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Bridgeport power feed

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Andrew Johnston27/01/2021 12:03:39
5930 forum posts
666 photos
Posted by Mark O'Callaghan on 27/01/2021 11:53:26:

Am I right in thinking that the neutral from the mains supply isn’t used?

Correct, none of my ex-industrial 3-phase machines use neutral, just 3 phases and earth.


Mark O'Callaghan27/01/2021 12:17:37
17 forum posts
1 photos

Thank you. All understood now. Thanks ever so much for all of the help and advice.

Alistair Robertson 127/01/2021 13:54:14
127 forum posts
6 photos

I have recently helped a friend to renovate a Bridgeport Varispeed. There was no table feed fitted but we used a new Align 110 volt kit and took the 110 volts off the existing transformer fitted in the control box. We also used the 25 volt feed through a rectitier to power a new control panel which works everything including motor, table feed, coolant and work light. So there are no high voltages in the hand controls everything is powered by solenoids. The DROs we powered from a 13amp socket which is also fed off the original Bridgeport Control box transformer.

Mark O'Callaghan27/01/2021 13:58:01
17 forum posts
1 photos

Mine also had a regular plug socket on the back. I now know how I’m going to wire it in. Hopefully should have it up and running in a few weeks.

Clive Foster27/01/2021 14:11:50
2596 forum posts
90 photos


Glad you have got the electrics thing sorted out.

PM me your e-mail address, can't send stuff via this site, and I'll put a data pack together for you covering the Erskine drive. Do you have a Bridgeport manual? If not I can include my wiring diagram and find an appropriate download link for you.

Easiest way to put the head on is to fix a stout bar pointing vertically up to the table and grab it with a collet in the spindle. Then the table and knee feeds can be used to align the head with the bolt holes. A few extra strategic supports (wood offcuts?) wouldn't hurt. As I recall it I used a 3/4" bar stuck into a 6" (ish) thick lump of timber clamped to the bed when I sorted my head out after buying the machine around 2004/05. I made sure the nose of the quill was set down hard on the wood. After so many years I no longer know if such a thick lump of timber is needed. This time round I shall use a special removing support made by welding a blank end R8 arbor to a 1" steel plate.

Sounds like Zan has done what I briefly considered doing when I got my Bridgeport back in 2004/05 but VFD boxes were way to expensive then. (Especially when a proper, commercial, 440 V output static converter surfaced at £35 and I had a motor in stock to make it a rotary.). I expect his circuit will do you just fine.

This is an example of a commercial VFD set up for a Bridgeport as supplied by Power Capacitors UK (the Transwave folk).

bridgeport vfd re.jpg

I grabbed it a few months back as the price was right and it was just down the road but I'm unsure if its exactly what I want so I might sell it on. That one is a vector drive unit and the guy who sold it to me said he didn't have any motor heating issues at low speeds despite not having an extra fan to help keep the motor cool. The man who used the Optidrive also made no mention of needing an extra fan.

I'm given to understand that modern Vector Drive inverters are much better at giving the motor exactly the right amount of power at exactly the right time so low speed heating isn't such an issue in machine tool use as it was with the older Voltage/Frequency types.

Considering the single speed toothed belt drive conversion I found a decent belt length and centre distance calculator at Technobots :- **LINK** and ran the American L section belt and pulley data through it.

For the two 40 tooth pulleys and 92 tooth belt the centre to centre distance comes out at 9.75”, 274.65 mm which looks right when waving a tape at my Bridgeport.

Can't quite get that with an 8M20 belt and pulley set. For same speed 1:1 drive nearest is two 36 tooth pulleys and a 105 tooth belt at 276 mm centre to centre distance. Closest I could fined was a 44 - 48 pair giving almost 10% speed reduction at 275.9 mm centres with 115 tooth belt.

I shall probably go for the 115 tooth 8M20 belt and 44 - 48 pulleys as having comfortably more load capacity and running the motor a little faster to help with low speed torque and cooling. Objectively the differences are pretty much academic really.


Edited By Clive Foster on 27/01/2021 14:20:14

Oily Rag28/01/2021 11:02:06
317 forum posts
126 photos


I'm interested in what you said in an earlier post about the L section timing belts only being able to transmit slightly less than 2 HP.

Some years back I was involved in manufacturing timing belt conversions for speedway bikes and these used a 2.47:1 reduction ratio on the primary drive. The belt we used was a 1.5" wide and this was perfectly good to handle 65 BHP from a standing start when the clutch was 'dropped' at the starting gate. There is a range of belts which are either polymer, polymer with glass fibre re-enforcing, or a polymer with glass and steel wire re-enforcing. It was the later specification belts we used.


Clive Foster28/01/2021 12:17:20
2596 forum posts
90 photos


Just over 2 hp is what the books say for classic timing belts. Which were designed for light duty work. Metric ones are HDT with different teeth for more power transmission.

OT digression

But there are lots of more sophisticated ones taking much more power. But not always enough. I remember Richard Negus telling me that Gates suggested one of their heavy duty belts would be up to the rear drive job on the Norton Commander rotary. But they got their sums wrong. Stripped the teeth straight off when he wheelied the test bike.


Rotary engine straight line, twist grip connected direct to the speedo needle, power curves can be nice or scary. Your choice.

Had a not so nice little earner changing batteries on speedway and grass track bikes for a while. Did an emergency change for a friends friend once and didn't charge enough so he bought all his mates round.


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