I’ll take some pictures and share here…to your point maybe the issue is the measurement
Revisiting weird gib issue
- Thread starter Eyecon
- Start date
Right on. That's all that matters. It's always possible that we will come back to the Gibb later on, but I think it's not likely. I really think it's just a red herring distraction. If the opposite dovetail is good, then the Gibb doesn't really need to do more than hold at one spot. In effect, it becomes the third leg of a 3 legged stool. Of course more is better, but might not be reasonably achievable. That said, nothing would thrill me more than having to say otherwise at some point.
Thank you. It is worth the effort.
Right on. That's all that matters. It's always possible that we will come back to the Gibb later on, but I think it's not likely. I really think it's just a red herring distraction. If the opposite dovetail is good, then the Gibb doesn't really need to do more than hold at one spot. In effect, it becomes the third leg of a 3 legged stool. Of course more is better, but might not be reasonably achievable. That said, nothing would thrill me more than having to say otherwise at some point.
Thank you. It is worth the effort
Former Member
Guest
Little late to the discussion, but if its tight enough to prevent movement in the desired direction and loose enough to allow up and down movement at the ends, something is worn wrong. pure and simple.
Clean everything and remove all oil from ways and gibbs. Apply a little prussian blue to all surfaces and re-assemble, adjust till movement binds.
Now carefully without further movement dis-assemble. and examine surfaces those bare are making contact, those blue aren't. (Basically you are blue printing your slide). It will give you a good indication of where your problem is.
Messy, but effective.
Clean everything and remove all oil from ways and gibbs. Apply a little prussian blue to all surfaces and re-assemble, adjust till movement binds.
Now carefully without further movement dis-assemble. and examine surfaces those bare are making contact, those blue aren't. (Basically you are blue printing your slide). It will give you a good indication of where your problem is.
Messy, but effective.
That’s exactly what I use on the surface plate, at some point I will do that with dovetails as the master reference as you suggested. I need to make something to spread the Prussian blue evenly though…I tried with a brush and it was a huge mess
The good lord made great applicators on the end of out fingers.... but then you've got blue finger for a long time and people will think that you were arrested and finger printed.
I use a brayer on the surface plate but that doesn’t work on dovetails
Dabbler
ersatz engineer
You need a pretty thin coat of Prussian blue to read anything well. Try to substitute cardboard and 'dab it' on your surface with only a little on (provided by a brayer).
You can and should measure your tapers.
But you can do an acid test *if* your dovetail in the gibb side is planar. It looks like your gibb is under 12" long is that right? For the purposes of proving the dovetail is planar, you can mill a 45 degree out of aluminum or 1075, and draw file it to 'print' on your surface plate. It doesn't have to be a 2 tenths straight edge - you are looking for anomalies in the 2-3 thousandths range. Then use your brayer to coat your improvised straight edge, and print the dovetail. I am concerned that your dovetail has a hollow in the centre, which will confuse your findings and complicate your measurements. Purely conjecture at this point, but it is what I'd try to check sooner rather than later.
You can and should measure your tapers.
But you can do an acid test *if* your dovetail in the gibb side is planar. It looks like your gibb is under 12" long is that right? For the purposes of proving the dovetail is planar, you can mill a 45 degree out of aluminum or 1075, and draw file it to 'print' on your surface plate. It doesn't have to be a 2 tenths straight edge - you are looking for anomalies in the 2-3 thousandths range. Then use your brayer to coat your improvised straight edge, and print the dovetail. I am concerned that your dovetail has a hollow in the centre, which will confuse your findings and complicate your measurements. Purely conjecture at this point, but it is what I'd try to check sooner rather than later.
I understand what you mean, a lower spot in the middle of the dovetail will cause the cross slide to be wedged on one end but loose on the other explaining the behaviour. I’m going to improvise a rig similar to what @Mcgyver suggested first since that doesn’t require the mill to fabricate and measure parallelism between two identical gage pins wedged in the dovetails…shouldn’t be too difficult since I’m only measuring relative distances. That should be a good indication of parallelism… assuming at least one dove tail is straight it would also give better assurances of flatness?
If the results are inconclusive, I’ll try and see if I can find a place nearby to machine a temporary straight edge since this is the only milling machine I have and the only other machine tool I have is a gantry router which is really not accurate. Worst case I’ll rough it on the router and file/scrape it against the surface plate as a way to get a temporary reference surface for the dovetails…again all this assuming the measurement rig doesn’t work. I’ll take pictures of everything and report back here.
You are on the right track, but there are a few key points to keep in mind.....
- forget about machining a straight edge, not flat enough, your instinct on scraping it to the plate is correct. beside even if you got it ground, the operating was good enough to get surface plate flat, no small task given the mag chuck's force, ground surfaces don't print very well compared to scraped.
- it should be cast iron, nothing else prints quite as well, nothing else is as easy to scrape
- it needs to be more acute than the dovetail and only one surface needs to be flat
- to mill the side away to create the acute angle, put the mill back together - this angle need not be accurate, you only use on surface of the reference flat
You kind have to check for flatness first, or guaranteed the results will be inclusive. A difference in the distance between the pins does not necessarily mean an irregularity with the angled dovetail surface. A perfectly flat dovetail surface and say concave shape in the horizontal surface will cause the distance between the pins to change.
AND.....you have to make sure the two horizontal surfaces are parallel (strictly speaking coplaner is not required) before the across the pins measure as a deviation will also change the result.
Philosophizing.....what you are doing is great and lots of guys can help you with it, but I keep mentioning the option of just using it as is. Reason is, identifying and correcting machine tool geometry problems is a big rabbit hole to fall down. There's not a quick and dirty way out of it....you need to do these test, in order, to figure it out and the test often require specialty tools as your finding out out (like the reference flat). I'm doubtful it is the gib which means you'll have scrape surfaces flat and for alignment to correct it, or as you say look to PM. That in itself might be worth a bag of popcorn, insisting they send you part until you get one that prints properly lol.
I see it like a guy sitting there with a basic Chev going, well I wanted the 911 performance, wtf? The good news is, unlike with cars, you can have that performance, but you get it with a lot of sweat equity; there is no free lunch. Me, I decided a long time ago to go for the performance so learned reconditioning.... but its not for everyone and at times I wonder what the ____ am doing this for lol.
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I second your advice as given @Mcgyver. All of it is sound.
Selfishly speaking, I'd like to see the machine put back together anyway because I'd like to see how the initial measurements were done such that they convinced @Eyecon that there is a problem. He seems to have more experience than others, but another set of eyes almost always sees more than just one pair does.
Im in Burlington Ontario
Burlington! Oh my! You are lucky as a pig in poop!
You have a virtual wealth of forum expertise right there in your own corner of the world! Literally an army of guys who can help you. Break out the steak and libations and do a few invites. Your problem today will be yesterday's problem the day after. Best of all, you will make some fantastic new friends too.
Apologies for the delay in getting back to this thread as I’ve been travelling for work.
I traced the source of my troubles to what appears to be a combination of an improperly ground saddle casting and a bowed/improperly finished gib.
Before making any changes to anything I measured the play in the saddle against the dovetails (or any stationary point on the machine such as the base or column) across the entire travel range using the setup pictured above. I pushed the gib far enough so it wedges the saddle to a point where it’s not perfectly easy to move along the Y axis and very lightly snug the back gib adjustment screw with no front retaining installed. I was getting a .0015-.0018” deflection on either end and no movement in the center. Again I verified this along the entire travel range and the readings were symmetric and consistent
First thing I did after this was to check the sliding surface of the gib against the surface plate. As before, contact point distribution on that surface was not very even, so I did a few hand scraping passes to get it as “flat” as possible.
Repeated the measurements from above, no change at all. At this point I turned my attention to the fixed side of the gib. It was so high in the middle (where the y axis lock screw makes contact) it was basically almost spinning freely on the surface plate. The point of rotation of the gib on the surface plate was basically dead center so I didn’t trust the blueing and did some rough scraping of the center just so I can actually read the blueing properly.
After a few passes of really rough scrapping, I started getting more contact very close to both ends, and the points of rotation on the surface place moved outwards towards the 1/3 point as they should.
At this point, I reinstalled the gib to measure slop as above and I repeated the play measurements. They were better (about half as much slop) but still the same amount on either side so the gib appeared to still be high in the middle although the surface plate blueing indicated otherwise.
Instead of taking more material off, I decided to disassemble the saddle and take a look at the inner dovetail that makes contact with the gib.
I didn’t take any specific measurements but to me it seems like there was something wrong with the casting to the extent they had to hand grind the center near the lock screw hole to get the gib to fit decently enough to ship the machine out…clearly they didn’t spend too much time fine tuning the fit. In fact, no too much care was taken doing this, you can see where the rotary tool probably got away from them and gouged a line in the dove tail.
At this point I decided to just fit the gib as much as I can. Knowing there was an issue around the center, I decided to do a few roughing passes on the center of the fixed side of the gib to get the gib to make proper contact on the ends of the dovetail saddle.
It didn’t take too many passes, once verified that my passes were indeed making the gib low in the center on the surface plate, I repeated slop test and managed to get rid of the slop completely. I switched to a 2um indicator to validate, and while there’s still maybe 1um of play, my 10um/.001” indicators can no longer detect it. The saddle is now solid with no slop while sliding smoothly by hand!
I think is good enough for now. I need to put the mill back together to work on a camelback straight edge that I just purchased and I plan to contact Precision Mathews about this problem. I don’t think a brand new machine should have such a fault so I hope they’ll send me a new saddle although I think the machine will work fine with the current fit of the gib.
I traced the source of my troubles to what appears to be a combination of an improperly ground saddle casting and a bowed/improperly finished gib.
Before making any changes to anything I measured the play in the saddle against the dovetails (or any stationary point on the machine such as the base or column) across the entire travel range using the setup pictured above. I pushed the gib far enough so it wedges the saddle to a point where it’s not perfectly easy to move along the Y axis and very lightly snug the back gib adjustment screw with no front retaining installed. I was getting a .0015-.0018” deflection on either end and no movement in the center. Again I verified this along the entire travel range and the readings were symmetric and consistent
First thing I did after this was to check the sliding surface of the gib against the surface plate. As before, contact point distribution on that surface was not very even, so I did a few hand scraping passes to get it as “flat” as possible.
Repeated the measurements from above, no change at all. At this point I turned my attention to the fixed side of the gib. It was so high in the middle (where the y axis lock screw makes contact) it was basically almost spinning freely on the surface plate. The point of rotation of the gib on the surface plate was basically dead center so I didn’t trust the blueing and did some rough scraping of the center just so I can actually read the blueing properly.
After a few passes of really rough scrapping, I started getting more contact very close to both ends, and the points of rotation on the surface place moved outwards towards the 1/3 point as they should.
At this point, I reinstalled the gib to measure slop as above and I repeated the play measurements. They were better (about half as much slop) but still the same amount on either side so the gib appeared to still be high in the middle although the surface plate blueing indicated otherwise.
Instead of taking more material off, I decided to disassemble the saddle and take a look at the inner dovetail that makes contact with the gib.
I didn’t take any specific measurements but to me it seems like there was something wrong with the casting to the extent they had to hand grind the center near the lock screw hole to get the gib to fit decently enough to ship the machine out…clearly they didn’t spend too much time fine tuning the fit. In fact, no too much care was taken doing this, you can see where the rotary tool probably got away from them and gouged a line in the dove tail.
At this point I decided to just fit the gib as much as I can. Knowing there was an issue around the center, I decided to do a few roughing passes on the center of the fixed side of the gib to get the gib to make proper contact on the ends of the dovetail saddle.
It didn’t take too many passes, once verified that my passes were indeed making the gib low in the center on the surface plate, I repeated slop test and managed to get rid of the slop completely. I switched to a 2um indicator to validate, and while there’s still maybe 1um of play, my 10um/.001” indicators can no longer detect it. The saddle is now solid with no slop while sliding smoothly by hand!
I think is good enough for now. I need to put the mill back together to work on a camelback straight edge that I just purchased and I plan to contact Precision Mathews about this problem. I don’t think a brand new machine should have such a fault so I hope they’ll send me a new saddle although I think the machine will work fine with the current fit of the gib.
Quick update: after a couple of phone and email conversations explaining the findings to PM, they asked me to take a video so they can show it to technicians at the factory. According to the latest communication, they are waiting for feedback from the factory to see if they’ll send me a new saddle or an entirely new machine
It surprises me how quality of a product produced in a factory could vary so much. I'd think processes, techniques and material would be consistent and the resulting product would also be consistent. I know there can still be the one-offs and that's probably what this one is. All I've heard is that PM is pretty good at ensuring their customers are happy with their machines.
