Small Milling/Drilling Spindle (again)

[Steve CrowParticipant @stevecrow46066]

Hello, I posted on this same subject a few months ago but I've struggled to come up with a compact design using bearings. These are my requirements:-

I want to make an 8mm watchmaker spindle for my Sherline lathe.

20mm dia. housing. (O/D of spindle is 1/2&quot

Housing no more than 3" long.

To be used for drilling and very light milling/engraving.

Max speed will be 5000 rpm.

How about using bronze bushes? Don't Potts and other spindles use them? Could I use Oilite bearings?

If anyone has any thoughts, advice or designs I would be very grateful.

Many thanks, Steve

[Steve CrowParticipant @stevecrow46066]

[HOWARDTParticipant @howardt]

Use a combined needle and ball thrust running on hardened shaft. Shaft would need to be ground to attain best fit.

[Steve CrowParticipant @stevecrow46066]

Sorry about the weird emoji thing in my post. It's meant to be a bracket.

[John HaineParticipant @johnhaine32865]

There's a useful book on "spindles" in the WS Practice series. I don't think you need to harden the shaft especially if you use rolling bearings. The Quorn spindle is 8mm I think, maybe that would be a good design, or basis for one? I think you'll find 5000 rpm limiting with small cutters, aim for much faster. Main problem is that common bearings with 1/2" bore will have too large an outside dia – Quorn uses magneto bearings. I got some special bearings from Ketan at Arc for an MT2 spindle where I needed a smaller OD housing, they had much smaller balls than normal so the difference between OD and ID was quite small – worth asking him perhaps?

[Steve CrowParticipant @stevecrow46066]

Posted by HOWARDT on 07/01/2019 17:29:59:

Use a combined needle and ball thrust running on hardened shaft. Shaft would need to be ground to attain best fit.

Thanks Howard but that set up would be too bulky. I'm looking for something more compact – hence bushes.

[Michael GilliganParticipant @michaelgilligan61133]

Sorry, Steve …It's your choice, of course, but I still can't understand why you decided against adapting the WW headstock.

[quote] Much as I agree with Michael's suggestion to use my existing headstock, I do feel the urge to make one from scratch.[/quote]

MichaelG.

.

Previous thread, for reference:

**LINK**

Edited By Michael Gilligan on 07/01/2019 18:14:48

[Enough!Participant @enough]

Posted by Steve Crow on 07/01/2019 17:48:48:

Sorry about the weird emoji thing in my post. It's meant to be a bracket.

Always use a space in front of a closing parenthesis on this site.

[HOWARDTParticipant @howardt]

Igus L250 flanged bearing, not read the specs but certainly smaller so long as thrust is light. Maintaining endfloat may need a spring loaded end thrust face but as diameters are small will work. Spring could be wavy washer.

[JasonBModerator @jasonb]

As was suggested a couple of months ago, use one of the designs from the spindle book and adjust sizes to suit your small needs.

Take this one for example

At the business end use a 61701RS thin bearing, 12mm ID and 18mm OD

Front nut threaded M19 x 0.5 will just fit your 20mm block.

Rear nut that bears against the inner race of the front bearing M12 x 0.5

11.3 OD spindle back to the rear bearing then step down to 10mm dia

Rear bearing 61700RS 10mm ID x 15mm OD

10mm ID drive pulley

M10 x 0.5 Nut to pull the pulley and bearing inner tight up against the 11.3mm dia.

Edited By JasonB on 07/01/2019 19:10:24

[geoff walker 1Participant @geoffwalker1]

Hi steve,

MEW issue number 73 has a article on a re designed Potts milling spindle..

The spindle and the housing have been modified, based on a illustration in a watchmakers book by Donald de Carle.

Looks very tricky to make but the author of article maintains it would be very accurate and suitable for high speed work.

It uses double opposed cone plain bearings. It is slimline design and like I say not easy to make but it may be of interest to you.

I have a copy of the magazine. You can have it for postage (paypal £2).

Message me if you are interested

Geoff

[Ady1Participant @ady1]

I like the upgraded Drummond system because it makes different bearings do different jobs

There are the sleeve bearings in the original design around the spindle

There are roller bearings at each end 90 degrees to the spindle

So all loading scenarios are covered

 

As an additional contribution to the mix I noticed a huge stiffness improvement when the front roller bearing was fitted at the front and rested on the housing, it made the spindle behave like a better machine with a bigger spindle

GL

Edit: The drummond bronze bearing adjustment system is worth considering too, it's very strong and reliable but it's not simple to replicate unless you have a good skill level

Edited By Ady1 on 07/01/2019 20:11:28

[Steve CrowParticipant @stevecrow46066]

Posted by Michael Gilligan on 07/01/2019 18:14:00:

Sorry, Steve …It's your choice, of course, but I still can't understand why you decided against adapting the WW headstock.

[quote] Much as I agree with Michael's suggestion to use my existing headstock, I do feel the urge to make one from scratch.[/quote]

MichaelG.

.

Previous thread, for reference:

**LINK**

Edited By Michael Gilligan on 07/01/2019 18:14:48

 

 

 

Hi Michael,

I did come round to your suggestion but the WW headstock is too big for the very limited space I have.I want to be able to mount the spindle horizontally on the cross slide at centre height as well as on my vertical slide. I only have 23mm from cross slide to centre height. I have a headstock riser block but that just complicates other things.Another problem with the WW is the pulley position would mean getting a drive to it would be a nightmare.

Thank you anyway.

 

 

Edited By JasonB on 08/01/2019 11:36:35

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for the clarification, Steve

I've been thinking about your question whilst out for my 'fresh air and exercise' walk.

As you are very limited on space … I would make the body from good cast iron, or bronze, and turn the bearing surfaces directly into it. Preferably a double cone each end [as per the classic watchmaker's lathe headstock bearing]; but realistically, I'm sure a single cone each end would do nicely.

Plain cone bearings tend to 'bed in' rather than wearing out.

MichaelG.

[Steve CrowParticipant @stevecrow46066]

Thank you Jason, I've reread the Spindle book and I like your idea very much. I think I can beef up the housing to 1" and go with slightly bigger bearings. Do you think there is much advantage in a double set of front bearing as in some designs in the book?

Michael, I'm also liking your idea for cone bearings into bronze. Any idea of the best angle for such cones?

Also, has anybody built the wheel cutting frame described in the Spindle book?

Cheers, Steve

[JasonBModerator @jasonb]

Two at the front would not hurt and if you stay with the thin section ones they are only 4mm wide.

[geoff walker 1Participant @geoffwalker1]

Steve

As you are very limited on space … I would make the body from good cast iron, or bronze, and turn the bearing surfaces directly into it. Preferably a double cone each end [as per the classic watchmaker's lathe headstock bearing]; but realistically, I'm sure a single cone each end would do nicely.

Plain cone bearings tend to 'bed in' rather than wearing out.

MichaelG.

Michael, I'm also liking your idea for cone bearings into bronze. Any idea of the best angle for such cones?

Steve.

In the potts article, MEW 73 the included angle is given as 8 degrees. It has a double cone at each end as per Michaels suggestion.

Geoff

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Steve Crow on 08/01/2019 18:33:23:

Michael, I'm also liking your idea for cone bearings into bronze. Any idea of the best angle for such cones?

.

Any 'self releasing' taper would probably do

… but I would guess 20° to 30° included angle. [*]

The optimum would depend upon the balance of axial and radial forces.

MichaelG.

.

.

[*] Edit: assuming that we are talking about single tapers at each end.

The double taper, of course, has one steeper and one shallower, to achieve the best of both.

Edit: This may help inform your choice:

http://www.dlindustrial.com/profiles/blogs/steep-tapers-fast-tapers-at3-and-what-it-means

Edited By Michael Gilligan on 08/01/2019 19:10:10

[Michael GilliganParticipant @michaelgilligan61133]

These might be useful, for the traditional angles :

.

.

MichaelG.

[Steve CrowParticipant @stevecrow46066]

I've a couple of more questions.

The bearing retention nuts. Is brass ok?

Also, what is the minimum "step" to retain a bearing on shaft? For example a 15mm bearing against a 5/8" diameter is less than half a mm each side. Do I need more meat?

Cheers, Steve

[duncan webster 1Participant @duncanwebster1]

If you go on SKF website they give all relevant mounting dimensions

[Kiwi BlokeParticipant @kiwibloke62605]

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 CrowParticipant @stevecrow46066]

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 BlokeParticipant @kiwibloke62605]

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 TriceParticipant @christrice43267]

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.

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