View Full Version : Threading bit grinding
07-24-2003, 02:35 PM
Hi all .. I bought a threading toolholder from Grizzley .. When I received it ..
I realized that the radius on the threading bit was too much for even a 16 TPI thread ...
i.e. rifle barrel threads
I can grind it down .. but I am very inexperienced in grinding bits and it would seem to me that a bit fomr making threads would be a bit critical in regards to the 60 degree angle ..
The bit is about 3/4 of an inch long v shaped on the bottom and fits into a notch
on the tool and the top of it is flat and there is a clamp on the tool that screws down on top of the bit to hold it in place .. along with a small allen screw ..
Part numbers G7030 tool and G7041 for the bits .. anyone have any suggestions and or a nother source for these bits.. Grizzly says they can't help me .. Thanks
07-24-2003, 03:39 PM
We must have the same list of projects ! I just went through the same thing last week with the same tool (although I purchased mine as part of a set from Lathemaster). I'm still playing with the preload issue but think I have it figured out.
Bob at Lathemaster told me the bits are proprietary. Any info contrary to this would be welcome for future reference though.
I used diamond hones to reduce the tip to almost a sharp V for 40 and 56 TPI threads. Used a center gage to maintain the 60 degree angle. Wasn't hard to hold since the hones were slow to cut.
It took a while but the results were gratifying. I have not cut a thread with it yet so can't comment on the carbide. It now has a nice finish on it and about 0.002" radius.
07-25-2003, 03:53 AM
Hence my comments an buying quality tools to begin with.
Send it back and order a standard threading tool with inserts form Kennametal, Sandvik-Coromant, Valenite, or Iscar and you will be much happier (and poorer). There are no free lunches in life - they all cost you SOMETHING in the end.
07-25-2003, 04:04 PM
Thrud: I've been looking but the options in a small shank (3/8" in my case) appeared rather limited. Looks like maybe a partial profile insert can get me to 48 TPI and beyond this, I guess you'd need to alter the radius. What do you use ?
07-25-2003, 04:44 PM
I've never seen a spec on the 60* angle for "V" threads (or any others for that matter) but I don't think the angle is that critical. I checked Machinery Handbook and just found 60*, no tolerance shown. You certainly don't need to hold it to anything in the +/- x seconds range and I would suspect that most threads, even "precision" ones are no better than +/- a quarter degree or so. Consider, at 16 TPI, the side of the thread is less than 0.062" long due to the fillet at the base and the radius or flat at the top. To produce an error of 0.0005" over this distance, you would need an angular error of arctan(0.0005/0.062) = 0.46
Just grind slowly and stick it into the "Vee" slot of a standard thread gauge and look for daylight. If your eyes are like mine, put your reading glasses on. No daylight = OK.
07-25-2003, 08:14 PM
nheng .. I did it .. I got out that carbide wheel and slowly ground it down .. so close
that when I put it in the fishtail I cant
see where its off any ... close enough for me ..
I tried the tool on a scrap piece of rifle barrel .. well .. let me tell ya .. it was worth the effort .. cut a very nice thread ..
Its a very solid and nice tool for the price .. just too bad the bits have such a
big radius ...
But I figure .. if ya don't mind putting the effort into it .. then I have 60.00 or so that I didn't spend on a more expensive model that I can spend on bits now .. * wink *
If I do find a bit that will work in there that we don't have to grind, I'll let you know ..
07-25-2003, 09:24 PM
That's good to hear. I can't wait to cut with it. Incidentally, the set I mentioned from Lathemaster (which he also sells on ebay) was a 6 or 7 piece set of indexable holders with standard designations like SCLCR, SCLCL, etc. (CCMT, TCMT and one trigon shape). So far, the other tools have been great and the entire set cost $69. Each tool would have been $50 to $70 for a better brand. The holders are actually finished off nicer than one US made one I bought a while back. They are 10mm though instead of 3/8" so you need to watch out if your toolpost is at its limit.
Thrud: I'm a fusspot and the set I mentioned really does look and behave well ... even the screws are ok. I'll still trade you for your swiss tools though http://bbs.homeshopmachinist.net//smile.gif
07-26-2003, 02:53 AM
For most of my external threading I use a Sharp V insert (3/8 shank top-notch type insert) for everything down to 80TPI, beyond that it is custom made tools. For interanl threads I use a laydown type insert. I do not use full thread form type inserts unless it is required and then I just buy what I need.
Larger threads with the 3/8 shank tool is a PITA, but it works great for fine threads (even in stainless).