Grinding a damaged tap

[BrokenBiker]

Chipped a few teeth on the first few turns of one of my better spiral taps, turns out the 10.2mm cobalt drill it bought specifically for tapping m12 is actually undersize at 10.04mm...

Anyway, tap is damaged.

It's a decent quality black nitrided spiral m12...can I just put it in a drill, slowly turn it and grind away the damage to effectively make it a taper tap? Or an i better just slicing the damage off and putting a small grind on like it had originally.

 

[Pete.]

You can try, but don't expect too much. If the application is non-critical just make life easier and drill a 10.5mm hole.

We're you tapping cast iron? Easy to chip your tap on that especially on a tight tapping size.

 

[dave h 2]

Same as the side flutes on a milling cutter. It's done by have the tap free moving and a pin to hold the tap in one flute at the point the stone touches it in another and pushing the tap along at the same time. Also whilst standing on one leg and rubbing your tummy anti clockwise.

No such thing as a taper spiral tap it seems.
No harm in trying free hand as the taper is scrap anyway.

 

[Agroshield]

Same as the side flutes on a milling cutter...

Are you not describing how to sharpen the cutting edges of a tap?

I think the OP only wants to remove the damaged portion, so it would be the same as grinding away the tapered flutes of a normal tap to make something that would reach right into the bottom of a blind hole.

That could be done as the OP suggests, but drill is perhaps not the tool to hold it in. A spin indexer would hold it more ridgidly and present it consistently to the abrasive wheel.

My instinct says that too much taper on a spiral flute tap could have the two features fighting each other. It could be better to reproduce the small factory grind and try it. Removing more material if that does not work is easier than putting it back.

 

[BrokenBiker]

You can try, but don't expect too much. If the application is non-critical just make life easier and drill a 10.5mm hole.

We're you tapping cast iron? Easy to chip your tap on that especially on a tight tapping size.

316 stainless, tapping by hand (tap in tailstock drill chuck, turning lathe chuck by hand), its a threaded bore in a mandrel for a polishing wheel to try and limit the damage my father does to himself with his current set up. the m12 hole is just for the bolt that runs through the buffing wheel and into the madrel, i was going to threadlock a bit of threaded bar into it, then make a beveled washer to it the other side of the wheel and use a nut as it isnt an accurate bore through the wheel but the metal plates the sides of the hweel seem pretty concentric.

 

[BrokenBiker]

Are you not describing how to sharpen the cutting edges of a tap?

I think the OP only wants to remove the damaged portion, so it would be the same as grinding away the tapered flutes of a normal tap to make something that would reach right into the bottom of a blind hole.

That could be done as the OP suggests, but drill is perhaps not the tool to hold it in. A spin indexer would hold it more ridgidly and present it consistently to the abrasive wheel.

My instinct says that too much taper on a spiral flute tap could have the two features fighting each other. It could be better to reproduce the small factory grind and try it. Removing more material if that does not work is easier than putting it back.

that seems like the option to try, i will cut the damage off with a slitting disc, make it as square ass i can and put a very small bevel on the end as if cleaning up a cut bolt, try it in some nylon or brass first and see what happens...i have other taps in that size, but they are straight flute and this spiral tap just seems to work better, faster adn feels nicer to use...no real reason to hold onto it apart from its one of those go to tools really...

 

[Pete.]

316 stainless, tapping by hand (tap in tailstock drill chuck, turning lathe chuck by hand), its a threaded bore in a mandrel for a polishing wheel to try and limit the damage my father does to himself with his current set up. the m12 hole is just for the bolt that runs through the buffing wheel and into the madrel, i was going to threadlock a bit of threaded bar into it, then make a beveled washer to it the other side of the wheel and use a nut as it isnt an accurate bore through the wheel but the metal plates the sides of the hweel seem pretty concentric.

You got no chance of tapping that with a 10.2 hole. You'll struggle with a 10.5.

 

[BrokenBiker]

You got no chance of tapping that with a 10.2 hole. You'll struggle with a 10.5.

10.2 is the tapping drill size? ive tapped harder stuff before, granted with much more expensive taps...anything bigger than 10.5 and the thread isnt going to be worth tapping surely?

 

[Onoff]

You got no chance of tapping that with a 10.2 hole. You'll struggle with a 10.5.

I've tapped plenty of 316 after drilling at 10.2 mm...

 

[Pete.]

It's the tapping size but you'll need all the clover leafs and horse shoes crossed to hand tap 316 without going over-size with the tapping hole.

It goes squeak - crunch - *&$£$%!

 

[Agroshield]

10.2 is the tapping drill size?

The major diameter minus pitch is a good rule of thumb. But there are options either side of that, ranging from 100% thread engagement on the smaller side to 50% thread engagement on the larger side. Guhring have some useful things to say, and provide a calculator here:

Tap Drill Calculator - GUHRING

 

[Hitch]

Buy a new tap... they're consumables.
Having a good quality sharp tap is key with stainless.

Keep that one to butcher when you need one to get right to the bottom of a blind hole.

Bang a 10.5 in it if its a 12x1.75

 

[Pete.]

According to the link above a 60% engagement thread would use a 10.64mm tapping size hole. for 316 and especially 304 I'd play it safe and go with 50% using a 10.86mm drill

 

[BrokenBiker]

Buy a new tap... they're consumables.
Having a good quality sharp tap is key with stainless.

Keep that one to butcher when you need one to get right to the bottom of a blind hole.

Bang a 10.5 in it if its a 12x1.75

i have a set of m12 taps that rarely come out the box, think they have only been used on mild steel maybe twice, will put a 10.5 in and try the taper tap first, if it cuts ill follow it with the plug tap afterwards.

new carbide tool holder has arrived now though so started a different piece at the mo for a little ring roller for myself, the mandrel is all but done now apart from that thread.

 

[Onoff]

Most of my metric taps are really old but good makes and all HSS.

 

[Alan Reynolds]

We get kart chassis powder coated and there are eight 3mm tapped holes that get filled.
I always pass the tapping drill through them and then run the tap through very carefully to clear the rest of the powder coat.
“Someone” decided they knew better and went straight in with a tap, because it was already a cut thread. Well the tap broke as I predicted.
Any ideas how to get a 3mm piece of tap, out?

 

[Agroshield]

Any ideas how to get a 3mm piece of tap out?

Through hole or blind hole? How many flutes does the tap have? Darning needles poked down the flutes and make a holder for the projecting parts might allow the tap to be unscrewed. It would be a home-brew version of this: https://www.waltontools.com/tap-extractors/

Presto make a spade drill for M3 tap removal: https://www.presto-tools.co.uk/Products/broken-tap-extractor-tool/090402.0

Vibration near or on the back of the hole (SDS drill with a wood block) could help.

Are there any chemical methods that could soften the powder coat - to what is it not resistant? Build a dam around the hole with blu-tak and use a syringe to apply carefully.

Would a brass or aluminium grub screw in the hole before coating be an option?

 

[BrokenBiker]

so, just put this madrel back in the lathe, drilled to 10.5 and ran a fresh m12 taper tap through, its put up a fight but eventually got it full depth, followed with a plug tap...threads a bit loose though so i have just coated them with 2 part epoxy before screwing the stud in...once thats set it should be solid enough for what its gotta do.

can now set up for a bit of milling tomorrow for the frame for my vice mounted ring roller

 

[Pete.]

An over-sized tapping hole should not on its own cause a very loose thread, unless it's silly over.

 

[BrokenBiker]

An over-sized tapping hole should not on its own cause a very loose thread, unless it's silly over.

well either way, the reuslt is loose threads in this case...to be honest, this stuff didnt really feel like stainless when tapping, felt a lot harder, similar to the driveshaft matrial i used for another part...if making it again, i would probably just machine the external threads in, but the gear needed for that is still on the list of things to modify, need to file a keyway in but waiting on a smaller file to arrive as nothing i have fits the bore or the keyway needed.

the original part had a left hand m12x1.0, which i could have cut, with a matching nut and a larger diameter on the end so the wheel slipped over the mandrel, down the threads to the stop, then the nut ran on, but that means removing the mandrel every time the wheel is changed, but wanting a larger diameter for the shank meant i would be limited to 10ish mm instead of the 16mm ive got now...at least this way its a fair bit stiffer and only 1 nut to remove and the wheel is free so changing wheels for different compoounds is now easier as well.

but before any gear mods or making another better part can happen i need to make another ring roller to finish yet another project thats over run for my father as my hydraulic one failed after 1 bearing on both non driven parts decided to bind up...so now its brass bushes in some 15mm plate that will be bolted to the vice in place of the jaws so a fair bit of milling to get things nice and accurate, bit of probably stick welding, more milling to true the top faces up and then a few 6mm counterbored holes, a few 16mm holes for the bushes and some rollers made from probably en16 as ive got a bar of 30mm sat doing nowt.