PDA

View Full Version : Source for oversized tap?



NAS-NAS
10-08-2008, 01:48 PM
I need to tap a hole about 4" deep for a Hot Dipped Galvanized bolt (3/4" -10)

From what I have read I need about 0.020" oversized.

Any ideas?

Thanks!

Just Bob Again
10-08-2008, 04:04 PM
Why? 40 threads engaged won't give you any more strength than 6. Tap and relieve the rest.

You won't find a tap that much oversize. Unless you buy a cheap import. Then it will be oversize, variable pitch, bent and dull, too!

madman
10-08-2008, 05:36 PM
Jam a piece of hardwood into one of the flutes to offset the tap a slight amount. Also drill the threaded hole a bit larger like whatever you need for galvanizing. FIRST try on a scrap piece of material and get it dialed and then try on the good part. I used to jam stuff into flutes to get it to wander and do what i wanted and it worked good. Good luck Mike aka madamn

dalee100
10-08-2008, 08:43 PM
Hi,

MSC has .005" oversized taps available. Check with them.

dalee

Teenage_Machinist
10-08-2008, 09:42 PM
As I understand taps are one of the easier tools for an HSM to make. Perhaps you could make an expanding tap for after use of a normal tap?

ulav8r
10-09-2008, 01:04 AM
Did you have a standard bolt hot dipped? If you bought a commercially plated bolt it should be a standard 2A class bolt and would fit a standard tapped hole.

Fasttrack
10-09-2008, 01:27 AM
Did you have a standard bolt hot dipped? If you bought a commercially plated bolt it should be a standard 2A class bolt and would fit a standard tapped hole.

erm... if it were "plated" yes. Hot dipped, no. Hot dipped bolts are always oversize. Your average hot-dipped bolt from HD or similiar will not thread into a normal hole worth a darn. You'd think you were using locking nuts or something and then the threads end up rusting because the plating was stripped off in the assembly proccess.

Don't know where you can get a tap that much oversize and not pay an arm and a leg. Most T&C shops will make custom tooling and I'm sure you can contact the big name guys and they will make you a custom tap but thats probably not what you want to pay for.

I've heard Madman's trick before and I second that approach. It works well.

NAS-NAS
10-09-2008, 08:43 AM
Thanks for the replies.

I might try offset technique.

Yes, the bolt is hot dipped galvanized and will not fit a standard nut. The reason so much thread is required is because a fixture bolts down to it that is fairly thick and when the fixture is removed, the bolt is stored in the hole.

jkilroy
10-09-2008, 12:36 PM
Helicoil it. Drill the hole oversize and Helicoil it with the insert at the top. The rest of the bolt can extend into the bottom of the hole. That way you don't have to try to tap down 4", you don't have to thread a bolt in that far, thats asking for galling, also, it can be fixed in the future if required. On fixture mounts Helicoils are often a good idea.

Scishopguy
10-09-2008, 12:54 PM
Thanks for the replies.

I might try offset technique.

Yes, the bolt is hot dipped galvanized and will not fit a standard nut. The reason so much thread is required is because a fixture bolts down to it that is fairly thick and when the fixture is removed, the bolt is stored in the hole.

Just a stupid thought here but why not just run a die on the threads of the bolt? As Fastrack said, the galvanize will be rubbed off and it will rust a little anyway. Besides, short of a grinder you will never remove all the zinc from the steel with a die unless you cut it undersized.