PDA

View Full Version : Tramming the mill using a rotor question.



Mr Fixit
04-16-2015, 07:12 PM
Hello Group.
I am in the process of setting up my new to me milling machine and have stopped at the local VW service shop and got a couple of brake rotors for tramming. When I was younger I had a VW bug and remembered that it had solid rotors not vented (the holes between the inside and outside) so I picked up one of these and a vented type.
So, the question is, is one better over the other as to trueness or any other issues I should be considering?
I will be truing them up on a friend's lathe and cutting the centers out.


TX
Mr fixit for the family
Chris :)

Doozer
04-16-2015, 07:35 PM
Skip the rotor completely and tram to the table
or vise bed. Tee slots are not a problem.
I don't know why people think a brake rotor
(or similar thing) is needed to tram.
In a real machine shop, you might tram the
head of a manual mill a few times a day
if you are milling angles and such.
And you better only take a minute or so
or the boss thinks you are lacking something.
Stop listening to people with this brake rotor
idea. If you suck at indicating, practice it.
Use a 4 jaw on the lathe and practice using it.
That also should take a minute or so to get a
part in less than .0005". Just get in the shop
and use your tools and practice. Leave the
brake rotors for the cars.

--Doozer

38_Cal
04-16-2015, 07:36 PM
I use a solid rotor. As for truing, make sure that the table is free from burrs and dirt, and set up an indicator in your mill. If you then carefully rotate the rotor under the indicator, you can see if there are any high or low spots on it. On my (new) rotor, the needle on the indicator barely moved throughout 360 degrees of rotation...using a very sensitive dial test indicator held in a collet.

oldtiffie
04-16-2015, 09:05 PM
If I need to know whether the quill spindle centre is square to the table in the "X and "Y" planes I check the tram.

If its near enough for the job in hand I use it "as is" and don't adjust the tram at all.

If it needs tramming I adjust the tram - and if it doesn't it doesn't get adjusted.

And not all tramming needs to be to an order of accuracy of say +/- 0.0002" (2 "tenths") over say 8" where as +/- 0.002" (or more) over say 8" will do.

There is no mention so far of checking the mill table for flatness, dents or burrs etc. as after all the mill table top face is just as much a reference plane in the event of tramming as a surface plate is in many other instances.

Doozer
04-16-2015, 11:16 PM
Yes, my point exactly.
If your "rotor" sits on a bump from a nick on the table,
it will throw things off, whereas just swiping the table is
immune to picking up a bump, because it is direct.
The needle will show the nick or bump, and you can
either ignore it and still be able to tram the mill, then
stone it out after.
I once tried a 6" Timken race to tram a mill, more farting
around than it was worth. It became evident in 10 seconds
that it was kinda useless and cumbersome.
Then there are those tram thingys with two indicators
permanently mounted to a tee-shaped affair.
Those are like training wheels on a motorcycle.
I saw in a video that even Starrett makes one.
Kind of superfluous in my opinion.


--Doozer

Honest Don
04-16-2015, 11:49 PM
I mount an indicator in/on the spindle and then check readings by sliding a 2" gauge block (on its side) between the stylus and the table. The gauge block helps factor out table imperfections.

Forrest Addy
04-17-2015, 12:28 AM
Big bearing races, brake roitors etc are all fine if that's what works for you. If you're a little bit practiced and know the nod ratio (how many thou to tweak the nod per thousandths of tram error for an 8" circle) you can tram a turret mill in a minute or three.

I tram directly on the table unless the vice is in the way. Then I use the 3" dimension of a cheap import no-hole 1-2-3 block to space up from the table. My "tramming block" has 15 degree lead belt sanded on it so the indicator doesn't "trip" as it ramps up to the reference surface. I use a version of the Indicol tool for most all tramming and location purposes.

Practice is the big thing. Otherwise it's over-shoot - crap! Under-shoot - crap!. Snug - move a bit - crap! etc. for a good part of an hour.

Thing is to pick a way to do things and get really practiced so all your moves are competant and automatic. Sieze on the latest fad then move on to another at the first frustration and you'll never get proficient - but you will get gadget poor.

oldtiffie
04-17-2015, 04:18 AM
It should be understood that when "tramming" a quill spindle centre to the mill table that both the mill table ("knee" mill or vertical head on a vertical mill) as well as the quill itself should be clamped.

You might be surprised how the tram will lose adjustment when the quill clamp is loosened and re-tightening and the same for the vertical dove-tail clamps - so be warned and be careful.

Same applies to tail-stock quills.

boslab
04-17-2015, 05:06 AM
Skip the rotor completely and tram to the table
or vise bed. Tee slots are not a problem.
I don't know why people think a brake rotor
(or similar thing) is needed to tram.
In a real machine shop, you might tram the
head of a manual mill a few times a day
if you are milling angles and such.
And you better only take a minute or so
or the boss thinks you are lacking something.
Stop listening to people with this brake rotor
idea. If you suck at indicating, practice it.
Use a 4 jaw on the lathe and practice using it.
That also should take a minute or so to get a
part in less than .0005". Just get in the shop
and use your tools and practice. Leave the
brake rotors for the cars.

--Doozer
I must agree, what your doing by using a rotor is adding another variable, what you want to know is not the relationship with the brake rotor and table, it's the spindle and table, so just measure that.
Mark

SGW
04-17-2015, 06:03 AM
I use the top of the rotary base for my vise (i.e. without the vise), if I want a uniform surface. Sometimes I just use the table. A brake rotor will work. Whatever you're comfortable with. After all, it's YOUR shop! Do what you please.

john hobdeclipe
04-17-2015, 09:11 AM
Clamp a piece of aluminum in your vise. Take a cut along the top. Look at the marks made by the end mill. Determine which side is low and adjust the mill head accordingly. Repeat as needed until you have both the leading and trailing cuts showing on the surface. At this point you may or may not be perfectly square to the table surface but you are square to the travel. Should be the same thing, no?

Fasttrack
04-17-2015, 12:01 PM
Skip the rotor completely and tram to the table
or vise bed. ...

--Doozer

^^ This!

I've never understood the need for a rotor.

Edit: For what it's worth, most lathes will cut a very slight cup when facing material. Even our rebuilt HLV at work cuts a very slight cup; about 0.0002" difference across a 2" radius. To get a good tram with a faced disc, you need to make sure the disc is centered on the spindle before starting.

If you use a rotor, you have to make sure the table is clean and nick free. Then you need to center the disc under the spindle. Then you tram.

I prefer my single step method: tram.

Bob Fisher
04-17-2015, 01:20 PM
I do what John does, with an ordinary end mill, better yet a flycutter. I also bought a "training wheel" device from Enco on sale. It works quite well for a quake check. Bob.

oldtiffie
04-17-2015, 09:38 PM
There is way too much emphasis on the need for super accuracy in setting up the tram when the real need of the accuracy of a tram for the job in hand may be somewhat (considerably) less.

Assuming for the moment that super high tramming is necessary it is a little unfortunate that any vertical face cut with the side of an end milling cutter will only be truly vertical to the horizontal trammed face if and only if there is no conical run-out of the milling cutter.

In the event of a conical run-out the vertical faces will be somewhat like (but less than) those on a dove-tail - on an "outside" vertical face the end of ther end milling cutter will describe and arc (and diameter) that is wider than that at the collet.

In short, and "outside" face will have a deeper cut than at the collet.

And an "inside" cut (as in a slot) the end milling cutter will also have a deeper cut than at the collet.

If that is the case, neither vertical will be square to the trammed (horizontal i.e. mill table) surface.

A similar situation arises in the cutter centre is running true but the helical (vertical) cutter edges are not true/parallel but are in fact conical.

So - I don't worry too much (or in many cases) or at all as I only need a tram to suit the job in hand.

CalM
04-17-2015, 09:49 PM
Skip the brake rotor, just another piece of crap.

Use 1-2-3 blocks on the table to allow a free sweep from one side to the other. Heck, lay your vise parallels on the table, they are well matched. (you can always mike them)

paul463
04-17-2015, 09:50 PM
Clamp a piece of aluminum in your vise. Take a cut along the top. Look at the marks made by the end mill. Determine which side is low and adjust the mill head accordingly. Repeat as needed until you have both the leading and trailing cuts showing on the surface. At this point you may or may not be perfectly square to the table surface but you are square to the travel. Should be the same thing, no?
When it gets that strange pattern down the center, I call it trammed good enough.
http://i149.photobucket.com/albums/s48/paulz463/jet%20lathe%20decals/IMG_20141218_175044227_HDR_zpsfvljfx9f.jpg (http://s149.photobucket.com/user/paulz463/media/jet%20lathe%20decals/IMG_20141218_175044227_HDR_zpsfvljfx9f.jpg.html)

CalM
04-17-2015, 09:57 PM
All right, Who has "tested" tram while taking a healthy cut ? I bet not many,

But if you did, you would be surprised at how cutting forces conspire to twist the machine.

Those beautiful "blanchard patterns" are most likely only available at one speed feed and DOC situation.

Not to dismiss the method as a great way to set the quill plumb with the table motion!

oldtiffie
04-17-2015, 10:53 PM
True.

I tend to "lift/raise" the "trailing" edge of the cutter/tram as the trailing edge is inclined to "drag" and leave "burrs" etc.

Her is one of my (home) jobs to "square up" my angle plates using "master/cylinder" squares with the trailing edge "up" a little -worked very well too as my angle plates are very square.

http://i200.photobucket.com/albums/aa294/oldtiffie/Master%20squares/Squaring6.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Master%20squares/Squaring5.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Master%20squares/Squaring4.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Master%20squares/Squaring3.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Master%20squares/Squaring2.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Master%20squares/Squaring1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Surface%20plate/Precgransqsheet1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Surface%20plate/630Platesheet1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Squaring-up/Machinist_Square1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Squaring-up/Machinist_Square2.jpg

My surface plates and squares rarely see the light of day as my mill table and float glass panels are quite sufficient for what I need and are quite good enough as regards flatness.

Lew Hartswick
04-18-2015, 11:52 AM
Well this has worked fine for several years.
http://i233.photobucket.com/albums/ee238/LewHartswick/100_3522_zpsvnbkalf1.jpg
...lew...

Mr Fixit
04-18-2015, 02:29 PM
Hello Group,
The OP question was is there any difference between a single thickness or a fluted rotor for trueness like the pic lew posted.
I know that time is of importance at a shop making money, but, for some of us HSM's it's the process for the same results that we look forward to.
You guys all make good point's Thanks for posting.

Mr fixit for the family
Chris :)

dp
04-18-2015, 06:18 PM
A rotor will work perfectly. Ignore the men behind the curtains. Rotors (new) are built to very good tolerances for parallelism. They're cheap, and flat. But - ask the sales person if you can measure them first. I've used a rotor for years with a co-ax indicator and it is one of the most perfect solutions for the job I've used. The combination is accurate and repeatable. It is also testable. After you've measured your tram with a DTI or co-ax, rotate the rotor 90 or 180 and measure again. My experience is nothing changes. If you want bad advice ask someone who's never tried it :). It's a target, simply put, and people tram with targets all the time.

Just don't do the last thing I did with my rotor - I dropped it while moving and it potato-chipped and went into the garbage. I'd measured it for flatness on a granite surface plate prior and it was off parallel by half a tenth. Much worse after dropping it.

TN Pat
04-18-2015, 07:37 PM
I don't see much use for using something between the table and indicator. I do see the tee slots as a problem. However - you can easily rough it in with a travel indicator first. Get it within a few thou, then you won't have to worry about a DTI jumping. Of course, a couple thou over a 6" swing would be adequate for a large variety of work to begin with.

Or, one could tram off the bed of the vise. Theoretically it should be the same, but even Kurts have a tolerance for parallelism.

peekaboobus
04-18-2015, 08:01 PM
Well this has worked fine for several years.
http://i233.photobucket.com/albums/ee238/LewHartswick/100_3522_zpsvnbkalf1.jpg
...lew...

what works for some may not work for others if they have higher standards though. For example, if that party does not want to hold a quite possibly flawed a priori assumption that the disk in question has the same thickness all around or is perfectly round.

Danl
04-18-2015, 08:09 PM
^^ This!

Edit: For what it's worth, most lathes will cut a very slight cup when facing material. Even our rebuilt HLV at work cuts a very slight cup; about 0.0002" difference across a 2" radius.



I believe most lathes are specifically designed to cut with a slight cup when facing. If yours doesn't indicate this cup, it may be either a very cheaply made lathe or possibly it has been damaged (spindle axis should not be at exactly 90 to the crossfeed axis).

Dan

dp
04-18-2015, 09:06 PM
Brake rotors are built to within 0.0005" parallelism and it doesn't matter if they're round. They would work if they were square. Many 1-2-3 blocks are parallel to 0.0002" over a span of only 3". Not as accurate or repeatable as a single rotor. For rocket parts this is probably not good enough but for home shop types it is probably better than they can get with their 0.001" indicator on the table. I think it would be difficult to buy a new rotor that was so bad it would not work for this purpose. I don't think it is impossible which is why one should test before buying.

For those of us with narrow tables the rotor gives us greater distance over which to measure the Y axis. When used with a CO-AX indicator it is the perfect setup. What's not to like?

CalM
04-18-2015, 09:39 PM
Brake rotors are built to within 0.0005" parallelism and it doesn't matter if they're round. They would work if they were square. Many 1-2-3 blocks are parallel to 0.0002" over a span of only 3". Not as accurate or repeatable as a single rotor. For rocket parts this is probably not good enough but for home shop types it is probably better than they can get with their 0.001" indicator on the table. I think it would be difficult to buy a new rotor that was so bad it would not work for this purpose. I don't think it is impossible which is why one should test before buying.

For those of us with narrow tables the rotor gives us greater distance over which to measure the Y axis. When used with a CO-AX indicator it is the perfect setup. What's not to like?

Which machines with "narrow tables" offer nod adjustment?

Just wondering what you are advising.

dp
04-19-2015, 12:12 AM
Which machines with "narrow tables" offer nod adjustment?

Just wondering what you are advising.

I use shims under the box post to correct vertical travel over a point. There are four attachment bolts that are available for correcting this problem. The head itself can also be shimmed to keep the spindle parallel to the direction of travel (nod). This is what you get with a tilting column mill, but it works. I don't have tram problems.

oldtiffie
04-19-2015, 12:23 AM
All right, Who has "tested" tram while taking a healthy cut ? I bet not many,

But if you did, you would be surprised at how cutting forces conspire to twist the machine.

Those beautiful "blanchard patterns" are most likely only available at one speed feed and DOC situation.

Not to dismiss the method as a great way to set the quill plumb with the table motion!

That comes as no surprise at all.

After all it is the same principle of "spring" under heavy loads on a lathe.

And providing the milling head has not permanently moved relative to its mating/mounting surface and that it "springs back" after a heavy cut and is still correctly trammed then the accuracy of the tramming will be there ready to use for lighter or finishing cuts.

For what its worth, tramming to say 0.001" between high and low over say 6" tramming circle diameter that is accurate to the order of 0.001"/6" = 0.000167" inches per inch of travel (0.000167 is 1 2/3 "tenths" per inch).

Its a good surface grinder that can do that well.

MaxxLagg
04-19-2015, 08:13 AM
If you're doing table work, tram to the table.
If you're doing vice work, tram to the vice. Adding another component just adds potential error no matter how flat it may be. If one rotor is good then two stacked would be better,right? After all, they're flat,right? Then why not 5 or 6? It would eliminate the need to raise the knee or lower the quill. They're all Flat,so it will work, right? Of course you wouldn't. Because you're inducing potential error between each rotor.

dp
04-19-2015, 09:15 AM
Stacking rotors does not make a lick of sense. One has to suspend reality to even conceive of a reason for stacking anything if the purpose is to test for tram. People have used 1-2-3 blocks for years for checking tram. Will anyone who has used stacked 1-2-3 blocks for checking tram please show us a picture?

While you're going through your photo albums please drag out the stacks of images of failed attempts to check tram with a rotor. Apparently there are some skeptics here who need proof that using such targets of known high precision for tramming has a history of failure. We have only seen images from those happy people who have used rotors successfully for this purpose and that success is looking mighty suspicious in the face of so much ardent but unsupported opinion the process is fatally flawed. With so much informed opinion there should be no problem finding some evidence that these opinions are based on solid evidence.

The following have trammed with rotors, and are happy with the results, and have provided photos:
Me: https://www.youtube.com/watch?v=ZfioLDhBNBQ
Lew: http://bbs.homeshopmachinist.net/threads/66656-Tramming-the-mill-using-a-rotor-question?p=978784#post978784
JTZshokunin: https://www.youtube.com/watch?v=OPMmGrdTqro
(JTZ is the recent winner of the Keith Fenner tool box contest)

If you have tried it and it didn't work then show your work.

TN Pat
04-19-2015, 11:09 AM
Perhaps the ultimate way to solve this would be - tram your mill with a rotor. Then remove the rotor and sweep around the table, or vise bed. If there's a difference, there's a problem with the methodology in that instance...

dp
04-19-2015, 03:09 PM
You can also put your rotor on a surface plate and map it. If it maps badly it's junk. If it is junk it is also not a part you'd want on your car. Take it back to the store and ask for a replacement.

MaxxLagg
04-19-2015, 09:19 PM
Stacking rotors does not make a lick of sense. One has to suspend reality to even conceive of a reason for stacking anything if the purpose is to test for tram. People have used 1-2-3 blocks for years for checking tram. Will anyone who has used stacked 1-2-3 blocks for checking tram please show us a picture?

While you're going through your photo albums please drag out the stacks of images of failed attempts to check tram with a rotor. Apparently there are some skeptics here who need proof that using such targets of known high precision for tramming has a history of failure. We have only seen images from those happy people who have used rotors successfully for this purpose and that success is looking mighty suspicious in the face of so much ardent but unsupported opinion the process is fatally flawed. With so much informed opinion there should be no problem finding some evidence that these opinions are based on solid evidence.

The following have trammed with rotors, and are happy with the results, and have provided photos:
Me: https://www.youtube.com/watch?v=ZfioLDhBNBQ
Lew: http://bbs.homeshopmachinist.net/threads/66656-Tramming-the-mill-using-a-rotor-question?p=978784#post978784
JTZshokunin: https://www.youtube.com/watch?v=OPMmGrdTqro
(JTZ is the recent winner of the Keith Fenner tool box contest)

If you have tried it and it didn't work then show your work.

I was being facetious. The point being, why not just tram to the surface you're going to br placing your workpiece on? Using a rotor or anything else is only showing you the relationship between said device and the table (plus any error in flatness of the surface of your device ). I am not dabating whether or not it CAN be successful, but rather the potential for error.

Forrest Addy
04-20-2015, 03:00 AM
TN Pat put his finger on it. Brake rotors may be safely assumed to be warped and out of parallel until proven otherwise. It's a car part not a precision gage. If new or freshly machined, the two sides may may still be dished or convex, etc. If used off the vehicle they may have any number of defects including heat checks, hard spots, distortion, and wear. Most geometry and wear defects are detectable by the tramming process but a few may confuse the user.

No way in the world would I use an unproved brake rotor as a tramming aid.

I strongly suggest setting the rotor up on 1-2-3 blocks on a granite flat and scan oth faces with an indicator and indicator stand or surface gage. Map the errorrs as you find them. If you know the errors you can compensate for them; and if needful, skim cut both faces to a plane and parallel to each other.

I suggest you prefer the vented brake rotors because they are thicker and stiffer than non-vented. While you're at it I suggest you machine off the hub and tidy uo the resulting bore until you have a ring. For that matter scrape it flat and parallel on both sides. Then you KNOW...

As for you fellows pointing our a machine tool's structure deflecting under cutting loads you are absolutely right. An old engineer's epigram is: everything is made of rubber which is a way of stating that all stuctures deflect in proportion to load even microscopically. The question is how much - and whether max stock removal deflections pose a problem when a subsequent light finish cut re-establishes part surface geometry.

philbur
04-20-2015, 04:41 PM
I think that the value of any tramming exercise is greatly influenced by how flat your table is. You are adjusting the perpendicularity of the spindle axis only to the points of contact of your measuring equipment on the table. You pays your money and takes your choice.

Phil:)

Boostinjdm
04-20-2015, 06:37 PM
You guys are analyzing the **** out of this one. If you're worried about the rotor being flat, tram the head, turn the rotor 180 degrees, and recheck. I don't trust an indicators reading once it has left the contact surface. Like when you have to pick it up to clear the slots or oil grooves. Any little ding in the table will give you a false reading also.

dp
04-20-2015, 08:23 PM
TN Pat put his finger on it. Brake rotors may be safely assumed to be warped and out of parallel until proven otherwise.

Brake rotors and Kurt vices have this in common. I don't know anyone who puts a new vice on their machine and uses it untested.

In my case and thanks to you and your scraping class I took, I have a surface plate, bluing, and a quality brayer and tested my rotor for surface contact. Definitely not a potato chip. Then I tested it for parallelism and the error was less than my instruments could repeat. I have better instruments now, but my point is I did all the right things and so have been happy with the results. I don't think anyone has mentioned using a rotor without first qualifying it. I even suggested it more than once in this thread. I think too the general audience here is certainly intelligent enough to know better than to trust anything that hasn't been verified, and if there are any here who don't then a rotor is not going to change much in their lives.

JoeLee
04-20-2015, 09:09 PM
I gave up on the rotor / ring idea a while ago for all the previous mentioned reasons. I just slide a six inch long x 1/2" wide parallel under the dial point as I rotate it. Works for me.

JL................

oldtiffie
04-20-2015, 09:20 PM
I do not use one of these types of indicator for tramming and similar as the long shaft projection could lead to "sticking" of the shaft and resultant "tram" errors.

http://www.machineryhouse.com.au/Q215

I prefer these - with the measuring/testing arm near to horizontal and being "dragged" against the surface to be trammed- and not "pushed":

http://www.machineryhouse.com.au/Dial-Test-Indicators

dp
04-20-2015, 09:42 PM
I used one of those to verify my CO-AX was working as expected (for tramming with a rotor). It was, so I quit screwing with the DTI.

dp
04-20-2015, 09:45 PM
I gave up on the rotor / ring idea a while ago for all the previous mentioned reasons. I just slide a six inch long x 1/2" wide parallel under the dial point as I rotate it. Works for me.

JL................

It is fascinating to read that placing one flat and parallel thing on a flat place creates an error, but a second flat and parallel thing doesn't. Maybe one of the flat things wasn't and shouldn't have been used.

oldtiffie
04-20-2015, 10:50 PM
I used one of those to verify my CO-AX was working as expected (for tramming with a rotor). It was, so I quit screwing with the DTI.

Good post dp (Dennis) - I remember the issues and some up-sets (and bloodied noses?) pertaining to it where for want of a better term the Co-ax came out of it all very well.

It should be pointed out though that the nominal accuracy of the Co-axial indicator decreases as one of the longer arms is fitter in lieu of the standard 2" arm.

For any that are not aware the co-axial indicator looks like this:

http://www.machineryhouse.com.au/Q515

https://www.google.com.au/search?q=co-axial+indicator&rls=com.microsoft:en-AU:IE-SearchBox&tbm=isch&tbo=u&source=univ&sa=X&ei=o7k1Veb1D6awmwWjrIGoBg&ved=0CDAQsAQ&biw=1536&bih=706

https://www.google.com.au/search?q=google&rls=com.microsoft:en-AU:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7IRFC_enAU360&gfe_rd=cr&ei=p7U1VdXWGa_u8wfNsoCwAQ&gws_rd=ssl#rls=com.microsoft:en-AU:IE-SearchBox&q=co-axial+indicator

http://www.blakemanufacturing.com/pages/aboutus.html

http://www.blakemanufacturing.com/pages/coaxvalues.html

The co-ax does not need accurate collets or mill/drill quill taper - it will work well in an ordinary drill chuck as it automatically centres/aligns itself to the mill/drill quill axis/centre.

DP has some excellent pics and YouTube files that are very interesting and I hope he will list/show them here.

dp
04-21-2015, 12:55 AM
Good post dp (Dennis) - I remember the issues and some up-sets (and bloodied noses?) pertaining to it where for want of a better term the Co-ax came out of it all very well.

It should be pointed out though that the nominal accuracy of the Co-axial indicator decreases as one of the longer arms is fitter in lieu of the standard 2" arm.

This isn't the case when the instrument is sweeping a horizontal surface but is true when measuring a pin or hole. This is not intuitively obvious, but the CO-AX is measuring the angle between the spindle and the target when exploring tram, not the vertical offset as a DTI does. The angle is constant regardless of the length of the probe. But - the goal is a differential indication of zero at all angles of rotation - again, it does not matter the length of the probe as zero is zero for all probe lengths.

This typically generates howls of derision, so...
https://www.youtube.com/watch?v=8ikS5aJeHG4

oldtiffie
04-21-2015, 01:45 AM
I do not use one of these types of indicator for tramming and similar as the long shaft projection could lead to "sticking" of the shaft and resultant "tram" errors.

http://www.machineryhouse.com.au/Q215

I prefer these - with the measuring/testing arm near to horizontal and being "dragged" against the surface to be trammed- and not "pushed":

http://www.machineryhouse.com.au/Dial-Test-Indicators

Example - but "drag" the pointer (rotate left) in this example as it will cross slots grooves etc. without damage whereas turning right will/may cause the pointer to strike the edge of the groove/slot too hard and damage the pointer (or the entire indicator):

http://i200.photobucket.com/albums/aa294/oldtiffie/measuring/Dialindicator8.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/measuring/Co-axialindicator1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/measuring/Dialindicator4.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/measuring/Dialindicator3.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/measuring/Co-axialindicator7.jpg