Tips/Techniques Tramming a vise, Gib tightness, accuracy etc.

[slow-poke]

Based on this thread... https://canadianhobbymetalworkers.com/threads/test-bar.12370/page-3

I imaging this question should invoke a few opinions ;-) I realize I'm splitting hairs here, but I'm here to learn.

In the previous thread I mentioned a rectangular straight edge and it seemed that most responders wanted to know what the heck do you need this straight edge for, well one example would be tramming a vise. Yes I can set up a sensitive lever dial indicator and run it the width of the vise (which is only 4" or 5") for the two that I use. So now imagine I tram that 4" vise and it's pretty close say a few tenths. If I had a really good straight edge so I could check it say 18" further away, that few tenths might be 0.002"? or more who knows?

Furthermore after tramming the vise to somewhere in the tenth's, if I grasp both ends of the table and twist it (depending on how tight the Gib's are), I can move that indicator perhaps by more than the tram error just from the slop in the Gib's. So there needs to be a compromise between:
- tight/loose Gib's
- easy table movement
- accuracy
- wear

My (Taiwan manufactured) 8x30 knee mill is old, used, not scraped so I don't really have a sense of what can be expected from a machine like this?
So for anyone that has taken the time to measure the error on their or preferable better quality work machines, I'm curious what you have found, I'm also curious how accurate a really good mill like a Mazak is in comparison?

Do I need this type of accuracy for what I'm doing, probably not most of the time, but I would prefer to at least have a clue as to how good or bad my machine is and what I can expect it to do if I do need to make something to a tight tolerance.

 

[Susquatch]

If I had a really good straight edge so I could check it say 18" further away, that few tenths might be 0.002"? or more who knows?

That may be true. But the only place it matters is where the work is being done - ie, under the spindle.

So even if the tram is out 2 thou over that 18" span, it might or might not be out when that 18" is under the spindle. Everything is rubber - so if the part or the bed bends or deforms when it is 18" away but straightens out when it's under the spindle. It's usually ok.

That said, I have come to like increasing my tram span just to take advantage of the geometry. Especially when I'm using a face mill where a little tram error is actually desirable.

I have not yet found a downside to using a 12" brake disk on blocks to tram my mill. 12" is plenty of span to increase the geometric advantage across. Plus it facilitates a very deep nod span too.

 

[ChazzC]

Based on this thread... https://canadianhobbymetalworkers.com/threads/test-bar.12370/page-3

I imaging this question should invoke a few opinions ;-) I realize I'm splitting hairs here, but I'm here to learn.

In the previous thread I mentioned a rectangular straight edge and it seemed that most responders wanted to know what the heck do you need this straight edge for, well one example would be tramming a vise. Yes I can set up a sensitive lever dial indicator and run it the width of the vise (which is only 4" or 5") for the two that I use. So now imagine I tram that 4" vise and it's pretty close say a few tenths. If I had a really good straight edge so I could check it say 18" further away, that few tenths might be 0.002"? or more who knows?

I have a pair (only because that is how they were sold: only one is required) of 9" x 1/2" x 1" parallels that I purchased for tramming my column in Y (table is only 4" deep). I haven't tried using one held in the vise to improve the accuracy of it's tramming, but I may do it as an experiment once I am 2-handed again.

 

[Mcgyver]

So now imagine I tram that 4" vise and it's pretty close say a few tenths. If I had a really good straight edge so I could check it say 18" further away, that few tenths might be 0.002"? or more who knows?

Not if you left the indicator in spindle and set up the second vise with it, tap the second vise in so you get the same indicator reading. In other words indicate across both vise fixed jaws to bring them into the same plane.

That is the most accurate way. My experience is most large work piece aren't going need sub thou accuracy nor is a mill usually capable of delivering it. In that case a long piece of cold rolled, say 4 x 3/4, clamped in the indicated vise then used to position the second should work. You could even quantify how straight one edge was lightly clamping it on the table and running an indicator along it. Better still, clamp the work to the table and use the Tslots as a referece (turn cylinders with a shoulder that fits the slot extends the T slot reference above the table) or just indicate the work directly. T slots are, or should be, quite accurately aligned with the table's axis of motion.

Far be it for me to deny you the pleasures of tool aquirement , It just that in close to 35 years of doing this and ofter to the highest levels of accuracy, the only compelling use I've found for a super accurate straight edge was scraping. For aligning two vises the above ideas are cheaper and provide greater accuracy (in the case of indicating).

If you want to get one, here's some that are good to 1/2 a thou per foot. You'll be well over 1000 cdn. Not as accurate as indicating and who knows what you'll get from China or India regardless of their claims (and if you don't know, is it any better than a piece of cold rolled?) You still should still indicate though to be sure, as just tightening everything up can move things.

CAST IRON STRAIGHT EDGES by Suburban Tool, Inc.

www.subtool.com