Thread Forming Taps

[EmilioG]

I need to tap a very short hole, about 4mm deep. I was thinking of using a thread forming tap.
Tap size is m5 x .80 (4.6mm drill for 70% DOT) I read that thread forming taps must be used on a DP or mill. Can they not be
hand/wrench tapped? Most taps don't have enough starting threads.,( like tapping sheet metal)
the hole is on a 1/2" OD x 8mm ID, so not a lot of tapping room. Not a blind hole.

If not a thread forming tap, what type of tap is best for such a short hole?

 

[Tony Wells]

Why do you believe a form tap to be suitable for this? If it is a shallow thread, the usual method is to use either a taper or plug tap, followed by a bottoming tap. There have been instances where I have had to modify even a bottoming tap to get a little deeper. It can get a little dicey once you start grinding even what little lead in that is on a bottoming tap, but it can be done.

Some parts I have had to run I said were tapped two turns past the bottom of the hole. At least it seemed like it.

 

[Wreck™Wreck]

Form taps produce no chips which helps with blind holes, this is a big plus in production work as cleaning chips from thousands of small tapped holes is time consuming.

Hole size is important, you are forming not cutting. I see no reason why hand tapping will not work if all parameters are met.

Forming a full thread all the way to the bottom of a shallow hole will require a non standard tool as Mr. Wells mentioned.

Good Luck

 

[2volts]

I'm trying to work out where the 4mm depth comes from.

Are you trying to thread a hole cross drilled through a tube 1/2" OD and 8mm ID?
so the wall thickness is only (12.7-8)/2 = 4.7/2 = 2.35mm
which isn't even 3 full threads at 0.8mm pitch



anyway this is a 5mm x 0,8 taper tap so you would get close to full thread and be able to finish with a bottom tap

pete

 

[EmilioG]

Thank you. Yes, you are correct. Wall thickness left after reaming will be 2.35mm. This is why I was thinking to use a roll form tap, but if I can use
a bottoming tap to finish, great. I will also need to put a flat on the part that slides into the 8mm ID so that I can use a set screw and clear the surface.
I don't want the set screw to stick out., unless I grind the set screw. The shortest set screw in m5 is 3mm L.

 

[Tony Wells]

Well, a very common cheat in this scenario is to drill into the far side of the hole to gain some length to work with. I have made extensions for air motors and taps and a few other things that needed a set screw to bear against the accessory being extended. It's not always, but never hurts, even to drill all the way thru the far side and just tap it all the way. That way you even have a backup in case the original tapped hole wears out. With such a short engagement, you should be careful of how much torque you put on the set screw. Of course, it's ideal not to have any set screw sticking out of the OD, but as long as it's not much, generally there isn't a problem with it, unless you are working in a very tight spot, or are in a bore that you can't risk putting some pretty heavy scratches and dings in.
Overall, I don't favor using these types of extension in high speed situations, but it can be done, with care.

 

[EmilioG]

The sleeve will be used for an Indicol mod. No high speed or high torque., so it will be fine.
I thought about drilling into the opposite side of the wall a bit to gain more threading room, but may take your suggestion and drill
all the way through for a secondary tap hole as a back up. The part will slip into an r8 collet so the set screw cannot stick out.,
hence the small weldon flat on the 8mm ID part. To get the squared off weldon flat, I need to use a square end/ end mill correct?

 

[Tony Wells]

That's probably the easiest way, and although genuine Weldon flats have beveled lead-outs, you shouldn't need them for what you are doing. You can just run an end mill across it at the desired location. Bear in mind that the length of set screw and depth of that flat work together.

 

[Jimsehr]

Remember form taps need a bigger id size then cut taps.
jimsehr

 

[EmilioG]

Remember form taps need a bigger id size then cut taps.
jimsehr

Yes they do. I have Guhrings' chart for roll form taps/drill size. Going with standard tap sets anyway.
Yes, Weldon flats do have bevel ends, but for my parts they are not necessary. Just a little dressing to remove sharp corners.