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Small Milling/Drilling Spindle (again)

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Kiwi Bloke 109/01/2019 19:29:29
110 forum posts
1 photos

Forgive me if I have misinterpreted the drawing, but the design illustrated in JasonB's post is seriously flawed. Both inner and outer races of the nose-end bearing are constrained. The pulley-end outer race is not constrained. This means that the bearings are not pre-loaded and therefore there is nothing 'taking up' the built-in clearance of either bearing, but particularly the nose-end bearing. The clearance is small, but enough to cause problems with milling.

A better design, and standard practice, is to have each outer race fitting into a stepped housing, with the nose-end bearing's inner race abutting an 'outboard' step on the spindle. The inner race of the pulley-end bearing is then located by the pulley hub, screwed and locked to the shaft (there is no shaft step at this end). Thus, bearing clearance can be removed.

There are other ways of taking up axial clearance, but the essential point is that the illustrated design has none.

Steve Crow12/01/2019 17:49:14
86 forum posts
21 photos
Posted by Kiwi Bloke 1 on 09/01/2019 19:29:29:

Forgive me if I have misinterpreted the drawing, but the design illustrated in JasonB's post is seriously flawed. Both inner and outer races of the nose-end bearing are constrained. The pulley-end outer race is not constrained. This means that the bearings are not pre-loaded and therefore there is nothing 'taking up' the built-in clearance of either bearing, but particularly the nose-end bearing. The clearance is small, but enough to cause problems with milling.

A better design, and standard practice, is to have each outer race fitting into a stepped housing, with the nose-end bearing's inner race abutting an 'outboard' step on the spindle. The inner race of the pulley-end bearing is then located by the pulley hub, screwed and locked to the shaft (there is no shaft step at this end). Thus, bearing clearance can be removed.

There are other ways of taking up axial clearance, but the essential point is that the illustrated design has none.

Now I am confused!

The drawing is from the Spindle book and a number of the spindles described use this design.

Can anyone enlighten me?

Steve

Kiwi Bloke 113/01/2019 10:59:45
110 forum posts
1 photos

Enlighten you? I don't know - possibly add to your confusion. For the sake of what follows, can I assume you have a copy of the Spindles book and that you're a beginner, with little engineering knowledge? It's safer not to assume knowledge sometimes.

I've just rooted out my copy of the book and am very surprised. The author does not discuss bearings in any detail and most of his designs are faulty. OK, they will almost certainly 'work', but they can be much better and at the same time simplified: bearing 'slack' can be minimized by simple re-thinking. You really shouldn't have to tolerate any avoidable free movement that can be designed out, and it's particularly important to avoid it in a spindle intended for milling, grinding, etc.

The problem is that the type of ball bearing used in most of his spindles are 'deep groove' ball bearings. These always have a tiny amount of radial clearance built-in. Clearance may be designed into the bearing to allow for expansion when whatever the bearing is fitted to heats up. Hot-running and high-speed bearings are designed slacker. Bearings come in different clearance grades. These are numbered, typically, C1 - C5, tighter being lower numbers, and more expensive. I haven't seen C1 grade available in 'our' sizes, from usual suppliers. CN is 'normal' clearance and is somewhere between C2 and C3. Aftermarket bearings are often C3 grade, to suit a wide range of hopefully-not-very-critical applications. CM is also somewhere between C2 and C3, and is supposed to be designed for electric motors, where quietness in operation is desired, so very smooth tracks, for low vibration, but I don't think 'tightness' is a design priority.

If you shake a typical C3 bearing set, with seals and all lubricant removed (bad for the bearing!), it will rattle surprisingly, and the inner track can be felt to move, in all directions, with respect to the outer. Figures for radial clearance can be found, buried in the manufacturers' data sheets, but axial clearance is rarely listed (as far as I can recall).

As the inner track is pushed axially into the outer track, the balls roll 'up the sides of the grooves' a little, and can thus react the applied axial force. This loading has removed the slack, and you'll note that the radial slack has gone, too. High radial forces can still, of course, cause the built-in radial slack to reveal itself, but, as axial force is increased, radial stiffness does also. What we want is to load each bearing in opposite axial directions, so the slack is taken up in each bearing. This is known as 'preloading', and is often done by adopting a mounting like the book's author does for his taper roller spindle: the outer tracks are prevented from moving deeper into the housing by shoulders, and the inner tracks are GENTLY pushed together by the nut at the pulley end.

All this is absolutely standard stuff, and I'm disappointed that the book got published, containing as it does so many poor designs. The Ch5 design allows the cutter in the chuck to 'see' all the slack in the bearing nearest to it: preload is impossible. The Ch6, 7, 8, and 11 designs are bad: the only thing holding the spindle into the housing is the friction fit of the outer tracks of the bearings (and the rear one should be free to slide with a bit of effort). The spindle might easily vibrate out of its housing under the influence of milling vibration, until the outer track of the rear bearing abuts the shoulder in the housing. Also, again, no preload provision. The Ch9 &10 designs have no provision for adjustable preload - both inner and outer tracks of both bearings are fully constrained - so incredible precision of manufacture would be needed for real success.

The design in Ch 13 is fine, and also the simplest. It embodies what I've been trying to explain, albeit with taper roller bearings. Note that the inner tracks are constrained at their outer face only - there is no spacer tube between them, so they can be moved together by tightening the nut outboard of the pulley. The outer tracks are constrained at their inner faces by the shouldered housing. If deep groove ball bearings (or angular contact bearings - but that's another subject...) are substituted into this style of design, you have a design in which bearing clearance can be miminized. You can also go for C2 bearings, if you can source them (I can't, in NZ).

Someone may post that I'm being over-fussy, but the 'proper' design I've described is the easiest to make and employs the bearings to their best advantage. Hope this is enlightening, rather than baffling...

Chris Trice13/01/2019 12:42:25
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1135 forum posts
2 photos

I have the book and agree with KB. I built the spindle in chapter 5 but arranged for the outer race of the rear bearing to sit firmly in the body rather than float so preload could be applied.

Steve Crow13/01/2019 13:13:38
86 forum posts
21 photos

Thank you KB, your reply did indeed enlighten me.

Your description of preload is exactly as I understood it. Since getting the spindle book and studying the designs I've been under the impression that I have been missing something regarding preload but I thought the book had to be correct.

How about the gearcutting frame in chapter 12? With the fully adjustable bearings at both ends, surely this is a legitimate design? I ask as I am just starting to build a slightly smaller version.

Cheers, Steve

Kiwi Bloke 113/01/2019 21:19:55
110 forum posts
1 photos

Re the cutting frame design: the bearing arrangement is OK, for the reason you suggest, but it's a pain to have to re-set preload every time the cutter is changed, isn't it? The design doesn't make it easy.

The design is, however, seriously flawed, for another reason. The weakest part of the shaft - the joint - is in the middle of the shaft, and under the cutter. It's exactly where it shouldn't be. It's easy enough to improve this aspect of the design, but why bother at all? I don't see any reason to bother with cutter frames these days. Perhaps I'm missing something...

My understanding is that they are a legacy of the days when you couldn't pop out and buy pre-made bearings. Early cutter frames had (I believe) conical-ended shafts, the cone running in a conical recess in the end of a screwed 'bolt' (for want of a better term) that screwed into the frame. (Hope that makes sense.) This is about the simplest bearing to make, and suitable for very light loads. The 'bolt' could be quickly unscrewed to free the shaft, and also facilitated bearing adjustment to eliminate end-float. One thing you really don't want, if you're cutting tiny gears, is any cutter end-float. With this design, the bearings were necessarily at the shaft ends, so the pulley and cutter had to be between the frame's arms. This can make things a bit cramped.

I can't really see why one shouldn't use a properly designed spindle, with the cutter at one end, and the pulley at t'other. Forget cutter frames - they're history!

Since you are laudably doing your own thing, rather than slavishly following published designs (which we now know to be untrustworthy...), bear in mind that you should only use sealed ball bearings if you are confident that you have sufficient torque to overcome the seals' drag, which can be surprisingly high. You can buy bearings with 'non-contact' or 'low friction' seals which are OK and shielded bearings are fine. Make sure your supplier understands your requirements - to some, a seal is a seal is a seal. Non-contact sealed bearings are no more expensive than 'conventional' sealed, but are perhaps not so readily available.

Edited By Kiwi Bloke 1 on 13/01/2019 21:25:07

Michael Gilligan14/01/2019 07:39:51
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12310 forum posts
538 photos

A good discussion, Kiwi Bloke yes

A ball-bearinged cutter frame does seem to introduce pointless complexity.

[ please forgive the inevitable pun ]

I would, however, make a case in favour of the cone-bearinged cutter frame:

It is a simple device [comparatively easy to make]; and the geometry is equivalent to turning between centres

... it therefore has inherent precision.

MichaelG.

Edited By Michael Gilligan on 14/01/2019 07:43:31

Kiwi Bloke 114/01/2019 10:15:50
110 forum posts
1 photos

Bearing in mind the 'inherent precision' aspect, I must concede that you've got a point there, MG.

...OK, I'll be off now.

Michael Gilligan14/01/2019 10:43:46
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12310 forum posts
538 photos

smiley

ega14/01/2019 10:58:45
1008 forum posts
85 photos

In the introduction to his 1997 book Harprit Sandhu invites corrections. I wonder whether it would be possible for him to be invited to comment?

I bought the book partly because, having faced the challenge of building the Quorn spindle I was interested in knowing if there were easier ways to achieve the desired result.

Edited By ega on 14/01/2019 10:59:05

Steve Crow14/01/2019 18:19:45
86 forum posts
21 photos

I agree on the cone bearing aspect but the appeal of cutter frame in the book is the outboard pulley.

I intend to use the frame horizontally on the vertical slide and the only way to drive it is outboard. I think ball-bearings might be the only way to go with that.

As regards the spindle book, I'm rather taken aback that a publication (and probably the only work on the subject available) which has been in print for over 20 years can be so flawed. Also, it made me question my own understanding of the subject for the last 3 months!

There must be other cases where published designs are flawed? Maybe there should be a thread warning of such things? People put a lot of time, effort and money into building such designs.

Michael Gilligan14/01/2019 18:33:50
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12310 forum posts
538 photos
Posted by Steve Crow on 14/01/2019 18:19:45:

I intend to use the frame horizontally on the vertical slide and the only way to drive it is outboard. I think ball-bearings might be the only way to go with that.

.

In which case, Bill Ooms is the man: **LINK**

https://www.billooms.com/resourceOT.html

MichaelG.

Steve Crow14/01/2019 18:52:22
86 forum posts
21 photos

Thank you Michael, some very nice, simple designs there. Such a contrast to the complexity of some others.

It's got me rethinking things again.

Steve

John Pace15/01/2019 12:07:04
109 forum posts
105 photos

Posted by ega 14/01/2019 10:58:45

In the introduction to his 1997 book Harprit Sandhu invites corrections.
I wonder whether it would be possible for him to be invited to comment?

I bought the book partly because, having faced the challenge of building
the Quorn spindle I was interested in knowing if there were easier ways
to achieve the desired result.

Edited By ega on 14/01/2019 10:59:05


I too have built a Quorn spindle and with care they run well ,it must have done
as i have had it 30 years and it has never been apart.


The photo shows 3 grinding spindles under construction,based on the layout
of the Quorn type spindles with modifications.

The outer sleeve is bored through for a close fit for the bearings,in this way
the bearings will be in line axially.

The shaft is made with bearing seats this gets rid of the bearing spacer tube
and makes the shaft much stronger as it is larger in diameter in the central
portion.

The two sleeves seen in the photo, the shorter one is eventually loctited into
the outer sleeve in the correct position ,the longer sleeve forms the spring box
to push on the rear bearing much the same as the Quorn design.The spacing
of the bearing seats on the shaft is a few thousanth's more than the total
length of the two sleeves limiting the amount of axial movement.

The two screwed end caps similar to the Quorn ,the front one traps the
outer raceway ,the rear cap is clear of the bearing.
In this way a nut fixing is required at both ends of the shaft to trap
the inner raceways to the shaft, as these form part of the labyrinth
seals these are much easier to do.

Perhaps the biggest advantage of constructing in this way is that all
of the turnings are single operations and require no turnarounds for all
of the important features.
The bearings are SKF C3 Eco 17x 35 mm bearings rated for 25,000 rpm
and are more than good enough for the expected loads,i expect that
angular contact bearings a better bet if the spindles were to be used
for milling.

John

01  29.jpg

ega15/01/2019 18:01:24
1008 forum posts
85 photos

John Pace:

Thanks for your very interesting post. If my spindle should fail then I shall know what to do!

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