Motion when locking the axis

[Eyecon]

I just received a PM-728 and working on setting it up. I’m fairly new to adjusting tapered gibs setup and I noticed that my X and Y axis move in the range of .0008-.0016” when I lock the axis. I’m assuming this is not normal and my gibs are too loose?

Also my Y axis gib moves up and down across the dovetail when I try to snug it with the thick side adjustment screw (I have a video but not sure how to share it here). I assume this means the gib is not making full contact on the matching dovetail or the taper angle is off and needs to be refitted?

 

[Dabbler]

The first thing you check is for sideways movement in the axis when locking the gibb. That is, you lock the X axis, but register the movement in the Y direction. Some movement is necessary. 8 tenths is not excessive. Excessive movement means the gibb is loose. On older machines .005-.010 is often seen. On a new machine, up to .002 is often seen in offshore machines. On a brand new Bridgeport, I think they hold it to a half thou or something. My Hartford was delivered with a similar tolerance. (but that was 43 years ago and 1000s of hours ago)

 

[Eyecon]

The first thing you check is for sideways movement in the axis when locking the gibb. That is, you lock the X axis, but register the movement in the Y direction. Some movement is necessary. 8 tenths is not excessive. Excessive movement means the gibb is loose. On older machines .005-.010 is often seen. On a new machine, up to .002 is often seen in offshore machines. On a brand new Bridgeport, I think they hold it to a half thou or something. My Hartford was delivered with a similar tolerance. (but that was 43 years ago and 1000s of hours ago)

Thank you this is very useful information. When measure the side ways movement, what’s the common place to take the measurements? Locking X and measuring movement in Y at the center of the table will result in much smaller number than on edges of the table for example

 

[Dabbler]

Your indicator has to mount somewhere and push against something.... I usually measure below the spindle - after all that;s where it matters - by putting a mag base on the table and pushing against a suitable surface on the head.

Try this at the centre of the table and about half way to the farthest extent, each way. that will tell you roughly how straight/worn/close fitting (of the dovetail/gibb arrangement).

[update] when you are moving away from the centre, use this same arrangement to measure the looseness by wiggling the table... Some ,mills were so worn out I'd get more than 10 thou 'wiggle'. Def a machine to walk away from!

 

[PeterT]

Maybe provide us a pic to confirm but I suspect what you are seeing is the end of the table lock bolt acting on the gib strip. It is providing oblique force against gib to lock table dovetail but unfortunately also imparts a torque effect which can displace the table. It also makes a bunch of pecker marks down the gib strip over time.

Here is my fix to give you some ideas. Maybe you can adopt a similar principle. I made a shoe that matches the dovetail angle on one side & half sphere cut on the other side (ball end mill). The lock screw was modified to have a bearing ball attached with Loctite (use same ball end mill for the pocket). The ball contact slips in the mating socket with negligible torque & the shoe imparts push force along its matching face.

 

[Susquatch]

I noticed that my X and Y axis move in the range of .0008-.0016” when I lock the axis. I’m assuming this is not normal and my gibs are too loose?

Further to what @Dabbler and @PeterT have said, your mill is new. I would fully expect the Gibbs to settle in a bit as they get used - think of it a bit like getting lapped with service. They will need very minor adjustment as that happens.

I qualified my machine when I first got it. (In my opinion, every new tool should be qualified before use.) I also yanked on the table forcibly to understand its limits as Dabbler suggests. There was some movement like you describe, but after I installed a DRO on my Hartford, one of the things I noticed is a very small consistent movement in each of the Gibbs as I tightened them even without applying any force. As you say, this shows up as movement in Y when you tighten X and X when you tighten Y. More often than not, it simply doesn't matter to me. But the nice thing about qualifying your machine as you are doing is that you know about it now and can take it into account if it ever does matter.

 

[Eyecon]

I would fully expect the Gibbs to settle in a bit as they get used

Precision Mathews are sending me a new gib blank to try and refit. Until it gets here(there are apparently huge backlogs at CBSA and my other package from PM has been stuck for 3 weeks!!), I plan to blue the Y dovetail to see how much of problem there is with the gib that keeps moving every time I try to snug it. I created a simple joint setup in Fusion 360 and I now understand that this movement is only possible if the taper angles are slightly off. If the gib is way off(which I don't think it is) I can try and scrape it in a little to so it makes better contact and it doesn't wiggle as much. While I understand that the gib will settle in, I didn't want to cause any uneven wear to the dovetail (I'm probably being unnecessarily cautious here ).

As for the movement, I'll need to repeat the tests more consistently and measure movement around the spindle where it matters as @Dabbler said. This would also allow me to qualify the machine and understand how it behaves as you suggested. You are probably are right though, it probably won't matter too much when I have the DRO installed and I can measure the in axis movement vs. the "twist".

I made a little gif of the gib movement here since I couldn't upload a video, hopefully my interpretation of the issue is correct.

Like I said I have little experience with tapered gibs and certainly with this size of a machine so I just wanted to make sure I have a good starting point for the setup. Thank you all for the great advice so far!

 

[Dabbler]

@Eyecon This gibb seems to be "ill fitted". I hope the new one from PM is better. There has to be some up/down play in the gibb or it will drag on the flat-way, scoring it. On my PM935 clone, I think the clearance is minimal. I can check tha once I get it unburied.

With this much clearance the gib can get twisted up (sorta diagonal) making clearances unpredictable.

 

[Eyecon]

@Eyecon This gibb seems to be "ill fitted". I hope the new one from PM is better. There has to be some up/down play in the gibb or it will drag on the flat-way, scoring it. On my PM935 clone, I think the clearance is minimal. I can check tha once I get it unburied.

With this much clearance the gib can get twisted up (sorta diagonal) making clearances unpredictable.

that's what I figured, I'm hoping there is enough "meat" in the gib that I can blue and scrap it for a better fit until I get the new full length gib blank to refit. I know this won't change the top and bottom clearances but at least if it's properly fit it shouldn't move as far as I understand. The gib on the X axis doesn't do that so as you said, I hope the factory just did a poor job hand fitting this gib vs. some other issue with the casting of the saddle.

 

[Dabbler]

vs. some other issue with the casting of the saddle.

the fitment should all be on machines surfaces. Sometimes they make a mistake or 2 when making the gibb, and that is fixable. I really don't think there is a high likelihood of a casting failure or significant dovetail problem.

- Does 'just received" mean "brand new" or is it "new to you"? It sounds like it is "brand new".

If it is new, I'm sure Matt will try hard to take care of you.

 

[Eyecon]

Yes brand new, I've been in touch with technical support who initially said this play is normal but then when I called they immediately offered to send a new gib blank. I have couple of other issues on this mill including a rusted spindle which they already send a replacement for and it's currently clearing customs for the past 3 weeks

I read that PM has great support and while they were responsive and willing to send replacement parts, they always seemed a little reluctant at first. I guess I was expecting a little more for the price point. I saw several videos about how great the 728 is before pulling the trigger so I'm a little disappointed.

 

[PeterT]

Did PM see your video & they said that was normal, or was it just from verbal description at that point? I only have my own machines to go off & no way the gib floats up like that. Hopefully the new one will be the cure. Not making excuses for them but QC issues slip through the cracks, its a function of where & how they are built. But every article I've read where there have been issues like this PM seemed to be very fair to remedy the situation. I hope that extends to your situation. Most of us have experienced teething pains if it makes you feel any better. The difference is if you have a receptive dealer.

I would have asked for what this dimension should be normally, but I guess you will get the new one eventually. Good luck & keep us posted.

 

[Eyecon]

Did PM see your video & they said that was normal, or was it just from verbal description at that point?

Yes it was the first thing I sent and the reply was “That motion is normal. If that is causing an issue with the motion of that end of the gib, once you get the gib where you want it, just leave that screw alone and take up the slack using the other gib screw.”

Then when I was arguing that two matching tapers shouldn’t allow for movement in this direction they said “
All of the gibs are hand-fit from longer gibs. They start with a gib of the correct taper, then cut it to length. If you're having issues like this on both ends, it could be that the gib is cut a little too far "down" the taper to the smaller end. I can send you a replacement gib, but you will still need to trim it to length. It will obviously be the same taper, but if you trim it so that it's larger on both ends (i.e. further "up" the taper), then this should fill that gap that is causing the play. You don't have to be very exact, because any variation is taken up by the gib adjustment screws, you just have to cut more off the skinny end and less off the fat end than they did at the factory”….which honestly didn’t make sense either because the gibs aren’t tapered in that direction. I eventually called this person and they agreed and sent me the replacement.

So yes they did the right thing but for example when I asked for the gib specs( taper ratio and width) they didn’t have them. Also when I asked them to use an alternate shipping method since Canada post was experiencing unreasonable delays they refused.

Given the fact that I have to take apart my spindle and replace it once the part eventually gets here, the fact that the ultra precision keyless chuck was completely out of tolerance(gave a me a full refund already), the paint chipping on several places on the mill(they offered to pay for the paint if I managed to get a same color mixed locally), I’m just a little disappointed that I have to wait that long and spend that much effort to get this mill to where it should be. This mill is advertised to be a little higher quality than the average asian import mill since it’s made in Taiwan not to mention a little more expensive than machines in this size range. Am I being unreasonable?

Anyway rant over and again I really appreciate everyone’s inputs and I’ll keep you all posted.

 

[DPittman]

and less off the fat end than they did at the factory”….which honestly didn’t make sense either because the gibs aren’t tapered in that direction.

Some gibs are. In a 12x36 lathe I owned the cross slide had a gib exactly like that, smaller on one end than the other and it had the two end adjusting screws. As far as I know (which ain't very far) all those gibs with end adjusting screws will be tapered like that with big and small ends. The gibs that are adjusted with side screw mechanisms are not and are parallel the entire length.
I believe my mill gibs are just like yours but I have not had them out. I also have movement when locking them up but my dro only shows maybe .0015" when doing so.

 

[Susquatch]

Precision Mathews are sending me a new gib blank to try and refit. Until it gets here(there are apparently huge backlogs at CBSA and my other package from PM has been stuck for 3 weeks!!), I plan to blue the Y dovetail to see how much of problem there is with the gib that keeps moving every time I try to snug it. I created a simple joint setup in Fusion 360 and I now understand that this movement is only possible if the taper angles are slightly off. If the gib is way off(which I don't think it is) I can try and scrape it in a little to so it makes better contact and it doesn't wiggle as much. While I understand that the gib will settle in, I didn't want to cause any uneven wear to the dovetail (I'm probably being unnecessarily cautious here ).

I don't think you are being overly cautious at all. Certainly LESS SO than I would be!

You don't know what you don't know! And for that matter what others don't know! Caution and patience is absolutely the order of the day! The consequences of a remedial mistake are monumental. MUCH better to take your time, go slow, question yourself and others, and don't do anything that makes a permanent change until you are absolutely certain beyond any shadow of doubt that it's the right thing to do.

And yes, settling in your ways to a bad Gibb would be a bad thing! A wise man (@Dabbler LOL! ) once told me that you don't just go get another table to replace the one you have, you go get another mill! So ya, don't Fk up your WAYS! Best to take your time and sort it all out first.

Anyway rant over and again I really appreciate everyone’s inputs and I’ll keep you all posted.

Don't be too hard on PM. They don't really know you. They can't just trust everyone who calls them to know what they are talking about. From my angle, it looks like you are getting the kind of service they are known for. Just keep working with them the way you have been but keep in mind that 90% of their calls are from owners who have no clue. My best advice is to latch onto a particular guy there and deal with him and him only. Even if that means waiting for time off, shifts, illness, and vacations. That way you can both learn to trust each other.

I have couple of other issues on this mill including a rusted spindle which they already send a replacement for and it's currently clearing customs for the past 3 weeks

Customs is not their problem. Try hard not to let their stupidity sour your feelings about PM.

So how bad is this spindle corrosion? Do you REALLY have to replace it? I'd be trying my best to clean up the rust before I took my spindle apart to replace it! It's HIGHLY unlikely that any rust there would affect the function.

 

[Eyecon]

Some gibs are. In a 12x36 lathe I owned the cross slide had a gib exactly like that, smaller on one end than the other and it had the two end adjusting screws. As far as I know (which ain't very far) all those gibs with end adjusting screws will be tapered like that with big and small ends. The gibs that are adjusted with side screw mechanisms are not and are parallel the entire length.
I believe my mill gibs are just like yours but I have not had them out. I also have movement when locking them up but my dro only shows maybe .0015" when doing so.

But if they are tapered along their width wouldn’t they act like a wedge and get stuck the more your try and push them in? Curious how this mechanism works

 

[Susquatch]

But if they are tapered along their width wouldn’t they act like a wedge and get stuck the more your try and push them in? Curious how this mechanism works

Stuck and broken Gibbs are a common problem...... Just sayin!

Most machinists prolly don't routinely maintain them, and I've never seen instructions for it, but I believe in routine removal and oiling. A stuck or broken Gibb is a nasty problem if you ask me! It's usually easy to do so why not......

 

[Eyecon]

I don't think you are being overly cautious at all. Certainly LESS SO than I would be!

You don't know what you don't know! And for that matter what others don't know! Caution and patience is absolutely the order of the day! The consequences of a remedial mistake are monumental. MUCH better to take your time, go slow, question yourself and others, and don't do anything that makes a permanent change until you are absolutely certain beyond any shadow of doubt that it's the right thing to do.

And yes, settling in your ways to a bad Gibb would be a bad thing! A wise man (@Dabbler LOL! ) once told me that you don't just go get another table to replace the one you have, you go get another mill! So ya, don't Fk up your WAYS! Best to take your time and sort it all out first.

Thank you, that makes me feel a lot better about taking apart the table and saddle to try and properly inspect the gib contacts (also want to try my new scraper!)

So how bad is this spindle corrosion? Do you REALLY have to replace it? I'd be trying my best to clean up the rust before I took my spindle apart to replace it! It's HIGHLY unlikely that any rust there would affect the function.

There was crusty corrosion on the inside of the spindle taper which I cleared with a brass brush. After cleaning, I measured less than 2um/80 millionth on the inside R8 taper on average when the quill is locked. However when I reported the excessive runout of the PM ultra precision chuck I ordered with the mill, the support person told I me need to replace the spindle and then measure the chuck runout again. Ultimately with the Canada post delays, I decided to mount the chuck on my lathe and true the R8 taper OD on my 4 jaws and sent PM a video of clamped gage pins runout and they just sent me a refund. So I guess I no longer need to replace a spindle to your point.

 

[Susquatch]

(also want to try my new scraper!)

Frankly.......... PUT THAT SCRAPER AWAY!

Seriously, I would not use that thing on anything till I knew a whole lot more about what's wrong.

Even then, I'd prolly practice for a few years on something I didn't care about.....

 

[Susquatch]

There was crusty corrosion on the inside of the spindle taper which I cleared with a brass brush. After cleaning, I measured less than 2um/80 millionth on the inside R8 taper on average when the quill is locked. However when I reported the excessive runout of the PM ultra precision chuck I ordered with the mill, the support person told I me need to replace the spindle and then measure the chuck runout again. Ultimately with the Canada post delays, I decided to mount the chuck on my lathe and true the R8 taper OD on my 4 jaws and sent PM a video of clamped gage pins runout and they just sent me a refund. So I guess I no longer need to replace a spindle to your point.

I see......

Ya, just put the replacement spindle away for now.

In the meantime, I'd check the spindle taper at several points along its length with a dial test indicator and then also mount a known good R8 arbour and check nose and axial runout of a mounted indicator rod.