[kendo]

This is a 1-3/8" rolled acme thread, long bars fed into a mazak lathe

I'm turning the threads off the end of these parts to a diameter just under the root of the thread, then cut off length is 1-3/8"

The turned diameter of some parts is not concentric to the thread, and the root of the thread is visible spiraling through when it gets too bad

Im clamping with fresh bored jaws and max pressure
This seems to be worse so far at the end of the bar with maybe 3' to go

Every part I've measured, good or bad, on the comparator the threads look perfect and stay right on the line when revolved

Im completely new to turning, so any advise at all is welcomed

thanks,

-Kenny

[kendo]

I realized this is a recurring problem with this machine.
Last time we ran material as big as 1-3/8", the bar feeder was vibrating very badly and the finish on the part was coming out like crap - and it got really bad half-way through the bar.

This material is even worse because it's 1-3/8" with a core diameter of .915 at the root of the acme thread. You can feel it sag when you pick up a full length bar in the middle.

This is an older machine and the feeder channel is pretty beat up and Im told nobody's ever done any real maintenance to the machine.

[307startup]

Are you using a spindle liner?
Is the bed of the bar feeder properly adjusted to the centerline?

[kendo]

no spindle liners
and no adjustment in the bar-feeder

I've been at this shop for 6 weeks and I've never heard of either
they run different size material all the time and the only change is the chuck jaws

[kendo]

if using a spindle liner,
the pusher diameter has to be same size or smaller than the material, right?
the pusher on this machine is 1-3/4", which is bigger than what Im running

[Geof]

When using a barfeed the best approcah is to have liners that are slightly larger than the diameter of the stock so it is kept on center. Natyrally this means that the pusher has to be smaller than the stock diameter.

Running without liners allows the bar to drop off center when the machine is stopped and the chuck is opened and then when the chuck is closed the end being turned is running off center and the bar inside the feeder tube is offcenter and vibrates the whole machine when it is up to speed.

One option to run bars without liners is to make the end of the pusher a good sliding fit inside the bar feed and bore a steep tapered hole in the end. This way the end of the bar is centered in the tapered hole when the pusher brings it up against the stop during feeding so when the chuck closes it is running closer to true.

If the pusher is the type that retracts after every push the unsupported bar inside the barfeed can still thrahs around off balance and shake the machine.

Alternatively sometimes it is possible to fasten a collar the same size as the ID of the bar feed on the end of the bar to keep it centralized all the time. This does have the disadvantage that the stub end has to be pushed backwards out of the barfeed rather than being taken out through the chuck.

[307startup]

Did your barfeeder come with interchangeable pusher rods? Almost all barfeeders that I have worked on/with do (granted, I have not worked with every model out there).

A spindle liner sounds like something that you should invest in with this job, as well as sourcing or building a smaller diameter pusher rod for the barfeeder.

As usual, Geof has provided good tech.

Just curious, what style of barfeeder are you running...servo, hydraulic/pneumatic?

Also, what length of bar stock do you run on your machine and barfeeder?

[kendo]

Its an old IEMCA CNC Boss 542
Its hydraulic running 12' bars

It has interchangeable collets on the end of the pusher but I've seen nothing interchangeable that would reduce the whole rod to 1-3/8"

This job was running with no collet on the pusher, just a flat rod pushing on the material allowing it to whip around
It was running like this because the 1-3/8" collet was broken.

I put it on anyway, it still had two "fingers" to grip the rod, and when I did, it almost cured the problem. Also, the rotating end of the pusher rod is completely worn out and has about 1/4" of play side to side. This is all being pointed out to me bit by bit by the russian speaking operator, as Im learning the machine as I go, just a few weeks on the job and completely new to cnc lathes.

[307startup]

You were running a CNC lathe with a broken collet? Running a barfeeder with a worn out pusher rod?

Normally I would say "Get out while you can", because this shop is running on borrowed time...

1. Does mgmt know that the equipment is in this state of disrepair?
2. Is mgmt willing to replace the defective collet?
3. Is mgmt willing to replace or make a replacement for the end of the pusher rod?

If the answer to 1 is no, you need to tell your foreman immediately.

If the answer to 2 is no, you are waiting for the collet to let go...possibly hurting you, the guy who is teaching you, or any other bystanders...along with receiving the blame when it does.

If the answer to 3 is no, see response above.

At the minimum, you need to make some "donuts" that fit over the stock, and within the thru-bore of the lathe. This will act as a floating liner. None of this will help however with you running this job on a broken collet. That is the primary source of your runout and slop. Fix that and follow everything else, will fall together to cure the problem.

My guess is that all of these conditions led to eachother...broke collet induced vibration in workpiece...contributing to pusher rod "finger" becoming sloppy...contributing further vibration to workpiece...exacerbating broken collet...

Geof...any input?

[Geof]

.......Geof...any input?

The place does sound a bit bush league.

Try and get permission to fix things correctly. It will take a bit of time upfront but will pay off in the long run and be safer.

If you get a negative response do the best you can, be careful and look around for a better opportunity; not that there are many going these days I think.

[kendo]

alright,
new pusher bearing,
new collet,
freshly bored chuck jaws,
chuck removed, cleaned, inspected, and greased

and the parts are still not concentric

I would guess the only thing left is the bearings inside the spindle itself?

?

[Geof]

Did you check the chuck jaws for true with a good piece of 1-3/8" diameter material?

It seems you have done everything correct now but you say still not true, which I guess you mean is the root of the thread spiralling as you mention in your first post.

It is possible that this is due to the rolled thread not being perfectly true. Have you checked them for straight both over the full length and over short distances of two to six inches. I have come across long threaed rods that were slightly wavy along the length. Also it is possible that the thread itself is not true to the crest diameter; although I guess I should phrase that the other way, the crest diameter is not true to the thread. With a rolled thread the crest sometimes has a groove with ridges on either side created by the rolling action and this is not always even. You must be gripping on this and it could be throwing you off.

[307startup]

Not so fast...if your bearings were the cause, none of the other parts turned on that machine would be concentric, and you would have many other tell-tale signs...noise, heat, chatter marks, etc.

You need to check the thread depth at various points on the stock...it is very possible that the threads themselves are not concentric with the base stock that the rod is made from. If this is the case, you will need to bore some appropriately offset jaws...or simply learn to live with the finished product (I would seek a new supplier if this is the case).

You also need to make some donuts to support the bar in the thru-bore. These can be made from something as cheap as appropriately sized PVC pipe or turned & bored hockey pucks with appropriate dimensions to allow fitting inside the thru-bore (-0.005" to -0.015" on the OD, +0.005" to +0.015" on ID)...this will ensure that the heavy end of the bar is supported and not putting a load on the chuck end. This would be evident with a slight taper being cut on the turned end...

By having the ID of the donut larger than the material it supports, and the OD smaller than the thru-bore, it will allow the material to advance when the bar feeder is in operation, without having the donut bind up the stock or push out with it.

Dang it Geof, you beat me to it...as usual

alright,
new pusher bearing,
new collet,
freshly bored chuck jaws,
chuck removed, cleaned, inspected, and greased

and the parts are still not concentric

I would guess the only thing left is the bearings inside the spindle itself?

?

[kendo]

the first thing we suspected was the material

I put a piece of it on the comparator that was about 4 feet long,
supported between two v-blocks. I checked the entire length of it while revolving. The crest and the root both stayed on the line, just like the short pieces did.

I see the imperfections in the thread you mention Geof, but its only the ends of the bar about 6-8" in both sides, which we manually cut-off each bar

the only kind of material I had to compare with was an 8" piece of 1-3/8" OD aluminum tubing
I chucked it up and indicated it spinning, it had a high side about .004", which also showed up when I turned a little bit of the end. This was done before any of the tear-down and new parts

I also indicated the threaded rod with the spindle turning 10rpm and me manually traversing in Z to keep the tip of the indicator on the thread crest. It had a high side as much as .015", which corresponds to the bad concentricity we're seeing in the parts

I plan on turning a solid piece to 1-3/8 on another machine monday and testing it, the donut is a good idea but I would think the collet on the end of the pusher would negate it, it keeps hold of the material the whole time and reaches all the way into the spindle through-bore right up behind the chuck

[kendo]

well the parts are running now,
the only way to make it run was choke the material up in the chuck so the cut-off is like a millimeter away from the jaw face, the part is only 1-3/8 long

this has me worried still,
this job ran last time at the same z-offset I've been battling all along
so now, the machine is incapable of running this job with the material extending maybe 4 inches out of the jaws

now that the machine is running along, Im not allowed to stop it and it'll be next week before I can do anything else

in the meantime, does any of this sound like tell-tale signs of some deeper issues in the machine spindle?

[extanker59]

and no adjustment in the bar-feeder

No different opinion about what the other guys are saying.

But all bar pushers are able to be adjusted. We had LNS hydraulic bar feeders that had to be re-adjusted every couple of years.
Remove the pusher and the back end. Make a bushing that fits the back end hole. The bushing will have a centerline hole through. Run a piece of string through it and all the way to the lathe chuck (with another bushing to hole it concentric and tight). The string should be running in the center of both the feeder opening and the lathe spindle opening. If it isn't, you have to loosen the mounting bolts holding it to the floor (and rarely the height adjusting clamp bolts too). Adjust (move) it until it's right.
Sorry for the long winded recall. I'm sure there's other ways.