FILENAMES: toolhold.gif DESCRIPTION: <<>> Note to users - there is now a newer/revised version with more pictures posted on MWN - http://www.loganact.com/mwn/howto/toolholder/toolholder.html <<>> This is drawing of a shopmade lathe toolholder for carbide insert cutters. Thanks to Ted Edwards . Ted provided the following description which includes the instructions to build: ============================================================================== TOOL HOLDER for CARBIDE INSERTS by E.M. (Ted) Edwards I have observed the following: You can buy some quite inexpensive tool holders for "positive" rake inserts (3 cutting edges for $5 or 6). They are actually 0° rake. You can buy some very expensive tool holders for negative rake inserts (6 cutting edges for $2 or 3). But never the twain shall meet. My solution: I buy TNMP 321 inserts from KBC or Travers. I'm sure there are lots of other sources. The TNMP has a built in positive rake of 10 or 11° (I forget which) when mounted flat. I make my own tool holders from 1/2" square mild steel with a 5° negative rake. This gives me about 5° clearance angles and 5 or 6° positive rake. I have tried negative rake inserts and my relatively light machine (Smithy) just doesn't like them - it works ok (sort of) but ... My tool holders are easy to make (about half an hour each) and are just as rigid as the expensive ones. See Machinery's Handbook 24 ed pp 448 - the modulus of elasticity is about the same for all steels. Sure, the clamping screws mar the top surface of the relatively soft material but so what! I can now use the economical inserts with out paying a fortune for holders and get the kind of results everyone is raving about in the NG . The accompanying drawing (tool holder.gif) shows the holder. BTW, at $0.35 to $0.50 per point, you really don't have to try to get the last possible grunt out of each point. Index as soon as you see the quality of cut degrading. Fabrication There are really only two tricks to this. First, you want the nose of the holder to droop down toward the point by about 5° to provide your negative rake. Second, the threaded hole is centered 0.185 from the cut edge rather than 0.1875. This offset of 2 1/2 thou and the use of a #6-32 flat head cap screw means that when the cap screw is snugged down into the countersunk hole on the insert (TNMP inserts are countersunk both sides), the insert is pulled firmly against the back edge of the mounting area. I make mine in sets of straight, 30° left and 30° right. I made two sets, one for C2 and one for C6 carbide. This way, I din't have to change inserts for ferrous and non-ferrous metals. I made two accurate 5° wedges of the same dimmensions. I set these square on the mill table with the thick end left. (You can do it other ways but it's easier to describe if I choose one.) Now I mount my rotary table on the wedges. If I now set the 1/2" square stock on the rotary table and mill off the end, the snoot will droop down 5ø when the holder is level. Move the mill table 0.0025 left and drill and tap the hole for the #6-32 screw. I now remove the tool from the mill and mount an insert. I then scribe lines on the holder along the sides of the insert, remove the insert and grind (off hand - it's not critical) the holder back a bit from these lines to have the insert stick out a bit from the holder. Make the left and right holders by rotating the rotary table left or right 30° and repeat the above. I expect to get flamed for not having carbide anvils and fancy clamps but remember these are for a light machine that is just not capable of very heavy cuts. The only times I have broken an insert (it's happened a couple of times) is when I screwed up big time. Much as I might like to, I can't blame the tooling for that! ------------------------------------------------------------------------------ Later in the rec.crafts.metalworking newsgroup Ted added: ============================================================================== I received an e-mail request to explain why my tool holders droop down. Maybe my drop box posting didn't explain it too well. I answered the requester by e-mail but maybe others have similar questions so her goes. Any single point cutting tool MUST have front and side clearances. Otherwise, the side or front of the tool just rubs on the work and can't get any bite. Inserts intended for so-called positve rake tool holders (they are really 0 rake) provide this clearance by having the sides of the insert slope in. As a result, triangular inserts such as TPUxxx's provide only three points. If you turn them over, the clearances are completely the wrong way - they just won't cut! Negative rake inserts have vertical sides - when you turn them over, nothing changes. Thus you get six usable points unless you REALLY break things. So now YOU have to provide the clearance angles for them. 5 degrees seems to be a good all around figure for most applications. Problem is that the usual insert (such as a TNUxxx) now slopes down so its top surface presents a negative rake. Negative rake is fine - good surface finishes, sturdy cutting edges that resist chipping, etc. etc. It does take considerably more power to cut and demands more rigid setups and machines. This seems to be OK for the big guys - Logans, Colchesters, etc. with bazillion horsepower and rigidy to burn but on smaller machines (e.g. my Smithy) It just doesn't work very well. Been there - done that. Negative rake cutters are almost a drug on the surplus market. I could have got dozens last time I was at Boeing surplus. So what can I do? I just dug out my KBC catalogue (Canadian branch). Some prices ($Cdn) in their Triumph brand inserts in grade C6 (General purpose for Steel) are: TNU321 $2.39 ($0.398/edge); TPU321 $3.74 ($1.247/edge); TNMG321 $2.13 ($0.355/edge); TNMP321 $3.72 ($0.62/edge). I'll come back to this in a moment. While I've got my catalgue out: KBC have some "quality import" toolhoders for positive rake inserts with a coutersunk hole - TPGs won't work. These are $27.13 for 1/2" square - the largest I can use without making a new tool post. KBC recommends a TT321 insert for these at $6.25 ($2.083/edge). I couldn't find (in a quick look) a 1/2" shank negative rake holder that would take 3/8" IC inserts. I have seen them somwhere (maybe Travers - I've requested an MSC catalog but haven't received it yet) for between $60 and $100. Back to the inserts. The TNMG321 has a chip control groove and a double countersunk hole (both sides). It looks (at a glance) as if it presents a positive rake - it doesn't. Look carefully with a good magnifier and you'll see that it is flat at the edge (side and top meet at 90 degrees). The TNMP321 is ground with a true positve rake at the edge. It's rake angle is 10 degrees. If we droop its nose down 5 degrees to get clearance angles, we still have 5 degrees of positive rake with the resultant lower cutting forces. With these and my holders you get to have your cake and eat it too. A foot or two of 1/2" square stock and a couple enjoyable hours making chips and you can have all the holders you want. Don't waste money on fancy steel - mild steel is just as rigid. It's not as strong but if bend/break a 1/2" square tool holder, the loss of the holder will be the least of your problems.