Mill Head Tram...How Close?

[glenj]

I've trammed the head of my mill for the first time and was wondering how close is close enough for most of you?

I used a .0005" indicator bolted to the end of a 1/2" rod that I bent 90deg and put in an R8 collet. I indicated directly off the table surface. The sweep radius of the indicator was just over 6". I checked along the X axis then along the Y (moving the table to reach) and when the head was tight I swept across every surface of the table I could reach. I ended up with less than 0.0015" of indicator travel all the way around.

Is .0015" in 12" considered "square" enough for most operations up to using a 2-1/2" face mill or 3" fly cutter?

[Paul Alciatore]

How good is good enough? That's a question you must answer.

You said you used a 6" sweep radius and mention a 3" fly cutter. A 3" fly cutter has a 1.5" sweep radius which is 1/4 of the radius you used to test with. Simple math or trig gives you a error of 0.0015/4 or about 0.0004". Is that good enough? It depends on the job. If you are making gyros for the space shuttle, I don't think I want to ride that one. If you are making a Christmas tree ornament, it'll be great.

But remember, head tram will also effect the squareness of holes yo drill and of any parts who's sides you mill. These may be more important considerations than the flatness of a surface milled with a fly cutter or face mill. Your error will be about 0.00025" per inch.

Paul A.

[spope14]

Your own personal preference, and your need for accuracy on the project at hand can affect this. I use 4 ground blocks (ground to within .00005) set at table witdth and equal length, and a .0001 indicator for my tramming. I tend to like .0005 or better over this distance, but shoot for perfect as much as possible. Takes time if things are not going your way (we all have our days as we know).

After tightening the head, I check again.

[glenj]

Thanks for the points of reference. I'll see if I can borrow a better indicator and try to get it closer.

When I poured the pad foundation for my shop last spring friends made fun of me for worrying about being out of square by 1/2" on the diagonals of an 18'x28' pad so I guess it's all relative.

[metal mite]

Heck,
You're close enough for most work.
When you run your table to extremes you'll have a different reading anyway.
Depends what you're doing.
Don't lose too much time tripping over knat turds.
mite

[sandman2234]

Just wondering how much that "rod bent 90 degrees" is adding or subtracting from the tram on the table. How did you decide that it was right? It is possible to move the centerline of the spindle over with the rod, which changes the tram...
Just something to keep in mind.
David from jax

[Forrest Addy]

Here's a few more points to consider.

Did you have the knee clamped? The knee when supported by its lead screw can droop away from the column. This causes the front of the table to tils down by an amount permitted by the knee column clearance. This is a common fault in all knee mills and it's severity depends on adjustment and wear.

Tram the table with the knee unclamped and a series of facing cuts taken with a shell mill the face will "shingle" - there will be a faint step perceptable to the fingernail when scratched a cross the surface away from the column.

When work requiring a lot of facing is in the hopper it's not uncommon to tram the head with a little "lead" in it so the exit side of the cutter has a small chearance. This keeps the idle teeth from picking up small chips and scratching the work with them.

When boring holes where accuracy is a major concern you want the hole to be square with the work reference. Thus you tram the reference face before proceding, shimming the work if necessary to bring it in.

While we're at it, clamping deflection can intoduce error. Support the work only on three points and position the clamps directly over them. Tighten the clamps with a couple of indicators on the part placed to detect deflection.

Hollow parts like transmission cases can deflect under clamping forces. Machine and bore a deflected part and what happens to your carefully wrought machined geometery when you release the clamps? The bore axes splay like the stems of a flower arrangement.

Many times I've machined hollow parts with small jacks (merely a bolt of the right length and with the nut flush with the end) placed just under the clamps supporting the case from the inside.

If the knee is sufficiently hour-glassed from wear the knee can tilt from the weight of the table being moved from far left to far right causing work placed flat on the table and faced on top to be thin on the ends.

A good machinist constantly "maps" the machines he works with, keeping the data in some part of his unconscious labeled "bag of tricks". Somehow he gets good work out of the shop dogs but when new guy off the street tries he produces nothing but junk. "I checked everything what's wrong?" he moans to the old hand. "Oh!" The data is accessed and the old hand rummages in the litter on his roll-away top. "You gotta stick these two shims under the dovetails on the bottom of the knee and you gotta set the knee clamp about 10 o'clock." There's a brief grovel under the knee with a flash light. The old hand places on the machine's table a 6" Starret 98 he put a 10 arc second vial in years ago and lifts each end of the table in turn. "See? Goes back to center bubble ever' time."

The new guy shakes his head in disbelief knowing full well he'd have to go back and forth extracting each little trick from the old hand like a firemen doomed in hell to carry water in a teaspoon from drippy faucet to violent conflagration. He goes back to work wishing he'd taken up real estate or selling false teeth door to door.

[This message has been edited by Forrest Addy (edited 04-28-2004).]

[wierdscience]

I would also add the following,I always plane the spindle in on the surface I intend to use as reference.If you are using the table then tram off it,but if you are using the vise tram off the vise jaw ways.Reason being the vises and other fixtures you use between the work piece and the table will be slightly out of parrallel,if the head is trammed in and the vise error is present,then the tramming will not be as good as it can be and effect results.

[glenj]

I have a question about tramming to the vice and/or to the table surface.

If the vice were at 10deg to the table would you still suggest tramming to the vice and not the table? If not, then why would you suggest doing so if it were at 0.1deg?

If the vice were at an angle and the head was at the same angle all the drilled holes in a block in the vice would be perpendicular to the surface you milled as would the end faces to the top surface of the work. The problem is that the top surface would not be flat because the bottom face of the milling cutter would not be perpendicular to the travel of the X or Y axis. Think of the case of a 3" face mill which would leave a scalloped surface.

I can see when it would be best to tram to the table and other situations were it would be best to tram to the vice. Perhaps you'd want to true the vice up to the table making it's bottom parallel with the table and tram to the table?

[wierdscience]

Yes you could tram to the table,then indicate the vise to see if there is an error then shim it to correct the error,but if your working in the vise 90% of the time then tram the vise,it makes life easier.

Now if your milling and want to be a purist,then screw the vise and do everything on the table.

To further clarify the logic,think of it this way,most of the time the vise is used to hold pieces of prepared stock(cold rolled flats and rounds,extruded aluminum etc)now lets say you want to drill and ream a hole in said stock at exactly 90*,how can you do this acurately if the vise is off parallel and the head is trammed to the table?

[This message has been edited by wierdscience (edited 04-29-2004).]