How to tap a plate to match motor mounting threads

Well, I would like to know how. It doesn't matter why if there is a way to do it, right? That is why I asked... If there is a procedure then someone had a reason. I would like to know how. I don't know what to look for in the engineering handbook.
I don't need it for my current motor situation as I have already committed to using a clearance hole and torquing the bolt to the plate. I was going to countersink the hole but the bolts I have are hex head. So I will raise the main plate up to allow the hex heads clearance.
 
So just for academic purposes.
The only way I can think to pick up the thread of a blind hole would be;

Drill the tap drill holes into the plate.

Take your tap and you will need to modify it. Many bigger, not all, taps have a reduced diameter shank that is at or slightly less than the root diameter.
If your tap is not a reduced shank type you will have to turn and or grind it down to be a reduced diameter so the whole shank end of the tap will fit thru the tap drill hole.

Then look close at the geometry at the tip of the tap, You need to duplicate that geometry at the other end of the threads on the tap.

Screw the tap into one of the holes in the motor, then clamp the plate to the motor using tap drill size pins to accurately align 2 of the other holes with the motor, and tap your way out of the hole. Rinse and repeat for the other holes. DO NOT forget to permanently mark the plate and motor for proper orientation so it is always put back on the same way or the threads will be way off. It will now fit properly only one way.

As mentioned above, this will get your threads timed correctly but will not permit actually tightening the motor to the plate. You would need to bolt the plate to the motor and then bolt the plate to the machine or really bad things will happen when the motor falls off while running.
 
mariner3302 said:
Well, I would like to know how. It doesn't matter why if there is a way to do it, right? That is why I asked... If there is a procedure then someone had a reason. I would like to know how. I don't know what to look for in the engineering handbook.
I don't need it for my current motor situation as I have already committed to using a clearance hole and torquing the bolt to the plate. I was going to countersink the hole but the bolts I have are hex head. So I will raise the main plate up to allow the hex heads clearance.
Click to expand...

The only way how is to either use a CNC machine (likely won't be perfectly accurate, but might be close enough), or to tap them together with them clamped tightly together. That is, tap the 'outermost' plate, then use those threads to keep the tap lined up for the 2nd hole.

From an assemblers perspective, its a terrible idea. From a 'will this hold these plates together right' perspective, it is also a bad idea.
 
Flyinfool said:
So just for academic purposes.
The only way I can think to pick up the thread of a blind hole would be;

Drill the tap drill holes into the plate.

Take your tap and you will need to modify it. Many bigger, not all, taps have a reduced diameter shank that is at or slightly less than the root diameter.
If your tap is not a reduced shank type you will have to turn and or grind it down to be a reduced diameter so the whole shank end of the tap will fit thru the tap drill hole.

Then look close at the geometry at the tip of the tap, You need to duplicate that geometry at the other end of the threads on the tap.

Screw the tap into one of the holes in the motor, then clamp the plate to the motor using tap drill size pins to accurately align 2 of the other holes with the motor, and tap your way out of the hole. Rinse and repeat for the other holes. DO NOT forget to permanently mark the plate and motor for proper orientation so it is always put back on the same way or the threads will be way off. It will now fit properly only one way.

As mentioned above, this will get your threads timed correctly but will not permit actually tightening the motor to the plate. You would need to bolt the plate to the motor and then bolt the plate to the machine or really bad things will happen when the motor falls off while running.
Click to expand...
Thanks, Flyinfool, that was kind of what I was thinking.. Use a bottoming tap, grind the top into a through bolt style tap, keeping the shank, and then bring it out. It would take a good bit of work and patience. It would also have to be at least an inch of thread or more the tap would be in. There have been times when I want to add a plate to something with closed threads. Kind of like the current motor situation. I see it isn't the way in that case but I like learning things.
 
Thanks, I will read it and I am sure add it to my reference library, as meager as it is.
I like to make things and like figuring out how to do what I am wanting to. I appreciate the comments and warnings but that isn't the point of the post. Pointing out why not to do something is fine but figuring out HOW to do something, regardless of why, is what I enjoy. I may never need to do a reverse tap operation but if I did, I want to have idea's on what it would take. I sail offshore singlehanded and sometimes you have to do stuff that isn't the "right way" but it works 'enough'. I ask these kinds of questions a lot so please don't feel I am disregarding your insight and experience.
 
As I previously indicated, you can counter bore the mounting holes under the plate and use button head or a low profile head type. The thickness of the head is less than 0.200" for 3/8" bolt so with a steel plate you would have ~0.290" of metal which is fine, a 1/2" bolt is has a head height of 0.265", I would trim off about 0.020". The head of the bolts will sit just shy of the surface when bolted in, use some blue Loctite to prevent them from loosing. an alternative if you need one would be to turn a 1/2" rod with a threaded end that goes into the motor mount holes. The other end forms an alignment dowel pin going into the mounting plate. This forms alignment pins to prevent motor rotation, then tap the plate and use fingers that hold the motor to the plate. This is also used to remount taper attachments to lathes, etc. But I see no reason to make it complicated. On the button bolts I have also trimmed them on the lathe if the head was a bit to big height or diameter.

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McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.
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I am not inclined to thread the plate and motor together, but if you want that approach you could clamp the two together and drill/tap the next size up tap. Use an allen screw thread with some loctite.
 
Since this is a theoretical discussion, the reverse tap idea is probably not a NASA-quality way to do it, owing to the slop in the blind threads and so forth.
If you were on a desert island and only had the tap, then fine. Gilligan and the professor would probably have little choice.
-Mark
 
another crude method:

1) use transfer screws (like Neilsen or Heimann) to mark the bolt locations on the plate,
2) drill clearance holes in the plate so that a hex nut of proper size just passes thru.
3) clamp the plate to the motor and thread in a bolt (or all-thread) with a nut already on it into the motor mount
4) once the bolt/all-thread is deep enough for you, tighten the nut into the hole against the motor mount.
5) weld the nuts to the plate.

This is essential using the nut as a thread donor so that you could get the threads between the two plates synchronized properly before making them a permanent part of the plate.

-brino
 
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