Help with hand tapping Invar 36

gibsonj · Dec 22, 2014

Hey all,
we work with Invar 36 for some baseplates (12" x 17" x .5"). We've had them done outside and are just starting to do them in house. I need some help, specifically with tapping the plate. The first one I did went fine and I could get about 18-20 holes out of a hand tap before it started to feel dull and I'd switch to be safe. After about 4 months since the first plate, I'm doing plates 2 and 3 and having a real hard time with tapping 1/4"-20 holes. I'm using the same Guhring blue ring taps that I used the first time (and for other invar projects), I'm using the same size drill hole (a .208" from a Guhring drill), and the material is from the same batch as the first plate. When I started tapping these holes, they were WAY more difficult than the first plate. I'd make it about 1/2 turn or less before chips would build and I'd have to back out and go again. That's fine, but I was making it much further with the last plate. After the first one I switched to another tap (of the same type) to see if it was possible that one was dull and I had the same issues. After 3 holes with that one I broke the tap off in the part.

I have around 90 holes to do, and the way it was going in the first plate I did a few months back it would have taken some time but would have been ok. This, however, is torture. I'm using tap wax and doing everything I can to make sure the tap is nice and straight.

Does anyone have any suggestions to help me get through this?

Thanks a ton in advance

Limy Sami · Dec 22, 2014

It's high nickel content will make it a bitch, look for taps made for high nickel etc applications. I found these http://www.guhring.com/Documents/Catalog/Taps/UNJThreads.pdf

More machining info here if it's any help High Temp Metals 800-500-2141 - Invar 36 Technical Data

CBlair · Dec 22, 2014

Welcome to Invar, the drilling cycle is pretty critical as it is easy to overheat the material and make it tough to work with. Using Moly Dee as a tapping fluid might help but I found that drilling was the trick. Slow speed normal drill feed and coolant plus keeping the drill sharp help a lot. But I always found that tapping was the slow part.

Charles

gibsonj · Dec 22, 2014

Limy Sami: do you think that tap will make a big difference over the Guhring blue ring? http://www.guhring.com/documents/catalog/taps/gbtapflyer.pdf

gibsonj · Dec 22, 2014

CBlair: I totally agree that the drilling cycle would have a lot to do with it. I used my old drilling cycle from the first plate (that tapped pretty well) until my drill broke about 30 holes in. Then I called guhring and got their recommended data and used that for the rest of the holes. So in theory the first bunch of holes should behave the same and the second bunch SHOULD be better...but it doesn't work out that way. (also, thanks for reminding me of moly dee)

adama · Dec 22, 2014

Do you u have the option of trying a coated tap. Some time i find it seams to really help in some materials like way more than i can explain.

I have also had more luck with the sharper higher clearance general purpose taps than i have with the supposed hard metal taps too in a similar hard to cut material.

Limy Sami · Dec 22, 2014

gibsonj said:
Limy Sami: do you think that tap will make a big difference over the Guhring blue ring? http://www.guhring.com/documents/catalog/taps/gbtapflyer.pdf

I ''know'' it will, a couple of years back a customer brought me drill and tap job in some hizoot nickel material (nice man, he supplies cutters!) the M6 & M8 taps he supplied were the Guhring I posted, after the job was done I tried them on Nickel Alu Bronze (a SOB to tap) & 316 not fun either! and really rated them. ........it's all to do with the geometry of the cutting edges.

I used a high sulphurised heavy cutting oil, the high nickel job was pretty much a doddle.

+ another on what has been said about the drilling, keep the speed down and feed up, AND DON'T LET IT RUB!! anything Nickel workhardens in less than a fruit fly's fart. Another point - if you can?? make the tapping drill as large as possible IME 0.002'' makes a lot of difference.

gibsonj · Dec 23, 2014

This is all awesome advice and I appreciate it. I have a machinist friend hooking me up with some thread forming information so I can avoid this in the future. Hopefully it'll turn up aces.

Thanks again guys

Limy Sami · Dec 23, 2014

gibsonj said:
This is all awesome advice and I appreciate it. I have a machinist friend hooking me up with some thread forming information so I can avoid this in the future. Hopefully it'll turn up aces.

Thanks again guys

I don't think you'll be thread forming Invar, could be wrong but to my way of thinking - with it's work hardening characteristics you'd be looking for trouble.

Oh and thread form taps aren't really suited to hand tapping.

DMF_TomB · Dec 23, 2014

.208 drill

gibsonj said:
Hey all,
we work with Invar 36 for some baseplates (12" x 17" x .5"). We've had them done outside and are just starting to do them in house. I need some help, specifically with tapping the plate. The first one I did went fine and I could get about 18-20 holes out of a hand tap before it started to feel dull and I'd switch to be safe. After about 4 months since the first plate, I'm doing plates 2 and 3 and having a real hard time with tapping 1/4"-20 holes. I'm using the same Guhring blue ring taps that I used the first time (and for other invar projects), I'm using the same size drill hole (a .208" from a Guhring drill), and the material is from the same batch as the first plate. When I started tapping these holes, they were WAY more difficult than the first plate. I'd make it about 1/2 turn or less before chips would build and I'd have to back out and go again. That's fine, but I was making it much further with the last plate. After the first one I switched to another tap (of the same type) to see if it was possible that one was dull and I had the same issues. After 3 holes with that one I broke the tap off in the part.

I have around 90 holes to do, and the way it was going in the first plate I did a few months back it would have taken some time but would have been ok. This, however, is torture. I'm using tap wax and doing everything I can to make sure the tap is nice and straight.

Does anyone have any suggestions to help me get through this?

Thanks a ton in advance

.
.208 drill for 1/4-20 ?? why so small ?

gibsonj · Dec 23, 2014

the thread forming is coming from someone who has experience. At this point in the process I'm totally cool with trusting him. It's a single point form with 3 passes per hole and he swears by it for almost exactly the same application. As for the hole size it's around %65 of the thread. I've never had a suggestion to go larger...should I?

DMF_TomB · Dec 23, 2014

gibsonj said:
the thread forming is coming from someone who has experience. At this point in the process I'm totally cool with trusting him. It's a single point form with 3 passes per hole and he swears by it for almost exactly the same application. As for the hole size it's around %65 of the thread. I've never had a suggestion to go larger...should I?

.
1/4-20 at 65% i see on chart 5.79mm or .228" (maybe .208 size you are using is a mistake??)
.
i do not use form taps but i would recommend you check your tap drill sizes

gibsonj · Dec 23, 2014

I used an incorrect term. I'm going to try thread milling with a single form cutter...not thread forming. That's my mistake and would make sense why it wouldn't seem likely to work in Invar.
Sorry, that's a beginner being dumb.

Limy Sami · Dec 23, 2014

Ah, threadmilling - that's more like it, - I don't threadmill, but I'd look up, ..........and speak to Vargus - I know they do ''silly small'' and ''exotic'' material apps Vardex Thread Turning, Thread Milling & Miniature Threading

Dumb? - nah, dumb is NOT asking in the first place ! ...............and he who's never made a mistake has never made anything.

CBlair · Dec 23, 2014

I have found Invar to cut very nicely with carbide tools so threadmilling should work very nice. I admit I have not done it in invar as most of my threads are pretty small so I tend to use taps. But otherwise if your machinery is capable of yielding good results then it should work just fine and solve most of your problems.

Charles

tomato coupe · Dec 23, 2014

We run an Invar part about once per year that includes (60) M6x0.5 thru holes and (144) M3x0.5 blind holes. We have successfully used hand taps, single point thread mills, and spiral point taps for the M6x0.5 holes. The spiral point taps are the quickest, but are hard to find.

We initially used spiral flute taps for the M3x0.5 holes, but it was easy to break them unless the tap drills were replaced often (I think dull drills led to work hardening?) We now prefer to use form taps. They last longer and we haven't broken any, but it took a few attempts to get the tap drill size dialed in.

Cheers

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