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Yes, when he said that, a lot of the bad things that people reported on this thread made a lot more sense. The friction from the taper should hold all of the load, but if you can't apply any normal force (because the bar has come to the end of the thread) it can't take any.
My machine has an MT3 and can't break screws like this, but I had to remake the stock draw bar for exactly this reason. I just figured that my machine was cheap
My machine has an MT3 and can't break screws like this, but I had to remake the stock draw bar for exactly this reason. I just figured that my machine was cheap
I just today made a collar spacer for @CWret. It serves two purposes. One is to make the drawbar seem a little shorter, and the second is to center the drawbar in the spindle. Most drawbars have a lot of slop inside and I find it sometimes causes them to vibrate and even to rattle in others. I first did that on my own old mill/drill which had an mt3 taper too. I've made one for quite a few other machines now.
Once you have one sleeve, you can just add washers if you need it even shorter or you can make a few other sizes of sleeves.
I am headed off to the post office this morning to mail this drawbar centering collar to @CWret. But I thought I would post a photo and a description first because this one is a bit different from others I have made and probably more typical of the average bench top mill out there.
As mentioned earlier, the collar serves two purposes - to center the draw bar inside the spindle, and space the drawbar up a bit so it doesn't thread as deeply into R8 Arbours. One can always add washers or custom spacer collars to add additional spacing if needed.
The main reason for adding this post is to briefly discuss the need to accommodate rolled threads on the drawbar.
Rolled threads are a fast and easy way to make threaded fasteners in industry that reduces waste. The threads are not cut and produces no swarf. Instead the rolling process displaces metal and the outside diameter of the threaded section gets bigger than the shaft they are rolled on. To accommodate this diameter growth, the shafts are deliberately undersized so they can grow to the target thread dimensions.
In the case of a drawbar with rolled threads, that means that the drawbar itself has a smaller shaft than the OD of the threads. As a result, one cannot simply slip a centering collar over the threads because anything that can clear the threads will be a loose fit at the top of the drawbar, which at least partially defeats the purpose of centering the drawbar inside the spindle.
The solution is to size the collar ID to fit the shaft, and then cut threads in it to clear the rolled threads of the shaft. The collar is then simply spun on over the threaded end until it clears the threaded section and then it will slide over the rest of the rod. Problem solved.
In CWret's case, the drawbar threads are cut, not rolled. Therefore the collar I made for him is not threaded. The threads on my Hartford Clone drawbar are rolled. So my collar is threaded.
Your solution above works fine, but as i understand it, the stationary collar threads would be rubbing on the rotating drawbar.
Another way, but a little more work, is to make split rings. For this solution you’d have to make 2 identical collars. Machine and discard half of both.
The collar, drawbar, spindle, and R8 Arbour all turn as one single unit. No thread rubbing to worry about.
The only time they might rub is when the drawbar is first rotated to engage the arbour threads.
But, since the collar is only threaded to a very shallow depth - always less than 50% of nominal, which itself is prolly only 75%, you are talking about something around 20 to 30% threaded and 70 to 80% cylindrical. That is more or less smooth cylinder on round drawbar. A little oil on the collar and the 20/30% spiral grooves will act like oil reservoirs.
In other words, its nothing I'm ever gunna worry about.
There is however an advantage to having split collars. The drawbar doesn't have to lifted right out to install a split collar, and any threads - rolled or otherwise, can't ever be an issue.
Yes - i very much appreciate @Susquatch making the part for me.
I certainly already thanked him but it was in a PM - so here I’m letting everyone know my appreciation. Thank you Susquatch!
If you turn it 2 pieces in the lateh, you just hold them in a 4J chuck without welding or gluing. After the turning and parting, they are ready.
*** if you are tapping using the 4J this way *** you use a hose clamp to help oppose the forces, as they are hanging out of the chuck. Or you can use a C clamp, or a kant-twist clamp. If you do this, then you drill/tap first, then turn to round. (after removing the clamp )
If you are talking about the pin that aligns with the slot on the R8 collet, I would beg to differ. Well, not me personally, because I know almost nothing. But as the pin on my "new" mill is severely buggered up, I've been researching options on fixing it.
In my "Ex-Cell-O Style 602 Ram Turret Milling Machine Manual", page 8, I found the instructions for "Replacing R8 Drive Key". This leads me to believe that the people who manufactured my mill believe that the key that engages the slot on the R8 collet is, in fact, a drive key. This leads me to believe that the key drives something.
BTW: They did an excellent job of designing this. It's an easy replace, which makes me think @Susquatch isn't the only one shearing it off.
In several PM conversations on this: I personally have used about 12 BP style mills 5 in professional settings and 7 in home shops. Not one was sheared. But @Susquatch has had the opposite experience in his scope. Others have had mixed results. So I think it is a wash.
But it should be a fairly simple fix. Some guys hate the pin. Some guys love the pin (me!) and some just don't care.
I have also seen references to the word 'key' pertaining to R8 slot on Bridgeport mills. Which has me wondering if some of the Asian clones have taken a shortcut with a dog point set screw & Bridgeports have something different? Or is it just that there are different interpretations/definitions of the word 'key'? I Googled your mill manual but not sure, is this the corresponding diagram that accompanies your description? It doesn't really show the part (52-1191) in much detail, but from what you know, is it circular section dog point looking part? Or does it have some kind of squared off section?
If a Bridgeport or Excello its essentially the same screw then I think this discussion is more about semantics. Key might mean something that slides within a slot & provides some resistance to torque. How much torque or what specific load purpose they don't say, they just give it a name & instructions to replace. There are keys like on shaft/gear assembly which are more tight fitting, significantly more surface area & not really designed to slide the parts off 100 times a day. They are designed to take up all or at least majority of drive torque. There are also keys like a padlock or door knob which are looser fit, repetitious in/out intending to rotate a tumbler when properly engaged (but not much more torque if its a mismatched key). I still think this R8 mill part qualifies as a looser fitting 'indexing/alignment key' designed to resist some lesser torque until the drawbar eventually forces the tapers together. I think if they really wanted a normal, rectangular key for the R8 slot, they would have integrated it into the spindle. But all I know for certain is my machine has a dog point.
