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Bridgeport power feed

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Mark O'Callaghan26/01/2021 16:05:25
17 forum posts
1 photos

I have a Bridgeport Vari speed with a 2j head. I got it for free so I’ve no complaints. I’m planning on using a VFD inverter to power the main 2hp motor. My problem is how to get power to the power feed on the X axis. It’s a genuine Bridgeport power feed and I believe it runs at 110v. Any suggestions on how I can fire it up? I’ve considered an alternative modern unit but have read that you need a suitable sleeve to go over the leadscrew end. What I’m concerned about in that instance is buying a new power feed that plugs into the wall but not able to find a sleeve to fit.

DC31k26/01/2021 16:19:23
686 forum posts
2 photos

Yellow building site transformer.

Mark O'Callaghan26/01/2021 16:32:02
17 forum posts
1 photos

Funny you should say that, I have considered it but wasn’t sure if the controls would be affected. It’s got an Erskine control unit, I suppose you feed the power directly to that?

Emgee26/01/2021 16:40:27
2426 forum posts
290 photos


If the original supply was 415v AC 3 phase check the transformer that provided the 110v supply to the table feed unit, where the 110v supply is connected you will need to connect your new 110v supply from the 230/110v transformer.
Remove the original 110v supply cables and insulate the ends separately to make them safe.


john fletcher 126/01/2021 16:48:41
794 forum posts

Several years ago I fixed a Bridgeport and the control box is lower right hand side and I think the transformer is in there as well as the PCB. You will need as similar size transformer to what is in there at present 240 volt input 115 out run via 13 amp socket or via a contactor. Unfortunately I don't know the rating of the transformer, but have a look at RS or Farnell. The motor is a shunt wound 110 volt DC and the PCB controls the field. Should you need it, I have a circuit diagram and parts list for the PCB for this machine, but some one on HERE re- drew the diagram in a much more clear manner, perhaps they will read your request and will send you a copy. John

Maurice Taylor26/01/2021 16:51:11
215 forum posts
36 photos

Hi, if you use a yellow site transformer ,make sure neither side of the items to be supplied are earthed,The 110volt from a site transformer is center tapped ,with center tap earthed.


gary26/01/2021 17:09:33
140 forum posts
28 photos

hi, sorted machine tool uk sell the adapters for about £50. they are easy to make i got the drawing off the internet but cant remember where. i just used a yellow transformer for my old unit

Mike London26/01/2021 17:26:44
28 forum posts
1 photos

Please correct me if I am missing something but you have a varispeed head with all the the torque benefits.
Why not just have a phase converter and you then you don't have to worry about the machine electrics.

Clive Foster26/01/2021 18:24:57
3135 forum posts
109 photos


The transformer in the original control box of UK built machines runs off one phase of the 440 V three phase supply. So, if you still have all the original electrics, you can just hook the transformer up to the single phase incomer and run the power feed off it in the standard manner.

My re-draw of the Erskine circuit diagram and all the other data I've managed to collect is in the files of the Bridgeport Group on :- **LINK** . If want the data and don't want to join up PM me and I'll get it sent over via an FTP substitute program. Way too big for E-Mail.

Even with a 2 hp spindle motor a VFD isn't terribly adequate as a direct speed control substitute for the Varispeed unit. The speed range is greater than ideal and Varispeed losses high so you run out of torque at lower speeds if you try to run with the Varispeed on a fixed setting.

At 1,000 rpm (ish) idle the drive power losses are said to be 550 W or so with only 200 due to the spindle. Under drive the Varispeed is probably taking something like 2/3 rd of a hp or more. Which is why the head casing gets hot. Using both VFD and Varispeed to control spindle rpm strikes me as being a quick way to major confusion. If you want to retain the Varispeed its probably easiest to use one of the Eaton DE-1 series inverters which are basically made as an intelligent substitute for ordinary contactor controls. No display or external controls so you just stick it in the box and hook-up switches.

I was told by a reliable source a that a 2 hp main spindle motor controlled by a modern VFD works well with a single speed belt drive of ratio of 0.75:1 or 1:1. Apparently when geared down at 0.75:1 virtually all jobs can be done in high range. Start to run out of torque around 300 spindle RPM. But gearing down makes things frantic at high speeds. Running 1:1 is much better at higher speeds but low range is needed at times. This was on a CNC converted machine (Linux CNC) earning its living in a professional workshop so driven fairly hard.

The VFD was an Optidrive. Belts were L section toothed belts. Unfortunately I don't have the pulley diameters.

Given the propensity for rattles with the Varispeed set-up and its need for regular rebuilds (mine is overdue, and loud) switching to a single speed belt drive seems a good way to go.



Mark O'Callaghan26/01/2021 19:04:54
17 forum posts
1 photos

Right, so a bit confused now. Clive Foster are you suggesting I use an Eaton DE-1 and a 110v yellow transformer, or convert to a single spindle and use a VFD and a110v transformer.

Mark O'Callaghan26/01/2021 19:14:20
17 forum posts
1 photos

Can I not use a 3kw phase converter, plus a 110v transformer? I have read that you lose a lot of power due to the capacitor bank taking up the 3rd phase.

Clive Foster26/01/2021 19:37:01
3135 forum posts
109 photos


Sorry for confusion.

Basically neither.

For the spindle


retain the Varispeed drive and use an Eaton DE-1 as the VFD (speed change is via the Varispeed as per standard)


replace the Varispeed with a single speed belt drive and use a modern VFD to control the speed (its reported that an Optidrive and 1:1 ratio belt works well).

For the Erskine drive

Connect a single phase input to the 220 V input on the standard transformer in the main box which will give you 110 Volts for the Erskine drive.

Transformer specification is:

Inputs :- 20 - 0 - 220 - 380 - 440 Volts at 50/60 Hz AC


a) 110 V at 750 VA (for power feed)

b) 5.5 V at 16 VA (?? possibly for the old vernier scale accessory illuminators)

c) 0 - 12 - 24 - 50 V at 24 - 48 - 100 VA respectively (for the low voltage light)

Earth - screen/terminal

Transformer data from Adcock-Shipley (Textron) wiring diagram WD 145/D dated 1977.

Talk to Inverter Drive Supermarket or a similar well regarded supplier before buying your VFD. IDS have sold me 4 drives so far, including 2 Eatons, which were just right for the jobs.

I plan to do a VFD and single speed drive conversion on mine with 220 Volts on the transformer later in the year.


Andrew Johnston26/01/2021 19:38:32
6602 forum posts
701 photos
Posted by Clive Foster on 26/01/2021 18:24:57:

The transformer in the original control box of UK built machines runs off one phase of the 440 V three phase supply.

On my Bridgeport at least, the transformer runs across 2 phases, ie, 415V. It can't run from a single phase as there is no neutral wire going to the machine.


Clive Foster26/01/2021 19:46:16
3135 forum posts
109 photos


Well spotted Andrew.

Didn't notice the error until too late to edit!

As noted above, according to the 1977 wiring diagram, the standard transformer has a 220 V input tapping. Mine certainly does.


Mark O'Callaghan26/01/2021 19:52:18
17 forum posts
1 photos

Ahh, that makes sense now thank you.

Mark O'Callaghan26/01/2021 19:54:20
17 forum posts
1 photos

Mines a 1979 with original electrical wiring and cabinet. So should be as you’ve described.

Clive Foster26/01/2021 22:19:59
3135 forum posts
109 photos


Just to give you some idea of the costs of doing a simple single speed belt conversion on the spindle I have some data from an American source about doing the job on a Bridgeport clone (Supermax?) with taperlock fitting L section pulleys and timing belt. Mislaid the link unfortunately.

Drive ratio 1:1 using 2 equal size pulleys.

Pulleys 40 tooth, 3/8in L pitch, 1" wide belt :- Bearing Shop UK price approx £20 each

Taperlock bushes size 1610, (I think 1" and 1 3/8" shaft size for my 1 1/2 hp machine ):- Bearing Shop UK price approx £5 each

Timing belt, 3/8in L pitch, 1in wide, 92 tooth, 34.5in pitch length (876.3 mm) Bearing Shop UK price approx £2.

Prices include VAT so call it £60 with delivery. Similar to a new Varispeed belt methinks.

I haven't yet verified that the belt length is correct for a real Bridgeport or whether there are more economic ways of doing the job. I suspect that the motor will need an intermediate mounting plate with eccentric slots so belt tension can be adjusted by twisting the motor. I imagine getting the pulleys at the same height so the belt runs true may be "challenging".

The American reference also mentions adding an inexpensive import digital tachometer to read spindle speed.

I plan to just calibrate the speed control knob. It would be proper posh to fix the potentiometer to the original indicator dial. With the added bonus of confusing the heck out of wannabe machine borrowers.


PS Not convinced about the power rating for L section belt, found book figures of 1.53 Kw at 1000 rpm, barely 2 hp.  8M, the nearest metric size seems to be rated at around 4 kw at 1000 rpm see a few pages around . But L section has been used and reported to be just fine so ...


Edited By Clive Foster on 26/01/2021 22:46:24

Zan26/01/2021 23:02:32
312 forum posts
20 photos

My Bridgeport was converted 10 years ago

ill post the circuit diagram in the morning, but

1 rip out all the fuses and anything between vdf and motor

2 wire the vdf directly to the motor use the reverse switch to control only the digital inputs to the vdf or you could inadvertently reverse while motor is running Don’t ask........... I did this while testing after fitting a new drive shaft and forgot.... new vdf a month later......

3 my transformer has a 220 v input and a 440 so former used to give power to the control contractors and the power feed the 12 v is great for led lighting

4 a single 240 contractor was wired up for main power on with a new pair of push buttons on the control panel  I don’t like 240 v going here, but there was no choice  all 240 v power is fed from here to vdf, DRO and an external drive socket 

5 the motor control contractor now just triggers the vdf enable digital . I like this because it enables a satisfying clone when turning on ans enables using my knee to turn off

6 (edit) both the micro switches in my power feed failed by age cracking. So replace these while you are at it!

Edited By Zan on 26/01/2021 23:06:27

ps I run the motor at a fixed speed using the vari speed unit, but it’s noisy and I’m thinking about changing to a direct drive as indicated above. Just worried about cooling the motor at low speeds. The fan at the moment draws a lot of air through the head to cool everything 

Edited By Zan on 26/01/2021 23:17:14

Mark O'Callaghan27/01/2021 11:01:40
17 forum posts
1 photos

@ Clive Foster I understand using the tap for the 220v and the earth shield. Where does the neutral from the single phase go to? Also could you PM me a copy of the wiring diagram please.

@Zan, could you PM me a copy of the diagram you used as well please? I may go down this route eventually. At the moment I have far to many things to do.

Mark O'Callaghan27/01/2021 11:53:26
17 forum posts
1 photos

Please excuse my ignorance. Domestic wiring I’m fine with. 3 phase I have very limited knowledge of. Am I right in thinking that the neutral from the mains supply isn’t used? I’ve looked in the main box and the transformer is as you’ve stated with a 220 point etc. Just want to make sure I understand it all before I connect it up, which maybe some time away as I’m struggling to get the head back on.

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