Bad surface finish 6x12 surface grinder spindle disassembly questions

[metric_taper]

I purchase new a Central machine DSG-612 model surface grinder (horrible fright). It has never had a good surface finish, and I have checked balance on the 7" wheels it uses.
I suspect poor quality manufacture, as there were some horrible manufacturing defects found many years ago in the cross slide front bearing mount (hammering of too thick of material, and it was made from scrap).
I want to disassemble the motor shaft rotor from it's custom housing to see what the bearings are, and if they are ABEC high tolerance or just minimal quality.
Has anyone here done this? Same model was sold as a low end surface grinder, elsewhere. Grizzly has this as G5963. Their manual has no disassembly or repair instructions.
As seen in the exploded view, the bearings are numbered 67-8,9. Is one of the spanner nuts (shown in below photos) a preload method to tighten the bearing slop? I don't want to go backwards and make the issue worse.


The spindle has a left hand thread M16x1.5, which appears to have been cut using a hand held threading die, as there is a swash plate wobble to the nut when I spin it on. Don't know why this was not single point threaded on the manufacturing machine, other then I'm guessing they had it mounted on centers and didn't have tooling to cut the threads correctly.


I do need to make the custom pin spanner for both of these face mounted nuts. These are 4mm size holes.


Below is a photo of the surface finish this machine has produced from day one (A2 tool steel scrap, hardened). It does grind flat to within tenths. But I want surface finish like the 3 inch grinding vise it is sitting on.

 

[DavidR8]

I may have missed it in the schematic but wouldn’t there also be a bearing at the wheel end of the spindle?

 

[metric_taper]

I may have missed it in the schematic but wouldn’t there also be a bearing at the wheel end of the spindle?

I think it's just behind the spanner nut plate, but I've not taken it apart, and looking for anyone that has on this type of machine. I've not see any posts on this site or elsewhere when I search for this machine type.
And that's a question only 2 bearings are shown, are these at the front of the spindle, or is the fan end have one also, or is that a sleeve bearing?
I know I'm going to have to tear this apart.

 

[Flyinfool]

Start by getting your very best indicator on the surfaces the wheel mounts to, to see if they are round and/or have run out.

How are you powering this machine? is it single phase, or manufactured 3PH. What are you using to get the 3PH? Static phase converters will do this. So will rotary to a lesser degree.

Did you dress the grinding wheel? You have to dress it every time you install it and you should every time you flip the part. I know some people that will dress the wheel every time they turn the machine on.

Do you let it run and warm up for a while to get the bearings up to operating temp before you start grinding?

That pattern can also come from moving the table to fast.

Lets be sure it is not something you are doing before you tear it apart.

 

[metric_taper]

Start by getting your very best indicator on the surfaces the wheel mounts to, to see if they are round and/or have run out.

How are you powering this machine? is it single phase, or manufactured 3PH. What are you using to get the 3PH? Static phase converters will do this. So will rotary to a lesser degree.

Did you dress the grinding wheel? You have to dress it every time you install it and you should every time you flip the part. I know some people that will dress the wheel every time they turn the machine on.

Do you let it run and warm up for a while to get the bearings up to operating temp before you start grinding?

Lets be sure it is not something you are doing before you tear it apart.

That was the first thing I did was put a tenths test indicator on the spindle. Can't see any runout, other then surface noise.
It's a single phase machine, running on 120VAC. I understand that 3 phase would give better surface finish. If I did have 3 phase motor I would use a VFD, so smaller wheels could maintain SFM.
And yes, I true the wheel every time I mount a new one.
Over the hours it took to grind all 6 sides of this cube, surface finish never got better, so I don't think bearing warmup was causal. And I retrued the wheel many times during this process. I tried 3 different wheel types with zero improvement in finish. I did see some wheels with a swash plate wobble, and that's probably why they were sold on eBay for $15.
I'm believing the bearings were bad from the start, or the motor rotor is not balanced. It may be the bearing housing is machined wrong, and there's slop.
For reference, I just got 3 new grinding hubs off eBay, and independent of them, this finish was consistently the same.
My tenths test indicator can't see these surface blemishes on a surface plate.
I've read through many forums and all your suggestions were looked at previously.
And it may be, this is such a cheap machine, there's no fixing it.
I've lived with this surface finish all these years, I'm figuring it's time to address it to make better looking parts. From a distance, it looks hand scraped.
As an oddity, the practical machinist forum has posts that say you don't ever need to balance 7" wheels. I don't think that's true if the machine is light weight as this one is. But I've never ran a high end surface grinder before.
The down feed on this machine is very poor, so I use a digital indicator, that displays in tenths or microns.

 

[benmychree]

Cheap grinding wheels will also do that, due to out of balance.

 

[metric_taper]

Cheap grinding wheels will also do that, due to out of balance.

I've tried many different wheels, with zero change in that surface finish.

 

[benmychree]

Balancing hubs are available for 7" wheels, but are quite expensive.

 

[metric_taper]

Balancing hubs are available for 7" wheels, but are quite expensive.

The one that came with the machine has adjustable weights. I have a wheel balancing fixture, bearing type. I've ruled that issue out.
I've been trying for a bit now to fix this. I'm at that point where I need to take the spindle apart and see what they did. For reference this surface grinder say's made in Taiwan, in 1991. I don't know what quality they pumped out then. My 1976 lathe 10x24 Jet brand was a disaster. But my Vertex rotary tables 8 and 12 inch are very good quality.
I have a 18x40 6000 lbs lathe, that was imported by Grizzly, and never was in their catalog, only online. 2004 vintage, I'm convinced it was made from every incorrectly made part that should have been thrown away. But it does turn true. I still have one project, the metric taper slug that fits into the head stock to adapt a #4 Morse taper center was made wrong, it swashes in the spindle, as the minor diameter is too small. All the manuals call it out as a #6 MT, but that won't fit in a 80mm through hole of this spindle (way too small). So I found it is a 90mm (gauge line) metric taper spindle taper. Which is common in Europe.

 

[Flyinfool]

Here are some pics of the first grinding on my grinder.
Notice the sparks are in clumps with an area of no sparks in the stream.
This can not be seen with the naked eye but the camera had no problem spotting it.
When this was happening I was getting the exact same surface finish that you are getting.
I could not measure the ripples in the part surface, but like you I could see them.

As I got better at dressing the wheel this went away.
Making sure that all 4 feet of the machine had equal weight on them also helped.
Going slower cranking that handle helped.
more overlap on the passes helped.

It is not just one thing, it is all of them together.