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Photo 1 – An ER-40 collet adapter aligned for turning. Note the dial indicator used to measure the slope of the compound slide.

With some simple trig tables it is very easy to compute the slope measured in decimal fractions per inch of carriage travel. Table 1 below shows the slope measured to ten thousands per angles between zero and twenty degrees and the tenths in between. To use this information place a dial indicator on “Y” axis and either using your DRO or another dial indicator move the carriage one inch and measure the slope of the compound slide. The slope of the ER-40 collet is 8.0 degrees for a depth of 1.377” From Table 1 this shows a slope of 0.1405 inches per inch of carriage travel. Dial indicators with 1 or 2 inches of travel will not measure in tenths but you should be able to extrapolate fairly closely.

[TABLE="width: 583"]

[TR]

[TD="colspan: 3"]Slope per inch of travel[/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD][/TD]

[TD="colspan: 3"]Tenths of degree[/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD]Degrees[/TD]

[TD]0[/TD]

[TD]0.1[/TD]

[TD]0.2[/TD]

[TD]0.3[/TD]

[TD]0.4[/TD]

[TD]0.5[/TD]

[TD]0.6[/TD]

[TD]0.7[/TD]

[TD]0.8[/TD]

[TD]0.9[/TD]

[/TR]

[TR]

[TD]1[/TD]

[TD]0.0175[/TD]

[TD]0.0192[/TD]

[TD]0.0209[/TD]

[TD]0.0227[/TD]

[TD]0.0244[/TD]

[TD]0.0262[/TD]

[TD]0.0279[/TD]

[TD]0.0297[/TD]

[TD]0.0314[/TD]

[TD]0.0332[/TD]

[/TR]

[TR]

[TD]2[/TD]

[TD]0.0349[/TD]

[TD]0.0367[/TD]

[TD]0.0384[/TD]

[TD]0.0402[/TD]

[TD]0.0419[/TD]

[TD]0.0437[/TD]

[TD]0.0454[/TD]

[TD]0.0472[/TD]

[TD]0.0489[/TD]

[TD]0.0507[/TD]

[/TR]

[TR]

[TD]3[/TD]

[TD]0.0524[/TD]

[TD]0.0542[/TD]

[TD]0.0559[/TD]

[TD]0.0577[/TD]

[TD]0.0594[/TD]

[TD]0.0612[/TD]

[TD]0.0629[/TD]

[TD]0.0647[/TD]

[TD]0.0664[/TD]

[TD]0.0682[/TD]

[/TR]

[TR]

[TD]4[/TD]

[TD]0.0699[/TD]

[TD]0.0717[/TD]

[TD]0.0734[/TD]

[TD]0.0752[/TD]

[TD]0.0769[/TD]

[TD]0.0787[/TD]

[TD]0.0805[/TD]

[TD]0.0822[/TD]

[TD]0.0840[/TD]

[TD]0.0857[/TD]

[/TR]

[TR]

[TD]5[/TD]

[TD]0.0875[/TD]

[TD]0.0892[/TD]

[TD]0.0910[/TD]

[TD]0.0928[/TD]

[TD]0.0945[/TD]

[TD]0.0963[/TD]

[TD]0.0981[/TD]

[TD]0.0998[/TD]

[TD]0.1016[/TD]

[TD]0.1033[/TD]

[/TR]

[TR]

[TD]6[/TD]

[TD]0.1051[/TD]

[TD]0.1069[/TD]

[TD]0.1086[/TD]

[TD]0.1104[/TD]

[TD]0.1122[/TD]

[TD]0.1139[/TD]

[TD]0.1157[/TD]

[TD]0.1175[/TD]

[TD]0.1192[/TD]

[TD]0.1210[/TD]

[/TR]

[TR]

[TD]7[/TD]

[TD]0.1228[/TD]

[TD]0.1246[/TD]

[TD]0.1263[/TD]

[TD]0.1281[/TD]

[TD]0.1299[/TD]

[TD]0.1317[/TD]

[TD]0.1334[/TD]

[TD]0.1352[/TD]

[TD]0.1370[/TD]

[TD]0.1388[/TD]

[/TR]

[TR]

[TD]8[/TD]

[TD]0.1405[/TD]

[TD]0.1423[/TD]

[TD]0.1441[/TD]

[TD]0.1459[/TD]

[TD]0.1477[/TD]

[TD]0.1495[/TD]

[TD]0.1512[/TD]

[TD]0.1530[/TD]

[TD]0.1548[/TD]

[TD]0.1566[/TD]

[/TR]

[TR]

[TD]9[/TD]

[TD]0.1584[/TD]

[TD]0.1602[/TD]

[TD]0.1620[/TD]

[TD]0.1638[/TD]

[TD]0.1655[/TD]

[TD]0.1673[/TD]

[TD]0.1691[/TD]

[TD]0.1709[/TD]

[TD]0.1727[/TD]

[TD]0.1745[/TD]

[/TR]

[TR]

[TD]10[/TD]

[TD]0.1763[/TD]

[TD]0.1781[/TD]

[TD]0.1799[/TD]

[TD]0.1817[/TD]

[TD]0.1835[/TD]

[TD]0.1853[/TD]

[TD]0.1871[/TD]

[TD]0.1890[/TD]

[TD]0.1908[/TD]

[TD]0.1926[/TD]

[/TR]

[TR]

[TD]11[/TD]

[TD]0.1944[/TD]

[TD]0.1962[/TD]

[TD]0.1980[/TD]

[TD]0.1998[/TD]

[TD]0.2016[/TD]

[TD]0.2035[/TD]

[TD]0.2053[/TD]

[TD]0.2071[/TD]

[TD]0.2089[/TD]

[TD]0.2107[/TD]

[/TR]

[TR]

[TD]12[/TD]

[TD]0.2126[/TD]

[TD]0.2144[/TD]

[TD]0.2162[/TD]

[TD]0.2180[/TD]

[TD]0.2199[/TD]

[TD]0.2217[/TD]

[TD]0.2235[/TD]

[TD]0.2254[/TD]

[TD]0.2272[/TD]

[TD]0.2290[/TD]

[/TR]

[TR]

[TD]13[/TD]

[TD]0.2309[/TD]

[TD]0.2327[/TD]

[TD]0.2345[/TD]

[TD]0.2364[/TD]

[TD]0.2382[/TD]

[TD]0.2401[/TD]

[TD]0.2419[/TD]

[TD]0.2438[/TD]

[TD]0.2456[/TD]

[TD]0.2475[/TD]

[/TR]

[TR]

[TD]14[/TD]

[TD]0.2493[/TD]

[TD]0.2512[/TD]

[TD]0.2530[/TD]

[TD]0.2549[/TD]

[TD]0.2568[/TD]

[TD]0.2586[/TD]

[TD]0.2605[/TD]

[TD]0.2623[/TD]

[TD]0.2642[/TD]

[TD]0.2661[/TD]

[/TR]

[TR]

[TD]15[/TD]

[TD]0.2679[/TD]

[TD]0.2698[/TD]

[TD]0.2717[/TD]

[TD]0.2736[/TD]

[TD]0.2754[/TD]

[TD]0.2773[/TD]

[TD]0.2792[/TD]

[TD]0.2811[/TD]

[TD]0.2830[/TD]

[TD]0.2849[/TD]

[/TR]

[TR]

[TD]16[/TD]

[TD]0.2867[/TD]

[TD]0.2886[/TD]

[TD]0.2905[/TD]

[TD]0.2924[/TD]

[TD]0.2943[/TD]

[TD]0.2962[/TD]

[TD]0.2981[/TD]

[TD]0.3000[/TD]

[TD]0.3019[/TD]

[TD]0.3038[/TD]

[/TR]

[TR]

[TD]17[/TD]

[TD]0.3057[/TD]

[TD]0.3076[/TD]

[TD]0.3096[/TD]

[TD]0.3115[/TD]

[TD]0.3134[/TD]

[TD]0.3153[/TD]

[TD]0.3172[/TD]

[TD]0.3191[/TD]

[TD]0.3211[/TD]

[TD]0.3230[/TD]

[/TR]

[TR]

[TD]18[/TD]

[TD]0.3249[/TD]

[TD]0.3269[/TD]

[TD]0.3288[/TD]

[TD]0.3307[/TD]

[TD]0.3327[/TD]

[TD]0.3346[/TD]

[TD]0.3365[/TD]

[TD]0.3385[/TD]

[TD]0.3404[/TD]

[TD]0.3424[/TD]

[/TR]

[TR]

[TD]19[/TD]

[TD]0.3443[/TD]

[TD]0.3463[/TD]

[TD]0.3482[/TD]

[TD]0.3502[/TD]

[TD]0.3522[/TD]

[TD]0.3541[/TD]

[TD]0.3561[/TD]

[TD]0.3581[/TD]

[TD]0.3600[/TD]

[TD]0.3620[/TD]

[/TR]

[TR]

[TD]20[/TD]

[TD]0.3640[/TD]

[TD]0.3659[/TD]

[TD]0.3679[/TD]

[TD]0.3699[/TD]

[TD]0.3719[/TD]

[TD]0.3739[/TD]

[TD]0.3759[/TD]

[TD]0.3779[/TD]

[TD]0.3799[/TD]

[TD]0.3819[/TD]

[/TR]

[/TABLE]

Table 1 – To determine the slope per inch of carriage travel select the degrees and tenths of a degree the value is in the intersecting box. Example, 8.0 degrees is 0.1405” per inch of travel. I keep a copy of this on the wall over my lathe.

After you have removed enough material from the sloped surface so that the width of the cut is half way or more to the final dimension use some high spot blue to confirm you taper adjustment. In this business you need to make some assumptions from time to time and in this case I assume that the collet is of the correct size and has the correct taper. If some adjustment is needed reattach the dial indicator to help guide your adjustment. Photo 2 shows this on an ER-32 collet and adapter.

Photo 2 – High spot blue used to confirm the angle of the collet adapter, this is a ER-32 collet.

Being able to construct collet adapters for your equipment is a handy skill. I find more and more uses for collets as venture into new areas. I am particularly happy with the double-cut ER style of collets. As show I have an ER-40 adapter on my 10x22 lathe and use it extensively. Photo 3 shows a R8 adapter that is part of the tool head for a cutter grinder I built.

Photo 3 – Tool head for a cutter grinder that uses R8 collets.

These collets holders are not ground or hardened but for the average hobbyist they should give many years of service. My oldest and most used is the R8 holder on a cutter grinder that has been service for three years. As always if you have any questions or comments feel free to contact me either on this post of via private message.

Jim