Bought a few QCTPs to compare quality in inexpensive brands.

Gnpenning said:
Clearly I didn't do a good job describing what I did. I was referring to the T nut. I removed to much material and need to start over with new material. I just don't know if I should use anything special for the new T nut.
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Ahh, T nut! of course! My bad...

I use whatever steel is available, usually 1018. This is how I make them...

I make the T nut 'just fit' in the top of the T slot and the wide part will have some clearance at each side. That's because the upper part of the t slot is usually finished or even ground. The wide part of the t slot is always left as milled. The contact part of the T nut has to make good, even contact. -- If you are a perfectionist, you will use prussian blue to confirm it. I never have, but probably should. If you are really a nit-picker, mill a bit of relief on the inside edges of the T nut... Here's a picture of what I mean:

Tnut.PNG
 
What is so important about a precise fit in a T nut? Isn't its sole purpose to be able to screw a bolt into it to hold the tool post to the compound. Does it matter if it has a loose fit. I often machine carriage bolts to fit in the slot. Trim down two sides of the head and machine the angled portion flat. What am I missing?
 
As you apply load to the cutting tool, any play in the T-nut can allow movement of the toolpost. That's all there is to it.
 
pontiac428 said:
As you apply load to the cutting tool, any play in the T-nut can allow movement of the toolpost. That's all there is to it.
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Isn't there always the long axis that could shift? If the tee-nut moved along the long axis, wasn't it by definition too loose? If it is tight enough not to move on the long axis, why might it move on it's cross axis? Not being semantic, just trying to understand your reasoning. Reducing the degrees of freedom of movement are good, but don't quite understand the argument for a super fit.
 
It is about reducing degrees of freedom. Put a dial indicator on the toolpost stud with the base on your compound and observe what it does as you work. Try different cuts and see what difference 75 lb-ft vs. 25 lb-ft on the toolpost nut does. See if your tool tip moves when switching from a profile tool (infeed load) to a boring bar (outfeed load), and ask if the resulting taper is acceptable.

I agree that you can clamp the snot out of the toolpost no matter how loose the T-nut fits, but under steady load and vibration, there will be movement. If you move the point of constraint from the top of the compound T-nut slot to the bottom of the compound slot, you will get more elastic deflection because you are lengthening the fix point of the moment arm.

It may seem small, but tool tip rigidity is everything. Since the fit of the T-nut is in our control, it makes sense to control it. That is the basis of precision.
 
I have no idea how the commercial tool posts fit on the compound. On my Norman the base of the post is machined to fit into the slot. This keeps the post from rotating. I guess the post could flex back when turning and flex in when boring. I'll put a DI on the next time I use my lathe to see what happens.
 
mickri said:
What is so important about a precise fit in a T nut?
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Good question.

I've been thinking about this for the last 2 hours. I like being challenged - it helps me to discover what assumptions I work with.

WobblyHand said:
If it is tight enough not to move on the long axis, why might it move on it's cross axis?
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You have in interesting point there, and I'll try to address it also.

pontiac428 said:
It may seem small, but tool tip rigidity is everything. Since the fit of the T-nut is in our control, it makes sense to control it. That is the basis of precision.
Click to expand...
well said.

Let me go into some detail here: All really top-of-the-line tool posts use multiple techniques to prevent movement of the tool post. Unfortunately Aloris and Aloris-style tool posts don't. Look they are good - I have one.

toolpostforces.PNG

The force on the tool post can result in rotation, and also translation - movement across the face of the compound rest. In a typical Aloris style QCTP the only way to resist rotation is by friction from the tension of the bolt and the surface area in grey. The other movement can in part be controlled by the fit of the t nut by having friction on the red dotted line as well as the geometry of the bolt itself. By only having .020 between the T nut and the bottom of the tool post, there is a strong structure created where bending of the bolt is far more difficult. any strong force will then try to slide the T nut into the red dotted line. There is an unavoidable component of force going down the length of the T slot, but the far greater forces act on the grey area and the red dotted line.

What happens if the T nut is not tight fitting? You lose the firm seat for the end of the bolt, or seen another way, gives another surface that can slip - that is the underside of the compound-to top of the T nut connection. For less than an hour's work, you can eliminate this potential slipping surface.
 
Dabbler said:
Ahh, T nut! of course! My bad...

I use whatever steel is available, usually 1018. This is how I make them...

I make the T nut 'just fit' in the top of the T slot and the wide part will have some clearance at each side. That's because the upper part of the t slot is usually finished or even ground. The wide part of the t slot is always left as milled. The contact part of the T nut has to make good, even contact. -- If you are a perfectionist, you will use prussian blue to confirm it. I never have, but probably should. If you are really a nit-picker, mill a bit of relief on the inside edges of the T nut... Here's a picture of what I mean:

View attachment 445855
Click to expand...

Wow, thanks for the detailed response with pictures to boot. I just need to learn to ask the right questions. Much appreciated.
 
Am I correct in my recolection that the T-nut is also prefered to be the total width of the compound slot? Similar reasoning. More clamping area and stability?
 
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