Fit between QCTP and it's T-nut/bolt? (Chinese = .020")

Hi all,
The results of a recent thread about parting chatter on my lathe have encouraged me to bump up a toolpost upgrade I'd been procrastinating on. I'm replacing a BXA sized Phase II wedge QCTP with a CXA sized Dorian on my Colchester Mk1.5 15x48. The Dorian didn't come with a t-bolt, which doesn't look like too big a project, and shipping one here would be expensive. It will be a good example project for my students anyway. I know that the Dorian official one is a 3/4-16 thread. The only question left in my mind is to figure out how much slop there should be between the unthreaded shaft and the ID of the toolpost bore. In the case of the Phase II, it's .020" (the bore is .010" over the nominal 5/8" and the shaft .010" under). Before I recreate this (possibly bad Chinese) engineering, I'd like to know from someone with a good Aloris or Dorian toolpost, how much should it be, what's yours? I'd assume it should be a pretty close slip fit, rather than this sloppy 20 thou, but maybe that would bind up too easily, or there's some other reason it's undesirable?

I'm planning to turn and thread a shaft with a slightly bigger head (which will be the underside) and then press it into the reamed hole in the piece of bar that will become the T-nut. Since I'm doing it, it might as well be right.

Jason, all the serious work is done by the T-bolt in tension. Clearance of 1/32" is fairly standard on a bolt this (5/8) size. What's more important to consider is this: the center-line of the bolt should be as close to normal (90 degrees) to the top of the compound slide as possible (using normal shop equipment like a bridgeport-type mill).
Also, the top of the compound should be flat (within a couple of thousandths) and parallel to the horizontal centerline of the lathe.
Before you mount the new tool-post, use a flat stone on the top the compound and then rig an indicator mounted in/on the chuck and check for parallelism. Ideally, this should be no more than about .002. Also, stone the bottom of the tool-post(even a new one) before you mount it because s**t happens.

Thanks, Malcolm.

I'll keep going with that design then. I don't know if it's overkill, but rather than have the bolt part be threaded top and bottom, I was planning to turn it with a round head and pressing it into a reamed hole in the "nut". I figure that since the flats on the nut will be the reference surface for the reamed hole, it will all but guarantee the bolt stays normal to the flats in the nut.

the base of the toolpost contacts the top of the compound and is held in place by
the pressure from the bolt and tee-nut . i would think any suitable fastener that
doesn't stretch under tension would work fine -regardless of fit .

if you decide to go with a tight tolerance - i suggest that you make sure your tee
slot is perfect as well . if the interior isn't flat, when you bind
everything up , and there's no more wiggle room to 'find' its center -the uneven tension might actually weaken the setup & could eventually distort the compound's casting .
(does that make sense?)

Relieving the center portion of the bottom of the tool post will help it sit properly on the compound and will put the compression force at the periphery of the tool post base and that is where you want it. Say both the tool post and the compound are "perfectly" flat prior to putting the TP on the compound. Now place the TP on the compound and draw up that 3/4" bolt good and snug. Should we expect the compound and TP bottoms to remain perfectly flat or did they warp a bit since steel should really be considered rubber for purposes of these kinds of discussions? Obviously both parts start to bend in such a way that metal-to-metal compression is maximized near the bolt and decreases as you get further from the bolt. Unfortunately, the friction nearest the bolt is least effective at preventing post rotation and that nearest the periphery has the greatest mechanical resistance to rotation.

So, if you put the post in your four-jaw and just remove 20 thou from the center to 2/3 of the way to the nearest side of the post, you will have reduced greatly any chance of post rocking and you will be able to use 1/2 the torque on the bolt to achieve excellent cinching of the post. It probably will offend your sensibilities to "mangle" a brand new tool post (and perhaps others won't agree with this suggestion) but that is what I would do.

Denis

Only problem with relieving center as above, is when you have to fudge the toolpost over the edge a bit, to get that last bit of reach/clearance..

In my shop, I make the T-nut to a close sliding fit in the T-slot, with the top of the nut very slightly below the top of the compound. The nut is made of tool steel, but not hardened and it is threaded coarse to agree with the bore of the tool post or the hole in the tool post washer. I then use a high grade standard hex head bolt. The length of the bolt is such that it just clears the bottom of the T-slot, assuring maximum engagement in the nut threads.

I find that design much easier to make than a T-bolt and it does work quite well.

Larry

abarnsley said:

Only problem with relieving center as above, is when you have to fudge the toolpost over the edge a bit, to get that last bit of reach/clearance..

Click to expand...

Hmmm. Not sure that the relief would actually pose a problem if the top of the compound is as wide as the base of the tool post and it should be. I drew this out and it looks like it does not make much difference even as the overhang approaches 50%. Not sure that so much overhang is a great idea with or without relief. I will confess to overhanging maybe 30% to take just a light cut in a fairly inaccessible location.

If you felt the relief would be a temporary problem, just fill in the relief with a shim that fits the relief for that special case. That would be easier to do if the relief was maybe a little deeper like 50 thou or so.

The benefit of center relief is obvious if you try it out. Nice thing is it's not irreversible as you could turn the base flat if you felt it necessary.

Denis