tramming with a DRO

[boatmadman]

Having read threads about tramming, I set my lonely brain cell to work and came up with this ....

I set up my DTI on the mill spindle and position the tip at the far left of the table, zero the DTI and Z axis of the DRO, raise the mill head so the DTI tip clears the table, rotate by 180 deg and lower the mill head until the DRO Z axis is at zero, the DTI gives the amount out of tram across the sweep of the DTI. It was within 1.5 thou.

I cant see any reason why this method shouldnt be ok, but would value the opinions of those more cleverer than wot I is!

Its an X2 mill by the way.

Ian

[75Plus]

Ian,

I may be wrong but I believe that by combining two measurements, DTI and DRO, you are doubling the chance of error. I believe using a thin card or piece of shim stock as a sliding surface for the DTI pointer would ensure batter results.
JMO

Joe

[raynerd]

I cant see any reason why this method shouldnt be ok, but would value the opinions of those more cleverer than wot I is!

I downt fink I am more cleverer than u but...... I explicitly concur with 75Plus, my most immediate instincts would suggest that one would be reading the error from both the mill and DRO.   

[DMIOM]

Distinctly not-cleverer here - but would suggest that that moving the head up&down as well could introduce errors due to any 'freedom' or backlash in the Z axis, or indeed the DRO scale connection. Far better to keep the head/quill/knee locked so you are truly only measuring one variable.

Dave

[boatmadman]

Ok, I see what you are all suggesting and it makes sense, thanks

Ian

[BobWarfield]

And now for a contrarian view...

This approach works fine, depending.  If your DRO measures the actual motion of the head regardless of backlash, as most quill DRO's would, you do not have a backlash problem to contend with.  That's one of the great reasons to have a quill DRO in the first place, and they're so easy to rig up.

If the combined error of your DTI and your DRO is so large as to impact tram, you probably need a new DTI and DRO.  Seriously, they should be able to be accurate to a thou with both tools in play, or close it enough so as not to matter.  In fact, many like to add a nice DTI to the end of one's height gage to increase the sensitivity of the read there and they don't worry that errors will suddenly stack up. 

I've used the method described and compared it to other methods of tramming--just as accurate.  Go with what works for you.  Sometimes it is helpful to know how much things are out rather than relatively speaking just that they're out either a little or a lot.  Just remember to adjust to "half the out" or you'll chase it quite a bit!

Cheers,

BW

[raynerd]

Bob - I don`t understand that, what do you mean:

"If your DRO measures the actual motion of the head regardless of backlash, as most quill DRO's would, you do not have a backlash problem to contend with.  That's one of the great reasons to have a quill DRO in the first place, and they're so easy to rig up.""

Surely if there is any error in the reading from the DRO you`ll end up measuring that as well as the actual tram? In theory, you could be in tram and your DRO out, but you read an error or "reading" on your DTI which shows your out of tram when actually it is the DRO. Are you saying that your DRO should be that accurate that it will not produce any error?
I`m not arguing, I`m trying to learn 

[75Plus]

Most of the quill DRO's I have seen, with the exception on the most expensive ones, have an accuracy of +/- .001". That in its self COULD induce up to a .002" error, plus one time and minus the next. You also should be tramming with the quill securely locked. It would require unlocking and relocking for each swing which could also contribute to error.

JOM

Joe

[BobWarfield]

The point is the source of the DRO error most likely is not backlash, and that the error is therefore likely very small.

If you connected the scale to the quill, it is separated from the backlash, just as if you connect a DRO to the axis of your table.  You can turn your handwheel all you want in the backlash zone, and because of the backlash, nothing moves, hence the DRO stays put and is not affected by that error.  As I say, this is why we like DRO's!

Now as to the accuracy of the DRO beyond backlash, that'll be a function of the scale you're using.  And it's worth checking it.  Certainly you can buy a really expensive scale that is particularly accurate, but the scale I've got has always read to about 0.0005 and it's just a cheap caliper I converted.  I'd be surprised if you're seeing 0.002 errors from such a caliper properly mounted, but again, it's all a function of the scale you're using.  If I found one out 0.002, they're cheap enough I suspect I would toss it in the trash, LOL!

As to the question of locking the quill, it's really in the same category as the backlash.  If the DRO measures the position of the quill, you can lock or unlock at any point and it will reflect whether that action moved the quill at all, and hence it is factored into your tramming.  It's a little frightening how much a lot of quills will move when locking and unlocking, so I regard that as yet another reason to want to have a quill DRO.  You'll eventually get good at knowing how far the lock will move it and you'll account for that in the position you select before locking.  All that becomes pretty second nature once you've used your quill DRO for a little while. 

In essence, then, this approach is kind of like using your quill as an erstwhile height gage. 

With all that said, I'm not advocating you tram this way.  Tram however you'd like (and I especially won't bring up tramming with a Coaxial indicator after reading that "other" board's thread, ROFL).  I'm just saying the Quill DRO will account for most of the errors people are bringing up as worries and won't affect your tram.  If you guys are reliably tramming closer than 0.001 on these cheap mills, my hat is off to you and I hope you've got your Interapid tenths indicator handy to do it with!

I've played at that end of the pool too when I squared the column on my mill:



It's an awful lot of trouble just to tram things though.

Cheers,

BW