Drilling accurate holes in Al

[BanduraMaker]

Hi there. I'm a pretty experienced woodworker but am a novice at metal working. Not a total noob but certainly not versed on accurate machining of metal. Anyhow, I'm the proud owner of a new CNC router and want to drill some holes in the T-slot table for indexing work holding fixtures. My plan is outlined here: http://www.cnczone.com/forums/diy-cn..._fixtures.html

The issue is that I'm very nervous about drilling holes in my pretty T-slot table so I want to make sure everything works right. I'm planning on using 1/4" dowel pins.

To drill the holes, I have at my disposal:

A large center drill like this: http://www.google.com/products/catalog? ... ps-sellers

My center drills are labeled F1 & F2. I have smaller ones but they won't quite make it to the table the way my spindle's set up.

Standard length split point drill bit. Length of the flutes is about 2-3/4" (Too long? will it be flexing all over?)

1/4" end mill.

I could also buy a stub length 1/4" drill bit for added rigidity.

18000rpm spindle that I could throttle down to about 5000rpm safely.

My plan was to drill with the center drill first and follow up with the 1/4" bit. The questions are:

How deep should I drill with the center drill?

Will the 1/4" bit end up where I want it to be? Position is pretty critical in this application. I'd really like to be within a thou of my intended position.

Edit: What RPM & Feed rate would be good?

What size dowel pin should I buy to get a tight but removable fit?

To make sure I'm exactly where I want to be, I was planning on using G81 for the center drilling cycles, G83 for the deep holes and writing the G-code by hand. Everything will be referenced from my home position.

Any and all help will be very much appreciated - I think you all know how difficult it is to un-drill something!

[easymike29]

You are not likely to achieve accurate size or location by drilling. Drilling and reaming will likely get you size but not accurate position.

Gene

[RICHARD ZASTROW]

Drill under-size then bore to size. Or drill under-size; bore for location, consistent light finish stock and roundness; then finish ream to accurate size. Granted, a bit of over-kill but provides about as accurate size and location as your machine can produce.

Dick Z

[Mark Housel]

Might give this a look?

Mill Tips

[BobWarfield]

The guys are right about the difficulties of accurately positioning the holes.

However, excellent repeatability and good size for dowel pins are likely good enough for a fixture plate. You can map the location where the holes wound up and then rely on repeatability and good sizing for the dowel pins to get you to where the map says the hole should be.

For that reason, it is often convenient to engrave a coordinate system for the holes. Perhaps label the Y rows with letters and the X holes with numbers. Then record exactly where each hole is located.

Here are some notes on how to build fixture plates that include links to some worthwhile threads:

Mill Fixture Plate

FWIW, you can use a center drill to spot the holes but I prefer spot drills. I'd also get screw machine length bits. You may find they don't need spotting and they'll flex less due to shorter length whatever the case. Load them in a collet rather than a drill chuck to cut the runout a bit. And then ream the holes for the dowel pins.

Cheers,

BW

[BanduraMaker]

Thanks guys, that's some fantastic info. I especially like the idea of using drill bushings to hold any pins - I was a bit worried about the holes in Al wearing out.

Looks like spot drill followed by a ream will do the trick.

I think what I'll do is wait until I have a little more experience with the machine before I do any real drilling in my table top and profiles.

I also have to re-think my pin sizes as the drill bushing will have to fit between the T-slots so maybe 3/16" or something.


Thanks again!

[BanduraMaker]

One more question - is putting steel drill bushings into an Al table a bad idea from a corrosion standpoint? I suppose I won't be taking them out so it should be o.k. no?

[BobWarfield]

If you're worried about electrolysis, you may rather be asking whether sticking an aluminum fixture plate on your non-aluminum table is a bad idea.

People do it all the time would be my answer.

Cheers,

BW

[BanduraMaker]

Point taken - My machine is all Aluminum but it's got plenty of steel screws in it.