Tapping threads in 7050 aluminum?

[Particle]

I'm looking for a bit of guidance on how to create some threaded holes in the side of some 1/2" thick 7050 aluminum plate, having not done it before.

The first question I'd ask is if it's realistic with this particular alloy.

The second question I'd ask is how to go about doing it if it turns out to be a realistic goal. What tools will I need, what is the general procedure for drilling into the thin side of plate, etc?

[mfgbydesign]

7050 has similar characteristics to 7075, however much more stable since it goes through a compression process. It can often be rough and finish machined on one side, flipped over, and fully machined on the other side with slight changes to the first side even with significant material removal.

7050 has 70,000+ psi of tensile strength. Many people see aluminum and try speeds & feeds they might try on softer alloys. It can't be done.

Treat the material like medium carbon steel for feeds / speeds.
If the threads are to have fasteners removed and inserted frequently, screw thread inserts (helicoils) should be used. Also use quality stainless fasteners along with anti-sieze lubricant or you will have problems with corrosion and galling.

[Geof]

Just to add a little bit to the above. Use a slightly larger than normal tap drill size. Many charts show tap drill sizes for 75% thread from and 65% thread from, go with the larger one. You will have plenty of strength and if you make sure the bolt length engaged is equal to about 1-1/2 bolt diameters the bolt will probably break before the thread pulls.

And the slightly larger tap drill makes tapping so much easier!

[bradleyban]

We cut 7050 all day and we don't do anything special for it. We use solid carbide endmills and HSS and colbalt drills and taps. I've never had issues from 6061 to 7050? Nor would I use feeds and speeds equivalent to carbon steel. You'd be wasting alot of time.

Here's a good feeds and speed chart I use quite often. For Aluminum according to this chart is 800 - 2000 SFM vs 100 - 400 for low alloy steel????


It's Aluminum let the chips fly!


Solid Carbide Speed & Feed

[Particle]

What tools do I need for tapping these threads? Any tips? It's a procedure I haven't ever done myself beyond some basic instances of threading small holes through the face of thin sheet. This time I'm trying to put a relatively large bolt into the thin side of plate.

[bradleyban]

What tools do I need for tapping these threads? Any tips? It's a procedure I haven't ever done myself beyond some basic instances of threading small holes through the face of thin sheet. This time I'm trying to put a relatively large bolt into the thin side of plate.

You need to drill a hole to the correct size for whatever sized thread you are going to make. You can google a "Tap Drill Chart" and it will give you a list of thread sizes and corrisponding drill size needed. Then you need a tap of whatever size of thread you want to make. You just thread the tap in just like a bolt except you wanna turn in 1/2 a turn at a time and then turn back about a half turn until you feel it break the chips inside. You wanna thread the tap in bit by bit other wise the tap will build up and ruin the threads. Also you want to use a little of oil on the tap to help it ease into the metal. As far as the material. Aluminum is a soft metal and tapping into aluminum is very easy but also easy to bind up the tap. So go little turns at a time and use oil.


Cheers and good luck

[mfgbydesign]

OK some specifics:
Start with a 90 degree spot drill and create a chamfer equal to the major diameter of the thread.
If the hole depth is not limited by a cross hole or unacceptable break-thru, drill your minor diameter (tap drill for 65% engagement as Geof recommends) at LEAST 1/2 inch if not an inch deeper than the minimum full thread depth required.
Again if you were able to drill deep, a plug tap (sometimes called "gun tap") is the strongest type but pushes chips ahead of the tap. It also has a long lead-in chamfer which decreases chip load and helps get the tap started. Tapping thru a block (jig) with a hole drilled thru will help alignment also, if tapping by hand. Use good quality cutting oil and besides the chip-breaking tips provided, you might need to pull the tap out periodically and blow the chips out before they pack into the bottom of the hole.
If space is limited use a spiral tap or follow the plug tap with a bottoming tap, which have 3 threads chamfer.
If pitch diameter is a concern you need to select a tap that has the proper H limits (inch terminology, sorry metric users), usually an H3 for tighter control (3B thread) and H5 toward the high side (say a 2B thread) if memory serves me. Each H increases the pitch diameter by .0005 inches.

[Particle]

Man, there's a lot more to this than I had anticipated. How do I know how much pitch diameter I want? When you say "increase" does that mean it's moved up or down versus the thread walls? I had to look up what it was to begin with, so please be patient with me.

[Geof]

I think you are getting 'over-answered'. Ignore comments about pitch diameter, H3, etc. This is applicable if you are making something to defined specifications of tolerance.

Just get your tap and a tap wrench, tap drill, drill the hole and turn the tap into it using some tapping lubricant.

For hand tapping it is easier if you start with a 'taper' tap which has the threads ground on a taper so it starts cutting the thread gradually.

The taper tap does not cut a full thread form until it is deeper than the length of taper so then you use a 'bottoming' tap which has practically no taper. The bottoming tap is difficult to start into just a drill hole but when you have started the thread with the taper tap the bottoming tap enters easily in the few turns of full thread cut by the taper tap.

Simply tap with the bottoming tap until you have the depth of thread you need for the bolt you are using.

One important thing is to keep the tap paralle with the hole as it is starting to cut. This is fairly simple with the taper tap because the long taper tends to align it with the hole but you do have to make sure you turn the tap wrench with even pressure on both handles. This way you rotate the tap rather than pushing it sideways.

[Particle]

Alright, I think I know what I need in order to get this done provided I can get a straight hole in the plate (tips?). I can get a set of 8mm taper and plug taps pretty inexpensively. Pair that with some stainless steel bolts and I should be set I think.

- Do stainless bolts and aluminum threads get along natively or is an anti-corrosive required?

- Does anyone have the experience to give me an idea on if this bolt arrangement will be substantially rigid? I'm planning to attach a gantry to a CNC machine's base plate. Both are 1/2" thick 7050 aluminum plate. The gantry side wall is 6" wide. The graphic below shows seven M8x25 bolts mating the two pieces.

[mfgbydesign]

You might consider overlapping the top plate by 1/2" and adding a 1/2" thick gusset on each side of the vertical (same material say 2 inches tall and cross bolted; don't try to weld 7050). That will give you more joint strength and decrease the load on the fasteners. 4 should be plenty but you need to add (ready?) dowels, 2 on each side to maintain accurate position of the top plate once it's all together. If you don't care that they show, can be done as an assembly, if you want them hidden that's another topic.

[txcncman]

To help with alignment of your drilled holes, set up and angle plate on your knee mill and drill them that way.