Accuracy of centre drill holes

[Yosiu]

I'm thinking about doing some fixture to face and drill centre holes on both ends of shafts on horizontal mill before turning between centres( by rotating shaft 180 degrees)
As I'm not familiar with turning between centres I've got this question - is it really important to get both centre drill holes spot on coaxial and parallel? How will it affect turning if they are skewed somehow?
Normally they are created by mounting in chuck ,drilling one end then chuck second one and machining another but I saw some guys doing them with handheld drills on shop floor so i can't be crucial,right?


Sorry for newbie question but this is far from my regular work

 

[FamilyTradition]

I would say they don't have to be within tenths, but you should get the center holes as close as is reasonable for your purpose. Especially if the part has already has features - you will need to hold whatever perpendicular, parallel, concentric, etc tolerances you have.

Also, center holes too far out of alignment and you risk not cleaning up on your stock. I would also assume that it puts undue stress on your centers - the axes of your centers are going to be damn near perfect in alignment, so mis-aligned center holes are going to ride on one spot on your centers. That's extra friction and heat, especially if you are working with dead centers.

I am not sure what the logic is for center drilling a (presumably) round shaft in a mill, I would just drill center hole using the lathe. Of course, I don't know the specifics about the process for your parts and/or what equipment you have access to.

I do a lot of grinding between centers, and I can't say I've ever noticed excessive mis-alignment, but it could be a factor in some issues I've had that I never quite figured out. As previously mentioned, it is important if there are already finished features on the component.

 

[L Vanice]

Decades ago, at a surplus equipment auction at the local General Electric motor plant, I saw a machine whose only function was grinding/truing the cones of pre-drilled center holes in the ends of motor shafts. The shaft was held vertically with the bottom hole resting on a male 60 degree center. The spindle at the top had a 60 degree pointed grinding point on a steel shaft. The object of the machine seemed to be to get the cones of the two center hole axes in perfect alignment so that subsequent operations on the shaft would have minimal runout when the shaft was turned or ground between centers. This facility made a wide range of motors from low cost items to expensive goods, including the motors for Hardinge HLV-H lathes. I expect the grinder was only used on the expensive motor shafts. One of the items I bought at one of the GE auctions was a Hardinge bench center set with the Hardinge indicator. The local GE die shop had about ten HLV-H lathes when they sold it all off.

Larry

 

[EmGo]

I'm thinking about doing some fixture to face and drill centre holes on both ends of shafts on horizontal mill before turning between centres( by rotating shaft 180 degrees)

You mean, like this ?

Doosan model HC-400 HMC 16" Twin pallet machine w/ Fanuc cnc controls New 2011 | Piselli Enterprises

Doosan model HC-400 HMC 16" Twin pallet machine w/ Fanuc cnc controls , This machine was set up for Facing and centering of shafts on the production line . Fixturing for the machine to do this operation may e available , Delivered in New 2011 2 machines available

pisellient.com

or if you want to offload that work

Nittoku Model NCR 400 mm twin spindle Facing & Centering machines New 2006 | Piselli Enterprises

Nittoku Model NCR 400 MM Twin spindle Facing & Centering Machines, 400 mm distance capacity , fully automatic , small footprint machines , High speed, chip conveyors , (2 Available) Excellent

pisellient.com

and for bigger

Hey #3 Facing & Centering Machine 6" x 51" capacity , Allen Bradley PLC controls, Auto cycle , New 2003 | Piselli Enterprises

Hey #3 Facing & Centering Machine 6" x 51" capacity , Allen Bradley PLC controls, auto cycle , New 2003

pisellient.com

It's a standard problem in shaft production and yes, please, get the centers as close to being inline as possible. It will make life much easier later on. Honest.

I saw some guys doing them with handheld drills on shop floor so i can't be crucial,right?

You know the big deadblow hammer in the maintenance shop ? Get that. Take it out it the floor. Tell those guys to put their dicks on the table, then hit them really hard with that hammer.

That's disgusting, that anyone would do that ! No, it's not acceptable. Period !

btw, the vises are the tricky part of this operation - you don't want small changes in bar diameter to be throwing your centers way off. Machines built for this have vises like v-blocks that automatically adjust for size.

 

[guythatbrews]

As long as you take a bit of care you'll be fine. Remember if you jam a shaft in a lathe chuck to face and center it how often do you really know how concentric the other end hidden in the spindle is. Usually it's taken for granted it's running true but it probably isn't.

 

[michiganbuck]

I don't trust a center drill to make dead center so with the part indicated at zero I scalp the part center with a lathe tool bit. If you have perhaps 1" stock and need a finished pert to be 1" you better not trust just the center drill out of the tail.

Qt (but I saw some guys doing them with handheld drills on shop floor so i can't be crucial,right?)

You can scribe and hand punch the center and then hand drill it, and if you have .010 or .020 extra shock likely you can make a part..if you are lucky

 

[Yosiu]

Material is 160mm (6.3")diameter and 550mm long (21.5") cold rolled bar. About 1000 of them in few variations.

It's too big to be chucked deep inside chuck so i would like to get it face and center drilled on horizontal mill.I was thinking about putting v-blocks on top of palette i can rotate 180 degrees .


My real question was what KIND of problem i can expect. If centers are coaxial but not In center of stock then raw outside diameter will have some runout but if you have enough stock that's not a big problem.
But what happens if centres are not coaxial? Or the are not parallel?
Will it affect reatability of clamping between centers?
I'm wondering how accurate my fixture has to be guarantee coaxial and parallel of centres. But of course same applies in drilling in lathe chuck and rotating especially if holding on rough stock.

 

[EmGo]

My real question was what KIND of problem i can expect.

You can expect every kind of problem you get with shitty centers, what else ? Trouble holding size, trouble with surface finish, trouble with roundness, just the usual.

I'm assuming that you want to put centers in these shafts for future processing. Well, go look up all the problems you can run into with turning between bad centers, grinding between bad centers, hobbing between bad centers, or anything else.

That's what you'll have, because if you don't put good centers in your facing and centering operation, you'll have shit to work with.

I'm puzzled by why this is even a question.

 

[Yosiu]

Well i simple cannot find any topic covering this. Writing "turning between bad centres" in Google usually brings up discussion on headstock and tailstock alignment which is not this case.

 

[john.k]

No different to putting centres in a rough casting .If you are doing super precision work,then when the item is rough machined,it may be necessary to reform the cones so the work runs perfectly true.

 

[FamilyTradition]

Well i simple cannot find any topic covering this. Writing "turning between bad centres" in Google usually brings up discussion on headstock and tailstock alignment which is not this case.

In a way, that is the case, just from a different perspective. Having your center holes out of alignment is just as bad as having centers out of alignment. You will run into the same issues.

As EmmanuelGoldstein said, you get out what you put in. You can't make good parts with a worn table, bed with burrs, fixturing that's clamped down on top of chips, dull tools, blown out spindle bearings, or... bad center holes (or at least it's not going to be easy or efficient to do so). If you have f'd up, lopsided, chatter, non-concentric center holes, you will receive f'd up, lopsided, chatter, non-concentric work.

I'm wondering how accurate my fixture has to be guarantee coaxial and parallel of centres. But of course same applies in drilling in lathe chuck and rotating especially if holding on rough stock.

I think I now understand what you intend to do. As long as your part does not have to leave the vee block between facing and center drilling each end, you are really relying on the pallet reliably and accurately rotating 180 degrees. If it does so, then your center holes will be concentric with each other (but not necessarily concentric with the OD of the stock - just make sure you have enough to remove). As long as you are not moving on your X and Y axes when the pallet rotates.

For best chance of success, I would just make sure that I have a good vee block (.0002 squareness and parallel between the sides, bottom and vee is standard for most vee blocks), and it isn't that hard to get that fixture indicated within .0002 parallel to the axis of your spindle.

 

[eKretz]

Listen. Emmanuel is right on the money. If you want to make the best parts that you can - that is to say, the most round, most concentric, straightest, etc. then you need good centers that are inline. Can you put centers into a shaft with a hand drill in a pinch just to get a shaft up in the machine? Yes. Does it work out well every time? No.

Generally that is done because one is in a hurry or has no better way to do it. Usually after doing that, the parts are put up in the lathe and spots are turned for a roller or steady rest to run on, then the centers are recut true and inline. Sometimes that fails though because the centers put in by hand weren't round - which means that when the spots are cut, they aren't round. It can snowball from there.

Consider too, a long thin shaft with centers that are crooked. Pressing a center into the crooked center hole with any force causes the shaft to bind and flex to try to align the center hole with the center. If the shaft is not substantial enough to resist this deflection it will be very crooked once machined after center pressure is released.

I could go on...

Best bet is to put in good centers from the start. That can be done on the mill or in the lathe with a cat head. You can also use one of the aforementioned machines that have a center at one end and a cutter or lap at the other - although they aren't much use except for pretty small stuff. Those are often called "center laps."

In a pinch you can make your own impromptu "center lapping machine" with the lathe too. Simply set the part up in a roller or steady rest and use a 60° center reamer that has a female center in the shank end - set the cutting end in the existing (maybe hand drilled, maybe just out of round) workpiece center, set the tailstock center in the end of the cutter, and rotate the cutter by hand with a wrench or similar. This method relies on good alignment of the machine and work though. It will be one HELL of a lot better than a hand drilled center though.

 

[EPAIII]

As with so much of machining, the choice of methods depends a lot on what you are starting with and where you need to go.

As I understand it, the idea of center drilling your stock on both ends is to ESTABLISH a center line. That center line is the line between the centers of those two center drill holes - wherever they may be.

After drilling the two center holes, the stock is mounted on centers and a lathe dog is used to impart motion to it and THEN the OD of the stock is turned as a true cylinder around the center line.

If you are starting with rough stock the only accuracy requirement is that there is enough meat on all sides of the center line (the line between the center holes) so that your part will fit inside the stock. So, many machining texts show a blade mounted on a centering attachment used to scribe an X for the center. This is then punched and drilled by whatever means. Those centering attachments are not all that accurate and, for this, they don't need to be.

And understand that if ALL of the features of the part, including the overall OD, are turned with the part mounted between the centers, then all of those features will be about as concentric (accurate) as your lathe will allow. This is the beauty of mounting even a piece of rough stock "between centers" for turning. All features turned in that same set-up will be concentric. Something along these lines is likely what you observed on the shop floor. Or, at least, I hope so.

BUT, and it is a big BUT, if you have a stock shaft that is ALREADY at the final diameter and you only need to machine additional features (shoulders, grooves, threads, etc.) on it that are concentric with that already established OD, then you will need whatever accuracy as is required for those additional features. Actually somewhat more because the subsequent machining will always have additional inaccuracies that will be added to the finished part.

So if that is the case, for most parts you will need an accuracy that is not achievable with marking the center by hand and using a hand drill, or a drill press. or most other manual or partially manual methods. In such a case the quick and easy way would be to hold the shaft in a collet in the lathe and mount the center drill in the tail stock with a drill chuck (Jacobs chuck). Drilling it in this manner is self centering so any inaccuracy in the drill chuck will not matter.

Even better accuracy would be achieved by using a four jaw chuck and "dialing" it in to the accuracy needed.

 

[eKretz]

As with so much of machining, the choice of methods depends a lot on what you are starting with and where you need to go.

As I understand it, the idea of center drilling your stock on both ends is to ESTABLISH a center line. That center line is the line between the centers of those two center drill holes - wherever they may be.

After drilling the two center holes, the stock is mounted on centers and a lathe dog is used to impart motion to it and THEN the OD of the stock is turned as a true cylinder around the center line.

If you are starting with rough stock the only accuracy requirement is that there is enough meat on all sides of the center line (the line between the center holes) so that your part will fit inside the stock. So, many machining texts show a blade mounted on a centering attachment used to scribe an X for the center. This is then punched and drilled by whatever means. Those centering attachments are not all that accurate and, for this, they don't need to be.

BUT, and it is a big BUT, if you have a stock shaft that is ALREADY at the final diameter and you only need to machine additional features (shoulders, grooves, threads, etc.) on it that are concentric with that already established OD, then you will need whatever accuracy as is required for those additional features. Actually somewhat more because the subsequent machining will always have additional inaccuracies that will be added to the finished part.

So if that is the case, for most parts you will need an accuracy that is not achievable with marking the center by hand and using a hand drill, or a drill press. or most other manual or partially manual methods. In such a case the quick and easy way would be to hold the shaft in a collet in the lathe and mount the center drill in the tail stock with a drill chuck (Jacobs chuck). Drilling it in this manner is self centering so any inaccuracy in the drill chuck will not matter.

Even better accuracy would be achieved by using a four jaw chuck and "dialing" it in to the accuracy needed.

Most shops these days do not often turn between centers. When they do, it's usually for OD grinding work. And if I'm not mistaken, the OP is not asking questions about a workpiece that he can fit in a collet, and almost certainly using a live center at one end and a chuck at the other...

And you are wrong about the centers not needing to be accurate. They absolutely do need to be - if the work to be done is to be of good quality. I've already explained that. Have you ever actually worked in a professional machine shop environment? Sometimes you seem to attempt to say things with an authoritative manner ("the only requirement is"...") that are very much at odds with my experience, anyway.

In addition - if you DO run between dead centers it's still important. Between dead centers it's less likely that center pressure will flex a shaft, but center misalignment means the center will contact on one side near its point and the other side near its base. Then you're going to revolve it around for thousands upon thousands of revolutions as that point contact pushes all the center lube out of the contact zone... Doing that over and over again will wear both your center point and your workpiece's center all to hell and gone. Getting things right from the start means more accurate work and longer lasting, more accurate tooling. Up to you if that's not an important factor in doing quality work.

 

[Overland]

Back in the UK auto industry in the 70's we used to turn forged camshafts, between centers. The first op was a Hey (spelling from memory) face and center machine - shaft clamped on vee blocks, and 2 heads that came in and did the deed with combination face and center cutters.
Then a new crankshaft line's first op was a German face and center machine that spun the crank and adjusted position of crank while spinning, that's how I remember it, to find the center of mass, then face and center both ends.

Back to the OP. If you're holding between a chuck and center, you need to be sure job is sitting correctly on the center before tightening chuck; maybe tricky to achieve with such a heavy work piece.
Bob

 

[Conrad Hoffman]

I've read about the center grinding machine described above, but never seen one. FWIW, I try to use radius center drills. That gives a more tolerant ring contact, but probably one that can't support as much pressure/load. Worked well when I was doing a lot of OD grinding on small tight tolerance stuff.

 

[Yosiu]

Thanks a lot guys for input - a lot of valuable information in this thread.

Actually the plan is to make faces and center drill then go for face driver in head stock and live centre in tailstock since whole shaft has same diameter on whole lenght.

 

[Peter S]

How about making a drill jig, eg a cap which fits on either end. A drill bush in the centre. Drill bush sized to suit centre drill. You might even get away with a hand drill but risky to mention it here any drill press otherwise.

I have seen a big shaft centre drilled on a mill, including offset centre's. It became a press crankshaft. I think they milled a flat longitudinally down the shaft first so both ends aligned

 

[eKretz]

My frame of reference may be a little different than some here I guess. I came up working with large machines and workpieces in the double digit multi ton range. A radiused type center is completely useless for that, would be wiped out in less than a few minutes. For something that weighs a couple pounds it'd probably be fine. The same goes for crooked centers or poorly formed angles etc. Those big workpieces will chew them right up tout suite. The small ones will do the same, it just takes longer.

These ideas for jigs and whatever else are interesting and all, but he's already mentioned having a mill with the capability to fit the parts and a plan to do them in that mill. I'd do them there. Getting them within a few thousandths or so of coaxial and inline would be plenty good enough, and quick and easy with a turntable and a set of v-blocks - especially with a decent quantity of parts. Don't forget that if you are removing a lot of material you will almost certainly have shaft movement due to stress relief. At that point, depending on how much things move it may be a good idea to recut the centers if you need a good straight, round and true finished part. It really doesn't take very long to do.

 

[Scottl]

Here's a thought; get a 3-jaw chuck large enough to accept the largest bars to be drilled but not so heavy that it will be awkward. Machine a threaded adapter with a reamed 3/8" bore so the guys on the floor can use a 6" long 3/8" shank center drill. That can be your jig.