Model Engineering Articles

Sharpening End Mill Cutters

Drawing 1,- drawings available on request

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 balk 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 half inch diameter 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.

Sharpening the end cutting edges is relatively simple. I made a jig (This was designed by Derek Brookes and was described in the MEW- No. 16 & 17.) to hold the {short description of image} 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 1º 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 (15º-16º) first on each cutting edge and then re-set the tool holder to a primary angle of 5º and grind each edge to visibly show a small land of about 0.020" thickness.

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 {short description of image} 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 cup 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. The following process seems to work well and produces good results.

Using the grinding disc-wheel. After dressing, the disc wheel is set at a small (5º) 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 90º and just off-set to the right hand side of the wheel disc 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 25º)and primary (usually 5º) 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 cutti {short description of image} ng 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. 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. Initially the lip rest is initially set such that the flute edge when it just starts to grind is exactly at 90 º and just clear to the right side of the cup wheel edge. The lip-rest is then lowered as appropriate, see chart below, for the required angle. Grinding proceeds as with the disc-wheel. for each cutting edge.

I have found that results from both methods produce equally good results. Recently I re-engineered the lip-rest lowering screw-thread to 40tpi making it is easier and more accurate to get the correct height adjustment distance. This has improved the success rate dramatically and I now find I am using the cup-wheel more frequently and has become the preferred choice. The key thing to remember is that the height of the lip-rest and subsequent adjustment in height of the rest or disc-wheel must be accurate. A small error in this regard can change the angle ground quite significantly plus it is important to grind the minimum of metal.

Use this table for the appropriate offset when grinding with the disc grinding wheel

Wheel Dia.	---------	---------	--angle--	--(deg)--	----/----	--offset-	---(in)--	---------	---------
	3	4	5	6	12	15	20	25	30

3.0	0.079	0.105	0.131	0.157	0.312	0.388	0.513	0.634	0.750
3.1	0.081	0.108	0.135	0.162	0.322	0.401	0.530	0.655	0.775
3.2	0.084	0.112	0.139	0.167	0.333	0.414	0.547	0.676	0.800
3.3	0.086	0.115	0.144	0.172	0.343	0.427	0.564	0.697	0.825
3.4	0.089	0.119	0.148	0.178	0.353	0.440	0.581	0.718	0.850
3.5	0.092	0.122	0.153	0.183	0.364	0.453	0.599	0.740	0.875

Use this table for the appropriate offset when grinding with thecup grinding wheel

Cutter Dia.	---------	---------	--angle--	--(deg)--	----/----	--offset-	---(in)--	---------	---------
(inches)	3	4	5	6	12	15	20	25	30

0.250	0.007	0.009	0.011	0.013	0.026	0.032	0.043	0.053	0.063
0.313	0.008	0.011	0.014	0.016	0.033	0.041	0.054	0.066	0.078
0.375	0.010	0.013	0.016	0.020	0.039	0.049	0.064	0.079	0.094
0.438	0.011	0.015	0.019	0.023	0.046	0.057	0.075	0.093	0.110
0.500	0.013	0.017	0.022	0.026	0.052	0.065	0.086	0.106	0.125
0.625	0.016	0.022	0.027	0.033	0.065	0.081	0.107	0.132	0.156
1.000	0.026	0.035	0.044	0.052	0.104	0.129	0.171	0.211	0.250

Metric

Cutter Dia.	---------	---------	--angle--	--(deg)--	----/----	--offset-	---(in)--	---------	---------
(mm)	3	4	5	6	12	15	20	25	30

6	0.006	0.008	0.010	0.012	0.025	0.031	0.040	0.050	0.059
8	0.008	0.011	0.014	0.016	0.033	0.041	0.054	0.067	0.079
10	0.010	0.014	0.017	0.021	0.041	0.051	0.067	0.083	0.098
12	0.012	0.016	0.021	0.025	0.049	0.061	0.081	0.100	0.118
14	0.014	0.019	0.024	0.029	0.057	0.071	0.094	0.116	0.138
16	0.016	0.022	0.027	0.033	0.065	0.082	0.108	0.133	0.157
18	0.019	0.025	0.031	0.037	0.074	0.092	0.121	0.150	0.177
20	0.021	0.027	0.034	0.041	0.082	0.102	0.136	0.166	0.197

It sounds difficult but is not and I am always surprised how good the resultant {short description of image} 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 25ºand 5ºresopectively. 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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