tapping a steel plate

Probably pretty tough if you don't keep the bit cool. I'd recommend a torch to blow the hole through it and if it's a little big, a washer on each side should do nicely.

Otherwise, drill and squirt wd40 on the bit as you're drilling. Don't drill more than about 15 seconds without a squirt and let the bit rest after about a minute of working on it.

BTW, I've ruined more than my share of bits for not adhering to the above rules.

If you use stainless, go to a machine shop to do the drilling and taping. They know how to deal with stanless and your 5/8" theaded stub will be perpindicular to the plate.
jud

The hole doesn't have to be perpendicular. The feet on the theodolite will allow it to level up. You can lo-tech this one.

Leveling the gun is not why you need to be perpendicular. If it is not, when you mount the gun on a non uniform surface you will distort the base because of uneven pressure. Using a torch to cut a hole will distort the plate from uneven heat, a machinist's nightmare.
jud

I guess it depends on how long you get it hot for.

I've got a small drill press that I'm sure could do the job.

I agree with Jud 100%

It's a job for a professional (or someone that has the EXPENSIVE equipment AND know's how to use it). Vertical mill (or very good drill press), cutting lubricants, HIGH-end taps, etc.

Loyal

What Jud and Loyal said. A machine shop should charge you less for this than the plate costs...

A 1/4 Inch thick plate may not give you a adequate number of threads. If I recall the "Standard thread" pattern is 5/8 by 8. You might need a finer thread on the plate end of your stud to get an adequate grip in the plate. As suggested a good fabrication shop could do this for you at a reasonable cost.

I have a guy just up the street who has done some work for the space program I know he could do it (oops wrong side of the state 😉

Don't Use Allthread

Get a 5/8" x 11 set screw, they come in different lengths. Cutting the allthread to a sort size is difficult because you bugger up the softthreads clamping it, then you have to file and cleanup the threads. The setscrew is a much better grade of steel.

Sorry but the hole must be absolutely perpendicular to the plate. Even if you drilled it you would have a hard time tapping it perpendicular, plus a 1/4" plate will give only close to 3 threads.

Lastly even if the screw is drilled and tapped perfectly, once it is fixed to the plate the only way to get the tribrach base on is to turn the tribrach like a nut. You will create excessive wear of the bottom of the tribrach against the steel, so I suggest using a cheaper tribrach that you leave secured to the plate permanently.

With all the negatives I suggect you instead have 4 pads fastened to the corners of the plate, drill a simple 11/16" hole and use a 5/8" machine bolt. With the corner pads there is plenty of room for the bolt head under the plate.

Paul in PA

> I'd recommend a torch to blow the hole through it

Stainless steel produces refractory oxides, which makes oxy-acetylene cutting ineffective. You might be able to blow a hole through it, but it'd be messy.

Stainless is generally harder than mild steel, but -- as Kris noted -- as long as you use a sharp bit and keep plenty of lubricant on it (I use a citrus-based cleaner rather than oil, it cleans up easier) and go slowly, you won't have a problem. Use a drill press to ensure perpendicularity, and with a modicum of care you can tap the threads square.

Regarding plate thickness, my guess is that John plans to set the plate, with all-thread attached and locked by a backing nut, into a newly-poured concrete post. That would obviate the need to have a thicker plate just to produce enough threads for structural integrity. If that's not the case, I second the call for a thicker plate.

I originally was going with 1/2" thick. But, a 12" square 1/4" plate is about $50, a 1/4" thick is about $150. I think I will take the advice to have the hole "professionally" drilled. I don't think it needs to be tapped, if they spot weld it then the plate should stay put.

I have seen survey pedestals where the bolt is well anchored, but the plate gets loose after some years and spins a little.

Don't Use Allthread

Thee are at least 75 of these around at monitoring projects that we screw the tribrach onto all the time. So yes, the tribrach does function like a screw, can't be helped. I do not want to put a used tribrach on permanently because then I couldn't use my gyro or my T-3, neither of which has a detachable tribrach.

Then I suggest A Standard 5/8" Cadmium Plated Bolt

Drill the hole, stick in the bolt and capture it with a 6 sided plate welded to the bottom of the base to keep the bolt from turning, with a light plate welded under the bolt head to keep it from falling out. That way the bolt is a bit loose which eases the wear and strain as you twist tighten the tribrach. Cadmium plated bolts take the weather very well.

5/8" standard coarse threads are 11 per inch.

Paul in PA

Is there such a great need to go stainless? Why not aluminum? And the benny is that aluminum won't pull your needle. 😉

I strongly agree with the aluminum plate.

Put 1/4 inch aluminum plate on inside and outside of the transom of my boat and it is really strong and resistant to most everything.

Use a solid brass bolt and you will not have to worry about that either.

I hadn't thought about aluminum.

I am not worried about the bolt itself turning, as I have a 10" steel rod that has 5/8 threads on the top 2" or so, and the bottom has a right angle bend with about 2" sticking out. That will NOT rotate. Yes, I am setting this in fresh concrete. I am more worried about the plate rotating and coming loose. I have seen several where the top plate can be turned by hand, and several have been stolen.

> I am not worried about the bolt itself turning, as I have a 10" steel rod that has 5/8 threads on the top 2" or so, and the bottom has a right angle bend with about 2" sticking out. That will NOT rotate. Yes, I am setting this in fresh concrete. I am more worried about the plate rotating and coming loose. I have seen several where the top plate can be turned by hand, and several have been stolen.

In that case I'd probably go with 3/8" aluminum, drill and tap a 1/4"x20 hole near each corner, and install a 1/4"x2" stainless bolt in each. Run the bolts in so that the leading ends are just short of flush with what will be the top of the plate and install in the wet concrete with bolt heads down. That'll anchor the plate in the concrete. The cost of the aluminum plate will be nominal, and the machining will be trivial.

I drilled and tapped a 1/4" stainless steel plate for a new Mag Mount on the roof of my truck without any problems at all and I'm surely not a machinist.

Some sort of tapping block will help to keep the threads straight. 5/8" x 11 is what your are looking for. (sorry, I haven't mastered making links on this site yet.)
http://littlemachineshop.com/products/product_view.php?ProductID=3427&category=935142539