Small Milling/Drilling Spindle (again)

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"

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

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

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

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?

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

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.

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Previous thread, for reference:

**LINK**

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

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

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.

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

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

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

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

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.

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

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

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

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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.

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[*] 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

These might be useful, for the traditional angles :

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MichaelG.

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

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