Making a Tool Holder for Carbide Inserts

I have a Smithy 3 in 1 lathe/mill/drill so this article is oriented toward relatively light machines. No 1/4″ cuts off old steel axles. Mind you, I do take 30 to 40 thou roughing cuts on mild steel. My machine came with a square block toolholder that can accomodate up to four cutters of up to 1/2″ in height with some space left for shims. I don’t wish to spend large amounts of money or time on Aloris style quick change tool posts so I use shims to set the cutting point to the correct height. More on this later. I wish to use triangular inserts because they have 3 points per side and yet still have less than 90 degrees included angle at the point. This lets me turn to a shoulder as well as turn and face without re-orienting the cutter. 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. In my opinion, carbide anvils and fancy clamps are not really necessary on a light machine that is just not capable of very heavy cuts. So I secure the insert to the tool with a single screw through the center of the insert. The TNMPxxx series of inserts is a so-called positive/negative rake insert. The insert is made so that if it was mounted flat, it would have +10 degrees rake. If it’s mounted so it’s nose droops down 5 degrees, it would then have 5 degrees clearance and 5 degrees rake – reasonable values for most machining. These inserts also have a hole through the center for mounting which is countersunk on both sides of the inserts. Some inserts such as the TNMG series look at first glance as if they are positive/negative type but a more carefull examination reveals that the chip control groove does not go right out to the edge. These are, in fact, 0 rake inserts. So. I want a tool holder that will fit in a tool post designed for 1/2″ high tools, will mount a TNMPxxx series insert with a single screw and provide a 5 degree droop along the axis of the insert. I have looked through several catalogs and the smallest TNMP series insert I could find is 3/8″ inscribed circle. The TNMP321 is the one I have chosen as I figure a 1/8″ thick insert is strong enough for use on a light machine and the 1/64″ radius tip reduces cutting forces compared to larger radius tips. You do have to feed slower on your final cut to get a good finish but this is acceptable to me. So TNMP321 is the insert of choice. Now I need some holders. Back to the catalogs. I could find no negative rake holders that are 1/2″ high and take 3/8″ inscribed circle inserts. What’s more, the available larger ones get pretty expensive if you want a half dozen or so. I make mine in sets of straight, 30 degrees left and 30 degrees right. See the final note if you don’t have a rotary table. I made two sets, one for C2 and one for C6 carbide plus an extra one for a threading insert. This way, I don’t have to change inserts for ferrous and non- ferrous metals. I make my tool holders from 1/2″ square mild steel HRS stock. A couple feet of stock and a few 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. See Machinery’s Handbook 24 ed pp 448 – the modulus of elasticity is about the same for all steels. Sure, the tool post clamping screws mar the top surface of the relatively soft material but so what! It’s not as strong as tool steel but if you bend/break a 1/2″ square tool holder, the loss of the holder will be the least of your problems. See below illustration:

I can now use the economical carbide inserts with out paying a fortune for holders and get the kind of results everyone is raving about in the NG .

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 degrees to provide your clearance. 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, the insert is pulled firmly against the back edge of the mounting area. I made two accurate 5 degree 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 this is how I did it.) Some thin double sided sticky tape holds them in place once set. 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 snout will droop down 5 degrees when the holder is level.

Figure 4 & 5 – Shows starting the mounting hole and threading it #6-32. The hole is drilled #34. This is a little large – the charts show #36 for 6-32 – but this reduces the likelyhood of breaking a tap and, given the length of engagement, gives plenty of strength.

Figure 8, 9, 10, 11 – Show the surfaces I get on steel, brass, nylon and aluminum. – Show the surfaces I get on steel, brass, nylon and aluminum. Final Note: If you are just starting out and don’t have a rotary table yet, you can still make the straight holder with only one wedge and a milling vise.: If you are just starting out and don’t have a rotary table yet, you can still make the straight holder with only one wedge and a milling vise. See also: Fixture for Milling Ted’s Toolholders on your Lathe By Ryan Flaherty Author: Ted Edwards