Model Engineering Articles

For some time now I have been regularly sharpening my stock of end mills. Inevitably the task of sharpening them with basic equipment presents something of a challenge. Whilst I am fortunate in having a Stent Tool and Cutter grinder it is still rather basic. The tool does not have pre-loaded bearings or precision made features but it is still able to perform an adequate job of sharpening end or slot milling cutters.

The trouble is that whilst there are many different designs of tool cutter grinding tools around there is little information about how to use them. As I see it is pointless spending an inordinate amount of time setting the tool up to sharpen a cutter knowing that when the next cutter needs sharpening it is a long job. I use a compromise solution, which many professionals would baulk at but then they simply buy new cutters and rarely sharpen the worn out ones. My approach is one which enables me to set up the cutter grinder in about five minutes and sharpen a cutter in three or so minutes. The results are quite reasonable and from experience the re-sharpened cutters seem to behave equally as well as new ones. The one area which suffers completely is any regard to standard cutter diameters. This is inevitable, as any re-sharpen of a standard commercially available cutter is going to reduce the diameter, but this rarely matters as I prefer to use a slightly undersized cutter and machine to the finished tolerance I am aiming at.

The other important issue is the cost involved to produce longevity for milling cutters.  Every re-grind significantly reduces the cost of the cutter!  There is little point in spending a large amount of money on making a TC&G just to sharpen a few cutters which can often be bought quite inexpensively, however, it is equally very frustrating to attempt precision work with a cutter that has dull cutting edges.  The solution is, as usual,  one of compromise  and in my case I wanted a simple and relatively inexpensive approach to give new life to my existing stock of cutters. 

Initially, I made collets and sleeves to fit into the holder jigs and these worked reasonably well and in most cases gave me several extra re-grinds to each cutter.  In the case of end edges on the cutter the problem of collet accuracy is less important but for side (fluted) edges concentrically is a lot more crucial.  The key objective when doing re-grinds is to remove the minimum amount of material possible to achieve a sharp and consistent cutting edge.  Whilst this may sound obvious and simple, in practice it is not.  If the collet and holder jig runs slightly eccentrically then extra material has to be ground to give each cutter side edges an equal sharpness.  Added to this of course, if the collet is running eccentrically then the cutter will also run eccentrically in the milling machine.  Whilst this may appear as a major problem in practice it is one you can live with as the cutter performs as a single edge fly-cutter.  This clearly presents a very reduced running life in the mill and reduced depth of cuts.

For many years my home made collets and holder jigs have managed to produce reasonable results but the use of sleeves to accommodate different cutter sizes was not ideal and the making of a full set of collets is no small task if they are to run accurately.  The other problem that presents is the now mixed availability of imperial and metric sizing of cutters.  The solution was obvious and required the purchase of a full set of ER25 collets and two collet chucks (with parallel shanks).  Normally, the purchase of collets and chucks are very expensive but I decided to compromise and buy a set direct from a Chinese supplier who had built up good feedback.  I did not expect absolute accuracy but then again I was not spending a lot of money but did hope that the accuracy would be at least as good as my home made collets and additionally provide a full range set of collets.  In the event the collets and chucks bought  were of a very acceptable standard and certainly better than my home made collets,  it is clear the Chinese have made significant progress in manufacturing these types of tools and I understand that many 'well known named' products are now sourced from the same suppliers.

Having received the new collet set and collet chucks I decided to strip down the Stent machine and 'improve' any wear  plus upgrade the jigs to now incorporate the new ER25 collets and chucks.  The picture below shows the 'revised' Stent TC&G after modification - note the main changes are the motor attachment and safety guard plus the addition of an adjustable Lip Rest.  The motor cover was now discarded the motor has been repositioned so that no open frame areas are affected by grinding dust.  The motor has quite strong internal fans at either end which blow air out so the chances of any ingress of grinding dust is minimal.

All parts were completely stripped down for cleaning and inspection.  

Dovetail slides:

Spindle:

Motor Attachment Brackets:

The original drawings seemed to assume this was probably by an overhead round belt so no on-board motor arrangement was considered.  I managed to obtain an electric motor from an old spin dryer which was a little on the large size relative to the Stent but provided 3000 rpm which meant that it could easily provide the required 5000 rpm drive to the spindle using a step down pulley and a 'rubber' round drive belt (dubious origin but looks like it came from a vacuum cleaner drive).   Attaching the motor is not an easy task and I had used a 'U' shaped bracket made from bending steel sheet.  Whilst this arrangement worked it was no that pleasing on the eyes and frankly looked wrong so I dispensed with the bent bracket and replaced it with a set of plates bolted together to provide a sort of fabricated bracket.  This new bracket now looks better and is quicker to dismantle the motor or remove one of the bracket side plates to give access to the gib adjustment screws but is really a cosmetic change.

Lip Rest:

The old design I used for a Lip Rest was crude by any standards and so this was radically changed to provide better height adjustment with a finer thread (40tpi).  Again, the original Stent drawings had no details of the Lip Rests and the main casting having sloping sides made attachment of fixtures such as a Lip Rest difficult.  

End Mill Holding Jigs:

End edge grinding jig:

Sharpening the end cutting edges is relatively simple. I originally made a jig (designed by Derek Brookes and was described in the MEW- No. 16 & 17.) to hold the cutters which allows me to index the tool as appropriate to the number of cutting edges and also alter the grinding angle. In use I set the cutter grinder with a cup-shaped grinding wheel which is always dressed with a diamond before use. The tool holder is set at a 1deg. acw so that the centre portion of the cutting edge to be sharpened is slightly angled to be lower at the centre of the cutter. This prevents the cutter 'skidding' or vibrating whilst cutting. I usually grind the secondary angle (15deg.-16deg.) first on each cutting edge and then re-set the tool holder to a primary angle of 5deg. and grind each edge to visibly show a small land of about 0.020" thickness.

In order to retain the functionality of this original jig (above picture) , which I very much liked, it needed some modification to incorporate the new ER25 collet chuck. The chuck had a parallel shank and this was ideal since it was 0.5" diameter and the use of standard ball bearings enabled it to be fitted into the existing jig body withou

Sharpening the cutters side (flute) edges is slightly more difficult and requires yet another tool holder jig. Unfortunately, no such ready designed jig existed and so I came up with this version. This jig enables the cutting tool to be easily rotated and also 'retracted' away from the grinding wheel at the end of each traverse on completion of grinding. For side flute edges I use a disc wheel or cup wheel, the disc wheel being the preferred option. The sharpening process depends on setting the correct relative heights of the cutter lip-rest, tool holder centre and the grinding wheel centre.  

Incorporating the new ER25 collet chuck was basically simple and used the base and upright but a new body fitted with two ball bearings and the ER25 chuck.

Also seen in the RH picture is a small height gauge jig.  This gauge is very simple and consists of a long base made from square bar with a screw attached at the end.  On top of the screw is a square block with a pointer rod fixed.  In use the screw is adjusted to obtain the relevant centre height (cutter edge) and as the pointer block can be easily rotated it can be positioned either at the cutter edge or the spindle centre.

Using the grinding disc-wheel: 

After dressing, the disc wheel is set at a small (5deg.) cw angle so that the corner of the wheel is responsible for the grinding and the lip-rest of the wheel is away from the cutting tool. The tool holder jig is set up so that it's travel path is parallel to the grinding wheel so that the cutter is not ground tapered. The rest is adjusted so that the cutting edge (best viewed from the end of the cutter) is a 90deg. and just off-set to the right hand side of the disc wheel so that it is clear (see sketch). The grinding disc wheel centre is initially brought to the same height as the rest and then raised according to the required angle to be ground - secondary angle (usually 20-25deg.) and primary (usually 5deg.) as shown in the table. The cutter edge is ground by traversing the table with the jig and cutter whilst at the same time rotating the cutter so that it's cutting edge remains in contact with the rest. A lever on the base of the jig enables the cutting  tool to be retracted away from the grinding wheel and tool rest at the end of grinding each edge. Without this feature it is almost impossible to withdraw the tool after traversing along the cutter edge, especially in the case of helical edges. The lever is reset for the commencement of the next edge grind.

Using the cup-wheel.  This is used primarily to grind the end edges of the cutter.  The process is very similar, only in this case the cup wheel height remains at a fixed height (centre at same as cutting tool centre). As before the cup-wheel is set at a small angle so that the corner only does the grinding.   The jig can now be adjusted to present the cutter at the correct grind angle.  Alternatively, the Stent spindle can be inclined however this is only effective for small relief angles.

The cup-wheel can also be used to grind the side edge and this method, like the disc wheel method describe previously, requires an off-set in either Lip Rest or Spindle to achieve the correct grinding angle.  In practice I find the disc-wheel method easier and more effective when grinding large relief angles as in the case of the secondary angle.

I have found that results from both methods produce equally good results. 

Grinding Procedure:

The following procedures describe the basic set-up steps required:

Sharpening End Mills – Side (flute) Cutting Edges

If necessary, dress the Grinding Wheel using a diamond.

            Using Grinding Disc Wheel

Set the table parallel at zero rotation setting. 

Set the grinding spindle parallel to the table and then slightly angled (5deg.) so that grinding is done on the leading edge only

Mount the Side-Grinding jig on the table at the RH end, use a set square.

Use the Height-Finder jig to determine the height of the cutter end teeth edge (as viewed from the end it is at 90deg. to the wheel)

            Set the Lip Rest to the same height as the Height-Finder jig (see note below)

            Position the Lip Rest to be just to the RH side and just in front of the Grinding Wheel

            Set Spindle centre to the same height as the Height-Finder jig

            Determine the required height off-set for grinding a primary or secondary angle

            Raise spindle to the correct clearance angle height (see note below)

Ensure Side-Grinding jig release lever is set to closed position

The Side-Grinding jig is re-positioned on the table, using a set square so that the cutter sits on the Lip Rest 

            Grind flute edge by moving table left and rotating tool cw. against the Lip Rest

            Use Side-Grinding jig release lever to open position and withdraw

            Grind primary angle to give visible land thickness (about 0.030”)

deg. Notes   Lip Rest – The thread on the Tool Rest is 40tpi. A complete turn is 0.025” so each index mark is 0.005”. 

Spindle – The thread on the Spindle is 20tpi. A complete turn is 0.050”.

                Use mirror to check results

                Wheel to rest height = Dia. Wheel * Sin a/2

                Wheel Dia.=3.2

              Mount End Tool-Holder jig on table

Set End Tool-Holder jig to datum 0 and release the spindle lock so that the cutter can be rotated to position the end teeth edge.

            Set Cup parallel to table (use rule to sight and then angle to cut only on the leading edge.

            Ensure spindle in vertical plane (Z) use set square

            Set Table to -1deg. (acw) to produce grind more towards centre of end-mill

            Set cutter to be sharpened in collet

            Set Tool-Holder to 15deg. (secondary angle)

            Raise Spindle to approximate tool edge height

            Set tool to be sharpened (sight cutting edges in vertical plane)

            Sharpen each cutting edge in turn

            Set End-Tool-Holder to 5deg. (primary angle)

Spindle – The thread on the Spindle is 20tpi. A complete turn is 0.050”.

Lathe Cutting Tools Clearance Angles      

The main function of the secondary clearance is to relieve the primary clearance when

repeated grinds have increased the width of the land.

The land width should be kept quite small, otherwise a wide land (depending on

the diameter of cutter) there will only be a small clearance for the chips. With too little

clearance the chips will score the work and affect the finish produced. Generally, land

widths on carbide tipped cutters are smaller than those for H.S.S. milling cutters.

Recommended Land Widths

         H.S.S. Cutters                0.8 - 1.5 mm (1/32" - 1/16")

         Carbide Cutters             0.4 - 0.8 mm (1/64" - 1/32")

When cutting Cast Iron with Carbide Cutters, if there is the horse-power available and

great stock removal is required, sometimes the Rake Angle (C) is made Negative as

opposed to the Positive Angles quoted and illustrated above. In such cases, it is usual

to have angles up to 8deg. which will make the included wedge angle of the cutter greater

than 90deg. for increased strength, compared with the more conventional cutter which

always has an included wedge angle less than 90deg..

It sounds difficult but is not and I am always surprised how good the resultant ground edge is. At the end of the travel the tool is retracted using the lever and the cutter rotated to present the next cutting edge to be ground. The same procedure is followed for grinding both the secondary and primary angles which I usually set at 25deg. and 5deg.  respectively. Inspection of a visible land is can easily be seen and should be about 0.020" thickness. The most difficult aspect of sharpening cutters is to remember to grind off only the bare minimum.

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