Lapping Machine Tool Spindles

[Cal Haines]

Over in the Heavy Iron forum we are having a discussion about correcting the spindle taper on a Van Norman milling machine. My suggestion was to lap the taper using one or more piloted laps, made on a lathe by setting the compound to match the taper of a known good tool (or tools). I am NOT suggesting using an actual tool to lap the taper.

The taper in question (VN ‘C’) has an included angle of 30°15’. It’s about 3/8” long and has a keyway slot the full length and two notches for drive dogs which are about 3/16” long (see attached photo of a generic 'C' spindle). The spindle is ground oversized by maybe 0.030” for about 1-1/4” behind the taper before dropping down to a smaller diameter that registers the tool.

My idea is to use several brass laps with a 1-1/4” long pilot, sized to be a close running fit in the oversized section of the spindle. The laps would be used in rotation to clean up and true the spindle taper. The laps would be returned to the lathe for truing as needed and the spindle would be checked against the “master” tool(s) on a frequent basis. The pilots would be kept clean to avoid lapping the oversized section of the spindle bore. Perhaps an O-ring would help keep abrasive and swarf from migrating into the bore?

It has been suggested that it is impossible to clean up a taper this way:

...
Hand scraping a CRITICAL internal taper, is NOT how I would do it....

Lapping is also NOT going to work right... Fine for finish improvement on a straight clean taper.. not one that is off, oval, not concentric, lumpy.......

Now mind you, the only things that I’ve lapped are the IDs of holes that I was trying to bring to a precise size, using commercial expanding brass laps, so I don’t know if what I propose will work, but I don’t see why it wouldn’t. I understand that grinding would be a better approach, but let’s ignore that for now, eh?

In my limited understanding, over time the rotating lap and the spindle will both wear (the harder spindle more so than the lap) until the surfaces conform to one another. The surfaces will be surfaces of rotation; every cross section, taken perpendicular to the common axis of the surfaces, will be circular. The surfaces will not, however, be perfectly conical. High spots on the spindle will be abraded down, but will produce a slightly depressed ring on the lap, which can, in turn create a raised ring on the spindle. Even if the spindle starts out oval in cross section, it will end up with very round cross sections. Of course, the more oval the cross section of the spindle, the more it will wear the lap out of a true cone and into something else, which is why the lap needs to be replaced or trued as necessary.

Concentricity is another matter. Over time both the spindle and the lap will develop a common center which will probably be eccentric to the true bore of the spindle to some degree. That’s where I’m hoping the long pilot will help.

In post #4 of this thread, Forrest Addy discusses lapping tapered marine propeller shafts for a permanent fit and touches on the idea of lapping spindle tapers: http://www.practicalmachinist.com/vb/general/lapping-tapers-189746/#post1206601

Here’s another post where Forrest mentions lapping spindle tapers: http://www.practicalmachinist.com/vb/general/rust-patch-spindle-taper-186514/#post1172289

In both of the above, he suggests using a brake cylinder hone, but I’m not sure I understand exactly what he has in mind or if it would work with a taper as steep as 30 degrees.

I would appreciate it very much if experts like Forrest could comment on the above and correct any misunderstandings that I have.

Cal

 

[Cal Haines]

Is anyone else having trouble with the "Go Advanced" button not working when you try to edit a post to add a photo?

Cal

 

[nateman]

The correct fix for a taper with an inproper shape is ID grinding.

I suppose that in theory you could use a shaped lap, but keeping the lap sturdily in place while you rotate the spindle would be difficult at best. You would have two members running against eachother under massive pressure when they make contact. This would lead to all sorts of problems.

Any decent grinderhand can regrind the taper.

The beauty of grinding is-

1. very light cutting pressure assures that the cutting pressure does not push the spindle away or "over in the bearings"
2. ease of setup. no tapered wheel or lap to dress or make. Simply set headstock to correct angle.
3. consistent and fine surface finish.
4. new taper will run concentric to bearings (might be time for new bearings?) if ground in the bearings.
5. can be ground in or out of machine.

 

[Robert Campbell Jr.]

The correct fix for a taper with an inproper shape is ID grinding.

I suppose that in theory you could use a shaped lap, but keeping the lap sturdily in place while you rotate the spindle would be difficult at best. You would have two members running against eachother under massive pressure when they make contact. This would lead to all sorts of problems.

Perhaps I didn't understand Cal but if it's a taper that he's proposing to lap, couldn't the "massive" pressure be demassified by judicious feed?

To me, Cal making the statement, "I understand that grinding would be a better approach, but let’s ignore that for now, eh?" means Cal understands that grinding is good but questions if it's the "only" way and for me, if lapping a taper under his stated regimen, IE, having a pilot at the far end, effort made to control abrasive migration, periodic checking, truing etc., isn't another, where is lapping an acceptable procedure?

In my experience*, light scoring and burrs clean up much faster than they in turn, can cut through the abrasives embedded in the soft lap, materially affecting it's profile. Periodic checking and truing of the lap, (more of Cal's stated regimen) making it an excellent in-shop procedure.
*#4 Morse taper in lathe tail stock.

Bob

 

[adama]

Lapping and tapers just don't go together. Rotating one part in another is not how lapping works. Like honing, it relys on the stroke of the tool - lap averaging things out. Rotory motion alone does not achieve this.

 

[Walt @ SGS Inc.]

I don't like to use the term never or always or things like that.
However, if you could lap tapered spindles and make them acceptable, I would carry only a tapered lap.
We carry a portable grinder and regrind tapered spindles in place.
If you were to use a lap, and you had any kind of buildup on one side of the taper, your lap would ride on the high spot. At the same time on the opposit side of the taper your lap would be touching only a line from the front to the back of the taper. The result would be wearing down the high spots on one side and wearing a groove in the other side.
None of this action would help to eliminate runout.
I would recommend regrinding the spindle taper in place. This would "restore" the taper to factory specifications regarding contact as well as runout.

Regards Walt Ammerman Jr.
Spindle Grinding Service Inc.
Albion, Michigan
"Have Grinder Will Travel"
517 629 9334

 

[Walt @ SGS Inc.]

Lapping Spindle Tapers

Cal,
How do I get to the heavy iron form?
Thanx Walt.

 

[abarnsley]

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[jdj]

If the deviation of the taper were slight enough to LAP, wouldn't it be possible to get it pretty close "back to true" by hitting the high spot with some appropriate grit emery cloth while the spindle is running near top speed? If we are going to try to get by without the professional method (grinding) of actually truing it up?

Jeff

 

[abarnsley]

No..

The lap will just move faster back forth... Still will run out...

Lapping and Taper just DO NOT get along...

Have you read ANY books on Machine tool rebuilding??? I could use a chainsaw to clean up the taper I guess, will get about the same results as anything other, than grinding or machining taper properly..

No way to generate an internal taper concentric with a spindle with a lap.

Once spindle taper is off, it must be machined/ground true to spindle axis .. taper lapping CAN'T do that..

Grinding can (roughly) be done in a home shop... A toolpost grinder (and an old lathe compound) is not that hard to find..

 

[Walt @ SGS Inc.]

Lapping Machine Tool Tapers

Grinding "roughly" Wait a minute.
Rough or presicion depends on the "compound" used.
We bring out "compounds" in about every 6 months for
regrinding and tuning up to stay away from that
"rough grinding".
99 percent of the time, we meet or exceed the specifications
on a new spindle. I could talk about grinding spindles at the
machine tool manufacturing plants but I will use no names.
Respectively, Walt Ammerman Jr.
President
Spindle Grinding Service Inc.

 

[DaveE907]

No quibble with what's been been said about the unsuitability and reasons why of lapping to restore the spindle taper.

However don't ignore the keyway in this example. Imagine attempting to lap a straight bore with a keyway. The lap will immediately go off center as it takes off more material on the side of the bore with the keyway due to unsymetrical support. One could attempt to make a filler of the same hardness and near perfect geometry to alleviate this but that would me more work than simply doing it the easy way in the first place: grinding.

If one doesn't want to grind the taper for whatever reason (it's not difficult with a good toolpost grinder and slide for a spindle of this class particularly with only a 3/8 in. long taper) a good and workable option is to scrape it in to a known good arbor.

 

[stoneaxe]

OK talk to me about grinding an ID. This will be a one time thing- are there any cheap alternatives to a tool post grinder ? How accurate are grinding stones? are they normally dressed on the arbor for concentricity before use?

Would a woodworking router at 20,000 rpm with a mounted stone on a 1/4" or 1/2" shank work? (I do have one around mounted on an angle plate....)

The VN head can be tilted to get the taper parallel with the table, so the grinder could be fed with the saddle, and depth of cut adjusted with the knee.

Although it would be a lot more precise to lay the head horizontal, adjust depth of cut with the saddle, and feed with a compound set at the taper angle (same principle as using the compound set at an angle on the lathe, to get a finer DOC adjustment.

Is this nuts?

 

[DaveE907]

No, it isn't nuts, it can and has been done successfully.

In situations of many years ago where little else was available and the spindle taper had to be improved on turret mills I've done the job similar to your description. Never used a router but I have used toolpost grinders, air and electric die grinders to accomplish it. Mind you, these were not die grinders from Harbor Freight or the like.

Kick the head so the taper is parallel to the table and the axis of the grinder is coplanar with and below the spindle axis (setup is crutial) and run the dressed stone point (if using a die grinder or what the heck, a router) in and out to grind the taper. Use a .0001 indicator to measure and set feed, take light cuts and keep the wheel dressed (always done running in the grinder of choice).

Gage with a good arbor. With careful work a very usable result is possible.