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tapping 1/2" alum jig plate 1/4-20 cut or forming tap??

R

richard newman

Titanium
Joined
Jul 28, 2006
Location
rochester, ny
Need to make a 1/2" x 12" x 24" tooling plate for a small cnc router (woodworking}, using MIC-6 or ATP-5. Need to tap a bunch of 1/4-20 thru holes, thinking to use a forming tap rather than a cutting tap to maximize thread strength and quality. Am I thinking right here?

Steel or cast iron would be much stronger of course, but this is a pretty light machine and the table moves, not the gantry, so I want to minimize the weight. The machine will just see occasional use, with 1/8" and smaller end mills in wood, hardly any cutting force.

Also, any advantage of MIC-6 vs ATP 5? Looks like the strength specs are higher for ATP 5
 
Form tap is definitely preferred from a no-chips standpoint. I don't know about higher thread strength, but I would guess in Al it wouldn't make much difference. You have to hold the hole size pretty tightly in order to get a proper form tapped thread.

Regards.

Mike
 
Either way your threads are not going to be the strongest. In 6061 form tapped threads have proven much stronger than cut in my bench tests, even with less thread percentage. No experience with ATP-5.
 
If it were me I would use screw thread inserts. Strength and wear would improve dramatically.
 
I'm not sure if cast aluminum will flow properly to form thread. I have never tried it. I look forward to learning something new today.
 
I think the MIC-6 plate is cast, not ideal for thread strength either way but cutting would be better IMHO. Agree with the use of some type of inserts, think they are called Keenserts or Kingserts and are easily replaced if they get worn out or stripped. I worked in a shop that used that type exclusively on everything, held up on fixtures and table sub-plates for decades.
Dan
 
Cast aluminum forms threads fine but does require more from the coolant as lubrication than other aluminums. You will be able to use the threads for hundreds or thousands of times before they wear out as long as you don't over tighten the screws. Helicoils work fine and are usually what I use when I insert the threads.
 
How are thread inserts made, that they would be stronger than simply cutting the threads? Is it a harder steel?

Or is it just that they would be stronger in aluminum, and that's why they are being recommended?
 
Two reasons they are stronger:
1)the thread cut into the aluminum to accept the insert is larger than the bolt accepted by the insert.
2)the insert distributes the load more equally over the engaged threads in the bolt to insert interface and from the insert to the aluminum. Because of bolt stretching as it is loaded, the topmost thread in straight aluminum (or steel for that matter)takes a large percentage of the load, the second one a fair bit and the third one only a little. By design, the inserts spread this load more evenly over about five threads. That is why they were first invented for use in aircraft in the thirties, not as a repair mechanism.

Denis
 
I have form tapped quite a few 1/4-20 holes in Mic-6 place, running the same canned cycle as I run in 6061 with the same coolant (Fuchs or Blaser at around 8-9%). Never worn out a tap, never had a bad thread that wasn't a drilling related issue. For fixturing purposes, using Mite-Bite claps or and a screw gun with a torque limiter set to a reasonable value, I expected to get several hundred cycles before the holes needed to be drilled and heli-coiled. I have never had to re heli-coil a hole due to wear. *I make no guarantee as to lifetime due to operator malfeasance.
 
kevin66 said:
If it were me I would use screw thread inserts. Strength and wear would improve dramatically.
Click to expand...

A strong yes on the thread inserts. We use 6061 for almost all of our fixtures and the threads fail more often than not, if I make the fixture I drill and tap for inserts.

Ymmv
 
Thanks for the advice guys. Sounds like form taps will work just fine.

I appreciate and agree with the suggestions re inserts. But this plate will see very little use, as will the router, so I feel safe going "bareback".

I'd thought about using 7075 and flattening it with a flycutter on my mill, but flatness is critical, and I'm just not convinced it wouldn't wander over time.
 
NO, do not use helicoils. Use a replaceable thread insert, they are more expensive but they are WAY more practical for a router table. Which is why CR Onsrud etc use these stock.

McMaster-Carr

that particular item is precisely what we use on our routers. M6x1 being a superior fastener to 1/4-20 (more threads).


or, go without entirely... seems like you have it figured out for low-use scenario.
 
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