How would you machine this?

We need to cut a .75" wide slot all the way down a 60" piece of .75" x 1.5" x .125" box aluminum. This slot needs to be as accurate as possible. What would be the most accurate way to do this?

A large milling machine could do it, or if you set up a smaller one so you can move the piece after you make part of the cut, then that should work too. It will be kind of challenging to set it up. You probably want to make sure the work is clamped in several places so it can’t flex, and that the clamps are set up so they won’t move as you reposition the work on the bed.

This year we are going to a lot of trouble to make our robot with a cordless drill and a hacksaw…just because of the fun problems like that we’ve faced in the past.

If you have access to a horizontal mill with a long enough table, you could just bolt the piece directly to the table and use a 3/4in cutter. You’d have it done in one pass and it’d be almost dead on. Just make sure the piece is square to cutter.

Yep. The problem is, you need 60" (5’) of travel. Most mills I’ve seen have nowhere near that length of travel in one pass.

Now, if you have the sides accurate enough, you could do it in two or maybe three passes. Pass one: one end to about the middle. Make sure the vise or other holder is straight first, though.
Pass two: Move the table back to start. Move the part to the middle, using the quill to help align. Repeat first pass. Do not have the tool contact the metal when you start!
Pass three: Repeat pass two.

How about a metal blade on a table saw? To get the 3/4" slot you may need to rip it twice, but it should be fairly accurate. You can always plug the extrusion with some wood after the first cut to keep it straight and true. Metal blades aren’t cheap, but neither are 5’ travel horizontal mills…

Bengineer

Is this to lighten the part, or is it a track for something to move in?

A manual mill may do it like eric said if you can keep the vice straight. I also like Ben’s idea of a table saw.

That was my first thought because we have already have a metal cutting blade for our table saw, but I’m just not sure if we could get a high enough level of accuracy. I’ll try this tonight on a piece of scrap.

The slot is for nylon rollers to run in. It’s supposed to be kind of like Bishop Wisecarver’s DualVee but cheaper and lighter (the rollers won’t be so “V” shaped so we won’t have to make the 45-degree bevel on the edge of the slot.)

Thanks for all of the quick replies!

I myself would not enven dare to try to make that big of a rip cut with a table unless its a saw you have no desire to use again for its proper use of cutting wood. The table motors arnt made for the kind of force need to cut aluminum. I tried this before on a dewalt table saw and didnt end very pretty not only is it bad for the machine its dangerous. I was trying to cut a 1/8 sheet of aluminum on a table saw and the piece vibrated uncontrolably and didnt leave a good cut what so ever.

Laxphan,

We regularly cut 1/8", 1/4", and 1/2" thick aluminum on our table saw here at work. It is, obviously, a little more heavy duty than your run-of-the-mill consumer table saw. But with the proper tooling and use, a table saw can certainly cut aluminum material.

I’m not saying that I’m the one doing the cutting, as I know that I am not experienced enough to work with the thicker materials. But, it can be done, and could actually provide the precision that is asked for in this application.

Bengineer

The shape you are looking for could presumably be formed from 3 pieces. A C-channel and two pieces of L angle. Rivited, welded, whatever.

I myself would not enven dare to try to make that big of a rip cut with a table saw unless its a saw you have no desire to use again for its proper use of cutting wood. The table saw motors arnt made for the kind of force need to cut aluminum. I tried this before on a dewalt table saw and didnt end very pretty not only is it bad for the machine its dangerous. I was trying to cut a 1/8 sheet of aluminum on a table saw and the piece vibrated uncontrolably and didnt leave a good cut what so ever. By the way the saw was securely mounted to the floor. Personally i would never dare to use my own table saw to rip metal. On top of that most abrasive cutting wheels are about 1/8 at most 3/16" which means you be making a lot more passes than two. Also to rip a 3/4" slot in wood you use a dado blades but there is no such instrument for cutting metal for the table saw.

As for my personal solution to machining this object you two option if you have access to mill you can mill the slot in two passes with 3/4" 2 flute mill bit by flipping the piece around. You should have no trouble doing this on any stage 1 or 2 bridgeports 9" x 41" tables. However if you dont have a nice mill. you have another option. We have a dewalt router about 4hp that we can put mill bits in for doing similar processes. If you go this route you need to create a jig to clamp the work piece and also a guide for the router to follow so it stays on the same cutting path. The mill is a far better option but we have used the router before and it works great much better than a table saw ever would. However you need the correct machine for any process therefore a 3/4 hp router is made for cutting wood not cutting metal.

On top of that most abrasive cutting wheels are about 1/8 at most 3/16" which means you be making a lot more passes than two.

Its not a very good Idea to use an abrasive blade to cut aluminum. There are carbide tipped blades that are specifically meant to cut aluminum and other non-ferrous metals. Here is a link to the blade that I’m talking about.Non-Ferrous Metal Saw Blade

The most accurate way to do this would be do mill it on a manual vertical mill.

Almost any way you do it you will have to do multiple passes. There are vertical mills out there with beds longer then 60" but they are few and far between.

If you want to minimize the number of passes that you wil have to make. You could make a really big vise out of two pieces of precision ground bar stock The first one you would drill holes in and then bolt to one of the slots of the mill. If you buy a precision ground flat stock bar you should be able to find one that will key into the t-slot of the vertical mill that you are using. The second bar you would use as the moveable jaw and use cam action clamps to move the moveable jaw into the fixed jaw. Like the ones found here http://www.miteebite.com/products/multi-fixture_e.html

The other approach is to use multiple vises. The big thing is that you need to set it up to be as even as possible across the vises. An easy way to do this would be to clamp the vises on to a piece of bar stock and then using a test indicator to run along the edge of the bar and adjust the vises until the dial does not move along the length of the bar.

The biggest problem with this machining task is that you will be turning a ridged piece of box tubing into a not as ridged piece of channel. So in order to compensate for the material that you are removing you will need to make a couple of blocks that you can slide inside the tubing so that you have something to clamp down to so that the tubing does not deform under the clamping pressure. If the sides of the tubing bend in under the clamping pressure that would cause the .75" slot the you mill to become wider once you release the clamping pressure.

I would also mill the slot in two passes. I would not use a .75" cutter I would use a .5" cutter over two passes. This allows you to compensate for clamping deformation, and actual cutter size.

It would be even more accurate to mill this on a horizontal mill using a slitting saw. But good luck on finding one I don’t know anyone that has one. But there are vertical to horizontal adapters for bridgeport mills that you can buy.

I have mill long slots in pieces of tubing and had to move and reclamp multiple times and it turned out fine it just took forever.

What type of tolerance are you trying to hold?

If you are planning on using this piece of box tubing in an elevator design it is not a bad idea to design in a degree of play so that the elevator does not bind as it goes up and down. Also please note that this piece of channel that you are making could easily deform under dynamic load and duing the competition.

I understand the desire to hold as tight of tolerance on machined parts as possible but just make sure that you don’t design yourself into a corner by demanding that they stay that way.

I guess the moral of this story is that it is possible to make it as accruate as you want to pay for.

Yeah most likely would be a large cnc mill or a horizontal mill with a little programming put into it. That would help.

You might look at this link. I am not suggesting using this but, it is designed for a table saw to cut aluminum

http://justsawblades.com/systimatic/hsr_gold.htm

We did something similar to this last year, on a shorter piece. The tubing will not want to stay rigid and hold the exact slot width that you machine. If you need the slot to be very close to 0.75, make sure to take more than one pass with a smaller bit so you can compensate for the spring effect. It may spring in or out, but be assured that if you take a single pass with a 0.750 mill, you will not end up with a 0.750 slot after you take the support blocks out. Otherwise, if you can leave the support blocks in the ends, that will help hold the tubing in it’s original position.

that blade is not for a table saw so esstentially your just using a radial arm saw or a miter saw as a chopsaw why do that when they have saws specifically made for metal called a chop saw.

I’ve been machining working with aluminum for five years now. Never once have i had a problem with an abrasive cutting wheel they work great. If ihad the money to spend the 460 to 540 bucks to buy a carbide tipped blade and dry cut saw i would but if your going to spend that kind of money on a saw you mind as well spend the 2000 and buy a nice wet saw which is the ultimate solution.