Gunsmithing Tapping 8-40 scope holes in stainless

[c1steve]

I am upgrading to 8-40 screws on some stainless M700's. The first hole went fine, but the tap became dull and will not complete the third hole.

I started with a #28 drill, then went to a #27 and a #26. My questions are:
-What drill size would you recommend?
-For the forward hole, would you recommend using a drill or having a gunsmith use an endmill?

I bought quality cutting tools from MSC, and am using Tap Magic/Pro Tap.

-Steve

 

[Raven 6]

Re: Tapping 8-40 scope holes in stainless

You should use an endmill for all the holes in case they are drilled slightly off center, a drill will follow the existing hole an endmill will not. Use a 9/64ths if I remember correctly, not at the shop right now. Use a good 4 flute tap, preferible coated or carbide.

Raven Rifles

 

[Mark Housel]

Re: Tapping 8-40 scope holes in stainless

I tap a <span style="font-weight: bold">lot</span> of holes in stainless, but 6-40 in my case.

I have had the best luck with Greenfield EM-NI taps.
Expensive as hell, but at least they tap dozens of holes before they slow down and then I can use them still quite well for a while in regular gun steel. They are coated taps.

Carbide taps scare the hell out of me since I have <span style="font-weight: bold">no recourse</span> if one breaks in a customers slide. HSS, I can deal with.
Yeah, EDM... but no one I know has one?

 

[PCR]

Re: Tapping 8-40 scope holes in stainless

Mark,

There is an EDM shop in PV on the main drag as you enter town from Prescott. Can't remember the name off the top of my head but I can get you there. Give me a call if you need.

 

[LRI]

Re: Tapping 8-40 scope holes in stainless

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: c1steve</div><div class="ubbcode-body">I am upgrading to 8-40 screws on some stainless M700's. The first hole went fine, but the tap became dull and will not complete the third hole.

I started with a #28 drill, then went to a #27 and a #26. My questions are:
-What drill size would you recommend?
-For the forward hole, would you recommend using a drill or having a gunsmith use an endmill?

I bought quality cutting tools from MSC, and am using Tap Magic/Pro Tap.

-Steve </div></div>


Tap drill formula:

American National Thread Form:

Tap Drill Size ='s <span style="text-decoration: underline">Tap Major Diameter</span>-(1.299 X percentage of thread / threads per inch)

Example:

What's the appropriate drill size for a 1/4-20 tap with 75% thread engagement?

TDS=.25-(1.299*.75/20)

TDS=.2013" OR a letter F drill. (you can safely ignore the 4th digit right of the decimal)

If the math bugs you there's several calculators available via Google.

As for SS. Sulfur seems to be a good lubricant for heat treated SS. Some black smelly pipe fitter oil would work well. I have really good success with Castrol Moly D tapping fluid. Tapping woes can almost always trace back to tooling issues. The cutting fluid generally aids in tool life but if there's fundamental problems to start it won't fix them.

Make sure drills/endmills are being used at appropriate speeds/feeds. This ensures your cutting properly and not work hardening the material as a by product. Use quality taps that are <span style="font-style: italic"><span style="font-weight: bold">sharp.</span></span> Once they go dull, make yourself toss them away. They'll only create problems for you at that point. A $20 tap can easily turn into a $1000 problem on a receiver.

If your plumbing for living remember this: The cst brought it to you the same as he/she would bring their car in for service. A dealership would have no hesitation on charging a cst for a tool unique to the job. That cost must be passed onto the customer. They may grumble but I promise you its a pittance compared to what the conversation is going to be like if you nuke a tap in their prized action. Tool grade steels used for gun parts are hard, tough, and abrasive.


Hope this helps.

C.

 

[AJ_Goddard]

Re: Tapping 8-40 scope holes in stainless

I also had issues tapping 6-48 and 8-40 holes with the taps purchased from MSC and Brownells. To say the least the Brownells taps were junk. The MSC taps were a little better but failing prematurely. The problem is they are odd enough sizes no one makes a premium tap. If it were a 1/4-20 or 8-32 you could get taps all day from MSC that would work great. I finally had taps custom made of a premium tool steel and coated. I have to buy in quantities of 2 dozen at a time to get the cost down to $30. each and I have a 2 month wait to get a batch made. I have done some thread milling on these small sizes and it works surprisingly well. The little carbide thread mills hold up great and when they do fail they are smaller than the hole so they come right out and are consistently made so you just touch off another tool and keep on cutting. I have never used a carbide tap in 30+ years and would be afraid of them being too brittle and trashing an action. I am sure the carbide taps have a place but I'm not going to be trying them any time soon. I use .140 diameter drills on the 8-40s.

 

[c1steve]

Re: Tapping 8-40 scope holes in stainless

This all helps. I am going back to MSC and buying some cobalt drills plus more taps. I note the problem with 8-40 being an odd size.

This makes me want to switch to 8-32, but obtaining the fasteners might be a problem. I have worked with stainless quite a bit, and was quite surprised at the toughness of the metal I am working on right now.

I will buy plenty of taps this time, and as C. Dixon recommended toss them as needed. These are my own receivers, so I have flexibility. Thanks to all for their professional advice.

-Steve

 

[hero's machine]

Re: Tapping 8-40 scope holes in stainless

Make sure you are not wasting any time when you drill the reciever....that stainless gets hard as wood pecker lips in no time.

 

[LRI]

Re: Tapping 8-40 scope holes in stainless

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AJ Goddard</div><div class="ubbcode-body">I also had issues tapping 6-48 and 8-40 holes with the taps purchased from MSC and Brownells. To say the least the Brownells taps were junk. The MSC taps were a little better but failing prematurely. The problem is they are odd enough sizes no one makes a premium tap. If it were a 1/4-20 or 8-32 you could get taps all day from MSC that would work great. I finally had taps custom made of a premium tool steel and coated. I have to buy in quantities of 2 dozen at a time to get the cost down to $30. each and I have a 2 month wait to get a batch made. I have done some thread milling on these small sizes and it works surprisingly well. The little carbide thread mills hold up great and when they do fail they are smaller than the hole so they come right out and are consistently made so you just touch off another tool and keep on cutting. I have never used a carbide tap in 30+ years and would be afraid of them being too brittle and trashing an action. I am sure the carbide taps have a place but I'm not going to be trying them any time soon. I use .140 diameter drills on the 8-40s. </div></div>



AJ,

Would you mind sharing where your buying your thread mills from? Been wanting to switch to this for some time.

Thanks.

C.

 

[c1steve]

Re: Tapping 8-40 scope holes in stainless

Yes, very quickly. High feed pressure, low cutting speed from what I remember.

 

[turbo54]

Re: Tapping 8-40 scope holes in stainless

Working with Titanium taught me the value of a machine doing cutting work, and not a human.

Try hand-tapping some 6/4 Ti, even just 1/4-28, and you'll be breaking taps left and right. Power-tapping, not one broken tap, and 20-30 holes later, tap still cutting beautifully.

 

[cal50]

Re: Tapping 8-40 scope holes in stainless

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Mark Housel</div><div class="ubbcode-body">I tap a <span style="font-weight: bold">lot</span> of holes in stainless, but 6-40 in my case.

I have had the best luck with Greenfield EM-NI taps.
Expensive as hell, but at least they tap dozens of holes before they slow down and then I can use them still quite well for a while in regular gun steel. They are coated taps.

Carbide taps scare the hell out of me since I have <span style="font-weight: bold">no recourse</span> if one breaks in a customers slide. HSS, I can deal with.
Yeah, EDM... but no one I know has one?


</div></div>


+1 for Greenfield & OSG taps. Excellent tooling and worth the cost. Just an FYI on fine pitch threads a small change in hole diameter makes a bigger difference in thread % over coarse pitch threads.Also if your parent material is tough you can run a lower thread % and be fine. High sulfur cutting fluids always worked well for me when machining SS.

A #28 drill gives you 75% thread and that's what I would use.

 

[boshine]

Re: Tapping 8-40 scope holes in stainless

Measure twice. Use lube to drill & tap. Most people use lube for a tight hole. But this takes a different lube, made for drilling & tapping(see above for types). Wait until you break a tap off in stainless. Runes your whole or hole day.
in-range

 

[BPaige]

Re: Tapping 8-40 scope holes in stainless



A F drill is .257"...drill size for 5/16-18 tap.

13/64" or #3 drill for 1/4 20

 

[LRI]

Re: Tapping 8-40 scope holes in stainless

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BPaige</div><div class="ubbcode-body">

A F drill is .257"...drill size for 5/16-18 tap.

13/64" or #3 drill for 1/4 20 </div></div>


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