Flat spot when facing

Hello guys,

I have this issue that every time I use a face mill (50mm) I get some flat spots in the surface. They are always at the same location, but I can‘t figure out where they are coming from. If I run a dial indicator over the surface they are about -0.05mm lower than the rest of the surface. My machine is a CNC’ed Optimum BF20 clone.
I recently had the machine completely rebuild and trammed in using a precision square.
My setup was using a 4“ vise, workpiece on parallels. I even get these spots when using an endmill.
What could be the reason?

Thanks in advance.

I guess your guide tilt over at this point. Try to adjust little closer. And measure with load on table.

Blue the Z axis gib and run the machine. Pull it back out and look at the contact. You should have full contact not just top or bottom. The pattern that you are displaying looks like head rock.

Where does this spot line up on the table? Does it occur on the same place on the x axis of the mill?

When the table is at full x axis travel can you move the table up and down and how much?

I have a G0704. These are the problems I have that create a similar pattern. Unless you scrape the whole thing I doubt you can get rid of it. The saddle on these machines is small and not very stable in design.

Hello,

Thanks for your answers!

I have just blued the gip and ran the Z axis up and down. I was surprised because there was good end even contact through out the whole travel!

Then I checked the table movement up and down at the end of the x axis travel. There is movement there, I would say at least 0.1mm, haven‘t put an indicator there.

Another thing I did was to move the vise around the table. When it was around 150mm to the right (where it was when I did the milling on the photos), I get a near perfect face mill finish! The vise sits now close to the right edge of the table. Maybe there is a high spot in the table?

My G0704 table had variations as much as .015" and a twist running through it. I got it all down to .0015". That was bad but compounded with a saddle that wasn't flat either made everything come out pretty far off.

Did you scraped your machine to achieve that?

"scrape" can be used as a loose term. It would take forever to scrape that much out. I have two surface plates a 12x12 and a 14x36 (old optical flat from a national lab). I have a scraper but not a camelback with a 60 bevel and the prices are a bit outside my range.

Since I didn't at the time have a machine big enough to mill it close I had to get creative. I used a belt sander to get rid of the high spots that the twist caused on the table face. Stopping to double check it with blue and a gauge very often. Switched to an air sander with 2" wheel to get more control. Once it was close I lapped the face with some lapping film against the larger granite. Deviation across the face was no more than .001" on any two points after hours (days?) of this.

The bottom dovetail of the table was fairly straight considering. It had some random high spots near the ends but it didn't really match the twist on the top. Once the top was flat I used a gauge to id the high spots and scraped them out and flat.

The saddle was done the same way as the table top but I did scrape in some oil grooves. I was never able to address the xy misalignment since I didn't have the tools I decided to leave it alone.

In no way am I advocating this as being correct. But my table was so bad I had to do something. They are now relatively parallel and the table doesn't lean much at full travel. I do use an SMW fixture plate that helped smooth things out on the table face.

Now that I have a larger machine I am probably going to properly address the errors. I may even turcite the saddles and gibs. And have also considered linear rails all around. But this is all down the road.