Spindle runout issue

Leadfootin · Jun 23, 2019

J head Bridgeport, and it has .0002" runout at collet but .001-.002" runout 1" from collet. This is quite inconsistent changing every time the collet is removed and replaced. I tried a friends draw bar and the problem was reduced by about half. Could this be the fault of an aftermarket drawbar or something else entirely? Does anyone have a drawing or dimensions for an OEM drawbar?

michiganbuck · Jun 23, 2019

If it changes each time then it may not be being pulled full tight and the draw bar may be too long for the length on the spindle. Taper or R-8 inside may be whacked out of precision or draw bar too long.
Guess you might make it tight and then light tap-tap on the end of a held round stock of that collet with a brass hammer. Might just add a washer to the draw bar. but better know the number of threads in use.

*Better yet start the draw bar out of the machine on the bench..turn the draw bar into the collet with counting the full turns. Perhaps you get ten full turns and it bottoms out..
Next put the collet on the machine and add the draw bar. Count the turns with a correct size part in the collet. The turns number should be perhaps nine when it goes tight on the held thing , but not ten when you make tight. (eight, seven and six are fine.)

If ten was the bench case and it only turned perhaps 4 then the draw bar would be too short. not likely the case with you problem. With 4 you should grind the stop shoulder or if having a washer reduce the washer one or two thread pith lengths perhaps .060" .

Yes it may me making tight and you spindle is whacked out ..but then the high and low wobble would always be of the same side, or with putting a marker mark on the machine spindle the out of position part would be on the same side of the spindle with taking it out and putting is back with turning the collet..

Milland · Jun 23, 2019

Your drawbar could have "drunken threads", where the helix of the thread is not axially aligned with the drawbar shaft. If you have a lathe than you can safely chuck the shaft of the drawbar through the headstock, do so, spin at a low RPM (100 or so), and observe the threads - if they "wobble", or don't progress uniformly as the shaft turns, they're defective and could cause the collet to be displaced.

Now, good collets should mitigate this to a degree by being constrained within the spindle at the upper cylinder, as well as at the tapered end, but cheap or worn collets (or a badly worn spindle) may not control locations well enough.

Conrad Hoffman · Jun 23, 2019

Speaking of collets, what kind of collets are they?

L Vanice · Jun 23, 2019

A J head will have had a key in the spindle when new. The key would assure the collet or other R8 tool in always inserted in the spindle in the same position. So a bad collet should give consistent runout readings, as far as the collet's own accuracy is concerned, unless the collet stem is loose in the spindle.

The key is not needed in most R8 machines, so they are often missing on older Bridgeports and clones. In that case, the collet is in a random position and the runout could be random.

When I check a collet for runout, I use an accurate test piece with a ground diameter of the rated size. In a 1/2" collet, I usually use a single ended end mill, for instance.

Larry

Leadfootin · Jun 23, 2019

Thanks for the quick replies:

Michiganbuck, engaged about 15, not bottoming, added 1/2" of washers with same result.

Milland, drunken threads a definite possibility with this KBC drawbar and rolled threads. Set drawbar in lathe, held threads in 7/16" collet and bar had some wobble, straightened out with no effect.

Conrad, Hardinge, KBC, assorted others all produced similar results.

L Vanice, used 1/2" dowel pin, fairly low hours machine, key still in spindle.

I suspect drunken threads as a substantial part of the problem. As well the upper straight portion of the R8 collets may be less than optimal diameter or the spindle worn, will measure. I gently reground the taper and it runs true, was only out about .0004" prior to grinding.

cyanidekid · Jun 23, 2019

Leadfootin said:
Thanks for the quick replies:

Milland, drunken threads a definite possibility with this KBC drawbar and rolled threads. Set drawbar in lathe, held threads in 7/16" collet and bar had some wobble, straightened out with no effect.

Conrad, Hardinge, KBC, assorted others all produced similar results.

what does "straightened out with no effect" mean?

as far as the collets, just because some are Hardinge doesn't mean the collets aren't the problem, are they dinged up, damaged? the "assorted others" makes it sound like they might be suspect.

when you say "used 1/2 dowl pin" does that mean you only tried it with 1/2 collets? have you tested the pin on a surface plate, or better yet, with a good V block and indicator?

metrology 101, what are you comparing to what?

Leadfootin · Jun 23, 2019

Numerous new and used collets were tried with similar results. I straightened the runout of the draw bar to less than .005" in the middle while holding the treads in a collet. I very much suspect that the rolled threads are not concentric with the drawbar shaft. A drawbar borrowed from a friend showed less than half the runout of mine with about a dozen tests including collet changes. Dowel pin was checked on a surface plate and another also gave nearly identical results. I even tried an R8 to ER32 adapter with 1/2" collet with worse results due to increased distance from taper.
Does anyone have the dimensions of a Bridgeport drawbar specifically the diameter of the shaft?

Thanks,
Peter

michiganbuck · Jun 23, 2019

QT: [Does anyone have the dimensions of a Bridgeport drawbar specifically the diameter of the shaft?]

It is the length of the draw bar that pulls the collet tight so, it is the length that is very important.

It is possible the id of the spindle is bad..but then the error would go to the same side of the spindle mark every time.

The 1/2 dowel in a 1/2 collet would line up with a sharpie mark made to the spindle with taking it out , turning it and putting it back in to show the error the same direction mand that it may be the taper in your spindle ID.

But if the draw bar bottoms out in the collet thread, it will be all over that place.

If the spindle tube is bent the error will go the same way every time to the sharpie mark on the spindle.

QT:[inconsistent changing every time the collet is removed and replaced.]
*Suggests it is not pulled tight IMHO.

cyanidekid · Jun 23, 2019

Leadfootin said:
I straightened the runout of the draw bar to less than .005" in the middle while holding the treads in a collet.

A drawbar borrowed from a friend showed less than half the runout of mine

Does anyone have the dimensions of a Bridgeport drawbar specifically the diameter of the shaft?

Thanks,
Peter

I wouldn't expect "straightening the drawbar in the middle" would help at all.

if the borrowed drawbar is 50% better, haven't you answered your own question, and shouldn't you just be ordering another, instead of asking the next irrelevant question?

Leadfootin · Jun 23, 2019

I am suspecting the drawbar thus asking for dimensions of a Bridgeport drawbar rather than an aftermarket. I will be trying a new replacement tomorrow and depending upon results either use it, make new, cutting threads on my HLV-H or head off in another direction in search of the runout cause. If the OEM shaft is considerably larger than the aftermarket this could be a substantial contributor to the problem.

michiganbuck · Jun 23, 2019

You can't even depend on the specifications draw bar length because you don't know if you spindle has been ground or the spindle has been altered.
It is only the length of your mill spindle that is important. The draw bar shoulder or washer if using one...to about 3/4 of the threads going into/inside a standard collet is the length your mill needs.. It would take me perhaps a half hour or less to confirm the needed length of your draw bar..with not spending a dime.

You may find adding a 1/16 thick washer is needed..but you should not guess you should know.

Back when I was building broach bars and the broach insert got less height with sharpening I would count the rotations of the bolts as they went in. 7 to 11 turns were safe for a 3/8-16 thread, more or less could be a problem. This an old journey man method of making an assembly..nothing new about it. likely guys were doing this since screw thread were invented.+

Ref; for 3/8-16 .. 1 / 16=.0625 x 7 =.438....x 11=.688.. 3/8= .375 so those lengths/and number of turns would be good for an assembly that specified 3/4 depth of tap/thread..

Bobw · Jun 23, 2019

I think I'll ask an obvious question.. Have you blued up a collet to
make sure you don't have a bunch of garbage up in your taper??

Whenever there is a runout problem, that is the first place I start.

iwananew10K · Jun 23, 2019

check the key too, if one shears it can leave just enough to seem OK but then when you tighten the bar it "cams" in the collet key way,binds and make things wonky...why many just remove them.

If it really your bar then you can't beat the rep of this company and for the price almost not worth making one, never hurts to have a spare anyway.
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michiganbuck · Jun 23, 2019

QT iwana: Good point the key... and be sure the key does not bottom out in the collet key way so the collet can't draw tight in the taper.

Guess you might blue up the collet key way and have a look-see..

Leadfootin · Jun 23, 2019

Thanks for all the good tips. Key is good and doesn't bind, taper was just ground but removed only about .0005", just enough to touch it with the stone. Will try a new bar and report back.

cyanidekid · Jun 23, 2019

michiganbuck said:
You can't even depend on the specifications draw bar length

If you look above, the OP is asking for the "DIAMETER of the shaft" (caps added) of a Bridgeport drawbar, so you could have saved your time posting that, for him at least.

Conrad Hoffman · Jun 23, 2019

How's the fit of the back of the collet in the spindle?

michiganbuck · Jun 24, 2019

cyanidekid said:
If you look above, the OP is asking for the "DIAMETER of the shaft" (caps added) of a Bridgeport drawbar, so you could have saved your time posting that, for him at least.

It should be clear that I am suggesting checking the length not the diameter. The total length and how many thread turns are into the collet to insure the collet is being pulled full into/tight in the taper. The number of turns also insure the nuber of threads on the draw bar are enough to not end bump to the bottom of the collet thread, or shoulder out at the top of the draw bar thread, or by thread quality not going into the collet with not binding. I'm not going to pull my draw bar but I would bet the OD has around .030 or so diameter clearance.

It looks like draw bars go from $20 to $80 and I bet/think they are all the same length by perhaps .03 difference and the length or the threads perhaps the same. Perhaps some are made to a length to include a washer at the top under the nut.

It is the mill spindle length that should be checked to know the draw bar is not long and the proper number of turns/threads are drawing the collet tight.

*Only if the OP has owned the machine since brand new would he know the spindle tube is to factory length from taper intersect on the collet, to the full length... or even the right spindle for his machine, or a repaired spindle, or a home made or after market spindle.

Guess I should just but-out if nobody wants to make this simple check with this the third or fourth time I have stated the same..

DMF_TomB · Jun 24, 2019

Leadfootin said:
J head Bridgeport, and it has .0002" runout at collet but .001-.002" runout 1" from collet. This is quite inconsistent changing every time the collet is removed and replaced. I tried a friends draw bar and the problem was reduced by about half. Could this be the fault of an aftermarket drawbar or something else entirely? Does anyone have a drawing or dimensions for an OEM drawbar?

.
runout is not that bad and it can vary based on tightening torque and how clean everything is. literally ability to self center tool without needing a little help by tapping it sideways at low torque and tightening, its not unusual to affect runout.
.
many tools have runout. even if you have a perfect spindle the tool itself can be ground with runout error and or not be perfectly straight. set screw weldon shank tool holders are famous for changing runout based on how tight they are.
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i wouldnt worry about .001 or .002" runout as I have never noticed it a problem.

Spindle runout issue

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