PDA

View Full Version : New lathe vibration(?) puzzlement



Just Bob Again
07-27-2008, 08:06 PM
Got a new lathe running. A 14x40 gear-head machine. Looks good. Runs smooth. Spindle indicates around 2 tenths. Gibs are tight. Nothing visibly flexes and I don't feel any unusual vibration. Brand new. Got 15 minutes on it. Camlock spindle. The chuck is tight.

Engage the feed gears at any spindle speed and use longitudinal power feed at a fine setting like .oo15/rev or .003/rev. Light cut or .002 or heavier at .020, same thing. I get grooves maybe .002 deep spaced around 50 to the inch. Not a nice finish like I expected. Run the spindle down at 100 RPM or up at 2000. Same thing. Turning a substantial chunk of 6061.

If I use the handwheel but the feed gears are engaged, still not great. If I disengage the feed gears and use the handwheel, not too bad. Not a mirror finish, but OK. There's a just bit of cogging in the handwheel but I expect that will go away.

Any ideas? IS this normal in break-in? The machine weighs a ton. I wouldn't have expected this.

tattoomike68
07-27-2008, 08:23 PM
.003" feed is super slow, I never use anything under.006" but like to run .010" to .012" most of the time. slow feed does not always mean fine finnish.

I think tool geometry is to blame for the finnish put a little radius on it and stone it smooth.

Doc Nickel
07-27-2008, 08:30 PM
If it's on aluminum, I suspect you're getting material buildup on the cutting edge, in a buildup/breakoff/buildup/breakoff cycle that produces the regular grooving.

You didn't say what tool you're using, so try an HSS bit with a classic grind, about 0.020" radius on the nose, and honed sharp. Then give it just a squirt or two of WD-40 and see how that comes out.

Doc.

Just Bob Again
07-27-2008, 09:25 PM
If it's on aluminum, I suspect you're getting material buildup on the cutting edge, in a buildup/breakoff/buildup/breakoff cycle that produces the regular grooving.

You didn't say what tool you're using, so try an HSS bit with a classic grind, about 0.020" radius on the nose, and honed sharp. Then give it just a squirt or two of WD-40 and see how that comes out.

Doc.

Basically what I'm using. HSS 5%Co, plenty of rake and radiused. I just touched it up on a diamond wheel. It's mirror-smooth. 1/2" bit in an Aloris holder. Chips are flowing off nice and clean. I even put a little Tap Magic Aluminum on it. The TCE stuff that works. I'm still puzzled. I'm not a newbie, just new to this forum. Been making chips for 30+ years. Can't say I'm an expert but I usually have a general idea. MAybe it's not really the 6061 that I think it is. I'll try some other stock.

Carld
07-27-2008, 09:42 PM
It's brand new so maybe it needs to be run in all the speeds and feeds for a while to smooth off the rough edges.

As tattoomike said, fine feeds don't always produce fine finishes as you already know. I have a 1985 13x40 Tiawan lathe that is very accurate and tight and feeds of .001" to .005" don't often look good and I have to leave some to file to finish. It really erks me to rough cut at .010"-.020" a get a nice finish and then do my finish cuts at .002-.004" and they turn out crappy. But that has happened on almost every lathe I have used. Some better, some worse.

I have learned to do real good file work on a lathe.

GadgetBuilder
07-27-2008, 10:16 PM
I have a 7x12 minilathe, not the same class of machine but these little lathes often have a similar problem to what you described so you might be able to translate the causes to your machine.

On the 7x12 the change gears are mostly plastic. However, a few gears are metal and they're not as precise as the plastic gears. I found one of the metal gears was binding slightly at one point in its rotation when meshing with the plastic gears; a minor adjustment with a needle file helped considerably. In addition, the gear mesh for the change gears is adjustable; if set too close the gears can intermittently bind because there is some minor runout. This binding causes the carriage to speed up and slow down just enough to cause an effect on the surface being turned.

A less common problem with the 7x12 is that the carriage drive gear mesh with the rack isn't deep enough. During auto feed a tooth of the carriage drive gear can ride up onto a tooth of the rack partially then drop into proper mesh. This causes the carriage to speed up and slow down slightly, leading to the "thready" appearance you described. Its easy to tell when this is happening because the carriage handle rotates at a varying speed rather than smoothly.

May not apply to your new machine, just something to check.

John

Bill Pace
07-27-2008, 10:34 PM
Gadget builder just may have something in his suggestion ---- I have a 12x36 gear head and if I switch from sae threading to metric & back I can have a pill sometime getting the gear mesh back to a nice quiet smooth operation.

Run in/break in time could also be contributing....

Keep us posted on what you find---

x39
07-27-2008, 10:44 PM
Is the machine single phase? I seem to remember a post on this forum some time back stating that single phase motors could have an effect on surface finish.

Mike Burdick
07-27-2008, 10:59 PM
Just a thought...

If it has two belts from the motor to the main shaft sometimes they are just a little bit different in length. This causes "competing belts" and sometimes this shows up on the work. Try removing one of the belts and see if any improvement is noticed.

Fasttrack
07-28-2008, 12:58 AM
Doesn't aluminum usually like pretty fast speeds? I think finishing speed for aluminum is given as something around 1000 fpm in machinerys handbook. A DOC of .002 to .010 for finishing and a high feed rate.

For most wrought aluminum alloys, the roughing cut is supposed to be at about 600 fpm and a feed on the order of .03 ipr - .006 seems very very slow for aluminum.

edit: ditto what Carld said. That is frusterating. I need to experiment more with my pacemaker and see how it does at small feeds. My smithy machine sucks. It does ok at .010 or .020 DOC. Anything more than that and it stalls, anything very much less than that and it looks ridged. Even with a razor sharp tool. My minimum practical DOC is .004, less than that and its a crapshoot whether it will turn out or not.

Just Bob Again
07-28-2008, 07:50 AM
The lathe is a PM1440 (Precision Matthews) much like most of the import machines. I've done a bit more playing and think the problem is in the apron. Maybe it will go away as the machine breaks in. I hope. When the feed screw is not turning, I get a decent finish using the handwheel. It feels smooth enough. No problem with the rack.

When the feed and leadscrew are turning (but the feed isn't engaged, of course), I feel a slight cogging in the handwheel. About 8 times per revolution. There isn't a feed clutch. The selector just engages either the cross slide or handwheel gear. Seems to me it isn't fully disengaging and maybe the cross slide is actually being jogged in and out a couple thou. I've never seen this happen. Got a call in to the dealer to see what they say. There isn't any carriage or cross slide lock (Why do idiots design something like this without them??). I can try making the gibs really tight and see if it goes away.

I'll run it in for a while and change the oil to something decent. Mobile probably. No unusual noise from the gears. It does have the double belts. I'll change them to a matched set of Gates when I can but don't think that's a problem yet. Or, I could just run it with one belt.

Bill Pace
07-28-2008, 08:16 AM
Hmmm, that does sound peculiar doesnt it? and certainly not something you wanna see on your brand new toy!

As you mentioned the PM 1440 should be very similar to the other 12x36s & 13x40s. I frequent the yahoo 12x36 forum ---
http://groups.yahoo.com/group/12x36importlathes/
and there have been a few of the PM lathes bought by members there. I seem to recall a couple glitches voiced by owners, but no details, you might want to post this over there and see if anything sounds familiar to someone.

I DO remember the praise of the dealer in assisting with problems---

Just Bob Again
07-28-2008, 09:31 AM
I figured it out. When all else fails, examine the obvious. Shipping damage. The feed rod is bent. I stepped back a couple feet and noticed it wasn't turning true. When the high spot is up, it cams the carriage up a few thou. Probably the carriage shims could be tighter, but shouldn't be necessary. Whenever the rod turns, whether the feed is engaged or not, it pushes the carriage around and makes grooves. Lead screw is bent a bit, too. Not as much, but more than I'd like. Should have thought of this earlier, but always looking for the more complicated answer. Can't see it when working up close since it's hidden by the bed and I never leave it running when I walk away.

Got a call in to the dealer to get replacements. Maybe they're repairable but I'd rather not do that on a new lathe. Not sure I could do it properly with a little arbor press. Thanks for everybody's input.

hardtail
07-28-2008, 11:37 AM
Don't know where you purchased it but Matt at qualitymachinetools handles this brand and is pretty knowledgable about their product, might want to give him a shout, good luck.

Just Bob Again
07-29-2008, 01:30 PM
Matt at Quality is where I got it. Parts are on the way. I can highly recommend them. Got a great price on a new scratch & dent machine and their support has been better than when I bought much more expensive items from other dealers.

pcarpenter
07-29-2008, 05:12 PM
I wonder how many machines have bent feed or control rods from someone improperly slinging around them when lifting the lathe. In many cases, the lower rod (often a control rod rather than feed screw) is below the bottom of the center of the bed casting. Lift there with forks on a forklift without proper blocking and the rod is sure to get bent. In fact, in some cases if the blocks are too long, they catch the lead screw or control rod.

Of course it could have happened when a shipper tied it down, too.

Paul