Need advice for grinding out-of-round spindle taper on mill.

[turbostew]

So I have a CNC Bridgeport Boss3 CNC mill I have retrofitted with servos, sweet little machine. The spindle is a quick switch 200 taper. There is about 0.0012 runout of the spindle taper, like it is bent. The smallest diameter of the taper ID is less than 1". My best idea for truing is to use an air die grinder clamped vertically on the table then have an XZ linear move with the same angle as the taper. The taper is like 2" per foot of taper. I have mocked this up and the linear motion seems great, tenths indicator is right on, no problem there. My problem is I have never precision ground anything. I bought a diamond point dresser and my plan was to mount it off the mill frame then have the same linear XZ move precisely make the taper on the grinding wheel then run the wheel up the spindle taper. I only get one shot at this and cant muck it up. Questions I have are:

1. Is this a viable plan?

2. What stone should I use? I want a really nice finish. Grit size? 120 grit seems to be the "fine" verity of the stones out there, seems I want finer than this. All the stones out there seem to be cheap China stuff with no specs? Where do I find good stones for this? What grit, material?

3. Are the die grinder bearings precise enough? Use a new Ingersol Rand die grinder?

4. how do I know when I contact the spindle? Will sparks fly with just a 0.0005 touch? Or do I have to cut a lot of air until I hear it?

5. What is a reasonable diamond dressing depth of cut and feedrate?

Thanks for your time,
Kent

 

[eKretz]

You almost certainly won't get a very nice finish with a die grinder. You're probably going to have a very difficult time getting that to work well. You need a very good die grinder and rigid mounting for that to have any hope of working. The next important consideration is ensuring that the center of the grinding spindle is as closely as possible coincident with the center of the machine spindle. This is doubly important when grinding a tapered/conical surface. Either way, it probably won't be a one-shot deal. You'll likely need to check fit and adjust a bit.

 

[akjeff]

C & M Precision Spindle

World-class Spindle Repair Services and Spindle Rebuilding since 1993

www.cm-spindle.com

Bill is awesome to deal with, and does great work at a fair price. If you send him the spindle in the quill, they'll grind your taper while it's as it would be in your mill. He put in new bearings and reground the R8 taper in my Lagun last year, and the runout barely makes a tenths indicator needle quiver. Turn around time was about 3 days. JMO.

 

[michiganbuck]

Good to first blue in a taper to see if there might be a high bug or build-up that may be selectively round-honed away like scraping in a bed way.

Getting the angle right, a first-time trying to grind a tapering ID may be a real bugger.
(My best idea for truing is to use an air die grinder) Most air die grinders I have seen are not up to that task.
Guess you should find what machine travel is accurate enough/most, the quill, vertical or long travel.

I have never ground a mill spindle ID.

 

[turbostew]

C & M Precision Spindle

World-class Spindle Repair Services and Spindle Rebuilding since 1993

www.cm-spindle.com

Bill is awesome to deal with, and does great work at a fair price. If you send him the spindle in the quill, they'll grind your taper while it's as it would be in your mill. He put in new bearings and reground the R8 taper in my Lagun last year, and the runout barely makes a tenths indicator needle quiver. Turn around time was about 3 days. JMO.

I will look into this.... Thanks All for the info.

 

[turbostew]

Thanks All,

I already did the bluing and it looks real good. Getting the angle is no problem. I indicate the spindle runout then position the spindle to max runout, then run Y axis around to make sure bore is centered about Y, rinse and repeat, then position spindle halfway between min and max (so bend is 90 degrees to indicator) then run the indicator up and town the taper with the XZ linear move and adjust move angle till 0 indicated movement. If I true the stone with the same linear movement and the grinder axis is at least planer with the XZ plane, seems all would be well....

But this is all mute if the die grinder is not up to the task.....

 

[Milling man]

1. Is this a viable plan?

Yes, it is quite.

3. Are the die grinder bearings precise enough? Use a new Ingersol Rand die grinder?

Probably not. Look at the spindles of internal grinding machines - and not just any internal grinding machines, but those on which the spindle cones are ground. The grinding wheel spindle on such a machine will rotate either in hydrodynamic (hydrostatic, as an option) bearings, or in duplex ball bearings of ABEC9 (7) accuracy class. It probably doesn't just happen

4. how do I know when I contact the spindle? Will sparks fly with just a 0.0005 touch? Or do I have to cut a lot of air until I hear it?

.0005 is a hell of a lot by grinder standards. On finishing passes, after wheel dressing, I often work with a depth of around 0.00007 - and I don't have much trouble detecting touch. The main thing is to do everything carefully, slowly, and not be lazy to make a full circle movement through the entire detail after each increase in depth.

5. What is a reasonable diamond dressing depth of cut and feedrate?

This will greatly depend on what kind of grinding wheel you are using and what tool will be used for dressing - with one diamond or with many small diamonds, if the latter - what size of diamonds.

I recently restored the cone on a milling machine in a similar way - first I bored its cutter with CBN, then rubbed it with cast-iron lapping.
As eKretz rightly pointed out, it is very important to ensure that the axes of the grinding wheel and the spindle that you are grinding coincide.

 

[implmex]

Hi turbostew:
You wrote:
"Getting the angle is no problem."

Sadly, that is a most naive statement...you have to get it within a tenth or so and you do not have a starting reference (indicating the trashed spindle bore SOUNDS good, but not to the tolerance you need to work to.

I've never reground a spindle taper myself either, but the pictures I've seen from professional rebuilders all have used a precision slide with a precision grinding head mounted on it and a sine plate to set the angle.

There is a poster on here named Terry Keeley who has posted pictures of a similar commercially made unit he has for ID grinding...I don't recall the brand (Falcon??) but it is a proper grinding spindle on a slide and he uses proper ID grinding wheels on it so far as I know.
Maybe Terry will chime in on this thread and post some pictures of his device.
Something like that is what you need to do a decent job.

In fact, I'll go so far as to say you will do better hard turning it with a carbide bar and a simple program you can adjust to blue in the taper than you will by trashing the spindle with a bodged together hacker grinder.
It'll still be shit compared to a properly ground spindle bore but it'll be a far better class of shit than a crap grinding job

So I'm with those who are unenthusiastic about you trying to do this at home with a quickie lashup.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com

 

[michiganbuck]

likely a Spindle grinder hand will/may bring his own travel device.
1. Using your machine's travel be sure it can travel to the needed length of travel and then return with no adverse movements. 2, be sure your machine spindle bearings are accurate and solid. 3 devise a way to be sure the grinding wheel is at the center of the to-be-ground spindle bore as because above pr below center will change the angle. 4. set a stop so you won't crash into the ID end shoulder of anything. 5. devise a way to tweak your angle by .ooo2 or so accurately. 6. perhaps set a iine plate to the desired angle and strike the sine with your indicator so to have very little tweeking..

 

[michiganbuck]

Q: (How do I know when I contact the spindle?)
Perhaps hand-feel the parked spindle at the beginning and the far of the taper, so to know the dial number where the dressed wheel is expected to hit. Belted spindles are great for doing that because one can take off the belt and then often get a one or two-tenths feel when the wheel touches/rubs the part.

* Good to have a setup that can pull the grinder out far enough the try a taper for a blue-in.
*Best to seriously check/inspect the taper for nicks/bugs and build-up that make it seem in error before grinding.

 

[spaeth]

Turbostew,
Twas years ago, (like 25) we ground a #40 taper in an Excello CNC mill. My task was to mount a Dumore tool post grinder parallel to the spindle. The taper in this one was beat up pretty bad as the machine used a power draw bar and was used in a production setting. After securing and indicating the grinder we ground the face of the spindle using a cup wheel. Then CNC guru took over and put a diamond on the quill and feeding XZ dressed a wheel to the proper angle. From there locate and grind while the spindle was running. We used a new 40 taper holder as a gage. Probably not within .0001, but it's a mill and after that repair it made profit. My 2 cents.
spaeth

 

[Milling man]

When I was boring the spindle taper on my machine, I first checked the accuracy of the movement of the machine using an indicator and a 7:24 reference taper.
I didn’t understand what kind of cone the author has on the spindle, but you can always use two indicators, school geometry and find out how accurately he moves to some angle.

 

[CarbideBob]

Somewhere here a bit back was a post with pics about using a CBN tool in a boring bar to "fix" a spindle.
It was from a guy who did spindle grinding on your machine for a living and well respected in the Midwest.
I believe he passed on and maybe his son now?
It is not just tool grinder on a slide with a super sine plate. You have to nail centerline just right to not make a hourglass.
With all the right equipment it is easy. With hacked up stuff the Devil may get you.
Me as I am DYI I thought this can not be that hard...... .. Fine, we will chrome it up and try again............
Bob

 

[michiganbuck]

Having a lathe and skills I would likely make a taper gauge between centers that would fit the taper and have an extended straight shaft about 3" or 4" long that would be near the minor diameter of the taper. with that, one could find the center in the bore/taper to be ground and set a sine bar on the shaft to make the taper angle near dead on.

Having a quick switch 200 taper tool or tool holder, one might use that for an angle and centering device/gauge..might run an indicator to the grind-side if the taper and check that with a sine bar, be sure the grinding spindle is straight and the at the exact bore center line...and the machine travel is adequate for the job....plus good to have a grind-method that allows checking the taper...and a way to knock it .0002 if that is needed (Two indicators is a good setup for doing that.)

*But first, be sure the existing taper has no bugs or such in the taper with a blue-in to a taper....

 

[michiganbuck]

Kwik Switch/Quick switch taper dimensions/specifications are not that easy to find.