same cutter! I've only had to offset my cutter a couple of thousands. Thread form is excellent!

We have a ground and hardened gage and it gages perfectly.

We recently bought an inserted threadmill--advent again ---for cutting 2"npt female threads. Again it worked beautifully. The thread form can't be beat.

Also advent has a new threadmilling software program you can get for free. Only takes a few minutes to input info and your done.

P.s. Thanks for your insight in this forum. I've learned alot.

[HuFlungDung]

Thanks Br,

I've done a bit of NPT thread milling in aluminum with an insert cutter, and liked it a lot. I just don't have big enough arms to turn a 2" pipe tap

I am much more nervous about milling smaller holes in steel, though. This means I have to buy the solid carbide thread mills, and I am worried about recutting chips and spoiling the tool. There is not a lot of room around the tool in these situations.

When I look at paying $300 Cdn for one to experiment with, I just know the first one is going to last about 2 seconds

Do you use an air blast to remove the chips?

I assume these are best used dry?

I also assume climb cutting is best?

[brtlatjgt]

Hu,

Had similar reservations too. On one repeat job, again in 316 s.s., we had to put around 15 -20 1/4-20 holes in it. Initially when we first got our mill we were tapping them with a floating lyndex head. Constantly breaking taps. So we got a solid carbide threadmill. I'd have to say I milled about 60 holes with no problem. Took it in two passes. I'm sold on ADVENT threadmills like your sold on ONECNC. Just flooded the blind hole with coolant and it worked great. I think you'll be surprised at how well they cut. Rpm's are your friend. There really not a lot of chips on a small diameter hole and they break very small. By the way if your tapping in stainless we found a spray on foam by Ashburn that works miracles. Rex supply carries it. Yes, climb mill and I don't think I would run it dry.