A Few Lathe Problems...

[Tiresius]

I've ran into some problems lately while machining...Well, three problems.

1.) Preventing the nub on my parted piece. Each time I finish up and wanted to part the piece, I get a small, 1mm nub that falls off with it. Is this a common phenomena with parting pieces off? I would much prefer the nub stays in the whole stock instead of the piece I wanted to part off.

2.) Centering a flat piece without any centering hole drilled for the centering chuck. I'm not sure how to fix this problem.

3.) Facing off the metal does not guarantee a perfectly flat surface. There's that tiny spec of metal that needs to be chiseled off to achieve a flat surface. Either chisel or drill it slightly with a small bit to remove it. (Have tried to change cutting bits and same results occurs)

Anyone ran into these problems as a noob on a lathe? If so, how do you fix the issue?

NOTE: The lathe I'm using has a three-jawed spindle.

 

[Davo J]

1) If using high speed steel tooling sharpen the tip of the tool 1-2 degree down on the left side (looking down on it) so it cuts off the part first and leaves the nib on the stock.

2) Put a center punch mark in the center, machine a piece of 6mm rod about 300mm long to a point at one end, then place the rod in a chuck in the tail stock. You then roughly center the plate in a 4 jaw and place the sharpened tip of the rod into the center punch mark. You then set up a dial indicator on the rod and start adjusting the plate to get it to run true.

3) It sounds like either your tool is to low or to high, or it could be either you carriage, cross slide or compound needs adjusting up as it's allowing the tool to go under the nib.

I hope I have explained it right, much harder to put in words than to do.

Dave

 

[Tiresius]

Thank you, Dave. I've got the idea of 1 and 2 down. It's 3 that is odd to me. The tool is exactly lined up to the center of the rod. I'll have to post a picture of it when i get to the garage.

 

[PCC]

If your cutting bit is too low you end up with a squared off nub. If its too high it's conical. Even a tiny bit off will give you either one.

 

[Bill Idaho]

An old machinist showed me a trick to find the correct height of your tool bit. Take your 6' rule, hold it up (vertically) to a piece of round stock that has been dialed in. GENTLY move the bit into the rule pinching it up against the round stock. A properly adjusted bit (in regards to height) will hold the rule just a shade off of perfectly vertical. You want it to lean ever so slightly with the top of the rule towards you.

 

[archer6817j]

The ruler method is a great way to get the center height close, however, the only way to get it dead on is to take test cuts and adjust the height by tiny increments. If there is a nub, then you aren't on center :)

 

[precisionworks]

... the only way to get it dead on is to take test cuts and adjust the height by tiny increments. If there is a nub, then you aren't on center :)

+1

A center height gage is a handy tool to make, as shown in this thread: http://www.candlepowerforums.com/vb...set-lathe-tooling&highlight=lathe+height+gage

 

[Davo J]

Try to watch the tool closely when it does it to see if anything is moving (springing under or over) if nothing moves, start shimming your tool height of adjust your QC tool post if you have one.

Another thing could be your spindle bearings are loose, but this should show up with bad surface finish on most jobs done on the lathe.

Dave

 

[archer6817j]

Another thing could be your spindle bearings are loose, but this should show up with bad surface finish on most jobs done on the lathe.

Dave

That's a good point. I started out on a shoptask 3-in-1 machine. I had a poor finish and heck of a time parting. I ended up replacing the bearings and found the original ones were installed loose. After the swap an tighten everything was fine :)

 

[brickbat]

...If there is a nub, then you aren't on center...

...unless the piece you're parting off is so heavy that it breaks off the nub before the parting tool cuts the nub. Maybe I'm missing something, but I don't see a way to avoid this.

 

[archer6817j]

...unless the piece you're parting off is so heavy that it breaks off the nub before the parting tool cuts the nub. Maybe I'm missing something, but I don't see a way to avoid this.

Another good point :) We are talking about two different situations. 1) actually parting off the part 2) determining if the tool is on center.

In case #1 there is no way to avoid having a nub on either the part or the stock. You can use a parting tool with an angled face (angled left or angled right) and that will leave a nub on the part or on the stock depending on which angle you choose. A "neutral" parting tool has no angle and is presented "square" to the work. It may leave a nub on one part or the other, depending on the particular piece.

Case #2 applies to the center height of any tool. Though a parting tool is not intended for facing cuts, you can take a very light pass to "see" how the blade is cutting relative to the actual center height of the part so you can make the proper adjustments.

 

[Tiresius]

Found out that parting nubs was caused by my recent negligence. The tool is crooked in X and Y axis. The center height of facing off is slightly lower...Very very slight difference. Fixed this problem by adding a thin cardboard piece under and VOLA!!!

Yes, the lathe itself is not perfectly even.

Thanks for the replies. Took a while to figure things out.

 

[Davo J]

Glad to hear you have it sorted.
Something better than cardboard is soft drink cans or beer cans. It can be cut into thin strips with a pair of scissors.

This is why a lot of people go to quick change tool posts so you can just adjust it up or down, but it comes at a cost so you have to way up time against money. I used packers for years before stepping up and just buying a quick change, I would never go back.


Dave