How to fixture and index a part with more than one plane to machine on.

[xitonecabs]

Hi All,

I am attaching a project that is presenting my some challenges. Please ignore all the warnings and drawing issues... this has been a work in progress and I have changed approaches a couple of different times so it kinda got out of hand a little.

So.. this project is a lap steel guitar that I want to CNC on my router, which is a Mach3 controlled 3D CNC that I built. 

I want to be able to do all of this on the CNC but I am struggling with how to index the part when there are two different angles that have to be machined. In effect, how can I index between these two setups so that when I turn the part over to cut the headstock profile and drill the tuning machine holes that it will all line up correctly?

I feel like this should be pretty simple and that I am just having a mental block on it.... but I could be wrong of course. 

Thanks in advance for any tips or advice.

Mick

[seth.madore]

For your "Left Fixture", is this a vacuum fixture? When do you plan on establishing the outside profile?

What I would do:

1) Accurate locating holes in the fixture, in areas of "waste" material that will still be there on the 2nd Op.

2) Bore accurate holes in the stock at those same locations.

3) Do all the work you can on OP1, leaving the material around the locating holes untouched

4) Flip the part over, using pins or steel dowel pins to locate the part, with the OP1 side down.

5) Locate your WCS origin accordingly. Pro-tip; use the same WCS for both sides. Specifically, I would make one of the locating holes your Datum, that way your OP1/OP2 Setup (in Fusion) would be accurate.

[Joshua.Aigen]

Hi @xitonecabs, Quick question are you trying to do a flip mill or are there other angles that you would like to do? For flip milling the tolerance of your machine is critical, if there is a ton of backlash you will never get a flip perfect. That being said you should be able to get as close as you can and maybe be able to tune the machine to be better. I would also say practice the more you do this kind of machining the better you will get. 

The things I have done to do a flip mill are,

1. Soft jaws or a machined out cavity that the part actually fits into.

2. Dowel pins (minimum 2 max 3) the pins are placed into holes in the part and the fixture therefor everything is aligned.

3. Leave material on the first side that is square to the part, this allows for alignment and fixturing although this is less accurate.

4. Last I've done the entire project on a rotary so when one side is done the rotary turns over and starts on the other side.

Looking at your part a flip should be pretty easy, maybe use the pocket/hole for the pick-up as an alignment pin along with a drill hole. I would also look at your work holding as that will be extremely important as the part is turned over you will need the part to hold securely as this will also determine the accuracy. Hope this helps and good luck.

[xitonecabs]

Hi Seth,

Thanks for your answer. I already have the alignment holes to align the fixture to the CNC table and the stock to the fixture. I use 1/4" steel pins in both cases.

[xitonecabs]

Hey Joshua,

Fortunately for me, my table is very accurate... nearly zero backlash and square and parallel wihin .0005.

I am not worried about the flip milling for the body. But the headstock is different. As you can tell, it is at an angle. I can easily mill the angle of the headstock from the top and back cuts. The challenge for me is to actually cut the profile of the headstock and drill the 6 holes for the tuning keys.. all of that geometry is at a 4.6 degree angle away from the flat plane of the table and the profile and holes need to be perpendicular to the 4.6 degree plane.

I have two fixtures and I already have the 4.6 degree slope cut into the end of them both. I will be making them both vacuum hold down capable.. as shown in the Left Fixture in the drawing. As said earlier, I already have alignment holes for fixture to table and stock to fixture alignment.

I can index the right fixure off the left one easily enough but I am drawing a blank on how to index the other fixture so that I can cut perpendicular to the 4.6 degree slope to cut the headstock profile and do the tuning key holes.

My initial thought was to mill the top slope flat all the way across and simply turn it over and do the headstock profile cut from the backside as well as the tuning key holes. But there is a problem with that.. how do I index for that? If I drill alignment holes in the stock from the top or bottom then those holes won't be perpendicular to the 4.6 degree slope of the headstock, which is required.

I will have one set of holes I need to index.. but they are at two different angles.. in the plane of the bed of the machine or at a 4.6 degree angle away from that....

I'm wondering if I will need to not only drill alignment holes in the stock but also cut the outside profile so thay maybe I can index from one of the ends of the stock????

I hope this make sense.. it does in my head... The reality is, I am probably over thinking it...

Thanks!

[seth.madore]

When it comes time to drilling the holes, you're going to need to lift your entire fixture and part 4.6 degrees. That is, unless you have a spindle that can be indexed to the correct angle. At which point, post processor edits will likely be needed to account for that rotation.