Author Topic: BCA mill/Jig borer ER collet spindle  (Read 1102 times)

Offline llionellis

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BCA mill/Jig borer ER collet spindle
« on: April 17, 2018, 10:27:38 AM »
In my race to furnish my little home workshop with all the equipment needed to fill my time as a fully fledged retire I came across a BCA mill/jig borer on that world famous auction website. The mill was duly collected and included in the sale was a plethora of useful equipment. Unfortunately the plethora included very few of the elusive BCA collets. A little internet research came up with a beautiful drawing of a ER 20 collet adapter courtesy of
http://www.unionsteammodels.co.uk/cgi-bin/yabb/YaBB.pl?num=1332458612

http://www.unionsteammodels.co.uk/files/BCA-ER20_collet_chuck-1.pdf
 and one was duly manufactured and collets from our friends in China were bought. This made the machine far more useful than when I only had the very limited range of original collets.
The ER collet adapter works fine and allows the use of many imperial and metric cutters up to 12 mm diameter and I would have been happy leaving it at that apart from the fact that the collet reduces the head height of the machine and the cutter is much further away from the machines spindle bottom bearing than in the original design.
For some time I wondered if manufacturing a new spindle incorporating a ER collet would be worth while.
Curiosity eventually got the better of me and now I have a BCA mill with a ER collet spindle. My Boxford lathe is a metric model and the BCA components are all imperial. The drawing produced is  metric annotated with imperial measurements. If any of you decide to make a similar spindle I strongly suggest that you check the journal diameters of your own machine before starting.
My chosen material for the new spindle was good old BDMS. As I had a length of 1 in stock. The original BCA spindle is hardened (case hardened possibly) I do not have the facility to harden and grind, so my spindle is left in the as found state.
As I only use the mill for model making and rarely is it run for more than a few minutes at a time I have not encountered any problems.
 


It is obvious that it is important to ensure that journals and the bore of the collet chuck are all concentric. This worried me when I made my first collet adapter, I believe that the method I eventually used overcame the problem and consequently the same approach was used to make the spindle.
Rough out the basic shape leaving about  1 mm on diameters and lengths for final machining.
Hold the material in a chuck with the collet end protruding out sufficiently to machine the threaded portion.
Machine the 16 degree internal taper using a spare collet to A) ensure that the taper is correct and  B) that the gauging dimension of 6.2 mm is obtained
Machine the external thread using the collet nut to ensure correct fit.
Once all this is done, have a cup of coffee before commencing on the next bit.
Find your largest collet and check the bore diameter.
Chuck a piece of scrap and turn a diameter equal to the bore diameter of the collet and a few mm longer than said collet. (do not remove this item from the chuck).
Fit the collet and clamping nut to the embryo spindle ensuring that the bore is spotlessly clean and slide the assembly into the mandrel that you have produced. Tighten the clamping nut up tight.
If you now spin this up it should be reasonably true but not spot on.
Center drill the end and support with your favourite center and machine all the external diameters and the thread for the thrust adjustment nuts.
With a bit of luck the bore and all the external diameters should be concentric (they were in my case with a Tir. Less than half a thou. (result)
All you have to do now is cut the key way and manufacture 2 nuts to suit the spindle threads and you are ready to Dismantle the BCA and refresh it with a new bespoke ER collet spindle.

Offline llionellis

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Re: BCA mill/Jig borer ER collet spindle
« Reply #1 on: April 17, 2018, 10:36:36 AM »
This is a picture of the finished article

Offline llionellis

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Re: BCA mill/Jig borer ER collet spindle
« Reply #2 on: April 17, 2018, 10:38:09 AM »
PDF of drawing.

Offline Jo

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Re: BCA mill/Jig borer ER collet spindle
« Reply #3 on: April 17, 2018, 11:40:37 AM »
:headscratch: sorry I'm a bit confused here (and I own a Mk 3 BCA) have you replaced the original high speed BCA spindle and this is a replacement? In which case what have you done with the spindle bearings?

The original spindle is very much hardened as are the standard BCA collets.

Llionellis would you like to post an introduction in the introduction area  :)

Jo
Usus est optimum magister

Offline llionellis

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Re: BCA mill/Jig borer ER collet spindle
« Reply #4 on: April 17, 2018, 12:04:42 PM »
:headscratch: sorry I'm a bit confused here (and I own a Mk 3 BCA) have you replaced the original high speed BCA spindle and this is a replacement? In which case what have you done with the spindle bearings?

The original spindle is very much hardened as are the standard BCA collets.

Llionellis would you like to post an introduction in the introduction area  :)

Jo
Hi Jo:
I have not changed the original BCA bronze bushes or the two thrust races. The replacement spindle is bright drawn mild steel unhardened. Prior to retirement I worked in engineering mostly as a supervisor in repair/maintenance shops. It is true the unhardened spindle if in continuation use would benefit from hardening, but in my application I am confident that it will outlast me. I have been using this setup for a number of months and have not witnessed any wear or experienced any vibration or overheating.

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