Interact 1 Mk II - Excessive cutter chatter

[Alfonzo]

Hi Guys,

I run a 1980's Bridgeport Interact 1, MkII mill, with a Heidenhain 155 controller. It is generally in good working condition, but seems to suffer from excessive cutter chatter, particularly (but not exclusively) in one axis (can't remember which one). The chatter takes the form of a very noisy high pitched 'scream' which leaves us knocking the speeds and feeds right back. This can't carry on, it's slowing us down so much. Can anyone suggest how we might go about analysing what is wrong with the ol' girl?

Thanks in advance,

Al

PS - I am confident this is machine related, not cutters or material.

[gus]

I would look at the tool holding

[Alfonzo]

As in the collets, etc? No - I'm confident it's not this, as I have a range of different tool holders and there is no difference, the machine still sings (loudly!). I'm forced to run silly slow through 6082 al alloy for instance.

[Alfonzo]

When the machine starts to 'sing' the vibration is very noticable if I touch the casting where the spindle quill emerges. Neither the X or Y axis seem to have any problems, both run smoothly. My instinct is that it's the spindle bearings, have you guys got any ideas on checks that can be made to verify where the problem originates from? Any help gratefully received

Al

[machintek]

Does this chatter occur in the corners? Or in a straight cut in a particular axis? How long is the end mill? Is it due to a dwell as the machine pauses to change direction?

George

[Alfonzo]

Hi George,

All good questions - I'll come back with answers tomorrow!

Thanks,

Al.

[jmelson]

Originally Posted by Alfonzo Hi George, All good questions - I'll come back with answers tomorrow! Thanks, Al.

With the machine off, grab the spindle nose and twist it back and forth to see how much play there is in the spindle spline. There should not be a great deal of play there. If you can click the spindle back and forth more than 10 - 15 degrees, the top of the spindle and the drive spline in the drive are worn. What happens is the spindle winds up in torsion a little as the cutter tooth is cutting and then jumps forward when the tooth comes out of the cut. You get this a lot when machining thin materials with the side of the cutter. When cutting thick materials there is always some tooth contact due to the flute helix, so you don't get it.

I'm not clear this is actually the problem you have, what I describe causes a huge clattering noise.
You describe chatter and also a screaming sound.

There are a bunch of bearings up in the head, in the belt drive mechanism and also in the back gear (low speed range drive). These are cheap utility grade bearings, and pretty easy to replace as well. None of these bearings should care whether cutting is being performed, though, so that may not be your problem, either.

Screaming may indicate too SLOW a feed rate! You may need to go FASTER when it screams, if that is really a high-pitched screeching sound, in the nails on a chalkboard pitch range. This couldn't be the belt slipping, could it?

If the spindle bearings are shot, it shouldn't make a difference which axis is feeding. The table is WAY too heavy to allow high pitched screeching due to table looseness. Now, it is possible your work holding is not rigid, and that could definitely depend on direction of cut. You might try bolting a big block of material directly to the table with heavy (1/2" -13) bolts and try a cut on that.
I got one of those 6" milling vises with swivel and quickly trashed the swivel base as I could actually SEE the vise rocking on top of the swivel!

As you can tell, your description of the problem confuses me. I've seen a lot of machine misbehavior over the years, but I can't match your description to anything in particular.

Anyway, the real test for the spindle bearings is to mount a magnetic-mount dial indicator on the main head casting and put the probe on the shank of an end mill. Apply about 50 Lbs radial force toward and away from the indicator. Note the excursions of the readings. Now repeat, with the probe on the quill itself. The quill should not move detectably, as the fit between the quill and the main housing is insanely tight. If it does move more than .001", you have a mess that is not easily fixed. Now, if the quill is steady, and the spindle nose is moving more than .001" from min to max reading, you may have spindle bearing problems. This amount of clearance is taken up by the spindle lube, so up to about .001" is OK. One other check is for the condition of the bearing balls and races. Throw the direct drive clutch out (move the lever on the ring on top of the 1J spindle housing to the right or operate the 2J speed handle to the mid position.) Spin the spindle rapidly by hand, with all other equipment in the shop turned off. A slight whirring noise is OK, but it should sound like a fine instrument-grade bearing. Major "noisy bearing" noises are a strong indication that the bearings may be bad. Is this a permanently lubed spindle or an oil drip system? If drip, does oil drip out of the spindle nose after oiling? If not, you aren't getting oil in there! It is SUPPOSED to drip.

Jon

[Alfonzo]

Firstly - Thank you for your lengthy response, it's people like you that make forum's like this so great.

I'm away from the workshop for a while now, but as soon as I get back (a few days) to the Interact I'll post some feedback to both Machintek & Jmelson's suggestions.

Regarding feed rates - I'm confident it is not as simple as this, unfortunately. As soon as it sings we have tried ramping up the feed rate, but it just starts to sound even angrier to be honest! (is it possible to attach a sound clip?) We end up running slow spindle speeds and slow feedrates to overcome the problem which is no good at all.

Obviously I could get a technician in to look at it, but the machine is not worth much, and neither am I! Plus, now I think about it - we did have someone in once, but was not a satisfactory conclusion.

Jmelson - regarding rigidly mounting a test piece, I think we tried that too a while back. I'll dig through my notes to see what happened.

Thanks guys, watch this space.

[machintek]

Interact 1 MK2, did not have a back gear. You are confusing it with a Interact 1 or 2.
It has a large 7 HP Italian motor with either a contraves or KTK spindle drive. On occasion (twice so far) I had a situation where the bearings that support the drive belt above the spindle did need changing. Never seen a bad spindle yet. If I remember correctly. the Spline drive was 2 piece that could be rotated from each other to take up play.

George

[Alfonzo]

I was re-reading Jmelson's comments wondering about the Interact Mk I / II thing. Thanks again guys.

[jmelson]

Originally Posted by machintek Interact 1 MK2, did not have a back gear. You are confusing it with a Interact 1 or 2. It has a large 7 HP Italian motor with either a contraves or KTK spindle drive. On occasion (twice so far) I had a situation where the bearings that support the drive belt above the spindle did need changing. Never seen a bad spindle yet. If I remember correctly. the Spline drive was 2 piece that could be rotated from each other to take up play. George

OK, is this based on a standard Bridgeport J head? I took Interact I to mean it was based on Bridgeport Series I, but with a 7 Hp motor that must not be true. So, is this a 4J head with different motor, or a totally different head not made by Bridgeport? I looked it up on the net, and it doesn't really look like a Bridgeport head, but the shroud covers up most of the details.

The adjustable spline sounds like a REALLY good idea! Must NOT be a Bridgeport head, then, I've never seen that on a Bpt.

Jon

[Alfonzo]

Mmm..reading between the lines here - but are we talking different machines on your side of the pond? The attached photo is the mill I have (Bridgeport Interact I Mk II). Not sure how it relates to the older Bridgeport stuff, or if it's the same as the ones you guys have in the US?