Making ER11 collet chuck

I am hoping someone on here can help me , I have the soft blank to make an ER11 collet chuck for my Pultra and I am unsure about the internal taper, according to google search the included angle should be 16deg but when I cut the taper in a bit of aluminium using Rocketronics, large end 11mm and 7.5mm small end overall length of 17mm so putting -Z17 and -X1.725 being half the difference between 11 and 7.5 I get 5deg ( so 10deg included) and not 8deg ( 16deg included) what am I doing wrong ?

I want to get this sorted out before I do it on the proper arbor.

Assuming 1.725 is a typo for 1.75??

1.75mm over 17mm is 5.88° by my maths. (tan^-1 (1.75/17) )

8° would be 2.389mm over 17mm ( tan(8° ) x 17mm ) or 11mm down to 6.22mm diameter.

(Only a few more days of smilie ambushes)

Edited By Andy_G on 03/10/2023 18:45:13

Well, according to your diagram the length of the tapered section is not 17mm. The dimension 17mm includes a parallel section. For an 8 degree taper with a difference in diameters of 3.5 mm, the taper length would be 12.452mm. Incidentally, half 3.5 is 1.75, not the 1.725 you show.

Hth

Mike

Thanks for replies the 1.725 was a typing error I don’t know how the 2 got in there, will blame it on my age anyway I have just found a calculator online and the taper length should be as Mike said 12.4mm giving a taper degree of 8.033 so that’s where I was going wrong , it just goes to show that it’s worth doing something that we have never done before using a bit of scrap, anyway I will cut another bit of scrap aluminium again tomorrow just to be sure.

Many thanks Dell

Okay now the next problem my controller will only accept 12.4 or 12.5 not enough digits to put in 12.452 but there is another setting for cone ratio that overrides other settings but I don’t have the first idea how to get the cone ratio, a google search didn’t help anyway not sure it would help without trying anyone know how to work out the cone ratio ?

Dell