Fitting a Variable Speed Motor to a Dore Westbury Milling Machine

Gallery, Projects and General > Project Logs

<< < (2/5) > >>

Bernd:
Ah well..... I ah......... not yet. No excuses. Just that darned round tuit the keeps me from finishing projects. :bang:

Bernd

andyf:
Yesterday, the job progressed a bit further, but my efforts ended with a bit of a disaster.

I needed to get the flywheel and poly-V pulley (both keyed) off the new motor, and attach the 3-step pulley from the old one. The flywheel came off easily, but the pulley needed persuasion using bolts and the long nuts from a clamping kit as jacks. The 3-step pulley from the old motor was easy to slip off its keyed shaft, but there was no keyway in the pulley itself. It was secured by a grubscrew going into the keyway in the shaft. I’m wondering if I should bother cutting a keyway into the pulley. The grubscrew method seems to have lasted 40 years without any problems.

The pulley was held in the 4-jaw and persuaded to run true. I couldn’t find either a suitable boring bar or the one I once improvised by grinding up an Allen key, so I used one of the good teeth left on a damaged ½” endmill to enlarge the bore from 5/8” to 16mm.

Next: mounting the new motor. Originally, it sat in a pressed steel cradle, but this couldn’t easily be modified to fit on the mill. The cradle on the mill is a casting, but contoured to fit the old motor which was about 1½ “ more in diameter than the new one. Reprofiling was needed, so I resorted to JB Weld. The previous night, I had wrapped my new motor in kitchen clingfilm to enable release, and stuck duct tape on the parts of the cradle where excess epoxy would need to be trimmed. JB Weld was applied liberally to the cradle, the new motor was given a good push down into it and left there overnight. In the morning, I removed the rolled-up paper plugs which had been put in the cradle’s four bolt holes, drilled the holes clear through the epoxy coating, and trimmed off the excess.
.
Of course, the original threaded fixing holes in the new motor casing were in the wrong places, and only two had been needed when the motor sat horizonally. On the mill, it would be vertical and two M8 bolts might not be sufficient, so the next task was drill and tap four new holes in the casing to match the cradle. The motor was taken apart so I could later clean swarf off the permanent magnets fixed inside the casing. Two of the holes were easy – they could go right through, in one of the gaps between the magnets. The other two holes were over one of the magnets, so are shallow and only allow about three threads of engagement for that pair of bolts. Still, adding another two slightly dodgy fixings to two good ones can do no harm.

I fitted the motor to the modified cradle, and bolted the cradle to the hinged arrangement which tensions the drive belt. The big boss expands inside the cross-tube of the mill, and the lettering on this photo is explained later.

Then I fixed my electronic floggle-toggle box to the mill. The spindle head has a couple of tapped holes on the front for fixing the pulley guard. I bent up and drilled some crap DIY store 4 x 20mm flat stock so I could bolt one end to the spindle head, and the other to the flange on the other end of the Dore Westbury’s cross-tube. The next pic shows this, with my box of electrickery fastened to it – the photo has been crudely edited to erase a confusing background, and the pulley and gearbox are chocked up to improve the view. The pulley guard is missing, but will return. I don’t want an unguarded drive belt, particularly at eye level.

The motor was mounted on the Dore Westbury and the wiring temporarily hooked up for a test run. All went well at first. The combination of the 3-speed pulley drive and the variable speed motor gives a range from dead slow to whirling round far too fast for the spindle bearings.

An earlier photo is repeated here, for convenience:

The hole for the bolt through lug A on the swinging plate has a lock-nut on it below the lug, and a conical end which goes into a correspondingly shaped hole on the fixed plate with the big boss sticking out of it. There’s a similar lug and bolt below, so the fixed plate is pinched between the two bolts, which act as a hinge. The black ball-handle turns a bar which is eccentrically hinged between lug B and a similar lug below, again using cone ended bolts. The bar acts as a cam to tighten up the drive belt.

Testing came to an abrupt halt when the motor, cradle and swinging plate fell off the hinge :bang: :bang:. It’s lucky the motor wasn’t running at the time, or there would have been mayhem. I thought I had located the hinge bolts properly in their holes; all seemed solid enough when given a good tug, but one of them must have missed its hole and have bitten just deep enough into the painted aluminium surface to hold temporarily. :bang: :bang: repeated ad infinitum....

The only damage was that lug B broke off when it hit the edge of the bench on the way down. The cable to the motor acted as a safety line and stopped its headlong plunge just short of the concrete floor. Things could have been worse – it might have been lug A which broke. That one takes the weight of the motor, whereas lug B isn’t subject to much strain.

So, yesterday afternoon’s work ended with re-attaching the lug using two steel dowels and more JB Weld. I’ll play safe and wait until this afternoon before disturbing it.

Andy

andyf:
Things went much better today :D. This afternoon, yesterday’s JB Weld and dowels repair had hardened after 24 hours basking in the warmth of the house. The reattached lug seems good and firm.

Because the cable had stopped the falling motor before it hit the floor, I dismantled the motor again and inspected the wires within. No apparent damage, but to be prudent I replaced the shortest wire, which will have taken the strain.

I continued on electrical safety by drilling and tapping holes to fix PVC brush covers in place of the originals. In the treadmill, the motor had been covered up and the original plastic brush covers were simply sprung into place. They could easily be dislodged.

More drilling and tapping provided fixing holes for a plastic box in which to cram the wires which emerge from the motor, the EMC ferrite ring they take a turn through, and the connector block coupling them to the DC supply cable from my control box.

Both the motor frame and the body of the control box (and thus the whole mill) are connected to earth (ground). That might prevent a short circuit causing a fire by blowing the 13A fuse in the mains plug, but it would only need a few tens of milliamps to kill me off. To protect myself (famous last words!) as far as I can from a major :zap: the supply to my entire shop runs through an RCD.

Earlier this evening I was ready for further tests. Having made doubly sure that the motor was secure (I do learn from my mistakes!) I did some speed trials using my only tacho, a pushbike speedo reading up to 2000 rpm. To actuate it, I cobbled up an arrangement with a magnet held in a slitting saw arbor in the spindle. With the drive belt on the “low speed” pair of the 3-step pulleys, the control knob gave speeds from 0 to 820 rpm. Using the medium-speed pulleys, I got 0 to 1820 rpm. Anything much faster would be beyond the range of the tacho, so I backed off until the spindle was at 910rpm (half of 1820) and stopped the motor with the off switch, leaving the knob where it was. After moving the belt to the high-speed pulleys, I switched the motor back on. Spindle speed was now 1810 rpm. If that’s half speed, top speed would be around 3600 rpm – way too fast for my old machine. Apart from anything else, that big epicyclic gearbox is sure to be slightly off-balance. For comparison, maximum on a Sieg X3 is around 2000 rpm, so I think I’ll forget the high-speed pulleys.

For really low speeds, I can bring the 8.8 to 1 gearbox into action. :smart: Arithmetic shows that the maximum speeds in the two ranges would then be around 90 and 200 rpm.

There’s nowhere on the spindle to fit a magnet or optical stripe to actuate the sensor for a tacho. I’ll have to fix up an rpm dial for the speed control knob, using a temporary tacho as above to calibrate it. The specs for the control board say that speed regulation under varying loads is within 1%, so a dial should be accurate enough.

One nice feature is the soft start on the new motor. Previously, the shop lighting dimmed for an instant when I switched on, but that isn’t happening any more.

I’ve given a bit of thought to a cooling fan. In the treadmill, the motor was horizontal, with a plastic fan on its shaft. The fan broke during removal of the original pulley. Now the motor is vertical (I hope its bearings are happy in that orientation) convection will help, and a fan on the shaft wouldn’t do much at low speeds. I’ll check how hot the motor gets, but I don’t expect trouble. The old motor was only about ½ or ¾ HP, so 2HP is overkill and should run cool.

I wanted to end with a pic of the complete mill, but my camera batteries have just given out. so that’ll have to wait until the morning for recharging.

Andy.

CrewCab:
Nice thread Andy, thanks :thumbup:

CC

andyf:
Here’s a pic of the entire machine with its new motor, control box and the car fan belt I had to get – the new motor being a different diameter, the old one (also a fan belt) no longer fitted. The local car accessory place was very understanding, letting me borrow a selection so I could pick the best size. This photo isn't the best size, though:

Apart from concocting a dial for the control knob so I can set the spindle to a chosen speed, the project is complete. Actually, as there are two useable speed ranges on the pulleys (the third is dangerously fast) and each range can if desired be geared down by the epicyclic gear box. I’ll probably end up making a little frame like those for labels on filing cabinets, and four different dials to slot into it.

Speaking of the epicyclic box, this is in the big canister on top of the spindle head. I found it quite interesting when I got the mill and looked inside. It uses six Myford changewheels, and no internally toothed gear. Very cunning.

If you have been, thanks for reading.

Andy.

Navigation

 Message Index

Go to full version