shotgun screw in choke

I am currenty working on some screw in chokes for a turkey shoot gun. I am having problems getting threads 32tpi to form uniform an symetrical, i am using a hand ground 60 degree cobolt cutter an spindel is running at 85 rpm. The lathe is a 2010 jet gh1440, compound angle is set at 29 degree. would a indexable laydown cutter make the difference? threads are way too bad to use in a gun..

Now you know why most smiths use piloted reamers and taps to cut choke threads.
You could rent the proper tooling here: 4-D Products Tool Rentals


Is the barrel springing away from the cutter? You might need to use a steady rest or reduce the protrusion from the chuck jaws.

RWO

RWO I believe he is trying to thread the choke tubes and not the barrel.

Fabman, is the tube material deforming slightly in the chuck? Have you tried using a HSS cutter?

shotgun screw-in choke

You don't say what material you're making the choke from, but I think that your spindle speed is too slow for almost any material.
Other factors would be type of cutting oil and angles on the tool itself.

Is the cart before the horse?

Seems like it would make a lot of sense to learn all about making really good threads way before trying to work on things that require really good threads.

I have been machining since 1964, and I still don't think my threads are good enough for putting in something I am going to set off an explosion in right next to my head.

J.O.

johnoder said:

Is the cart before the horse?

Seems like it would make a lot of sense to learn all about making really good threads way before trying to work on things that require really good threads.

I have been machining since 1964, and I still don't think my threads are good enough for putting in something I am going to set off an explosion in right next to my head.

J.O.

Click to expand...

Most things gunsmiths thread are within the skill set of any decent machinist. Many very successful gunsmiths are self taught machinists.

Piloted taps and such are often used because it simplifies the process...but there is no reason the work could not be done with a single point tool. The OP did the right thing and is trying to make it all work out on scrap material before working on his gun....a smart way of going about things. The tap required can be bought at MSC....I might bore the threads to get them concentric then tap with the tap being guided by the tailstock center.

Firearms do not have an explosion going off inside the the barrel, the smokeless powder is a propellant, a controlled, engineered flammable solid that generates tops about 60,000 PSI, often quite a bit less. In the case of a shotgun 15,000 psi, At the muzzle of a shotgun, only a few thousand psi tops.

I would guess that some pressure on the carriage handwheel to create some drag while the threading is going on might improve the threads greatly.

The stress on the threads where a rifle barrel screws into the receiver at 60,000 psi with a .473 case dia by my math is 10,500 lbs...that is 1/10 the yield strength of an equal size grade 8 bolt...so that threaded connection is NOT highly stressed by any means...in fact it is only about 1/3 the yield strength of a grade TWO bolt that size. Any decent machinist would not be afraid to make a threaded part that only had to hold 1/3 the load of a 1-1/16" grade two bolt would they ?

yes i am threading the choke itsself not the barrrel. the choke is being turned from 4130 cromoly, the diameter is 0.810 where the threads are to be formed. I have no problems running threads to close tolorences what so ever, im going to try increasing the spindle speed by a 100 rpm or so an see if that cures the chunks missing from the tops of the threads. I think thread rolling would probably be best way but the cost of those thread rolling fixtures its out of the question.

What kind of threading tool are you using ? For very fine threads I typically use a very sharp tool. If your tool is not perfect it will show up a lot more in fine threads I think than course ones. If your nose radius is too big for example you will have to go smaller on the pitch dia to actually get it to thread in. And a crappy looking thread will be the result.


Also I suppose you could just buy a die from MSC and again chase to get concentric, then run the die over to get perfect form. I do that with 1/2-28 for muzzle brakes and suppressors. Also some people here and there have used geometric die chasers as threading tools, that might give a perfect thread form too.

There are also full profile threading inserts for inserted holders,that cut the whole profile...you would leave the dia big and the insert cuts the top of the thread as well as the valley.

http://en.wikipedia.org/wiki/File:ISO_and_UTS_Thread_Dimensions.svg

Try Chroma Tap or Cool Tool cutting fluid.

fabman13 said:

I am currenty working on some screw in chokes for a turkey shoot gun. I am having problems getting threads 32tpi to form uniform an symetrical, i am using a hand ground 60 degree cobolt cutter an spindel is running at 85 rpm. The lathe is a 2010 jet gh1440, compound angle is set at 29 degree. would a indexable laydown cutter make the difference? threads are way too bad to use in a gun..

Click to expand...

Speed it up(a bunch) and use some sort of lube if your not already. Near as i can nompute, your only running 13 or 14 sfm. Cobalt is only a bit better than plain ole HSS, which would like no more than 100 SFM. Have you ground a positive top angle on your tool? Sounds like you have it flat or negative, and the material is tearing rather than cutting. The lube would probably help most if your running dry. Try hi sulphur cutting oil if you have it, Mic-O-Cut would be better, but if you have that, you'd most likely already be using it.
I know it's more difficult to catch that lead at a faster rpm, but too slow and the material wants to build up at the cutting edge, causing a tearing action. The positive angle will reduce the heat somewhat, relieving some of your ailment. The lube will help some more.
What kind of DOC are you trying?
A carbide cutting tool will only exacerbate the speed issues you having. Indexable are all carbide as far as I've seen. You'll need at least 300 spm in most cases with carbide.
SFM =(dia x pi)divided by 12) x rpm. Try to keep HSS below 100SFM, carbide above 300SFM is a general rule off thumb. There is a max SFM that applies to carbide, but it depends on the material and hardness.
Also, a tiny radius on the tool tip will help it last longer. Probably easiest to hone that with a handheld diamond hone.
All this is subject to final destination charges and dealer prep. Your mileage may vary. Always be aware of current driving conditions and always use a designated driver.

4130 tubing? Probably annealed?

I'd try some pre-hardened 4140 barstock, drilled out 1/16 undersized before threading and then bored to size. Probably machine a bunch better. Most alloys like this machine better at a higher hardness.

Is there a consideration here for the type of shot to be used in the gun? I have a Beretta choke that is stamped not for steel shot. The metal alloy used for the choke tube may be critical.

Just curious.

rough threads

hone cutting edge to remove all grinding roughness, use liberal amounts of cutting oil, and run at an RPM you feel comfortable with.