Use of Mercer dti

Could someone please give me advice on using a dial test indicator (or what I think is a dti !)?

I bought my Mercer instrument on e-bay. It reads in 0001’’ increments. When it’s at rest in its case it has the needle poised at about 13 gradations anti-clockwise of zero, that, I assume, is to allow pre-load back to nought. In the box is a flat rectangular bar about two-and-a-half-inches long with an attachment at one end to take the stalk of the instrument. I assume (again) this can be used somehow to increase the ‘sweep’ but I don’t know where you would hold the bar.

I’m trying to use my instrument to sweep a 23mm hole in the centre of a lathe chuck adaptor plate I’m making. I have the work held in a self-centring chuck on a rotary table on a milling machine and I want to centre the quill over the workpiece hole.

I begin by putting a 22 mm drill bit in a Jacob’s chuck (nearest I have to 23) and with everything unlocked on the mill (head – it’s a Warco Economy – X/Y axis) trying to centre the drill in the hole. Thinking is, this will roughly position the quill.

I now put the Mercer in the Jacob’s chuck (I know I should use a collet but at the moment I don’t have one that will fit its stalk. I use Posilock collets in a Vertex collet chuck for endmills etc.) I push the Mercer’s stylus into contact with the interior of the hole with the tip of a scriber (nothing clever intended, just seems convenient way to touch it). Then, mill belts slack, Mercer zeroed, I rotate the quill by hand. But the stylus loses contact with the hole by a huge amount (i.e. so much I can clearly see it with the naked eye) I just don’t understand why.

If I do manage continuous contact of the stylus I have wild swings of the needle (easily more than one revolution of the dial). I try to proceed in the same way as I use my dial gauge (‘clock&rsquo. I half the value by manipulating the axis (X or Y) from the highest reading, zero again and proceed until I’m getting a zero to modest (5-10 Mercer gradations) discrepancy on each rotation, but that’s almost always because the stylus is losing contact!

How do you correct the discrepancy on a dti – even if you are doing it right !?

Do you measure in one plane only? Say the stylus is pointing along the X axis of the mill table, and showing an eccentricity, is that when you half the value, zero, and take a reading 90 degrees on, when it’s lying on the Y axis and so on until (hopefully) you have zero all round (with, of course, continuous stylus contact !)

Wow! This is a really long question for which I profusely apologise. If someone says s-d o-f you fool, I won’t be the least bit offended! If someone says, ‘there’s an easier way to do this’, and I suspect there is, I’ll accept that unreservedly. It’s just they make using a dti look so easy on the internet and I do feel a fool! Very many thanks in advance.

Martyn N

Difficult to know without pics. However the stylus is not guaranteed to be on the centreline of the body. That can introduce errors.

The dial can be zeroed at any time by rotating the bezel and lens.

You may be looking at a loose quill in the drill shaft if I understand correctly. Mine rattles about like a milk bottle in a crate.

Most of the film on utube is set up so don't take too much notice, they may have had a couple of runs at it.

Edited By Jesse Hancock 1 on 14/05/2015 16:23:05

If you have someone near you, to show you. your question will take seconds to answer and will seem obvious once demonstrated. It will take less time to answer, than for the kettle to boil to make tea for your newly acquired best friend.

Your problem is in your paragraph 3. Have you centred your rotary table, then centred your self centering chuck to the rotary table by your dti on the same settings. And I am assuming the self centering chuck has minimum run out.

Oh for a cup of tea. Its not difficult and it doesn't need to be difficult.

Ask and ask again and you will get help, best of luck

are you anywhere near Chelmsford this week end

I am assuming that you have a mercer lever type indicator,so that it will go down the bore,and not a plunger type indicator with a 90 degree attachment. For a start it is not good practice to use a .0001 ie a tenth thou clock as they are known,on the rough surface of a drilled hole,a .001 inch dial indicator is more usual practice. .0001 d/gauges are too sensitive. you are going in the right direction but just need more practice. Now why are you trying to centre the drilled hole? its the centre vertical axis of the rotary table that needs to be set true. Then hold the work in the chuck and it does not matter if the drilled hole is a bit off centre,bore the hole to size ie 22mm . The piece of of flat metal in the kit is used to mount the dial indicator in the lathe .the metal bar is held in the toolpost,and the dial indicator mounted on the end fitting to "clock up" work in the lathe chuck. .0001 indicators were more often used for inspection rather than setting up work unless you were working to very tight limits.

I think you are underestimating the sensitivity of your instrument.

If the stylus is losing contact with the edge of the hole, then the hole is far from centred. As it is a 0.0001" sensitivity the hole only needs to be around 1/200 of an inch off centre for the needle to be whirling around by one full rotation.

Your DTI can show if your hole is two or three wavelengths of red light off centre!

If you need the backplate to be accurate to a more realistic 0.0005", then that allows you to have the needle moving by 10 divisions.

Neil

Your 22mm drill won't be dead straight. A drill chuck isn't that true either.

How did you get self Centering chuck true tothe centre of rotation of the rotary table? You either have to do that first and then move on to aligning the chuck to the mill , or start by aligning the bare rotary table to the mill and then move on to setting up the chuck. You can't do both together.

The centring of the hole on the rotary table is irrelevant as he's just centring the hole under the spindle and not rotating the work.

A 0.001" or 0.0005"sensitivity DTI is better suited to this sort of task.

By the sound of it, you have what is usually known as a "Finger Clock" (Often referred to as a "Verdict", one of the other main manufacturers of this type of device).

This "tenths" type of clock has a limited range of movement. It is intended to be used to measure small differences. As you correctly say, there should be a small preload. Most finger clocks have a range of about 0.020" or 0.040", so the preload should be about 0.005 - 0.010", because these are the sort of eccentricities or height differences that you should be expecting to measure.

In your case the stylus is coming clear of the work, because the eccentricity exceeds the range of movement of the clock.

Finger clocks can be obtained which measure in 0.001" increments.

Firstly, presumably, you wish to centralise the work on the Rotary table?

1)Set the clock so that it cannot move relative to the Rotary table. (A Magnetic base will make this much easier.)

2)Move the clock to give a small preload and adjust the Rim to bring the Zero to the Needle.

3)Rotate the Rotary Table until a maximun reading is obtained. If the stylus comes clear of the work, move the workpiece on the Rotary table by HALF the apparent error.

4)Reset and reZero.

5)Rotate the Rotary Table and remeasure the eccentricity.

6)At the point of maximum reading, move the workpiece by HALF the eccentricity,

(You may well need to do this in both planes).

7)Repeat these operations until the workpiece is centred on the Rotary Table to within an acceptable run out.

8) Ensure that the workpiece is firmly secured to the Rotary Table, and recheck that the workpiece is acceptably central. (Your decision as to whether within 0.0001" or 0.001" unless the drawing specifies the tolerance)

Once the work is set centrally on the Rotary table, you can begin to set the Rotary Table under the Spindle of the Mill.

1) Fix the clock to the head of the Mill and proceed as below.

You need to move the work piece, (Held on a Rotary Table on the Table of the Milling Machine) by moving the Table to reduce the amount by which the stylus fails to contact the work.

Again, this will need to be done in BOTH planes (X axis - along the table, Y axis to and from the column)

2)Reset the DTI to a small preload, and and adjust the outer rim to read Zero .

3)Rotate the Rotary Table, until a maximum reading is obtained.

4) Move the Table of the Mill to reduce the reading to HALF of the original value.

5) Repeat steps 1 to 3 until the DTI shows an eccentricity ("run out" which is acceptable to you.

6) Lock the Table on BOTH axes.

You are now ready to bore the hole to the finished size. Hopefully, the eccentricity of the already drilled hole is less than the depth of cut need to bring to finished size. If you are using a Boring Head, my advice is to take small cuts, and to take "spring cuts", (without altering anything), regularly, checking size after every cut. You may be surprised how much material is removed on a spring cut.

If you are reaming, check the run out of the reamer, when fitted to the Spindle.

A spindle which runs eccentric (not unknown) or a bent reamer will result in an oversize hole.

(Which is why, if possible, you bore or ream first, and then turn the male component to give the required fit.)

Sorry to go on at such length, but taking short cuts (PUN not intended) is likely to help fill the scrap bin rather than deliver a part to the required size and surface finish.

Hope that all this is of some help.

Remember the old adage "Measure twice, cut once" Wish that I did that regularly!

Howard

Many, many thanks to everyone for the comments and extensive advice.I shall be studying this closely and then implementing. Thank you again.

Martyn

I think you meant to put 'rotate the spindle' in the second set of instructions.

Neil

Have seen this problem many a time and fully understand.

On my cheap beefy driller If I use sub 3/16" drills it drills off centre towards the column. 1/4" roughly straight line minimal deflection, above 1/4" drills it deflects away from column. Can see table move with larger drills.

Current Chester Super Lux always drills towards the upright column, in rotary its common to see 1.5 to 2mm deflection. So if put a DTI in looking in realms of moving off centre 1/16"+ to centre up. Also additional problem when locking the quill, theres 0.3mm deflection towards column, that's close on 12 thou.

Next time drilling the hole have a visual, bet you see the drill wanting to veer off axis before cutting and when it does near on impossible to stop.

Jon, Seems to me that you are not centre drilling your hole location before drilling with a standard jobber drill? You need to use a centre drill first which is very stiff and has a smaller point to start then an increase in size, you only need to drill a shallow hole with this then follow up with the size you want.

Locking the quill, there is almost always some movement except on very high precision machines e.g. jig borers, however in my opinion .012" is very excessive -- how old is your machine? Maybe a call to the importers is in order?

John

I'm repeating my sincere thank you to everybody. Some of this I'm doing, some is a revelation. It's all extremely valuable and I'm studying it and taking it on board. Thank you.

Martyn

Drill I bought in 1991 DP16-12, had about 3hrs use total absolute junk. The table flexes and beefy column which is larger than current offerings, think 83mm+ dia rings a bell. Cant even give it away but does have an English 3/4hp Brooke Compton motor. Just tried stationary not spinning, pull handle down small drills deflect in to column as if its drilling at an angle and gliding off. Drills can deflect left, right, back and forward before biting and may need a centre drill or punch mark.

Mills had it 5 years ago bought mid 2004, progressively getting worse. Just a bad design, rack in quill drops in a cutout at rear of quill bore, subsequently its oval.

Drills fine ultra rare i use any centre drill and I do drill a lot of holes on tapers and contours without veering off about 57000 holes so far just for one high class product on this machine excluding other stuff. **LINK** RF25 prior did around 25000 holes on tapers and was still good after 4 years. Quills worn it rattles and creates chaff marks boring since cant use spindle lock. Can visually see quill deflect towards column with lock on, others will deflect sideways since other machines have spindle lock side on. Even the chrome worn off the handles years ago down to the copper plating, its had some use.
Its earnt its keep threefold just calling for a 942 or Bridgy.

Hole saws circa 7/8" to 1 1/2" have a tendency to torque off centre 1/16" towards column, but true up with boring head.