Contouring - chattering.

[ammohead]

I am attempting to match the contour for a model 99 project, and I am getting chattering. What are the most likely causes. 600 rpm, slow feed rate, carbide tooling.

[M-Tecs]

Lack of rigidity. Without a hydraulic follower rest contouring a full length barrel is a challenge. The lower cutting pressure tooling like HSS helps but when I have done it I have had to chuck up short sections and follow out with hand blending with a file and a spin fixture and abrasives.

[ammohead]

Thanks.

[ammohead]

Wow 3 inches at a time then blending with a 4" grinder and a 120 grit flapdisc, finishing with 220 grit emory strip. Looks pretty good, but I am glad I can breathe again. What an intiation! Hope it shoots as good as it is looking.
I am chambering a model 99 Savage take down in 6.5 grendel. I believe it will be the only one in existence. Still have to hand fit the locking groove, and then dovetail for the hanger, but it is looking good so far.

[country gent]

Play with speeds and feeds and depth of cut. Use a hand sharpened and honed HSS cobalt tool. experiment with angles sometimes a sharper point helps. Another is a live center that has a heavy spring loaded point. This sets the pressure and maintains it as the part and machine warm up to temp. Let the machine Idle for 1/2 hour to 45 mins to warm up every thing. If your machine is a flat belt machine foe to a skived and glued endless belt. the wire and cat gut splice can induce a bump that sets up chatter. stiff belts can do this also a reason for warming them up.

[HollowPoint]

Lack of rigidity. Without a hydraulic follower rest contouring a full length barrel is a challenge. The lower cutting pressure tooling like HSS helps but when I have done it I have had to chuck up short sections and follow out with hand blending with a file and a spin fixture and abrasives.

Exactly. This is how I ended up doing it; mostly out of necessity.

I've only contoured three different barrels in my day and each one had it's share of chatter. After the first barrel I tried different things to eliminate the chatter but no matter what I did I still had that problem. I hated it because I know that chatter means that something isn't being done right. Just what is that "Right" way of doing it? Hell, I don't know. I'm a DIY guy and all my lessons are learned the hard way but, now that you mentioned a "Hydraulic Follower Rest" I now know what one of my next projects will be. I never really thought of that as a possible solution.

The little arms that hold the rollers on my existing follow rest can be easily slipped out and replaced with some small diameter strut-like gas springs. I'm just wondering what might be a good amount of pressure needed for this application to ensure rigidity in a given work piece to mitigate or eliminate that chatter all together? Depending on how my present rebarreling project goes, I may be rebarreling one of my other rifles later this year. I'd love to be able to do my lathe work without that chatter.

If the OP comes up with some other solution I hope he'll post it here cause this is a lesson I haven't yet learned.

HollowPoint

[ASSASSIN]

1 - a properly ground and sharp tool.

2 - 600 to 1,200 rpm

3 - take .030 cuts at a time and with plenty of coolant.

4 - if your barrel is set up properly in your lathe, and you are off-setting the tailstock for your desired taper, you should be using a fixed steady to keep the barrel from flexing away from the cutter, which in turn, with everything else combined, you should be getting NO chatter, and a finish that would require very little work after machining, especially if you make the last 2 passes at a cutting depth of less than .010 per cut.

5 - make sure of your tool bit height as well. If it sits too high or too low, it will definitely induce chatter into the workpiece!

I just finished turning down a 1.300" straight tube to a #6 barrel contour with a muzzle diameter of .750 and it took less than 5 minutes to put a very fine and smooth satin finish on it after turning off a LARGE pile of metal chips!

[deltaenterprizes]

Google spring loaded follow rest, I made one and it worked like a charm!
I may still have pictures of the one I made.

[Dapaki]

Harmonics are present in all materials but steel seems to be the most prone to this phenomenon, lack of support, too much pressure on the live center or it's dinged or not concentric, stresses in the stock that seek relief, cross-feed stiffness, etc. I find a spring loaded follower rest to be the most handy, hydraulics can be spendy and what many call "hydraulic follower rests" are actually air charged pistons like a shock with a preload.

You want to (like the gents above said already) find the right feed, cut and speed that works best but don'r forget dampening, grab, stickiness (what type of carbide are you using?), example; C-2 is fine for cast-iron. C-5 & C-6 should be used on stainless and tool steel. Using a cutter that has too aggressive a chip hook can introduce chatter as well as dull tooling.

I would love to see some pix!

[john.k]

If using inserts ,dont use negative rake ,get a positive rake tool ,and check the insert has a sharp edge .Carbide/coated is always a better bet because the smooth surface lets the chip flow easily.....if using HSS ,the top surface must have a high polish for chip flow.

[AJB]

Ammohead,

Have you tried a steady rest. It has been a while since I contoured a blank, but when I did, I would taper one section at a time using the steady rest to reduce chatter and bending. Basically, I would start by setting up the steady rest about 10 inches from the tail stock and taper that section, then move the steady rest about 8-10 inches toward the headstock and do the next section, etc. It is also important to check the tailstock frequently and loosen it up as the barrel heats up and expands, otherwise the barrel will bend as it is being tapered.

[Tokarev]

Keep your tools razor sharp!

[Willbird]

Over the decades I have run into some wicked chatter situations. A few things that have helped sometimes.
Do not be afraid to crank up the feedrate, all the rules are out the window, maybe .015-.03" per rev will work with a smaller depth of cut, like .01" per size depth of cut.
Reverse directions of feed every cut.
Wrap anything rubber around portions you are not machining, surgical tubing worked once, strips of inner tube another time. Some of those ideas were on parts being bored on a boring mill or milling machine, same concept tho.

Bill

[oldred]

Not sure how much help it might be on a smaller bore but I filled my 32" 45 caliber barrel Green Mountain blank with no.11 lead shot and it helped a lot, after a bit of experimentation I found that the shot packed loosely seemed to work a bit better than packing tightly against the oiled cloth plugs. I am not suggesting that this is simple solution that by itself would solve the problem but it did make a noticeable difference for me, simple to do and causes no problems so it may be worth considering?

[Willbird]

Not sure how much help it might be on a smaller bore but I filled my 32" 45 caliber barrel Green Mountain blank with no.11 lead shot and it helped a lot, after a bit of experimentation I found that the shot packed loosely seemed to work a bit better than packing tightly against the oiled cloth plugs. I am not suggesting that this is simple solution that by itself would solve the problem but it did make a noticeable difference for me, simple to do and causes no problems so it may be worth considering?

I will have to try to remember that :-). Might be that filling it with boolits that are a slip fit might help too :-).

Bill