Possible Spindle Bearing Replacement

All info relating to the Denford Starmill CNC milling machines

Moderators: Martin, Steve, Mr Magoo

Post Reply
NylonAdmiral
CNC Apprentice
CNC Apprentice
Posts: 48
Joined: Wed 03 Dec , 2008 13:43 pm

Possible Spindle Bearing Replacement

Post by NylonAdmiral » Fri 17 Feb , 2012 15:57 pm

I recently found that I was getting excessive runout when indicating tools on the mill.

Using a DTi gauge I indicated the mill equivalent of points 1, 2 and 3 on the diagram posted in this thread:

viewtopic.php?f=45&t=3293

I found that they were all within tolerance except 3 (the equivalent of the tool runout).

Regardless of the collet or tool that I use, I was always getting high runout.

I indicated pretty much every moving part of the mill and couldn’t find the problem.

However, I have since tried indicating the tool whilst “wobbling” the top of the spindle to see if there was any play in the bearing.

I have recorded a video to illustrate the point below. You will see that I’m not applying a lot of force but that it is having a significant impact on movement of the tool.

http://www.youtube.com/watch?v=CxeGRMCk ... e=youtu.be

Before I embark on replacing the bearing, does anyone have any other ideas that may be worth exploring first?

Also is this likely to be one of those painful jobs that I’ll wish I never started and is there anything I need to be wary of whilst doing so?

As always, thank you for the excellent help provided on this forum.

angel-tech
CNC Guru
CNC Guru
Posts: 525
Joined: Tue 14 Nov , 2006 12:12 pm
Hardware/Software: .....
4 Quatros
3 Oracs
2 Easiturn atc
12 station lathe atc
Mach 3
Location: Quatroland

Re: Possible Spindle Bearing Replacement

Post by angel-tech » Sat 18 Feb , 2012 0:29 am

Have you tried adjusting the bearings

NylonAdmiral
CNC Apprentice
CNC Apprentice
Posts: 48
Joined: Wed 03 Dec , 2008 13:43 pm

Re: Possible Spindle Bearing Replacement

Post by NylonAdmiral » Sat 18 Feb , 2012 9:19 am

Hi angel-tech, thanks for the reply.

I'm not really sure what you mean by adjust the bearings? How would I adjust them in order to remove the play which you can see in the video?

Thanks.

angel-tech
CNC Guru
CNC Guru
Posts: 525
Joined: Tue 14 Nov , 2006 12:12 pm
Hardware/Software: .....
4 Quatros
3 Oracs
2 Easiturn atc
12 station lathe atc
Mach 3
Location: Quatroland

Re: Possible Spindle Bearing Replacement

Post by angel-tech » Sat 18 Feb , 2012 11:05 am

There is a tab washer holding the nut in position, fold the tab back down and tighten the nut until the play is eliminated. Be careful not to over tighten the nut, and when you're happy fold the correct tab back into the nut to lock it.

NylonAdmiral
CNC Apprentice
CNC Apprentice
Posts: 48
Joined: Wed 03 Dec , 2008 13:43 pm

Re: Possible Spindle Bearing Replacement

Post by NylonAdmiral » Sat 18 Feb , 2012 13:13 pm

Thanks, following your suggestion I tried that but the amount of play remains constant.

I've tightened the nut about as much as I think is safe to. I marked the ring before tightening to see how far it moved and it must have only done up a few degrees.

I also removed that nut to have a look at the pulley. From looking at the attached assembly drawing (from this forum) and the picture I've attached showing the pulley (minus tab washer and retaining nut), it seems to me that the nut and tab washer are there to stop the pulley unwinding itself. It also looks like the pulley sits straight onto the top of the two bearings, so really it would be the pulley I would be wanting to tighten rather than the retaining nut?

Whether tightening solves the problem or whether I end up having to replace the bearings, I'm still going to have to tighten/loosen this pulley in one way or another and I can't see any obvious way of doing so! It has two recesses on the internal thread (like the channels you would expect to see for a woodruff key), I'm wondering if there is some sort of special tool that maybe locates into these enabling you to tighten/loosen the pulley?

Thanks again!
Attachments
Pulley.JPG
Pulley.JPG (153.11 KiB) Viewed 15529 times
Diagram.JPG
Diagram.JPG (40.19 KiB) Viewed 15529 times

angel-tech
CNC Guru
CNC Guru
Posts: 525
Joined: Tue 14 Nov , 2006 12:12 pm
Hardware/Software: .....
4 Quatros
3 Oracs
2 Easiturn atc
12 station lathe atc
Mach 3
Location: Quatroland

Re: Possible Spindle Bearing Replacement

Post by angel-tech » Sat 18 Feb , 2012 13:44 pm

the pulley isn't threaded in that diagram, so it will slide up and down when you loosen or tighten the nut.

NylonAdmiral
CNC Apprentice
CNC Apprentice
Posts: 48
Joined: Wed 03 Dec , 2008 13:43 pm

Re: Possible Spindle Bearing Replacement

Post by NylonAdmiral » Sat 18 Feb , 2012 14:03 pm

:oops:

Sometimes you really need a second pair of eyes after looking at something too long! That was stupid of me!

OK, well as I previously mentioned, I did the retaining nut up really pretty tightly and it didn't help so I think I'm going to try and take a closer look at these bearings.

With the belt removed I can run the spindle more freely by hand and the bearings don't sound as smooth as they could. Admittedly they're not too crunchy but the fact that they are making a bit of noise and the fact that tightening the retaining nut right up didn't help leads me to think they may need replacing :(

Thanks again for all your help angel-tech. I'll post back once I've gone a bit further with this.

NylonAdmiral
CNC Apprentice
CNC Apprentice
Posts: 48
Joined: Wed 03 Dec , 2008 13:43 pm

Re: Possible Spindle Bearing Replacement

Post by NylonAdmiral » Sat 18 Feb , 2012 14:43 pm

OK, now that I've come this far I feel like I might as well replace the bearings anyway, especially as I'm pretty sure that they are the problem.

Now I just need to source the correct bearings.

From this thread:

viewtopic.php?f=45&t=712

I think I need an SKF 7208B. Like this one:

http://simplybearings.co.uk/shop/produc ... s_id=45552

Can anyone confirm this before I order them?

Many thanks.

angel-tech
CNC Guru
CNC Guru
Posts: 525
Joined: Tue 14 Nov , 2006 12:12 pm
Hardware/Software: .....
4 Quatros
3 Oracs
2 Easiturn atc
12 station lathe atc
Mach 3
Location: Quatroland

Re: Possible Spindle Bearing Replacement

Post by angel-tech » Sat 18 Feb , 2012 15:17 pm

I would pull the bearings out first to be sure they are the correct ones. The original bearings will be marked.

if the originals are noisy they should be replaced.

NylonAdmiral
CNC Apprentice
CNC Apprentice
Posts: 48
Joined: Wed 03 Dec , 2008 13:43 pm

Re: Possible Spindle Bearing Replacement

Post by NylonAdmiral » Sat 18 Feb , 2012 16:17 pm

Thanks, will do.

I've got the whole spindle head off now so will take it to my local engineers to get the bearings pressed out.

NylonAdmiral
CNC Apprentice
CNC Apprentice
Posts: 48
Joined: Wed 03 Dec , 2008 13:43 pm

Re: Possible Spindle Bearing Replacement

Post by NylonAdmiral » Sat 28 Apr , 2012 15:45 pm

Thanks for all the help so far,

I've been away for a while but finally got everything back together and working.

However, I still have a problem I need to figure out. I've replaced the bearings and everything seems to be OK, except for the fact that I'm getting a poor quality surface finish.

I machined a flat face on to a block of mild steel, however, you can see from the attached picture that I'm getting "rings" or "swirls".

They are very slight and can only just be felt when running a finger over them so essentially they look a lot worse then they are. Logically I can only think that they are caused by some small degree of play in the Z axis and I can only think it is because the retaining collar that clamps the spindle bearings in place is not done up tight enough?

I am thinking then that I need to torque the collar up more tightly. Does anyone know what sort of torque should be applied?

I also wonder if anyone knows if there is a proper tool that locates into the slots (pictured) in the top of the spindle in order to secure it when tightening the collar?

I have so far managed to put a flat bar between the slots and tightened the collar up pretty tightly however I can only think that it needs to be tighter in order to secure the spindle and prevent any movement at all in an effort to remove the "swirls". I don't want to just rant it up and end up damaging the new bearings.

As always, any help is greatly appreciated.
Attachments
Spindle.jpg
Is there a proper tool that locates into these slots to secure the spindle?
Spindle.jpg (90.46 KiB) Viewed 15468 times
Rings.JPG
The rings that leave the poor finish.
Rings.JPG (163.06 KiB) Viewed 15468 times

Post Reply