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Vintage Dumore Model PD Precision Drill Press motor repair

Art Vandalay

Plastic
Joined
May 30, 2024
Location
USA
I am new to this site and found it while searching for information on refurbishing the motor of a vintage (about 1927) model PD Dumore drill press. The wires that exit the housing and become the power cord frayed and broke without leaving a length for splicing.
Being a hobbyist, I am hoping someone here can give me tips and information on how I can splice a new power cord onto this connection. Many thanks!

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I would start by doing some exploratory surgery. Using something quite sharp & expendable (single edge razor blades), cut and strip the fabric taping. Try not to cut into the wires. The fabric tape may have been coated with insulating ("armature") varnish. The single edge razor blades will dull a bit , particularly if you run into the copper wiring, and the insulating materials may be somewhat abrasive. When you have the fabric tape stripped off, you may discover that the wires in the original cord (which broke off) were spliced to the field wires by soldering. I am guessing the cord which broke off had two (2) conductors in it. When you have exposed the two individual wires, you can try to unsolder the splices to the cord wires. Or, you can cut back a little ways on the two field wires. Be very careful in flexing and handling the field wires as the old insulation is likely to be crumbly.

When you have the field wires cleared of tape wrappings and the old solder splices (which may have a knot of twisted wire that was soldered), you can proceed with rewiring. I'd suggest getting some Kevlar insulating sleeve material. This is used in motor rewind shops and resists higher temperatures and oils. Get a new piece of lead cord, size based on amperage the motor draws (nameplate if it is on the drill will have this). Off the top of my head, a lead cord with 10 gauge stranded wires should be OK. You can take the opportunity to go to a grounded lead cord (white, black, green wire insulation). Run the green (ground) in the new lead cord to the frame of the motor.

As for splices, I'd avoid using crimped butt splices. Get some heat shrink tubing and slip it far back on the wires so as to be away from the splices. The old splices were likely soldered, and in the day, were done with lead-tin solder. Twist the wires together neatly at each splice, flux with something like "Nokorode" ( a rosin flux for electrical work), and solder the splices. Work the shrink tube up over each splice when the soldering has cooled, and apply heat (a hot air gun or even a butane cigarette lighter will work for this).

After the shrink tubing is on each splice, you can bundle the splices together. The original splices were wrapped with a fabric tape (possibly 'cambric tape' used by motor builders). Nowadays, a fiberglass tape is used in motor winding and rewind shops. It may get kind of warm inside the motor housing, so you need a tape that is better at heat resistance and has good electrical insulating value.

There is an 'aircraft grade' of heat shrink tubing which has an adhesive inside it. I bought some from McMaster Carr years ago for motorcycle wiring work. This tubing seals against oil and moisture getting into the splices. My own opinion is to avoid crimped butt splices. Vibration, strain and thermal cycling tend to loosen the crimped splices or lugs. I put a drop of solder into each crimped lug or terminal end when running wiring, particularly on engines, vehicles, and machinery.

Another good thing to use is "Machine Tool Grade" wire. This is a stranded wire which is made with more/finer strands to be more flexible and stand up to movement and vibration. Its insulation is rated to resist oils and is good for a fair amount of heat. For snaking wiring in tight places like inside motor housings or small control boxes, machine tool grade wire works a lot better than THHN or THNN grade wire.
 
I would start by doing some exploratory surgery. Using something quite sharp & expendable (single edge razor blades), cut and strip the fabric taping. Try not to cut into the wires. The fabric tape may have been coated with insulating ("armature") varnish. The single edge razor blades will dull a bit , particularly if you run into the copper wiring, and the insulating materials may be somewhat abrasive. When you have the fabric tape stripped off, you may discover that the wires in the original cord (which broke off) were spliced to the field wires by soldering. I am guessing the cord which broke off had two (2) conductors in it. When you have exposed the two individual wires, you can try to unsolder the splices to the cord wires. Or, you can cut back a little ways on the two field wires. Be very careful in flexing and handling the field wires as the old insulation is likely to be crumbly.

When you have the field wires cleared of tape wrappings and the old solder splices (which may have a knot of twisted wire that was soldered), you can proceed with rewiring. I'd suggest getting some Kevlar insulating sleeve material. This is used in motor rewind shops and resists higher temperatures and oils. Get a new piece of lead cord, size based on amperage the motor draws (nameplate if it is on the drill will have this). Off the top of my head, a lead cord with 10 gauge stranded wires should be OK. You can take the opportunity to go to a grounded lead cord (white, black, green wire insulation). Run the green (ground) in the new lead cord to the frame of the motor.

As for splices, I'd avoid using crimped butt splices. Get some heat shrink tubing and slip it far back on the wires so as to be away from the splices. The old splices were likely soldered, and in the day, were done with lead-tin solder. Twist the wires together neatly at each splice, flux with something like "Nokorode" ( a rosin flux for electrical work), and solder the splices. Work the shrink tube up over each splice when the soldering has cooled, and apply heat (a hot air gun or even a butane cigarette lighter will work for this).

After the shrink tubing is on each splice, you can bundle the splices together. The original splices were wrapped with a fabric tape (possibly 'cambric tape' used by motor builders). Nowadays, a fiberglass tape is used in motor winding and rewind shops. It may get kind of warm inside the motor housing, so you need a tape that is better at heat resistance and has good electrical insulating value.

There is an 'aircraft grade' of heat shrink tubing which has an adhesive inside it. I bought some from McMaster Carr years ago for motorcycle wiring work. This tubing seals against oil and moisture getting into the splices. My own opinion is to avoid crimped butt splices. Vibration, strain and thermal cycling tend to loosen the crimped splices or lugs. I put a drop of solder into each crimped lug or terminal end when running wiring, particularly on engines, vehicles, and machinery.

Another good thing to use is "Machine Tool Grade" wire. This is a stranded wire which is made with more/finer strands to be more flexible and stand up to movement and vibration. Its insulation is rated to resist oils and is good for a fair amount of heat. For snaking wiring in tight places like inside motor housings or small control boxes, machine tool grade wire works a lot better than THHN or THNN grade wire.
Thank you very much for that valuable information!
 
I would point out that Dumore is still in business and does provide spare parts for their products. I was able to replace the speciality bearings on my more modern drill press at work. My guess is they did not change their designs much so a brand new armature assembly might be possible. If this were mine I would do exactly as Joe's excellent suggestions go, other than the additional comment that simple non-shrink teflon tubing is another way to go if one were concerned about the heat of a heat gun harming the older fabric/rubber insulation. Also that *gentle* heating can temporarilly soften rigid or brittle old insulation like that - a fact that is often used by antique radio folks.
 
Art:

I am glad you appreciate what I posted. I did check McMaster-Carr's online catalog. They have the supplies you will need for this repair. Surprisingly, even 'cambric tape' for securing and insulating wiring and windings is still available. McMaster-Carr is an excellent source for people like ourselves who need small quantities of specialized materials or hardware.

As grungy and old as the field and wiring look, I'd be hesitant to do any real cleaning. As I wrote, keep handling, and resultant flexing of the old wiring to a minimum. I note the brass clips for the brush holders are tarnished. A careful cleaning of those clips to get bright metal should be done. Something like a fine Scotchbrite pad cut into small pieces, or some 600 grit silicon-carbide paper would work. If you do find the insulation on the wires to the brush holder clips is cracked or damaged, you can un-solder the brush holder clips and slip insulating sleeving over the wires. You could avoid the use of shrink tubing by going with insulating sleeving, and McMaster has a good selection of it.

I found that a soldering gun usually does not have enough heat for this kind of work, particularly un-soldering. I use an oldtime soldering 'iron', quite a bit more heat. I have a vintage "Weller" soldering gun, and, as I mentioned, it is way too light for this class of work. I have a couple of the old wood-handled soldering irons, quite a bit more wattage.

I would not recommend cleaning the field windings and wires with any solvents. Solvents can dissolve the old insulating materials. I would also not recommend using compressed air to clean the windings. This will drive dirt and oils further into the insulating materials. A vacuum cleaner with a piece of smaller diameter hose taped to the nozzle would work to suction some of the dirt and grunge off the windings.
 








 
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