milldrill tilting fix

I have been tramming the head of my Tom Senior mill for years just using a 10mm diameter “L” shaped rod, short leg in the collet chuck, & long enough to reach close to the table ends when it is set centrally under the head. I then set the head roughly vertical using the engraved scale provided on the head of the machine and just nip up the fixing bolts enough to hold the head in position. I them set the DTI at the end of the rod to just touch the table at one end by about 3 thou – i.e. just enough to get a reading then I swing the rod round carefully and check the deflection at the other end of the bed.

Of course it is never right first time so I tap the head lightly with a plastic hammer in the appropriate direction, test again, and repeat the process until identical readings are obtained at both ends of the table. Tighten the head fixings and check again. It is important to maintain the very small deflection on the DTI as this enables it to ride over the slightly chamfered edges of the T slots without damage provided the movement is slow.

A few years ago I made a twin headed tramming device cloned from commercial models using pair of identical DTIs but I found it most confusing to use compared with the simple single DTI method and less accurate as its “measuring base” was much less.

So I agree with the other contributors that twin DTI tramming gauges are a total waste of money.

Eric

As Heraclitus said "you can not put your foot in the same river twice", The double headed tramming device breaks the rule.

It is not a practical resolution of the problem, that is unless your spindle taper is perfectly aligned to the spindle, many are a few tenths out, and there is not a spec of dirt in the taper interface when you insert the tool, and the double headed tramming device is set perfectly.

I guess you could always insert it in exactly the same place to an index point and keep it in a velvet lined case for protection from bumping. but even then I would have to check it every time i used it. however you are still breaking measurement rules. the two dial indicators may not be perfectly matched. They are indicators not measuring tools. they are not designed to be perfectly linear compared to a standard.

Clearly the twin head device is a bad design.

Swinging a single indicator solves all of the above problems.

Cheers
John

I don't think these twin headed tramming devices rely on each indicator being perfectly matched. As I understand it, each indicator is set to zero on the same test block (without moving the block). This then sets the plane of the indicators at exactly 90 degrees to the spindle and eliminates any misalignment of the tool in the holding collet or chuck.

Doug

The problem's not with the twin-dial device itself — it may well be perfectly formed and calibrated. The problem is with the way it's mounted on the machine under test.

As I understand it, it's simply mounted via a drill chuck or collet. But we all know the ills chucks/collets are potentially heir to, notably angular runout. So if your first use of the device shows a deflection, you're left with the question — is the spindle really out-of-square with the table or is the deflection the product of the chuck/collet's angular runout? And the runout won't be constant but variable dependent on how you happened to mount the chuck/collet for that particular test session.

So, you might say, make sure you use a chuck/collet of known angular accuracy when using the two-dial device. That's all very well but, already, you're introducing pre-conditions and uncertainties into a process that's supposed to be a 'simplified' form of tramming.

The single-dial method has none of this uncertainty. The dial's pointer revolves around the spindle's centre of rotation and the plane it generates in space is truly perpendicular to that centre. So, if by adjusting the mill's head with the pointer in contact with the mill's table you manage matters so that the pointer's deflection is the same at 3 or more reasonably well-spaced points in its rotation, you've established that the mill table is likewise perpendicular to the spindle's centre of rotation in both axes. You may well use a chuck or collet to hold the 'L' arm but the accuracy/deficiencies of those devices won't affect the accuracy of the test at all.

Joe

It is interesting how geometry can work with us or against us.

Assuming a 100mm (8" radius circle for the indicator, a one-degree error in the spindle alignment gives us a deflection of 3.5mm (1/8" As we can easily measure to 1/100 of this, we cal correct the spindle angle very accurately. with 0.02mm difference between the sides, the error is 0.005 of a degree.

Obviously if the error over the 200mm diameter is 0.02mm, then across a 10mm cutter it is 1/200 of that - only .0001mm - tiny and less than the flex in a very rigid machine.

As the 'dual tramming machines' have measuring points about 100mm from the spindle nose, they inmtoduce an error at the measurement point equivalent to the runout at 100mm from the holder.

If held in a collet chuck with an accuracy in the region of 0.02mm runout at 100mm, they wil roughly triple the error in the system , about 0.0003mm across the 10mm cutter.

If you hold the trammer in a drill chuck with could have 0.2mm runout at 100mm the runout across our 10mm cutter is still only 0.0011mm.

Perhaps the real message here is that we may be congratulating oursleves on getting our mill heads perfectly aligned, when what we should be doing is giving more attention to good rigid setups, accurate toolholders and sharp tools.

...and here endeth the sermon!

Neil

And there speaks a wise man!!!

Brian

I stand by my previous post, the accuracy of the collet or chuck is irrelevant if the tool is set up the way I mentioned.

Doug

I think Doug is right, but only if both DTIs are set up on the same block in the same place and teh arrangement is trued in this way every time it is used..

Neil