Milling a Slot in Stainless Tubing

pak · Oct 11, 2009

I am in the middle of a project that you all helped me with a while back. Here is the thread. http://www.practicalmachinist.com/vb/showthread.php/ot-fuel-level-round-184875.html

I am using a sight tube as some suggested. The sight glass will be protected with a ¾” stainless tube about 26 inches long. I need to cut slots in the SS tubing in order to see fuel level. See attached pictures.

Using my CNC mill, I tested a couple of bits that I had on hand. I used a range of feeds and speeds and the results were the same - less than desirable. Although they both cut fine, they left burrs on the top that will have to be removed by hand.

Is there a type of bit that I should be using that would not leave burrs on top? In wood working there is something called a down-cut bit where it leaves the surface very clean? I presume that there is something like this in the metal working field as well.

My question is; what is the best bit to use to leave a clean surface on the tubing? If I knew this, I could then experiment with the proper feeds and speeds.

Regards,

Drew

vettepicking · Oct 11, 2009

im in fla, where would one need a clubhouse in the middle of a bay and a generator to run it? maybe the jetport. let me know, sounds interesting.

Walter A · Oct 11, 2009

Yes you can get an endmill with a built in chamfer tool but for a one shot job a file (or hand deburrer) is faster and cheaper.

Milling leaves burrs. It's expected which is why we have deburring tools and files.

Walter A.

jackalope · Oct 11, 2009

Do as you have done and then run a chamfer bit right back over it.

PATEGW · Oct 11, 2009

Must be Stiltsville.

Walter A · Oct 11, 2009

The only problem with running a chamfer bit on tubing is alignment. Where an imperfect outside diameter or slight off center will not change a slot the chamfer will be uneven as it connects two adjacent surfaces.

Walter A.

Mickey_D · Oct 11, 2009

I mill a lot of slots in 16 gauge 316 stainless tubing (840 this weekend alone) and the stuff just sucks but is doable. It looks like your spindle is turning too slow and your feed rate is too low, but without knowing what the material is and what tool/rpm/feed you used it is hard to be sure. I use a 3/16th HSS 4FL CC (I have to plunge into the tubes) at about 950 rpm at 2ipm plunge and 4.2 ipm feed with the coolant at around 10% to 12%. This lets me get around 250 slots before the tool is trashed. I have tried several different carbide tools, and they all seem to chip on the plunge, so I keep going back to HSS just because it is more predictable. As far as the burr that you are getting, it should come off quickly with a file and a deburr tool. Some of the 316 that I get machines beautifully and some is like cutting abrasive bubble gum that just wants to weld to your tool. The bad stuff comes in with no markings, just a very pretty brushed finish - I suspect that it is from China. The good stuff has lot and mill numbers and labeled USA.

Troup · Oct 12, 2009

Sometimes you can minimise the final thickness through the 'hinge' (connecting the burr to the parent workpiece) by using a slightly smaller slotting cutter (you can perhaps use an in between metric size) and then programming a cleanup cut to orbit round the slot, taking maybe 0.005 - 0.010" per orbit (the thicker the preexisting hinge, the more orbits it will take)

Do these orbits in the downcut (climb) direction, and try to find a 'sweet' spindle speed (probably faster than when you cut the slot) where the spindle and cutter and structure are not at, or near, a resonance.

There are such things as right-hand cut, left-hand helix cutters for metal, similar to a down-cut router bit, but that would just throw the burr to the inside of the tube, where it's a lot harder to get at.

In theory what would be ideal would be a straight-fluted slot drill, but I think you should do fine if you use a high quality, sharp cutter of a more conventional type, and perhaps give it the best chance of remaining in that condition by predrilling a hole at each end of the slot, ideally with a spotting drill.

If you go the undersize cutter route, this hole can be big enough that the end mill won't have to do any end cutting at all - which will make its life easier in stainless.

pak · Oct 12, 2009

Follow up

Vettepicking,

PateGW has it right. I am a member of the Miami-Springs Power Boat Club; thus a caretaker of one of the most beautiful historical sites in Miami, Florida - Stiltsville. It is owned by the National Parks Service and has a very long and colorful history. Here is a link. http://www.stiltsville.org/pages/history.html We have an agreement with National Parks that we can use it at anytime as long as we maintain it at our expense and provide community service events for organizations such as Boys Club, Optimist, Church groups. Etc.

The particular house that I am working on has a 35kw generator that serves both the house power and the many boats that routinely dock there. For the boats, there are five fifty amp shore power connections and three thirty amp connections.

The house itself requires very little power – paddle fans, satellite TV, microwave, toaster/oven, outlets, etc. As a rule, these appliances get their power from an inverter whose power is supplied by 8D batteries. The batteries charge from solar panels on the roof. The generator is only run when the inverter cannot supply the demand. Some of the boats require much more. Boats with a 50 amp service is not uncommon.

My objective is to create a better way to gauge the fuel level for the generator without significantly modifying the fuel tank, thus my previous post.

Walter A & Jackalope,

While I could chamfer the sides of the slit with little problem, I cannot figure out how to do the ends of the slot. My CAM program has only horizontal roughing, profiling and parallel finishing. None of these will work as needed. I would have to debur the ends by hand and they would look different than the sides.

Mickey-D,

I am using a 4 flute ¼” HSS coated bit @ 1500 rpm and 6 ipm. That’s about 98 sfpm and .001 ipt. It looks like you are running at about 46.6 sfpm and .001 ipt. At that speed I would run @ 712 rpm. The SS is labeled inside, but I can’t read it without cutting more of a slot. It looks like a 300 series.

Troup,

My thoughts exactly. I am doing a multi-pass slot as suggested. I am also spot drilling then oversize drilling the entry point so I don’t have to plunge mill. The bit is ¼” while the slot is 3/8”. I am roughing the slot counter clockwise to .01 with a conventional cut then finishing the same way with a light cut. I have not yet tried climb cutting the finish, though I will give that a shot as you suggest.

I guess that I will try some other feeds and speeds and buy a deburring tool. What type of tool would you recommend for my application.

Regards,

Drew

Walter A · Oct 12, 2009

http://www.noga.com/nogaCategories.php?catID=deb

Several styles to choose from. With a steady hand and good blade all will be equal.

Walter A.

henrya · Oct 12, 2009

Handwork!

After cutting the slots I'd draw file the tube carefully at the cut then finish the entire outside with wet or dry paper glued to a hard rubber or even a wooden block. For final "color" I'd give it a longwise rubbing with Scotchbrite or SS steel wool. Or you could beadblast it to get a uniform look to the part.

pak · Oct 12, 2009

Walter A said:
http://www.noga.com/nogaCategories.php?catID=deb

Several styles to choose from. With a steady hand and good blade all will be equal.

Walter A.

Thank you Walter. The NG1003 with the S20 blade is on its way from MSC.

Regards,

Drew

pak · Oct 12, 2009

henrya said:
After cutting the slots I'd draw file the tube carefully at the cut then finish the entire outside with wet or dry paper glued to a hard rubber or even a wooden block. For final "color" I'd give it a longwise rubbing with Scotchbrite or SS steel wool. Or you could beadblast it to get a uniform look to the part.

Henrya,

The outside of the tube already has an extremely bright finish. Much like a mirror. I only need to concetrate on the edges of the slot. Thanks though.

Regards,

Drew

jim rozen · Oct 12, 2009

Polished brass would look a lot better than stainless and
will cut a lot easier.

Don't use an end mill. Use a woodruff key cutter. Run
slow, use sulfurized cutting oil if you insist on stainless.

Jim

Troup · Oct 12, 2009

pak said:
I am roughing the slot counter clockwise to .01 with a conventional cut then finishing the same way with a light cut. I have not yet tried climb cutting the finish....

If you're orbiting counterclockwise, you'd be climbing- unless you have a left-hand-cutting tool?

Actually you can make GREAT burrs by spinning the spindle the wrong way for the cutter- DAMHIKT !

pak · Oct 13, 2009

Follow Up

jim rozen said:
Don't use an end mill. Use a woodruff key cutter. Run
slow, use sulfurized cutting oil if you insist on stainless.

Jim

Jim,

While I have completed the stainless part of my project, I would be interested in the woodruff key cutter option for future projects. Can you elaborate?

Troup said:
If you're orbiting counterclockwise, you'd be climbing- unless you have a left-hand-cutting tool?

Troup,

Good catch. I think my head was spinning counter clockwise. The orbit was clockwise as was the bit’s rotation on the roughing operation. As you pointed out earlier, I changed to a climb cut on the cleanup. It came out much better, but still left a little ragged edge.

I wasn’t sure how to secure the tube without crushing it, so I milled a 2 x 4 clamped to the table (see pic) just undersized so the tube would fit tight. I then clamped a half inch round bar to the wood in the open end of the tube and used a wood shim just past where the end of cut would be on the other side. Using the same feed and speed, the difference in tone was remarkable. It almost sounded like I was cutting aluminum instead of stainless. If I had a lot of these to do, I would consider a fixture something like this made out of plastic to withstand the coolant.

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Milling a Slot in Stainless Tubing

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