Thread Mic and Internal Threading

[mikey]

Forgot to respond about the level. I think the minimum accuracy for use on machine tools is about 0.0005"/foot. There are a lot of import levels that fit this criteria and they are pretty cheap. Here is one that might work, for $88.00. There are many levels that are better than this but not for the money.

 

[gun410]

Okay, cutting speed for 17-4 stainless is roughly 175-300 sfm, so you need to speed up the lathe. RPM = Cutting speed in SFM X 3.82 / D, where D = diameter of the work piece. So, say your work piece is 1.5" OD. The calculated speed would be somewhere near 764 rpm; this would be a good starting point for external turning. The problem is that we are not external turning; we're boring. Your boring bar is not seeing that 1.5" OD; it is seeing the size of the bore. So, say you started with that 0.385" hole. This would give us 300 X 3.82 / 0.385" = 2976 rpm. Note that as the bore size increases, rpm drops (EDIT: rpm is supposed to drop) until you reach your 1.24" ID, where lathe speed would be somewhere near 1000 rpm. Not only that but your feed is supposed to change with rpm as well. It gets complicated, and this is the stuff I left out of the boring article because it confuses folks.

So, how do you bore a stupid hole? The way I do it is to start with the starting ID and calculate rpm; in this case our starting rpm would be near 3000. Since my lathe cannot go that fast, I would start at the max speed my lathe can run, which is 2400 rpm. I would manually feed the bar because the hole is shallow, blind and because I can feed at whatever feed rate I need to keep up with the speed to produce coiled chips. As the hole gets larger, I cut my speed down IF I have trouble producing the chips I want. Sometimes in harder materials it can be difficult to keep up with the speed and this will cause poor finishes and inaccuracies in the bore. If this is the case, slow the lathe down a gearing step or two and that will allow you to keep up with your feed.

As I keep saying, the lathe teaches us to respond to the cut and this is one example of that. If you cannot feed fast enough to produce the chips you need, slow down the speed until you can.

One thing you must do when boring to precise depths is to use a carriage stop. Do not rely on a DRO; it is not precise enough. Use a stop. If you do not have a stop, buy or make one but get one and use it. I'm not trying to seem draconic here; not only will a stop improve the accuracy and appearance of your bore; it will prevent you from damaging your bar.


Not sure how the gun guys do this. Maybe post this in that subforum?

That clears up surface speed for me thanks, but how about finding desired DOC based off NR? I currently don'y have a carriage stop, I thought my dro would do the same purpose. I would like to purchase a carriage stop for my lathe if i can find one, but what happens if the carriage's half nut is engaged and feeds into the stop?

This baffle is ready to be finished but I'm concerned about not having enough metal sticking out of the chuck. The height of the baffle is 1.25" and after measuring off the chuck jaws i have 1.285" sickout leaving me only .035" extra room for the parting tool and my 2mm parting tool is .07" wide, that's a problem... And I don't want to pull out the metal because even moving it a little will change the origin.

I could profile the inside of the cone, turn down outside of material to be concentric with the inside of cone, cut metal to 1.5" long, flip the jaws around in the chuck, grip inside of baffle and then i could profile the outside of cone, and machine OD of baffle to have a nice slip fit.
Usually baffles are made in 1 setup to ensure concentrically, the steps bellow are the common practice for machining baffles and which i was originally planning on. But the method i explains above is very similar to Practical Renaissance's method in the video bellow.

1-Drill the bore
2-Use a boring bar to profile the inside of the cone
3-Profile the outside of the cone
4-Part off cone

 

[mikey]

Depth of cut when boring depends on a number of factors - material being cut, desired finish, horsepower and rigidity of the lathe and the cross slide/compound, etc. My lathe, with my CCBI bar, can easily take 0.020" deep cuts in 304 SS but I am not sure what your lathe can do. You will have to test it to see how big a roughing cut your lathe can handle with that bar in whatever material you're working with. While inserts can take big cuts, that is not what they are designed to do. They work best with a depth of cut that is just a bit larger than the NR. If you need to go deep, get a CDCD 505 insert with a 0.007" NR.

Ask Grizzly for a carriage stop. If one is not available, make one. For precision boring, it is almost a must have. A DRO will work for occasional bores but for a gunsmith doing precision boring, no, too slow. When you bore, you have to stay focused because you are moving really fast when using inserted carbide bars. If you are using power feed then you will mark your bar so you know when you're approaching your final depth, then you will kick the lathe out of power feed and feed manually into your carriage stop. If your lathe has a clutch on the feed rod then no problem if you run into the stop under power; if it does not have a clutch then something will break, like your feed gears. I don't recommend power feed for shallow blind bores unless you are experienced at boring under power and can stop in time. Most of the time it is just as fast to manually feed, especially in shallow bores.

For your baffles, please bring it to the gunsmithing subforum so the guys there can give you a hand.