View Full Version : Machinist question


Don Cowles
11-18-2002, 05:37 PM
I have dug myself into a hole. I have committed a cardinal sin: I have 5 knives to deliver wednesday night, and I did not make a prototype. They are my basic straight blade, but the carbon fiber scales will be held on with 4-40 screws, for which the blades were tapped before heat treat.

THE PROBLEM: Although there does not appear to be any scale in the threaded holes, I can only get a screw (or a tap) part way into the holes- so I am assuming they shrunk during heat treat.

QUESTION ONE: can I fix it, and how?

QUESTION TWO: what can I do to prevent this in the future?

Many thanks -

Tom Anderson
11-18-2002, 08:11 PM
I wouldn't try tapping, because the tap would probably just break.

Localized bead blasting might remove enough material that the screws will eventually go in.

DC KNIVES
11-18-2002, 10:17 PM
Just a guess, but would spot annealling work? I have done this on carbon steel,but not stainless.Good luck Don,Dave.

Geno
11-18-2002, 10:38 PM
Don, seal up the holes with vinegar overnight, and let it eat that black off the iside(it is a microscopic scale).
Come back later and blow it out with high air pressure real good, then tap it using oil. It should come right out, but don't force it.
Try a used tap first if you have any old ones.
Don't use a tapping machine now either, do it by hand so you can feel it. Hope it gets you out in time.
Be blessed.

neil charity
11-19-2002, 01:40 AM
I've had a similar problem in the past Don, I get around it by running the screws I'm going to use in the final setup before the blades are heat treated. Seems to work OK however Gene's solution should work also.

Regards from Australia.

Don Cowles
11-19-2002, 06:26 AM
What's baffling here is that I did exactly what Neil suggests - and all the screws went into all the holes. After HT, they were all bead blasted. I have spot annealed, and broke off a tap in one of the blades.

Next step is the vinegar, then the trash can. Thanks guys.

Jerry Shorter
11-19-2002, 10:33 AM
Don,
Two things you can do in the future. Use a (.003 ) larger tap drill, #43 for 4-40 . For the purpose of knife handles a 60% thread will work fine.( tap drill charts are set for 75% thread) 2. Get an oversize tap. Over sized taps are Ideal for steel that must be tapped befor heat treat. Spiral point (gun) taps are much stronger than spiral flute taps. the best way to use gun taps is with a tapping head.
Hope this helps in the future
Jerry

SharpByCoop
11-19-2002, 11:05 AM
I like Gene's answer. Going further, since you are getting down to the wire, I'd try sealing some acid (ferric?) in that hole for a short while. I can't imaging it not working.

At my shop I also get success with buggered-up threads by using a fine lapping compound on both the thread and the bores. In and out, in and out, like a tap. That works, albeit slowly.

Keep us posted.

Coop

Colin KC
11-19-2002, 11:07 AM
Another option (all be it an extreme & decidedly fiddly one) is to tighten up a die real good & cut the thread on the screws a bit more. (just make sure you use blue loctite;) )

Don Cowles
11-19-2002, 11:14 AM
Thanks for the suggestions, guys. Jerry, good tip on the oversize tap drill/gun tap. I will call Brownells today!

Rob Frink
11-19-2002, 11:39 AM
Don,

In the future, I think the solution may be simple. It sounds like you may be using the wrong "thread class" tap...or "h-limit". Taps have tolerances and are denoted with : H1, H2,...H6. H1 is the smallest tap size and H6 is the largest size for a given thread.

So look at the tap that you are using....look for the "H" number on the shank and make a note of it...then order a larger "H" next time. For example, if you are using an H2 tap.....move up to an H4,5 or 6. Premium taps from from industrial suppliers are offered this way. The purpose of the sizes is to acount for plating build-up.

I'm very curious...so if you look...please let me know what you find.

Good luck,
Rob

Jerry Shorter
11-19-2002, 12:13 PM
Don, What Rob says is true . the oversized tap that I am referring to is grade H11. Most taps that you buy are grade H3 for inch sized taps and H4 for metric taps. ( grade refers to size limit). When using acid to etch out the threads check it often because it degrads the entire thread form including the thread angle. and you will get a very sloppy fit. If not extremly sloppy the blue loctite will make it better. but loctite is no replacement for threads.If it does get extremly sloppy I dought if you would be happy with the results.
Jerry

correction: 4-40 taps are generally grade H2

P.s. if Brownell's doesn't have them give McMaster Carr a call or check their web site, they stock them #2612A11 for a 4-40 oversized tap.
Just found this excelent information
http://www.morsecuttingtools.com/reference/engineering.html

Don Cowles
11-19-2002, 12:57 PM
Guys, what blows me away is that the screws fit the holes just fine before HT. There is no visible scale on the threads, either.

Jerry Shorter
11-19-2002, 04:03 PM
Don, it very common for screw threads to distort during heat treat, Using an oversized taps is common practice. the threads will still distort but the diference in size makes up for the distortion.
Jerry
......a little later, I was just looking in the Mcmaster Carr catalogue and I see where they have heavy duty taps that will tap hardened steel (30-48C) Your tang may be quite a bit harder than 48C but one of these might work to clean up the threads. They are pricey but might be worth it to save the blades. 4-40 part #25215A51 about $14.00

Don Cowles
11-19-2002, 07:35 PM
A gunsmith in the neighborhood cleaned them up with a TI coated cobalt tap, so for a $20 bill, I am out of the woods for now. I will surely avoid this problem in the future by using oversize taps!!@#$%!!

caveman
11-19-2002, 08:29 PM
Don, I'm no machinist, but you might want to try a product called "Never Seize" you can find at automotive parts houses, it's a silvery colored stuff you can brush on threads that are subjected to high heat, like exhaust bolts.....works great & does exactly what it says, a little on a cue tip in each tapped hole before heat treatment may brighten your day from now on......

Nifmakr
11-20-2002, 11:17 PM
Don,
If you have an ajustable 4-40 die, you can also adjust it to cut the threads slightly undersize to fit the holes. Then if you get a sloppy fit, you can take another screw and back the die back out and get a perfect fit. Most all tool steel, SS and carbon, shrink slightly during heat-treat. And a few expand slightly.
Sorry I didn't see this sooner.

Steve

tom mayo
11-25-2002, 07:13 PM
seems to me all of you are way too smart.............why cant you just run the threads of the screws on a dull 400 grit belt so they are a bit smaller??????????????

Colin KC
11-25-2002, 07:41 PM
Because we're way too smart;)

Colin KC
11-25-2002, 07:41 PM
Smarty pants:p

Nifmakr
11-25-2002, 08:05 PM
Tom,
It's not necesisarily the outside of the thread holding it off but the littlebitty grooves that are keeping it from threading in. ;)

Steve

Colin KC
11-26-2002, 03:16 PM
See, someone even smarter:p

tom mayo
12-10-2002, 12:24 AM
Hes from the hills in Tennessee, he doesnt know nothin'! :D

Nifmakr
12-10-2002, 06:49 PM
"Pot calling the Kettle black".

Steve :D

Ken Onion
12-18-2002, 02:56 AM
I have also had this problem in the past the best and simplest thing to do is to get a couple of carbide drill bits from 1/8-3/16and a 3/16 carbide reamer and drill out your tapped holes and ream to 3/16 then buy or make a couple of 3/16 straight folder pivot tubes that are already threaded and insert into the holes in your blade trim excess length and you are back in business .these tubes are usually threaded to 6-32 but you will find the occasional 4-40 . Good luck!

Don Cowles
12-18-2002, 07:29 AM
Thanks, Ken. In this case, I did wind up just drilling them out with a carbide bit and using pins on the scales. I have also bought some oversize 4-40 taps. The threaded tube is a good solution.

Bob Warner
12-18-2002, 08:35 AM
I have a question.

If you were to run screws into the tapped holes and left them there during heat treat, do you think you could get them out afterwards?

I see several possible answers here but I really don't know what would happen.

They could be stuck there forever.
They could shrink more than the hole and fall out.
They could not change and the holes shrink around them and deform the holes.
They could weld to the tang.
They could be removed and new screws fit perfectly.

Any speculation as to what would REALLY happen?

Don Cowles
12-18-2002, 10:04 AM
Bob, let us know what happens when *you* try it. :evil :eek:

Bob Warner
12-18-2002, 10:07 AM
And I was hoping for an educated guess, instead I get homework.

:)

Geno
12-20-2002, 10:53 AM
A bi-matalic fit up should not weld, for example S/S screws and C/S blade.
The holes could shrink around the screw, worst case senerio.
Black soot fron acetylene can coat the screws to ensure no fusion occures inside the holes.
S/S should not be affected by the heating and cooling processes,
The tang holes can be.
I use threaded barrels and drive them into the tang for threads.
Havn't lost a tap sinse I started this way."IF" the threads got buggered up, just knock it out and replace the barrel instead.
(Unicorp in New Jersy-800 526 1389 has these fittings)
(they are called threaeded stand offs)
Heat treating with screws in place is not a good idea, just asking for extra problems.
It depends most on what kind of screw you put in what kind of steel, of what would happen.My best suggestion-Don't.
Don, try these fittings and see if they help.
I use the P105's in S/S with a 2-56 thread.
It just may eliminate this problem forever.
Be blessed.

Don Cowles
12-20-2002, 01:05 PM
Gene, I have tried this, and the concept doesn't work out well with attaching scales to tapered tangs- especially on smallish knives, where I might only be able to get one thread of engagement on each side. I even went so far as to design a two-part fastener something like the corby bolts that Rick Hinderer was going to make for me, but his South Bend lathe wouldn't cooperate. I'm still working on alternate methods, but I'm not there yet.

tom mayo
12-24-2002, 03:44 AM
I do the same thing, using the pivots from halpern with the 4-40 screws..........having them sit up above the handle and countersink the inside of the handle....works great for me. if you have a problem with tapered tangs then drill the hole square before you taper the tang.

Don Cowles
12-24-2002, 07:05 AM
Tom, counterboring the handle slabs for the threaded sleeves may be just the ticket. Thanks for the great idea!

I do my drilling wile the flats are still parallel (before tapering), so that's not the issue.

Merry Christmas!

Ken Onion
12-25-2002, 02:01 AM
I thought I said that !

Don Cowles
12-26-2002, 10:00 AM
Ken, you did, but the only thing missing was the concept of recessing the threaded sleeves into the handle material.

Merry Christmas!

Don Robinson
01-20-2003, 06:23 PM
Hi Don, long time no see. Sorry to hear about your problem, but I see the gun smith fixed it for you.

Take what the guys said about using oversize taps to heart. I suggest you buy them from your closest industrial supplier and specify the oversize "H"pitch diameter limit you want.

One reason the holes got smaller is that the metal grew into them a little when you heated it. Metal always grows when heated, and it goes where there is least resistance. Outside profiles, thickness, holes and voids.

When the steel is quenched, it contracts violently and the amount it contracts varies depending on the steel chemistry. Some tool steels return to near original size, and some don't.

Don't be a stranger.

Don Robinson
01-20-2003, 06:23 PM
Hi Don, long time no see. Sorry to hear about your problem, but I see the gun smith fixed it for you.

Take what the guys said about using oversize taps to heart. I suggest you buy them from your closest industrial supplier and specify the oversize "H"pitch diameter limit you want.

One reason the holes got smaller is that the metal grew into them a little when you heated it. Metal always grows when heated, and it goes where there is least resistance. Outside profiles, thickness, holes and voids.

When the steel is quenched, it contracts violently and the amount it contracts varies depending on the steel chemistry. Some tool steels return to near original size, and some don't.

Don't be a stranger.