Thursday, October 11, 2012

25K RPM Bridgeport Mill

On one of my many visits to see my toolmaker friend Charlie we ended up on the subject of high rpm drilling and milling. He would always be curious about what was going on in the job shop I managed, and many times offered solutions to real problems we encountered. I don't remember the job but we must have had a bunch of small holes to drill in some tough material. At the time our maximum spindle speed was probably 6K rpm on one of the Matsuura mills. I think he loaned me a contraption with a high speed motor on it that we tried but didn't work for us for a reason lost in time. This story is typical of my friendship with this master toolmaker.

Charlie and his wife lived in a modest two story house in South San Francisco. "City of Industry" is literally painted on the side of the local mountain. Charlie's shop was in the "Basement" which was just the street level entrance of the house. It amounted to a large three car garage with a house on top. If you have been following the blog then you have seen some of the machines he had in the shop. I would come over and the door to the shop would be open a crack in anticipation of my arrival. No formalities just jump right in and hit the shop. Charlie was a good scrounger and serious packrat. There was more material squirreled away than many job shops keep around. In this modest basement shop he had surface grinding, cylindrical grinding, tool and cutter grinding, heat treating, milling and turning, sawing and all the stuff in between. He rarely needed to go out for anything mechanical except plating. This was probably good because he had a hard time sleeping which I hear is a by product of getting old. He might get up at three in the morning and get dressed and head to the shop for a little heat treating and grinding before breakfast.

I would typically visit on Sunday afternoon and stay for several hours talking about all manner of subjects. The following week after the visit all the thoughts settled and gelled I might write him a letter or send a copy of an article and some bit of material I spoke about during the weekend visit. Late the next week one of the guys tells me there is a box on UPS for me. I wasn't expecting anything so I went to see what is was. When I got sight of the box I immediately knew it was from Charlie. He had this way overdo it thing with shipping and packaging and he would use black spray paint to blot out the old labels on his recycled boxes. He must have had a problem with UPS in the past.

I opened the box and this is what was inside.
I found the letter that preceded this shipment of a high speed spindle attachment. For the life of me I cant remember what the job was that we needed some very high spindle speeds for. There is a series of letters that touch on the subject of small high speed milling and then one letter where Charlie describes his mental picture of this very tool that showed up in the mail.
Ongoing organization in my shop exposed the box to the light of day once again. I got curious to see if the unit would fit the milling machine I have now which is a Bridgeport knockoff.
There are two heads for the unit. One takes a ER-8 collet and the picture shows the second unit with a hand feed drill chuck. One of the belts has gone missing (upper)  so I tried to track some replacements down on the web.
The way this thing works is the R-8 insert is connected to the pulley at the top. It drives the smaller pulley on the top of the idler arm. This in turn drives the large pulley on the bottom of the idler arm and finally makes it back to the lower section of the spindle. The drill chuck assembly is connected to the pulley but has a bearing on the OD that is housed in the R-8 insert. This allows the R-8 adapter to provide all the driving with the pulleys acting as a two step speed increaser. The ratio on the first step is 1:1.6 and the second step is 1:3.95. If you do the math your 4000 rpm Bridgeport can now sport 25,000 rpm at the drill chuck or collet adapter.
Included with the speeder were a couple of nice cross section drawings of the unit. Charlie was a pretty decent draftsman. For many years he designed injection molds at a small tool and die shop in an alley on Heron street in San Francisco. Its all a snooty artsy area now but the die shop looks almost the same as it did back then. When I worked there it was down and dirty industrial area and you had to roust drunks out of the alley if you needed to move equipment around.
This cross section shows the clamp that attaches to the bottom of the quill housing. You can see the chuck shank rides in bearings that isolate it from the R-8 adapter and the machine spindle rotation.
The entire assembly moves up and down with the quill. In this shot you can see the v belt tensioner and the bearing housing for the idler pulleys. When I  used this many years ago the whole deal was rotated around to face away from the operator. Guarding is non existent and definitely close to the operators hands. The belts are very small and would probably not nip a finger off but I'm not too excited to try my theory out.

I fiddled around on the web trying to find these small cross section v belts without any luck. I called my drive component connection Ralph at Bearing Engineering in San Leandro and got the scoop on the belts. He found them easily. I guess they are a metric cross section or angle so I probably wasn't searching with the right search string. Ralph got me some prices right away but before I order the belts I wanted to measure the pulleys more accurately than I had previously by using a soft seamstress tape instead of a regular tape measure. It dawned on me that this might make a good article to write up so I took a few pictures this evening. Guess what I found on the box when I set up to take the pictures?
 Charlie put the belt numbers right on the box for me. I didn't even notice it until now. This is typical toolmaker behavior in case you don't know. In his letter that I found he mentioned his nephew had given him a nifty labeler. Direct quote from the letter,  

"When the keys are punched, it somehow prints the letters (looks like jet printing) in whatever size has been selected, and then runs the finished label out of the machine, where it can be separated from its backing, leaving a good glue for sticking in place." 

The world was changing pretty fast technologically in the early nineties. In many ways Charlie had a child like wonder at things we take for granted now. The first time I sent him a letter with color pictures imbedded in the letter he thought that was almost magic. I tried to get him playing with computers but he always resisted and rightfully so in retrospect. I think he realized that he would become obsessed with them at the expense of what he really loved to do which was work in the shop when ever the mood hit him. 

When I get the new belts I'll shoot some video of the high speed spindle in action and put them on the blog.

Thanks for looking


  1. Hmm, interesting! Never thought of such a thing.

  2. Hi Andre,

    Pretty cool. I shot a video of the unit running. Its on my channel somewhere.



  3. here I find myself researching CNC mill spindle cross sections after waking up at 4:45am and come across your fascinating article. Thanks for sharing this, I felt more than a little kinship.