Our series continued last month with Quality Is Still King, which placed emphasis on not burning tools. Quality Is Still King examined how to keep top rake angles as acute as possible and highly polished, along with how to reduce relief angles to an absolute minimum to give the best edge support and grind life.
In this issue of FM Digest, we conclude our series by considering how such relief can best be produced, as well as examining a few of the grinding tricks that can save time and money for any shop.
So far, we have considered tool geometry as if it were the same for all materials. This is not the case, as shop men know. Because of this, the empirical formula allows the 2½ to 10 times clearance for form relief as a broad basis within which most materials will fall. Softer materials, being more apt to compress, call for greater relief angles. Then, too, tougher materials, due to springback, require higher relief than normal materials. In drilling, there is the tool expansion factor, which must be considered.
Expansion is controllable to some extent by keeping rake angles high, and top faces of tools polished. However, for the same reason, it must be accepted that more energy is needed to cut a chip away from some materials than from others. This energy, originating as machine horsepower, converts to heat. No matter how well the tool is lubricated by the coolant, and how slippery the tool faces may be, heat will be created.
In deep drilling, the drill, usually having smaller mass than the workpiece, is apt to expand faster, and can, as shop men know, result in a squeal-fit of the tool to the hole. The important thing is to give those who use and grind tools an intimate knowledge of what actually goes on in the cut so that, by looking at a tool, they can diagnose its weak points. We are convinced that a tool grinder will be a better man for having spent some time using cutting tools, and that a machine operator will produce more and better work if at some time in his training he has had tool grinding experience.
It is the teamwork of machinist and tool grinder which puts tough jobs on a paying basis. Going back to the know-how of the old time troubleshooter mentioned earlier, let us make one point again: Though he may not be able to put it into words, the old-timer knows that the chemistry, grain size, structure and hardness of a material has a direct effect on the condition of the chip and the amount of energy needed to push a tool through a given piece of metal.
In more cases than not, tools are underfed rather than overfed. The rule should be to give the tool edge all the support possible, using the principles mentioned earlier, and then to feed it up to the limit of the tool?s strength, or the capacity of the machine. As an evaluation of whether tools are being fed at what might be called a ?national average,? the accompanying Table 3 shows reference examples of average HSS twist drill feeds for endworking tools (similar tables can be made for other types of tool material and work material). If careful flute and face polishing techniques are followed, these feeds can be greatly exceeded, as this table shows.
While everyone dreams of a condition in which the engineers lay out the tool geometry and send the specifications to the tool grinder . . . and they are right . . . we are still quite a way from this ideal. Production
shops depend in a large degree on the imagination and knowledge of the cutter grinder and the machinist. And right here, let it be said one of the most profitable things a tool grinder can know, from the standpoint of his management?s costs, is his ability to tell whether or not a tool is worth regrinding.
If unusable tools can be scrapped before they are resharpened and tried, time and money are saved in both the crib and the production area.
TAPS, FOR EXAMPLE
Let?s take taps as an example, since probably more useless taps are reground than any other cutting tool. One should do a bit of measuring before even considering regrinding them. The major diameter of a tap must always be larger than the major diameter of the go gage, or the bolt, which will be used in the hole. This is to allow for corner wear on the top of the tap?s thread crest.
As the tap?s thread crest rounds off through wear, it reaches a point at which the wear radius extends below the minimum diameter needed to clear the go gage. The logic involved here works out so that, unless there are threads behind the chamfer on which the cross-tooth diameter measure is considerably larger than the major diameter of the screw, do not spend time resharpening the tap. (see Table 4 for example).
The go gage will not enter the part if an undersized tap is chamfer ground (on the OD) centrally. If you grind off center to get a bigger hole, the not-go gage will always enter because what you need is a thread form in which the go gage will not interfere, not one with larger root size. Literally hours of tap trouble on the job can be saved if the tool grinder is aware of this condition and will avoid resharpening and reissuing taps on which the major diameter is too small.
To repeat once again, the way to know whether or not a tap should be scrapped, or whether it will fall within tolerance, is by referring its dimensions to the American Standard tap tolerances listed in various tool handbooks.
It will be noted a ¼-20 tap ground variety has a basic major minimum diameter of .254 in and maximum of .2555 in. A ¼-20 cut thread tap having a diameter between .254 in and .2555 in will hold tolerance. Any used tap having a diameter less than.254 in will not let the go gage enter. This does not mean taps cannot be resharpened. They should be, so long as somewhere back on the tap?s threads there are tooth crests which come up to the major minimum diameter. This, in some cases, may mean a worn length of tap must be cut off before a new chamfer is ground. Also, on tap chamfers, we recommend a .006 in to .010 in relief drop from cutting face to the back edge of the land.
As in all other tools, what we are seeking is maximum backing for the cutting edge, coupled with good rake, or flute hook, to get the chips away. It is understood that the height of all tap tooth chamfers should be alike. If they are not, the tap will cut oversize.
EQUAL TOOTH HEIGHTS
To get equal tooth heights, one must control the concentricity of the tap while the chamfer is ground, as well as the grinding of the hook. It is a good idea, if possible, to put both axial and radial clearance on the chamfer. We will discuss more about this later. It is sufficient to say that, unlike a drill, for a full thread, a tap must first enlarge the drill or reamed hole, and then form a thread in it.
Therefore, the chamfer is very important. Note also that a way of making the chamfer angle of a reamer hold up longer, is to put a bit of axial, as well as radial relief on it too.
If one can regrind a tap so that the TIR, tooth-to-tooth, is within .0003 in, the number of holes possible between each sharpening can frequently be doubled. It is also wise to avoid grinding the flutes of ordinary thread taps as long as possible. The rake angle generated in them by the manufacturer is usually correct, and few know how it is designed, or how to reproduce it in the tool grinding room. On the other hand, pipe taps may require flute grinding to maintain cone diameter.
Earlier it was mentioned that sharp corners can best be dressed on grinding wheels by dressing below center, and away from the corner. It is equally important that when a tool is ground, the grinding be done away from the cutting edge. So far as possible, it is best to wipe the heat generated by the wheel back into the thicker portion of the tool, and at the same time avoid slurring a burr out onto the cutting edge.
TOOL SHARPENING BURRS
If burrs are left, they should be on the heel of the land rather than up front, where they matter. If certain equipment is used, this is a built in feature, for the tool is fixtured to rotate top-coming against a top-coming wheel.
In Part 5 we shall be getting down to cases and considering the problems of grinding various geometries onto various tools. But for now, just one more tip: Often a very slim tool must be held on its shank and ground in a manner which allows less than maximum support for the cutting edge. If possible, a bushing should be mounted as an outboard support.
But at times, even this cannot be done easily, and the usual result is chatter on the ground finish. Much of this chatter can be avoided if a wad of modeling clay (or something similar) is wrapped around the tool over all its exposed length except that which is to be ground. A good fistful wadded about the tool?s shank damps off the vibration, and tool finishes can be cut to half the RMS otherwise possible.
Often these vibrations cause a portion of the tool to snap off. This is caused by the teeth of the grinding wheel pulling the drill, tap or another type tool into the wheel. The normal resistance to bending causes the tool to snap back. The subsequent tool undulations dress the wheel, destroying wheel shape and causing pilots to be broken off. The modeling clay trick is an old fashioned remedy, of course, but it is one worth trying if you have not found a solution.
If a grinding wheel needs only a sharp corner or a simple angle, put it on with a diamond dressing tool. If a more complicated shape is required, try crush dressing. In almost every case, it will be faster and easier. There is nothing difficult or mysterious about crush dressing if a few rules . . . and a few tricks . . . are known.
Usually, the shop can make its own crushing rolls. Work finish will be as good as the finish on the roll and the skill of the man using it. In diamond dressing, some of the grain is flattened, some is torn out, and some is fractured and resharpened. Crush dressing works just the opposite. The grit is crumbled free from its bond by a rolling, compressive motion. Crush dressing breaks down large flatted clumps of grain and provides a sharper wheel with more teeth. Therefore, more stock is removed at a lesser heat, reducing tempering of the cutter as well. The wheel surface cuts freer and holds shape longer.
In some crush dressing applications, both the wheel and the roll are driven at synchronized speed. On certain grinders the roll drives a free-running wheel. The crush roll is placed in the grinding fixture, rotated slowly, and brought against the wheel to drive it by friction. The roll is then advanced into the wheel by infeed until the proper wheel profile has been obtained.
The first thing to keep in mind is that a lot of the crumbled grit from the wheel tends to stick to either the wheel or the roll. If this is not wiped away it will have a ballmilling action, and the wheel will be impacted, destroying shape. Scrub the grit out by laying a brass wire brush with bristles no larger than .005 in diameter across both the wheel and the roll. A steel brush will work, but if a hard steel bristle comes loose, being polarized, it may cling to the wheel, coming between the wheel and form, thus destroying the form.
On certain grinders it is best to crush dress dry since grit can fall away more easily, but a coolant can be used. It is not a bad idea, when the form is almost done, to back the wheel away and put it under power to fling off loose grit before coming back to crush for the last few tenths of shape. Of course, there is no ?spark out? in crush dressing because there are no sparks but one must still feed in very gently for the last pass to get a shape which duplicates that of the roll. When crushing is complete, the wheel will skip on the crusher. Any wheel may be crush dressed, generally 80-220 grain, hardness H to O. Friable bonds are in order.
Crushing rolls can be made from any material. Hardened crushers have longer life. Case hardenable steel works well. The usual procedure is to drill and ream a center hole in the blank to fit whatever driving arbor will be used and then to turn the OD profile on the roll in a lathe. After hardening, the shape is polished or ground to exact shape. The shape on the roll will be duplicated on the wheel, and also on the workpiece, and so it must be
accurate. If form tools are to be ground, corrections for offset must be figured and put into the roll?s shape.
There are two very good things about crush dressing. The first is that, once a roll is on hand, it is an easy matter to dress a wheel and duplicate a tool form made in the past. The other is that, with a little ingenuity, rolls can be self-conditioning. We suggest that, if a roll is to be used for any great number of dressings, a spare blank be turned out when the first roll is made. With the finished form in the grinder, dress the wheel to shape. Then use the wheel?s profile to circle grind the hardened spare blank.
Now there are two rolls. Put one aside until the first begins to show wear. At that time, crush dress with the second roll and use the dressed wheel to recondition the form of the first. If the operator is careful not to let both rolls become worn at once, this reconditioning can be done repeatedly without loss of form. Since crush dressing, as recommended here, calls for the roll to drive the wheel, there is no exact requirement for either wheel or roll size.
It is best though, to make the roll at least one-third as large in diameter as the wheel it will be used with. Any wheel-to-roll ratio between 3:1 and 1:1 works well. Obviously, larger rolls wear slower, since they have greater working surface. Also, one must keep in mind that if reconditioning of the rolls is to be done, a case hardened skin may not work as well. A deep hardening steel is recommended if long use is expected.
When many tools of a kind are to be sharpened on a crush dressed wheel, it saves time to make a combination arbor, which holds both tool and roll. For example, a small milling cutter is mounted on the end of a crush dressing roll arbor in such a manner that either the tool or the roll can be brought up to the wheel without disturbing the setup. In this case, the procedure is to crush dress, circle grind the grooves in the cutter, and then use the same wheel to give radial relief to the cutting edges.
In order that the roll?s form will be duplicated on the wheel, crush dressing must be done at the same height above or below center as grinding will be done. To make this simpler, we suggest crushing and grinding both be done at the wheel?s center line.
FLAT FORM TOOLS
When a flat forming tool has been circle ground on a crush-dressed wheel it must either be gashed on top and set high, or given front clearance. We suggest the latter, since this can be done in the same setup and with the same wheel which produces the cutting profile.
The relief, if produced on certain grinders, will be radial rather than angular. This means the operator must convert angular values into decimal drop per-degree-of-arc. This is a simple matter, and is explained in the caption on Figure 12.
Tapping is always improved when drilling and countersinking are done with the same tool, since if the countersink is not centered with the hole, the tap follows the eccentricity as it starts and then has to correct its position as it gets deeper into the work. For this reason, among others, many shops find the need to make their own combination centering and chamfering tools for some work.
One approach is to put the required profile on the wheel either by crush dressing or with a diamond, using a spin grind on the drill?s pilot diameter and chamfer angle with the tool held in the grinding fixture. Then, without disturbing drill location, switch to cam control on the form relieving fixture and give the tool a combination of radial and axial relief. The reason: The most vulnerable spot on a shopmade center drill is the junction of the pilot diameter and the chamfer. If this is just circularly ground, it has no axial, or endwise clearance, no matter how well it is backed off radially.
There is always the chance it will rub. You can solve this by swinging the base of the fixture while keeping the tool in line with the wheel, as shown in Figure 13. When the cam-controlled base is swung in this manner, there is a combination of endwise and crosswise movement. As the tool is fed toward the wheel, the first contact is at the cam?s high point, and if the tool has been correctly related to the cam?s position when chucked up, first grinding takes place on the trailing edge of the drill land.
As the tool rotates, the fixture is moving axially toward the left as seen in Figure 13, and at the same time is moving radially away from the wheel. The result is that the junction of the two cutting surfaces on the tool are given form relieved clearance at ?X? equal to the ?X? setting of the fixture.
Getting both lips of a square-cutting drill or piloted counter-boring tool exactly the same height and at 90 deg is an almost impossible job if done by hand. Without taking anything away from the old timers who made those tools off-hand for years, one of their grinding jobs is very apt to look pretty crude if viewed on an optical comparator. In almost every case the hand-sharpened flat bottom drill will (1) have lips of unequal height, (2) have one or both lips off angle, (3) be undercut at the corner, or (4) have an objectionable radius in the corner.
The time-honored way to lick this problem was to undercut at the neck enough to make sure there was no radius which would cut, and after grinding, to lap the two cutting lips square on a piece of emery cloth while
the tool is rotated in a drill press. There is a way which gives more predictable results faster. Mount the tool in the grinding fixture and circle grind on the pilot in the conventional way to establish pilot size and length. Then, without disturbing the tool?s position, swing the grinding wheel head to another position, and using a small wheel, relieve the square faces as shown in Figure 15.
Here again, use of a combination of axial and radial motion in the fixture clears the cutting faces and the
corners. If the wheel has been dressed squarely and sharply, there will be no measureable difference in height or angle of the two lips when viewed on the optical comparator. If one leaves a generous margin at the cutting edge, and if a tool is ground in this manner, it can be sharpened time and again without cutting off the pilot length each time, by careful flute grinding.
Trepanning tools are ground in much the same way, except that there the wheel, which should not exceed twice the major diameter of the tool, just does the undercutting work. It should be redressed using diamond particle grit tool or phono point dressing tool for form, before attempting to finish the profile. To sum up, the first problem facing every shop is finding a tool shape which will do the job. The second problem is finding a means of consistently duplicating that shape at a reasonable cost.
In complex forms, crush dressing is well worth a careful look. It, in effect, provides a ?master? which can be laid on the shelf and used whenever needed in the future. If there is a question of whether the job will run again, it is less costly to keep a crushing roll on hand than to stock finished tools. As for relieving the cutting edges of profiled tools, the matter is largely one of finding a way to get the right size and shape of grinding wheel into the proper location, and then having a means of holding the tool in relation to it.
We hope that the discussion given thus far will open up new areas of investigation for the reader.
To get high quality tools out of the grinding room and back into use at reasonable cost, we need an accurate, versatile, simple grinding machine. The machines must be accurate, for as we have tried to show, the tools that work best and last longest are the ones that are correctly ground. The machine should be versatile and simple so that it can handle just about any job that comes along.
Our approach to the machine design problem is to mount a toolholding fixture, which can circle grind and form relieve, and an optical comparator on a basic grinder designed for them. In this way a tool can be sized for diameter, backed off, and inspected without being disturbed or relocated. If you follow some rather simple charts, the tool which comes off the grinder should work.
Finally, some odds and ends not covered elsewhere in this article. Let?s start with tool thinning. It makes good sense to always thin drills and flute-grind taps or reamers before end sharpening them. If you try to thin or face grind after sharpening, the chamfer or endworking angles are bound to be unequal, and smear metal often flows over the cutting edge. Most shops know this, but occasionally a plant attempts to thin as the last operation. It does not work as well.
If a hole already exists in a part, and just has to be opened up, try a three-lip core drill. It is more difficult to measure than a four-lip type, but it will be stronger. On a long run job, use a three-flute drill, buy the necessary measuring tools and charge them to the job. A plug soft-soldered in one flute will allow normal measuring tools to be used and can easily be knocked off after inspection. You will come out better in the end.
On small drills, chip breakers, are a problem. If one gets enough hook in the cutting face to curl the chip, the drill point is weakened. Sometimes you have to use chip breakers, but on a drill smaller than 3/8 in diameter, it is often better to experiment with different spirals. Chipbreakers, at best, require more of the point to be ground back when the drill is resharpened, calling for faster replacement of the drill, and added grinding time.
Chips in drilling do not cause trouble if you can get them to move out of the hole; that is why the change in the drill spiral. Somehow, you must get coolant to the drill point. The ideal chip on most work looks like a figure ?6? or if you prefer, a figure ?9?. On deep holes, a woodpecker attachment may help, but it is entirely possible to break chips so short they will not come out of the hole as well as if they were longer.
A drill point thinned to one-half the production standard will cut better, and last longer, and as mentioned earlier, the crankshaft point cuts freer and lasts longer.
With the proper cam on certain fixtures, triangular, hexagonal, square, or elliptical shapes can be made in a single operation by continually rotating the part. This factor is part of the ?versatility? feature mentioned earlier so that dies and punch shapes can be made. Customers produce cold heading tools in this manner, and it is a benefit whenever a shape has to be made for some tool room item.
Tool grinding is now changing from art to science. A great deal of credit is due the ?old timers? who discovered how to make tools, and made them by hand. The need today is to automate as simply as possible the geometries they have discovered.
We hope that the information in this series has opened up new fields of investigation and experimentation for the reader. The better each individual understands tooling concepts, the more profitable the shop will be.
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