Drawbar Thread Engagement

D

ddickey

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H-M Supporter Gold Member
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My mill has an NMTB40 spindle and to my delight Cat 40 collets fit just fine but I noticed for some reason I can not get many turns of thread engagement, although the drawbar is more than long enough. A 5/8-11 bolt screwed into the Cat40 toolholder shows ~ 1.125 thread engagement. When I place into the spindle it is considerably less. When I get back to the shop I will have to count how many turn I can get. My drawbar is a little odd in that you screw the bar into the toolholder then it is drawn up by a captured nut. My question is what would be the minimum thread engagement for a secure tool?
 
Engineering analysis says 3 turns makes the nut as strong as the bolt threads, more is gravy. That said, I would want 5 or 6 turns for something that is constantly engaging and disengaging.
 
There is a metric thread that is very close to that size. I have some R8 collets that are that size. Three threads are a "book number", I'm like @Asm109 in that I would want about double that. Check if they are metric. . .

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I'm not familiar with your setup, but is the captive nut possibly limiting the drawbar's travel? With the drawbar removed from the mill, how far will it screw into the toolholder?

Tom
 
@Bi11Hudson definitely 5/8"-11. All the NMTB and ISO40 work great. I have several Cat40 from Haas that seemed to work normally now I'm not sure.
The drawbar has 1/2"-13 on the end, not sure why but I wonder if it is interfering with the Cat40 holders not allowing to thread very deep.
@higgite I don't think so. The threaded drawbar from what I have figured out is first screwed into the holder while holding the nut or captive nut I call it. Once you get the bar screwed some you then lock the spindle and tighten the captive nut and that raises the drawbar and pulls the holder up into the spindle. When you take it out you loosen that nut and that breaks the hold of the taper, then unscrew. I am not familiar with this type of setup. I can not pull the drawbar out although I have not spent much time trying to figure that out. Will post some pics later today.
 
I think I figured it out. The holder I've been using is an ER32. I did not realize there is a backup screw behind the collet, this was interfering with the drawbar engaging enough threads. Case closed.
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