How to machine 2 sided parts on a entry level CNC mill

Hi All!

Firstly I love how much information can be found here and how helpful the members are.

I am new to the MIlling world and would like to ask you guys for your ideas or best practises on a particular scenario.

I have a Chinese 3020 CNC mill and only machine wood with it. It works well and produces nice results.
My problem is that I don't really know how to machine 2 sided parts accurately.
I can machine the top and the bottom part very accurately separately but when I try to machine it on the same stock, I can't alight the stock perfectly enough so the top and bottom operations are perfectly aligned ans I end up with drifted dimensions due to me not placing placing the stock back on the machine perfectly after turning it upside down or not zero-ing the machine perfectly.

Here is some info that might help:
- Stock is always 50mm(x) x 50mm(Y) by 7mm(z)
- I always set the Gcode to start from the centre of the Stock on the X,Y axis
- I find the centre of the stock with a caliper, mark it on the stock
- I place the stock on the machine and fix it in place with a flat vise
- I manually move the machine to the marked centre on the stock and visually check that it is on the mark, then zero the machine and regenerate the G-code so the machine knows where it is positioned compared to the stock
- I then machine the top out of the stock

When its done machining the top part from the stock I turn the stock upside down and mark the centre again as I did before on the top side then place it back on the machine top facing up this time
- then I again find the centre of the stock manually by moving the machine spindle over the mark and zero the machine and regenerate the gcode
- I machine the bottom part

the problem is that the above method does not give me perfect precision and I end up with a part that has the top part perfectly centred while the bottom part is misaligned compared to the top operation.

How can I get around this?

I thought about fixing a 90 degree corer onto the machine that would always stay in the same place. This would ensure that the stock is always in the same place on the machine
then I was thinking of somehow defining a zero point on mach 3 relative to the stationary corner placed on the machine. Since my stock is always the same size, in theory this could work.

Do you guys have any suggestions that does not involve me spending a lot of money?

Thank you

This is really easy.
Google gage pins.

You drill a hole somewhere, then ream it, to about 5.00 mm, for 2 of 5.00 mm gage pin.
Drill 4.7 mm, ream 5.00 m.
Then you put your blank on the gage pins, and proceed.

When you flip your part, it will repeat to about 0.01 mm, every time.

The part may be oriented differently for op2, wont matter.
Just program accordingly.

You can flip and align the part on the gage pins, then clamp a floating vice to it, then remove the gage pins.
Or machine out the gage pins as last step.

I think there are really two key aspects to what you're talking about. The first is fixturing, and hanermo has some good ideas for you. There are lots of creative ways to fixture parts, and I've been learning that learning how to fixture parts throughout multiple machining operations is an ongoing learning experience for machinists. For many parts, it's worth taking time to either machine areas in your table or subplate for registration pins, or even making custom fixtures to register and hold your work piece throughout different machining steps.

The second thing is that it's difficult to visually home your mill. I generally use an edge finder such as a Starrett 827A. I can generally get reproducible results with it if I go slowly up to maybe 0.0005 in. If I have a part that is oddly shaped, I will generally machine a fixture that has registration surfaces for my x and y axes, and then I will home z using the sheet of paper method.

Based on what you're talking about, I would invest in a sacrificial subplate that stay mounted on the mill. I would first fixture it to the bed and then have the mill cut holes for gage pins that you align your work to. I may also cut a feature that can be used to register your x and y origin that is a known distance from the registration points, and then I would use an edge finder to make sure the machine is properly zeroed prior to starting your work.

Good luck!

Thank you for your responses.

I guess the gape pins are there to push the stock against on the X and Y axis? in that case I need something that pushed against those pins.
Lets say that I have the gape pins in place and I managed to secure the stock against them, I still have the issue of zero-ing the machine to the centre of the stock.
Currently I don't think I have a home set in mach 3 for my machine that I can return to when I want.
I guess with the gape pins and a saved home position in mach 3 I could do what I want.
First I need to look into gape pins and find a way somehow to save a reusable zero position on mach 3 for my machine. Once I have that, I can set the pins in place and based on their position define a zero position.

Since my stock will always be the same size I can just set it once and I should be fine

In my mind, you would use the gage pins to make sure your work was parallel to the x and y axes during the flipping. The best bet may be to home your machine to the gage pins, not to the work itself because they shouldn't change with respect to the machine.

Tooling Tabs
https://www.google.com/search?client...30.pFBJuY3qWHw

I usually drill a small through hole in the stock at known coordinates. After I machine side 1, I turn the part over and set the new X origin using that hole.

As for the Y axis, I simply align the stock with the bottom edge of the table. So the flipped part stays parallel to itself and the Y origin point remains unchanged.

Make sure your spindle is trammed (perpendicular to the X-Y plane), otherwise your side 1 and side 2 will never align with each other.