Need some help troubleshooting micro-lathe chatter

[calipsoii]

Hi machinists of CPF!

Well I guess the hobby has progressed for me to the point where I'm turning a lot of attention towards this subforum, so it's time for my first post here.

I acquired from a coworker a Taig micro-lathe. He wasn't using it and knows I'm currently saving for a PM1236 (or one of it's ilk) so he offered it for my use in the meantime. This works out ok since I have a small house and no room for a 12x36 here right now.

I built a solid bench for it, mounted the lathe, and finally decided last weekend to take it for a spin. It was in storage at his house for quite some time and shows it. What I've done so far:

1. Disassembled all the components I can get apart, save for the sealed headstock.
2. Cleaned everything with acetone, rubbing alcohol and an old toothbrush. There was lots of dust and packing grease in the gibs and cross-slide screws, etc.
3. Lightly lapped the bed with motor oil and Comet, just enough to remove some of the surface rust and give a much smoother slide to the very sticky carriage.
4. Cleaned everything again to get the lapping compound off.
5. Lightly oiled everything that the manual says should be oiled with 10W non-detergent motor oil.
6. Reassembled everything, tightening the gibs until there is zero play in the carriage or cross-slide (the handwheels are a little stiff as a result but still turn smoothly)
7. Confirmed that as best I can tell, the lathe is absolutely as rigid as I can get it.

I'm attempting to practice on a piece of 1" brass round bar. It's factory-rolled but I don't know the grade. It creates tiny square confetti swarf instead of chips, so I believe it to be a decent grade.

My issue is that anytime I take a cut deeper than 0.001", I get chatter that escalates in intensity the deeper the cut is. Taig's website says it can hog 1/8" cuts - I can't even get 0.003" ones without the lathe vibrating and shaking the workbench.

I am working on a 5" long piece of brass. It is supported at the end by a live center. It is held by a steel 4 jaw chuck and is centered to 0.002-0.003" runout (as good as I can get it). The toolpost is a AZ2CNC QCTP. It is holding A.R. Warner tools with A.R. Warner CCMW-2-1.5-2 HSS inserts. The inserts have a 7 degree relief angle, 0 degree rake and 1/64" nose. I'm cutting at the recommended 500 RPM, though at 800 RPM the vibration is still there and just higher in frequency.

There was no vibration when center-drilling the end; the vibration only occurs when cutting by toolpost. Light 0.001" passes make a pleasing 'hissing' noise and throw a tiny fountain of chips, but dig it in a little deeper and its bang-bang-BANG-BANG. When I try cutting through the vibration, all I get are tiny gouges in the surface all the way around the bar, like the cutting tip is skipping along and digging in.

I finally turned my attention to the headstock and noticed that the pulley seemed to be wobbling when spun by hand. I slid the pulley half-way off and noticed that the preload nut and the shaft it's attached to appears to be slightly crooked!

See the little gap at the top with the light shining through?

Now rotate the pulley 180 degrees... and it's at the bottom.

To make sure I wasn't seeing things, I dropped a dial indicator on the pulley and it's wobbling ~0.005" (enough to see with the eye for sure).

More distressingly, I've noticed that when turning the spindle slowly by hand, it will rotate maybe 5-20x very smoothly, then suddenly it will lightly "seize" in my fingers. Additional pressure will push it past the seize point but after another few rotations it does it again. It feels like something is either sticking inside or an oblong shape is turning sideways and causing more pressure. I've also noticed that if the spindle is lightly spun, it will "seek" a certain position before coming to rest. Reminds me of when you turn a bent rod and it always orients itself so that the bend is towards the ground.

Things I've tried to eliminate in troubleshooting:

• I'm using 0 degree rake angle HSS inserts, which is recommended for brass.
• I've tried varying RPM to no results.
• I've tightened all gibs - there is nothing I can find that's loose.
• The bed is very securely attached to a heavy, level workbench.
• I've supported the end with a live and dead center with little effect.

Anything anyone else can recommend before I purchase another headstock to see if my current one is the issue?

Thanks all! :o

 

[archimedes]

I have absolutely no advice, and frankly don't even understand half of your post ... but am really looking forward to your next project :)

 

[gadget_lover]

It does sound like a bearing problem. The spindle should never catch.

If the spindle preload is too loose the spindle can move under load. This will cause chatter. You get the same effect if you have a bad bearing roller too.

Can you see movement of the chuck with a dial indicator? There should be no lateral play to speak of.

Dan

 

[calipsoii]

It does sound like a bearing problem. The spindle should never catch.

If the spindle preload is too loose the spindle can move under load. This will cause chatter. You get the same effect if you have a bad bearing roller too.

Can you see movement of the chuck with a dial indicator? There should be no lateral play to speak of.

Dan

I'll check that out with the dial indicator this evening! I didn't notice any visible wobble in the chuck but I'm not certain. I may have the opportunity this evening to pick up a new headstock, so if I can, I'll try that out and see whether the issue persists. I'm hoping that clears it up because if not I'm running out of ideas.

I have absolutely no advice, and frankly don't even understand half of your post ... but am really looking forward to your next project :)

Hah, thanks archimedes. If anything, this exercise has taught me to temper my expectations a bit more. I sort of foolishly expected I'd run out to the garage and bang out pretty workpiece in a couple hours, not spend 2 weeks tweaking and testing and reading machinist forums. I've learned an awful lot while troubleshooting though, so perhaps this wasn't such an unfortunate turn of events. :)

 

[Anglepoise]

You are getting an incredible amount of experience that will hold you in good stead when you eventually get a larger lathe. Seems to me you are doing everything right. Swapping temporarily the headstock will tell you more about the headstock bearing.Can the headstock be taken apart. I don't like that nut not making contact evenly. Keep us posted and good luck

 

[precisionworks]

It does sound like a bearing problem.

+1

With those symptoms it's time to look for new & better bearings. Standard grade lathe bearings are often the same class as utility trailer & boat trailer bearings - not great when new, awful as they wear. Bearings come in standard sizes designated by OD-ID-bore and any decent bearing shop can help you out. Ask for the best ISO class bearings available. That should help a ton.

 

[calipsoii]

+1

With those symptoms it's time to look for new & better bearings. Standard grade lathe bearings are often the same class as utility trailer & boat trailer bearings - not great when new, awful as they wear. Bearings come in standard sizes designated by OD-ID-bore and any decent bearing shop can help you out. Ask for the best ISO class bearings available. That should help a ton.

Great advice precisionworks, thank you. From what I've been reading about the Taig, its headstock innards are press-fit into place and you basically need a 2-ton press to squeeze them all back out. I don't know if I'm setup to do such an operation right now; I have the bare minimum of shop tools in my tiny garage. Maybe a local workshop would be willing to take a look for me...

In the meantime, I did get a chance to pick up a new headstock!

This one has literally ZERO wobble on either end of the spindle. I think I saw the needle quiver once but that's it.

It also feels different than the old one. It's stiffer to turn but feels very consistent in its rotations. Sort of feels like hand-turning the chuck on an electric drill - some resistance but smooth consistent movement. I mounted it to the bed this evening, re-aligned the tailstock center and turned it on only to make sure the belt grooves on the pulleys line up. Will try chucking up that hunk of brass tomorrow and report back with whether things have improved. I'm hopeful! If it's much better I will look into replacing the bearings as you've suggested.

 

[calipsoii]

I got a few hours today to practice on the Taig and I think I've made some real progress.

The new headstock makes a big difference when cutting close to the chuck but I was still getting the vibration when cutting out near the live center. I decided to try the steady rest to see if that would improve things and it made a huge difference. Mars up the workpiece and makes a grinding noise but the excessive vibration was gone and I got a nice cut with no chatter.

The carriage is SO wide that the live center hangs out of the tailstock kind of precariously (you can see this in the above picture) and I think that's part of the vibration problem. About halfway through the bar (2.5") I can drop the steady rest, pull the live center back into the tailstock more (now that the carriage isn't banging into it), and cut without any vibration. Deeper passes than before (around 0.010") seem ok so long as I keep the carriage moving and the chips flying off.

I decided to try my hand at knurling with my little scissor knurling tool. The minimum RPM is 500 so instead I chose to rotate the spindle by hand. This was my first attempt, but I felt the knurling wheels slip out of their grooves while I was wrenching on the carriage handwheel and it mucked up the finish a bit.

The second attempt went better. I jammed the knurling wheels in hard before rotating the spindle and they left clean grooves, then I started working my way across. The ugly marks were on the workpiece from the steady rest, I should have cleaned them up before knurling.

I cleaned up the area in between the knurls to basically finish the outside, except for one pass to remove the marks from the steady rest.

The problem I'm having now is drilling. I'd been told that drilling brass is interesting so I tried to follow all the tips I was given but this is basically where I had to stop for the day.

My instructor told me to take the rake edge off any brass drill bits but never actually showed me how. I believe I did this correctly but I could be wrong (the shiny line across the bit at the top of the image).

My problem right now is that:

• The center drill worked perfectly.
• The pilot hole (5/32") worked fairly well. I set the RPM, lubricated throughout the process, and with a hard feed the swarf spiraled out and left a small hole.
• The largest drill I have that fits in my Jacob's chuck is 1/4" and this didn't work at all. The second I started feeding it SCREAMED at a high-pitched frequency. Less feed pressure didn't help and with more feed pressure the bit grabbed in the hole and stalled the lathe.
• I tried a middle-sized bit (7/16"?) and it worked only mildly better. Still screeched but didn't get stuck. With the longer drill bit the chips didn't clear out of the hole though, and pecking didn't seem to help.

For the chip problem I think I need a simple coolant flow setup. Maybe a small pump that flows coolant along the bit to help clear the chips out of the hole?

I have no idea what to do about the squealing with larger bits. My best guesses at this point:

• I should have drilled a smaller pilot hole and gone immediately to the large drill (though I think this would stall the belt-driven lathe)
• My rake angle is wrong on the end of the drill bit (not sure if this is the cause)
• The 1/4"+ drill bits are just too long; they need to be shorter and more rigid in my setup (I think this is the most likely scenario)

I'm going to look tomorrow for a set of drills with shorter business ends and see if that helps the squealing.

Before calling 'er quits for the day I did finagle the boring bar in there though and it works great. If I can just get the hole drilled enough for clearance I can bore the rest of it out.

If anyone has any suggestions on why the drill bits squeal the second I touch them to the metal I'm all ears! :thinking:

 

[StrikerDown]

I'm not familiar with this lathe but with all machining rigidity is key to reducing chatter. It looks like you have way too much stick out on your live center. Is there a way to get the clamp/tail stock closer to the work piece so there will be less flex/more rigidity?

On the head stock binding, was the drive belt attached to both the motor and headstock when you noticed the binding? If it was, check the run out on the motor pulley. If there is run out on both the headstock and motor pulleys then depending on the ratio they will synchronize both "high" spots in opposite directions every so many rotations and cause a bind as it has to stretch the belt past the high spots. It will follow a regular pattern and that pattern will change with different positions of the belt on the pulleys.

Hope this makes sense. Somewhere in my head it makes sense to me!:)

 

[StrikerDown]

On to the drilling, try a smaller or no pilot hole. When the pilot is too close to the next drill bit the drill tries to screw into the hole when drilling pressure is added. If you try to keep from stalling the motor with very little pressure then the bit bounces around grabbing one cutting edge at a time. I'm sure there is a rule of thumb somewhere but I like to have 2/3 rds or more of the cutting edges engaged with the work.

 

[calipsoii]

Awesome suggestions Ray, thank you!

The binding was actually happening inside the headstock without the belt or pulley attached. The new headstock seems much better so I think that's fixed. As for the live center I think the Taig is just poorly designed in that regard. The tailstock should really overhang the bed so the carriage can slide under it. Instead it just rises vertically and you need to hang the center way out. Lots of people have modded their tailstocks though so I'll see if there's good suggestions online.

I think you're bang on with the drilling problem and I'm excited to try your suggestions next time I practice. :) Thanks again!

 

[gadget_lover]

Another thing you can do for the tailstock overhang is to rotate the toolpost and mount your tool on the right side of the post. This will get the saddle out of the way so the tailstock can be close to the end of the work.

Dan

 

[edgar]

the curling look pretty good , do you have a picture of your curling wheel ?
I am looking to buy such a tool myself .

 

[calipsoii]

the curling look pretty good , do you have a picture of your curling wheel ?
I am looking to buy such a tool myself .

Hey edgar, I am using the 1911 knurling tool from LittleMachineShop.

 

[PhotonFanatic]

Maybe this will help:

 

[calipsoii]

Hey guys, just a quick post to thank you so much for all your advice. I tried all of the suggestions while practicing today and eliminated the issues I was having while drilling. Much obliged!