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Something new for me: a piece so hard, it can not be tapped?

chale4incolo

Plastic
Joined
Jan 26, 2014
Location
Colorado USA
Hi all, so yep, something new under the sun for me today; a small COTS plate that I needed to modify with a simple 6-32 tapped thru hole. It’s the front mounting plate of a beautiful all-stainless steel 3-axis mic translation stage made by Line Tool (sadly now out of business, but the best such stages that I’ve ever used. Lots of them can be seen on eBay). My first indication that this piece was HARD, was the total inability to center-punch the thing for the tap drill hole. O—kay; lemme grab a file and confirm that I can scratch this thing! And I saw that, yes, it was JUST able to scratch the thing…gulp.
Ok, onto the mill and let’s center drill it; nope, NOT gonna happen (a good KEO HSS center drill…). Well then, go ahead and hopefully just drill the tap hole with the ‘special’ #36 carbide drill I decided I better buy. That actually went very well, slow but sure. OK.
I then found that none of my 6-32 taps were even remotely going to start and have a hope of cutting that thread. Felt like I was twisting the tap in glass or something. I ultimately decided to mill a 1/4” hole on thru the thing with a real good chamfered carbide end mill, and arbor-pressed a nice little custom 6-32 brass insert into it, which will work fine in this app. So anyhow, wow, never thought I’d see anything quite like this. I’m beginning to believe there really are materials that are just pretty much not tappable. Eh?
 
I'd say that's just about the long and the short of it. The harder the material, (usually) the higher its strength. So, in addition to requiring a harder tool (like carbide) just to start cut, it also requires more force. Tiny taps have a limit as to how much force they can handle.

Frequently folks will threadmill in hard materials. You can take lighter chiploads, achieve better surface finish, and if the threadmill busts off, it can be removed from the hole.
 
Spot anneal? Carbide tap? Send out for thread milling?
I thought about thread milling it (do have a CNC mill), but those darn things are so $$, and I truly expected I’d break the little guy in this stuff. But I reckon that is probably the very best way to do it. I’m happy enough with the insert, tho I will admit it’s a little low rent…
 
Shove a hole, stick a top hat insert in? Sounds nitrided or carbonitrided as you like , if it’s a block, replace with something workable perhaps, I don’t feel that if it’s not an engagement or wear part that the excessive hardness is useful.
Mark
 
I thought about thread milling it (do have a CNC mill), but those darn things are so $$, and I truly expected I’d break the little guy in this stuff. But I reckon that is probably the very best way to do it. I’m happy enough with the insert, tho I will admit it’s a little low rent…

If it will work, sounds like you did fine to me.
 
I thought about thread milling it (do have a CNC mill), but those darn things are so $$, and I truly expected I’d break the little guy in this stuff. But I reckon that is probably the very best way to do it. I’m happy enough with the insert, tho I will admit it’s a little low rent…
Thread mill. I thread mill M3 holes 11mm deep in 60 HRC guide rods no problem, I think the milling cutter has done close to 200 holes so far.
 
Customer gave me a piece of bronze, silicon-bronze perhaps, can't remember details.
It was for a mandrel for a CNC bender bending square tube on a very tight radius. The square mandrel had a round groove on one face that controlled how the inside face of the bend collapsed inwards.
I did OK with HSS steel end mill and ball end mill to mill the groove on my BP, using Y,Z coordinates to get the curve.
Then called for a 25 mm or so coarse thread in the end for a drawbar. Had some difficulty with the drilling, but I simply couldn't get the tap to start. It kept just blowing out the threads. Lots of downward pressure and a big wrench on the tap, on my BP and no luck at all.
In discussions with customer decided to make a simple adapter with female thread on one end for the drawbar and a straight diameter other end and use a roll pin to secure to mandrel. Customer had roll pins, so he said he would drill hole, 3/16" or so from memory.
Well he had a huge job drilling hole as material so hard ! Finally got a hole and assembled the mandrel in the bender to prove he could bend the 2" thin wall square tube to this very tight radius. (He was under time pressure to prove he could achieve this bend).
Then pulling the mandrel out of the tube he blew out end; it failed at the roll pin hole. But he proved the concept.
He then got the correct grade of bronze and the new mandrel machined up nice and did well.
First time I've come across such a hard grade of bronze.
Pics show the first one blown out, then the good one
Bob
 

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Probably Ampco. If you use HSS it had *better* be brand-new level sharp. It won't hold up long, either. Carbide works a lot better and *sharp* carbide is way better than a honed edge insert.
 
Customer gave me a piece of bronze, silicon-bronze perhaps, can't remember details.
It was for a mandrel for a CNC bender bending square tube on a very tight radius. The square mandrel had a round groove on one face that controlled how the inside face of the bend collapsed inwards.
I did OK with HSS steel end mill and ball end mill to mill the groove on my BP, using Y,Z coordinates to get the curve.
Then called for a 25 mm or so coarse thread in the end for a drawbar. Had some difficulty with the drilling, but I simply couldn't get the tap to start. It kept just blowing out the threads. Lots of downward pressure and a big wrench on the tap, on my BP and no luck at all.
In discussions with customer decided to make a simple adapter with female thread on one end for the drawbar and a straight diameter other end and use a roll pin to secure to mandrel. Customer had roll pins, so he said he would drill hole, 3/16" or so from memory.
Well he had a huge job drilling hole as material so hard ! Finally got a hole and assembled the mandrel in the bender to prove he could bend the 2" thin wall square tube to this very tight radius. (He was under time pressure to prove he could achieve this bend).
Then pulling the mandrel out of the tube he blew out end; it failed at the roll pin hole. But he proved the concept.
He then got the correct grade of bronze and the new mandrel machined up nice and did well.
First time I've come across such a hard grade of bronze.
Pics show the first one blown out, then the good one
Bob
Ahh, good 'ole Ampco 18. I've cut tons of that stuff over the years. It can be very stubborn, but if you have things dialed in just right it can cut relatively "easily".

In my experience it likes a little bit of heat to cut nice, coolant seems to work against ya. Oil or WD-40 is typically a better choice or simple air blast.

That being said, if I never have to cut that shit ever again I won't be sad.
 
Ahh, good 'ole Ampco 18. I've cut tons of that stuff over the years. It can be very stubborn, but if you have things dialed in just right it can cut relatively "easily".

In my experience it likes a little bit of heat to cut nice, coolant seems to work against ya. Oil or WD-40 is typically a better choice or simple air blast.

That being said, if I never have to cut that shit ever again I won't be sad.
The Ampco 18 was good to work with. The other alloy he sent was the nasty stuff; much harder !
Bob.
 
I recently encountered this same problem. I was able to drill the holes using carbide, but there was no way I was going to break a tap off in that part. Someone recently suggested what should have been obvious, spot anneal with TIG.
 








 
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