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DRO----How accurate

Dro Quil/Spindle Mill

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Chris Mate22/03/2022 20:29:50
151 forum posts
32 photos

This is just a friendly question looking for opinions/thoughts around this, after thinking a bit different about the subject: I think I May have identified an accuracy standoff........

-----Ills of accuracy locking the quil on a DRill/Mill.......

With a Lathe the subject spins , the tool stand still. The ways are horizontal.

With a Mill the tool spins, the object stands still compared normally.
With a Mill the X/Y ways are horizontal, while the Z1/Z2 are vertical with some weight involved with Mill/Drills.

DRO: Now if I fit a DRO to ether a Mill or Lathe way which is horizontal or vertical it will probably give an accurate representation of the position it represents and can ignore backlash.

Now when it comes to connect a DRO to represent a mill's spindle height, is impossible seeing the spindle is spinning. The nearest you will get to the spindle is the Quil. Now the further you are mounted from the centre of the spindle the more an ignorable FLT in spindle from centre perspective will be amplified as you go further away with your mount, and this may go up or down measuring with a dial indicator or DRO fitted to quil, which can confuse you.

Now today I went futher with my mill experimentation with dial indicator and and trying to read the spindle movement.

So I fit an extra dial test indicator from below(Vice) measuring at the center of the spindle, not the quil. Now when operating the mill, this is of course not possible to do, because theres going to be a cutter in operation at that position.

--SO I found that if I adjust the fine adjustment handwheel, If I adjust it down say 0.01mm and lock the quil, it everytime stay there, if theres a FLT, I cannot see it with this dial test indicator.
--For the same action, my other dial indicator mounted, I would say at least much more sturdy that any flex/arm type of magnetic dial indicator holder.
Now think about this, while I know know the fine adjustment does not influence the spindle movemet(Just do not mess with it and then unlock the quil, it will move the spindle)....This indicator may or may not stay in the same place if I just unlock/lock the quil....

So I want you to think about this situation....
1-I cannot trust thr fine adjustment indicator by factory, but I can trust as log as I move it foreward only(Cut deeper and deeper), I cannot trust my dial indicator linked up to the quil.
2-So I have a standoff here....

So from all this in concideration, the fact you cannot mount anything to measure to the spindle directly, do you think a DRO representing the spindle centre itself can ever be considred accurate to eliminate the backlash effect, so you can mill the depth you think you are milling without quessing to the end between cutter and part-?

bernard towers22/03/2022 21:04:49
691 forum posts
141 photos

Sorry Chris Mate what is this all about?

Robert Atkinson 222/03/2022 21:40:41
1246 forum posts
20 photos

You can mount something to measure the spindle. A Linear Variable Differential Transformer (LVDT) connected to the drawbar. The core of an LVDT can rotate without affecting the accuracy. Not very practical for everday use and it needs a lot of head room above the mill.
A no-rotable sensor could be connected to the drawbar via a bearing. A pull string sensor is one option. For small movements a DTI would do.

Robert G8RPI

Nigel Graham 222/03/2022 21:41:24
2287 forum posts
33 photos

It may help if you tell us the machine make and model, or supply a photo, as there are two basic layouts of small milling-machine; and to show how your DRO is fitted.

- One keeps the table at fixed height, on the machine's plinth, and raises / lowers the head.

- The other is more like the industrial pattern with the table on a "knee" that can be raised and lowered.

Both have a quill as well, but that is normally for drilling, not high-accuracy depth-milling.


The DRO's vertical axis may measure the movement of the quill, but usually is for the knee or the head..

Not the spindle.

This does depend on machine type and how the DRO is installed, of course. For example, my mill is a Myford VMC and I have fitted the Z-reader to measure the knee travel.

Backlash is less of a problem with the Z-axis because the moving unit's weight tends to keep the same thread-flank in permanent engagement, but in any case a normal DRO's long and cross-travel detectors measure the table not the lead-screw.


Other than that I am really rather puzzled what you are asking, but To Mill to Depth accurately, using a DRO or handwheel dial:

- If possible work with the quill fully raised, for maximum rigidity.

- Use the head or knee control to approach the cutter tip to just touch the work. You can use a witness cut but a safer way, more likely to err on the side of shallow rather than over-depth, is to "glue" a slip of thin paper to the surface with oil, and bring the cutter down to just remove that.

- Set the DRO's 'Z' reading or the elevating hand-wheel dial to 0.

Now cut down from that 0 setting.

Mike Poole22/03/2022 22:41:17
3380 forum posts
77 photos

I am not quite clear as to what the issue is that you are describing but a DRO should be attached so it measures the actual quill movement and unless their is a serious problem with the spindle bearings it should represent the cutter position. If the mill is a knee type mill then it is possible to have a scale measuring the knee movement and a scale measuring the quill movement, a summing module can be used to give the true position of the cutter in Z whether you move the quill or the knee. The same method applies even if the table is fixed and Z movement is effected by moving the head or the quill. I think most people will lock the quill for normal milling operations for maximising rigidity. For drilling it can be simpler to use the quill or boring work with the head at an angle will mean using the quill.


Hopper22/03/2022 22:52:08
6690 forum posts
347 photos

Easy peasy. Hold a ball bearing mounted on an arbor in the dpindle. The inner race will rotate eith the spindle. The outer race can be held still by the pressure from a dial indicator plunger which can measure running spindle movement. Compare that with your DRO and you can see any error between the two and make allowances.

But worrying about such precision on a milling machine is a waste of time. They are not a super precision machine. If you can get within a thou or two thats all you need.

JasonB23/03/2022 07:02:56
23058 forum posts
2769 photos
1 articles

I just leave my quill locked so that takes any movement due to locking out of the equasion

There is also the argument that on a lathe the DRO measures the carriage and cross slide position, put on a heavy cut and there will be some deflection so you may not take off as much as the DRO shows. People don't seem to have a problem with that either.

SillyOldDuffer23/03/2022 09:56:20
8898 forum posts
1998 photos

Apologies if I've misunderstood what Chris is saying.

I come at this another way. I don't bother with the absolute position of the table (X,Y) or of the spindle (Z). Instead, I measure relative to the cutting edges of the tool.


  • I plonk a vice on the table without worrying about it's exact position, apart from making sure the jaws are aligned parallel with table. This is done with a DTI held in the spindle.
  • I position the job in the vice, again not worrying about its exact position. The vice ensures alignment, and height is fixed by parallels or by vice features.
  • A fixed point on the job is selected as a datum, from which the DRO is set to 0,0,0. Corners are convenient when the job is a flat plate:
    • With an edge-finder in the spindle, the table is positioned to move the corner under the vertical axis and the DRO set to X=0 and Y=0. Thereafter all X,Y movements measured by the DRO take place relative to this reference. (Until it's convenient to work from a different datum on a complicated workpiece.)
    • Z (Spindle) is different because changing the cutter on a manual machine requires the Z DRO to be reset again from a reference datum. I usually set Z=0 by tacking a piece of hand-rolled cigarette paper to the job at the datum with a dab of light oil, spinning the cutter slowly, and gently lowering the head/quill until it just whips the paper off. As cigarette paper is less than 0.02mm / 1thou thick, this is usually 'good-enough' to set the DRO Z=0. All Z movements with this particular cutter are then done relative to this DRO setting.

The process uses the DRO to move cutters relative to fixed references on the job. Doesn't matter what the quill and head do provided the operator knows where the cutter is relative to a datum, as understood by the DRO. If the cutter or table moves because of backlash, the DRO is still correct and the operator can trust it to reposition the job under the cutter.

The DRO can also be reset on a reference datum at any time to make sure accuracy is maintained: errors can accumulate! On a simple job, the DRO might be set once but anything remotely complicated will have several datums and much shifting between them.

In short, always locate the job under the spindle relative to a fixed point on the job, as touched by the tool, resetting X,Y and Z to new zeros as necessary.


Circlip23/03/2022 10:08:00
1531 forum posts

Gerra Mic and a set of clapperlipers, the battery never runs out.

Regards Ian.

Baz23/03/2022 11:04:26
756 forum posts
2 photos

Blimey I’m still trying to work out what the question is, but DRO——how accurate? I can answer that, good enough for industry to use for the past 40 years so plenty good enough for chasing microns in your garden shed.

Neil Lickfold23/03/2022 18:56:53
892 forum posts
195 photos

Most DRO mounts are not very rigid and are often just piece of ali plate screwed together and then that is to transfer the movement of the axis it is connected with. The very short plate to the machine is never an issue, but on some installs there is 3 or 4 pieces to connect. Once the axis is moving it will show where it went to. But just moving back and forwards a minimum amount , like +-0.1 on an indicator and then watch the DRO read out. Some installs can have 0.03 mm difference. Cast iron is a better choice because of its higher rigidity. Other option is to make the connection from a single piece of steel or cast iron.

Chris Mate23/03/2022 21:09:49
151 forum posts
32 photos

This is a ZAY705 ZX-45 Mill DRill setup...

Sorry for confusion, I am trying to illustrate that trying to tap the spindle up/down movement(Z2) is much different than the usual others like: I am trying to understand the differences trying to tap the movement.

Lathe:The spinning part don't slide but spin.(Chuck/Collet etc)

1-The apron movement along the bed(Horizontal) Not spinning.
2-Crosslide movent(Horizontal) Not spinning

Mill:The spinng part(Spindle can slide with help of the Quil)

1-The Quil/Spindle=Z2 Can move Vertical up/down.
-------Spindle spinning. (Slide with Quil)...You cannot tap the spindle itself)
-------Quil Not spinning. (Not spinning but can slide)-Vertically...You can tap the quil, with some consequences other than with the other horizontal types.

2-The Headstock=Z1 axis(Column) Not spinning.
3-The Bed/table X-Axis Slide horizontally.Not spinning.
3-The bed Table Y-Axis Slide horizontally. Not spinning.

I see a big difference trying to tap the movement from a Mill Spindle compared to tapping the movements of those sliding horizontally as well as the Mill/Drill headstock sliding vertically.

Just curious if you see my point(s)..........I never thought about this like this ever before untill recently.

Hopper23/03/2022 21:34:44
6690 forum posts
347 photos

So the spinning spindle will move the same amount as the non spinning quill it is mounted in. Plus or minus the negligible clearance in the ball bearings holding the spindle into the quill. One thou or less. Not enough to be of concern. Or if you have taper roller bearings that clearance will be zero. So how much the easily measured quill moves, the spinning spindle also moves.

Edited By Hopper on 23/03/2022 21:38:32

Pete Rimmer23/03/2022 23:43:15
1257 forum posts
69 photos

Put the DRO on the quill like the rest of the world and read the quill movement. If the spindle moves any differently in the vertical direction to the quill at all then the problem isn't with the DRO placement it's with the spindle bearings.

Chris Mate24/03/2022 03:17:06
151 forum posts
32 photos

Maybe how I am trying to say something is confusing. I used 2x indicators at the same time.

1-I have proved already with a another Dial Test Indicator in the vice clamed upwards to the spindle pressing on the spindle, that if I Lock-The-Quil, there is zero change in vertical position that I can see at .01mm type of Dial Test Indicator...This reading is constant at zero, does not matter how I move the quil up or down with the Fine-Adjustment. ...So it seems the machine can cut accurate from the machine's spindle centrepoint perspective.

2-However at the same time as I move the quil up or down, my fitted Dial Indicator does not always agree with the Dial-Test-Indicator, this one may or may not show up to .5mm difference. Now this can maybe only due to the quil moving sideways, and this Dial Indicator(Also .01mm) picking that up because its not pointed to the spindle centre.

3-My argument is if I fit a DRO to quil it would be similar to how I fitted the Dial-Indicator and it may be inaccurate, in the sense it can mislead me trusting it------?

Maybe a long time german machinist visit me next week, I will show him exactly what I see(Had discussed it with him on telephone, he did say he never use his quil adjustment), and hear his opinion on this. He has a knee mill.

Hopper24/03/2022 04:08:50
6690 forum posts
347 photos

No idea what you are talking about now. Pictures needed to explain your problem. I am totally unclear on where you have the two dial indicators mounted and where they are measuring.

Are you having problems with your mill not producing accurate work? If not, then you are probably overthinking the problem.

Best to have your machinist friend look at it in person. It seems whatever your problem is, it is not commonly known about among the mill owners on here or one would have jumped in with a solution by now.

Pete Rimmer24/03/2022 06:52:27
1257 forum posts
69 photos

Now it sounds like the quill is loose in the head. Chris, post a photo of how you have your dial gauges arranged.

Chris Mate24/03/2022 07:45:29
151 forum posts
32 photos

Here is an illustration: This is about using the Fine Adjustment for quil/spindle.

I can get the "Red" indicator close by repeatedly locking unlocking the quil, but if I unlock it, the Red indicator sags away from locked reading...If I actually mill this is worse.

I am pretty sure one can trust the spindle behaviour when quil is locked and you mill, but that is not where I can mount a DRO pick up of course.


Edited By Chris Mate on 24/03/2022 07:49:48

Edited By Chris Mate on 24/03/2022 07:50:27

Nicholas Farr24/03/2022 08:06:40
3421 forum posts
1592 photos

Hi Chris, all I can think of is there is too much clearance in your spindle bearings, the quill and spindle should move as one, but perhaps you are chasing microns.

Regards Nick.

Nicholas Wheeler 124/03/2022 09:12:10
958 forum posts
88 photos

This all sounds like an excuse for not getting any work done to me.

The only time I've put a DTI near either of my lathes is to dial a part into the 4 jaw chuck.

What is the OP intending to make that justifies this sort of inspection?

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