Member postings for John Baron

You need a bigger magnet

Hi Martin,

You have all the motor info, why not just call Brooks and get a new original one !

http://brookcrompton.com/distributors.aspx

Hi Guys,

Thanks for your posts !

Nobody pointed out that I had forgotten to do the drawing of the fastening/adjusting screw holes and positions

Edited By John Baron on 20/03/2021 15:14:05

Edited By John Baron on 20/03/2021 15:14:32

Edited By John Baron on 20/03/2021 15:14:48

Hi Guys,

I managed to strip the thread in my cheap Chinese die stock recently, so I decided to make a new one.

I did a drawing to give me something to follow with my new DRO on the mill. I've placed a copy here.

The bits in blue are just in case somebody would like to make one with the corners milled off. The 7 mm holes are for threading M8 for handles to screw into. My existing handles are threaded M8.

Hi Keith,

Your motor may not be identical to mine. The brushes in mine are 3.6 by 10 mm and 15 mm long. I had similar problems getting hold of some brushes, so I made my own.

If you measure the brass sleeve that will tell you the size.

As far as the semiconductors on the power board are concerned, failed diodes almost invariably go short circuit, the more likely things to fail are the SCR's.

If its any help I'm near York.

Edited By John Baron on 13/03/2021 19:49:36

Hi Guys,

I've just fitted a DRO to my Chinese copy of an Optimum mill. I put a glass scale on the column so that I can read the head height. My mill also has a readout on the quill for drilling depth.

I used a pair of aluminium brackets to mount the scale. I also used a spacer plate between the read head and mill head to match the thickness of the brackets.

I think the protective plastic dust cover is a nice touch ! Keeps greasy fingers off the membrane buttons.

Like all things, I should have got one years ago...

That is why they changed it on the S7 so that it rotates through a full 360 degrees.

On the S7 it is secured by two screws, one on each side of the cross slide.

Hi Peter, Guys,

When I refurbished mine I made a blocking piece to fill that gap ! The short end cannot now disappear through the gap and the material is supported right to the end. However beware that very thin pieces may get dragged into the gap between the blade and guide, jamming the blade.

Before !

After !

No more lost or unsupported pieces.

True !

Hi Andrew, Guys,

The Device known as a voltage-dependent resistor (VDR), is not a resistor at all It is a semiconductor device more akin to a voltage sensitive diode. A similar mechanism as a Zenner diode but it has the same electrical characteristics for current in both directions. In other words it is not unidirectional as a diode is but Bidirectional.

FWIW a resistor is a linear device. The behaviour is according to Ohms Law.

As far as discussing motor brushes is concerned, I've better things to do than going round in ever decreasing circles !

The theory and the math is there on the Internet !

Hi Malcolm, Guys,

Resistivity "resistance" is a constant ! It does not vary with voltage. There are however resistive devices that increase or decrease in resistance with temperature.

For any given resistance the current through it will increase with voltage.

I do agree that the current drawn by a motor tends to increase as the motor load increases. Motor brushes are very low resistance devices, fractions of an Ohm in most cases, loading them with copper increases the hardness of the brush, in turn improving the wear resistance, usually at the expense of the copper segments of the commutator. Actually the arcing that one often gets at the brush/commutator interface can wear the brush as much as just the friction between them.

But as you imply quite a complex subject, much of which has been alleviated with the advent of brushless DC motors.

No it isn't ! In fact it has absolutely nothing to do with voltage. Material resistivity is a function of the material ! It is a measurement of conductivity, the ability to conduct electricity.

Edited By John Baron on 09/03/2021 14:54:09

Hi Stevei,

Very nice protectors for the mill table ! I'm just about to make some new ones for mine !

I've just fitted a set of scales for the DRO on mine and am doing some servicing whilst I'm at it. Surprising what you find when you start stripping a Chinese mill to bits.

I'm about to investigate sorting out the quill bearings ! They feel fine in the machine, but quite lumpy with the quill out and turned by hand.

Edited By John Baron on 09/03/2021 07:28:48

Hi Keith, Guys,

Thanks for the picture, that is what I expected to see ! just curious, did you measure the dimensions of yours ? I wonder weather they are the same size.

As far as the brush hardness is concerned there is no difference between AC and DC brushes, as seen in universal motor use. There are however quite marked different grades and composition of brushes intended for different applications. For instance, some brushes are copper loaded and some are very soft, like the carbon material that I am using for making my replacements.

I'm not saying that the stuff I'm using is correct, but it works for me. The wear on the brush is basically due to two causes, brush pressure and arcing. Higher pressure on the brush reduces arcing but increases wear hence the reason for harder brush material.

I would remove the brush cap and inspect the commutator for wear and blow out any dust. Inspect the wiring to the brush holders for any heating damage, particularly loose push on fix connectors. Any looseness will cause heating leading to failure of the insulation of the brush holder.

DO NOT attempt to remove the armature !

Apart from the fact that it will be very difficult, it will cause the field magnets to loose their strength.

HTH.

Hi Dr_GMJN, Guys,

You could make a holder for the blade that clamps into the slot in the tool block and use shims under it ! There is no way I would want to start shimming under the whole tool post.

Hi Keith,

Good question ! Those big brushes were given to me by a motor re-winders in Sheffield quite a while ago. They came out of their scrap bin. I haven't a clue what they would have belonged to, but they are enormous compared to the ones that I make from them.

The one on the right is the one that I've made, it also requires cutting in half. That big brush is 50 mm by 32 mm by 6 mm thick. Manufactured by Morganite I believe. The brushes in the mill motor are 3.6 mm thick by 10 mm wide and 15 mm long. Everybody I've tried for replacements wants to sell me an expensive motor costing an arm and a leg !

Hi Noel, Guys,

No ! The little pigtails don't exist on my home brew brushes, they did on the originals, but they simply fell out allowing the motor current to destroy the springs.

I think anything 3/8" inch or thicker will be quite satisfactory. Don't forget that the nut on top will also be adding to the security of the plate.

For what its worth, and I'm sure you have seen the picture of mine, I use a cheap Chinese parting blade that is 2 mm thick, 12 mm wide and 200 mm long. It is perfectly rectangular in section. And is clamped by two screws with a third used to clamp the supporting piece.

I don't have any packings or shims under the blade, the height of the whole tool block being used to set the blade centre height.

Hi Dr_GMJN,

Just putting a steel plate on to replace the broken lip will not change the tool height or be any weaker than it would have been had it not failed.

You could of course do a much better job which would require far more work for very little benefit ! What I've and I noticed other suggested the same would produce a quick and easy fix.

Lets see piece of plate, five holes to drill four holes to tap. Two M8 two M6. Saw the top off, mill it clean, mark out for the five holes, drill. Tap the M6 holes, spot through the M8 ones. Drill 7 mm, thread the block M8. Fit the plate. Job done !  Opps forgot the hole for the long clamp screw.

Edited By John Baron on 07/03/2021 21:02:25

Don't mess about ! Just break that lip off, mill a flat across the top and bolt a steel plate on there to replace the broken bit. That failure is about what you would expect from cast iron. Use a pair of M8 coarse threaded bolts. to fasten the new plate.  Shouldn't take more than an hour !

Edited By John Baron on 07/03/2021 20:39:11