Making a broach

[kd0afk]

I am embarking on making a bolt action rifle and the first step is to manufacture a broaching cutter to cut the lug raceways in the reciever.
I have the tool steel and a plan and I'm pretty sure I can make the tool. It will be used a maximum of 5 times and It needs to be at a 68C Rockwell.
Is this something I can make at home or should I send it out to be done? Also how crooked is the thing going to be when it comes back?
If needing to send it out is the answer, can anyone suggest an inexpensive shop? I've tried a few places via Google and nobody is returning my estimate requests. Maybe because they all do massive work and one offs aren't worth thier time but nevertheless I need to get this done. There are workarounds but this is the most sensible and frankly, will be more fun to make it that the workaround.
Anyway, you guys haven't let me down in the past, thanks in advance on this one.

 

[Wizard69]

I am embarking on making a bolt action rifle and the first step is to manufacture a broaching cutter to cut the lug raceways in the reciever.
I have the tool steel and a plan and I'm pretty sure I can make the tool. It will be used a maximum of 5 times and It needs to be at a 68C Rockwell.
Is this something I can make at home or should I send it out to be done? Also how crooked is the thing going to be when it comes back?
If needing to send it out is the answer, can anyone suggest an inexpensive shop? I've tried a few places via Google and nobody is returning my estimate requests. Maybe because they all do massive work and one offs aren't worth thier time but nevertheless I need to get this done. There are workarounds but this is the most sensible and frankly, will be more fun to make it that the workaround.
Anyway, you guys haven't let me down in the past, thanks in advance on this one.

I can't say I can help with this based on experience, but a few things come to mind.

You should be able to do a broach that is good enough for this in a home shop. It might not be a durable professionally done broach but if your goal is a few receivers I don't think you will have a problem.

As for warping I'm not to certain you can prevent it 100% but you should be able to minimize it by dropping the blank vertical, fast and straight into the oil. Think of a ballistic misleading leaving its silo in reverse.

Like everything else the broach will deflect some as it is pushed through the bore. So to some extent a little imperfection is possible.

That being said it might pay to be a bit over sized prehardening and the grind to final diameters. This with the goal of a suitably straight broach.


Take the above with a grain of salt, the only things I've broached are pulleys and hubs for various things. One thing to watch out for though is press stroke and force requirements to actually force the broach through your receiver.

 

[kd0afk]

I have a buddy with a big hydrolic press so the actual broaching shouldn't be a problem I'm going to go for it. No to learn as much about heat treating as I can. Lol.

 

[Dave Sohlstrom]

So what shape of broach are you talking about. How long of a hole are you broaching. What is the ID of the receiver.

Dave

 

[kd0afk]

Id of the reciever is .8" the broach has three lugs spaced 120 apart. Total length to broach will be 1/2 - 1"

 

[Dave Sohlstrom]

OK that helps. Now what is the depth of the 3 lugs. What material are you broaching.

Dave

 

[kd0afk]

Here is the plan

View attachment 1437189452702.jpg

 

[kd0afk]

The raceways are .325" wide and .14" deep. Broaching 4140

 

[kd0afk]

Speedy metals website says to soak in 1450f heat for 1 hour per linear inch and quench in water or brine. Then temper at 300 degrees for a 65c hardness.

 

[Dave Sohlstrom]

That is going to be a very hard broach to make in the home shop. I have all the machinery that it would take to make one but I would not want to. Just sharping it after heat treating would be a real job.
I expect if they are sold by some one that they are very expensive but it may be cheaper and easier in the long run to buy one or find a gunsmith that has one that would do the broaching for you.

Dave

 

[kd0afk]

I realize it will take a while to make the tool and yes the sharpening is going to be intense but I have made a pair of centers for my mill to hold it whilst I sharpen it. It should take the better part of a day. The good part is that it's round so really the only setup will be tramming the thing.
It will be kind of tricky but it should work.

 

[GailInNM]

I think your brooch as drawn is much too aggressive. A quick calculation by counting the number of teeth versus the depth shows about 0.010 depth of cut her tooth. A commercial keyway brooch in this approximate size range will only have a depth of cut of about 0.002 inch per tooth maximum. A 3/8 inch wide brooch is about a foot long and takes three passes using two shims to cut a key way.

I would buy a 3/8 inch wide key way brooch and grind (or have someone do it) it to a 0.325 width. Make a bushing with three grooves for the brooch. Then using two shims make three passes and cut one groove. Put a key in the groove you just cut and then repeat for a second and third groove. You will have to make two custom shims as well as the bushing to get the spacing and depth correct.

Gail in NM

 

[kd0afk]

Broaches aren't cheap. I could always hand broach the raceways on the lathe the old way. It would accomplish the same thing and save me money

 

[GailInNM]

Broaches aren't cheap. I could always hand broach the raceways on the lathe the old way. It would accomplish the same thing and save me money

True enough. A 3/8 inch import broaoch is about $35 and a top-quality Dumont one will set you back about $75. To plane a key way in the lathe will probably take about 100 passes per keyway for your size. But, it would be the cheapest way out. If you hold your stock in a three jaw chuck you can use the jaws for indexing the three key ways. Doing it in the lathe works quite well but it does take a lot of time.

By the way, I did a spline with 3/32 wide keyways using the method of broaching I described in the last post. It would have taken a long time to do in the lathe but was a fairly short job using a broach. Being square the spine ways were of the wrong shape but it worked well enough for my application.

Gail in NM

 

[kd0afk]

I also have a mill so I can pare out the bulk of material before broaching.

 

[kd0afk]

I have a question which might solve my problem. I saw a rifle the other day and it's bolt screwed closed on a deep coarse thread and the threads had gaps cut so the threads became the lugs. I'm thinking that an interface like that could be done on the mill and hand filing.
I like the idea of the bolt screwing down on the barrel and may do it that way.

 

[Wizard69]

I have a question which might solve my problem. I saw a rifle the other day and it's bolt screwed closed on a deep coarse thread and the threads had gaps cut so the threads became the lugs.

Interrupted threads aren't Uncommon in breach loading guns. Usually larger cannons and the like.

I'm thinking that an interface like that could be done on the mill and hand filing.

I'm not sure how you figure this will reduce the need for broaching. The lugs are still there unless you have an unusually large diameter bolt. You also need to consider how the ejector and extractors will work.

I like the idea of the bolt screwing down on the barrel and may do it that way.

In effect even a two lugged bolt screws down. Someplace in the bolt you need the action of a cam to pull the spent round from the breech. The threads acting as lugs could do this for you.

Here is the problem I see though, how will you model this so that you know you have a safe design? All else considered this should be the primary goal. I'm simply not up on the engineering need to help you with that. I'm not even sure which thread form would be best, though I like the idea of trapezoidal threads.

Another option to simplify things would be to consider an AR15 style bolt assembly where the bolt head is carried in a larger bolt assembly. This has the potential to simplify machining at least for some parts.

It should be noted that all of these designs still require some sort of guiding arrangement to keep the parts that shouldn't rotate from rotating while allowing those parts that should the freedom to rotate.

 

[kd0afk]

The design is from a book called: The bolt action rifle by Stuart Otteson.