BP M Head Rebuild by Daniel K


Site Founder
Sep 22, 2010
Original Here

Daniel gave us permission to post this.

Bridgeport M Head Rebuild (for the Benchmaster Mill) – Pt. 1

This post will chronicle the rebuilding of my Bridgeport M Head milling head that will be put onto my Benchmaster mill. I’ve heard of the “Benchport” but I’ve never seen photos anywhere online. Most people seem to say it won’t work, but none of them have said they tried or have seen it done and failed. So with that, my plan is to rebuild this M Head that I bought on Craigslist and put it on my 1948 Benchmaster mill. I may post about the BM mill later – that was a worthwhile restoration and I enjoy using the mill often.

Benchmaster Mill (BM) 2016-04-09-12.38.25-225x300.jpg

(A few other things I may post about later are my Light timpani project, new welding hobby, and the ring roller I built to make timpani hoops and repair existing ones. Be prepared for a technical geek-out.)

The Bridgeport M Head was original built in the mid-20th century and featured a 1/2 HP motor and several tapers for holding tools in the spindle. Mine has a Morse Taper no. 2. This is nice because so is my BM. No additional tooling is needed to get started. The spindle (lower) section of the head is made of cast iron and the upper portion is (usually polished) aluminum. Mine came painted and gunked up. The gunk also hid several broken portions which must be fixed.

Pulley casting as received: 2017-01-06-18.32.10-300x225.jpg

Under the badge was the original finish:
Cracked motor mounting area:http://www.dkpercussion.com/blog/wp-content/uploads/2017/01/2017-01-06-20.13.20.jpg

Stripped thread on other motor mount:http://www.dkpercussion.com/blog/wp-content/uploads/2017/01/2017-01-06-20.13.20.jpg
Oiler port cracked and stripper (and Gits oiler missing): 2017-01-06-18.32.23-300x225.jpg
The first part of the project involved checking the motor to make sure it was still functioning. I bought a Variable Frequency Drive (VFD) to run the 3 phase motor and it tested fine, but it was obvious it needed new bearings. So the whole thing came apart and was cleaned using paint stripper, Purple Industrial Cleaner, and a fine wire wheel. It will get new sealed bearings when put back together.


The castings have been primed, but I’ll be sanding them later, re-priming, and then putting a coat or few of Rust-oleum Charcoal Grey on them. Once the paint has hardened (I’ll give it a week or more), I’ll reassemble the motor with new bearings and hook it up to the VFD again to test it. Once done, it will be sitting on the shelf waiting for when the whole head is rebuilt.


The next (concurrent) part of the project is the aluminum pulley casting. When I bought this on CL, the seller sent me a picture of ANOTHER head, leading me to believe I was getting something a (good) bit different. The picture had a nicely kept appearance, no missing parts, and no stripped bolts head with a mounting bracket. The package I received contained a grimy mess, missing parts, stripped and missing bolts, and pieces missing from the casting. Oh, and no mounting bracket. It was so bad that the casting couldn’t have held a motor or oiler. So… a quick run down of what I did:

Removed all the paint and cleaned the casting in the Purple cleaner. This was dangerous since the cleaner is not meant for aluminum. However, it was worth it to get the 1/16″ of oily grime off in addition to some paint. Removing all the paint and grime allowed me to see any additional cracks or missing parts. Luckily, no more surprises were found.

Cracked motor mount: 2017-01-06-20.13.36-300x225.jpg

Extra holes (some will be filled, some may be used…) 2017-01-07-19.05.12-300x225.jpg http://www.dkpercussion.com/blog/wp-content/uploads/2017/01/2017-01-07-19.04.38.jpg
Oiler port hole: 2017-01-07-19.04.23-300x225.jpg

All cleaned up and ready to fix:

I milled the cracked and stripped oiler port down. I messed it up a little by moving the wrong handle on the mill. Not a huge deal. (The irony of working on one mill using another one isn’t lost on me…)


Milled a 3/4″ flat hole, drilled a 7/16″ hole farther down: 2017-01-07-20.10.31-300x225.jpg

Made a 1/8-27 NPT threaded aluminum piece to be press fit into the casting: 2017-01-07-20.10.37-300x225.jpg

Installed: 2017-01-07-20.20.47-300x225.jpg

Side view: 2017-01-07-20.20.36-300x225.jpg

Next, the motor mounting “threaded” holes were milled out and new aluminum press fit and Loctited in.

The first side was simple – bore out the stripped thread to 3/4″, press fit and Loctite a piece of aluminum stock. I peened it in a bit with a ball-peen hammer to mushroom the top a little. It was a tight fit but there was a tiny wobble in the cut when starting. This covered that up nicely. 2017-01-08-11.17.18-300x225.jpg

Filed and drilled to 3/8″, later threaded to 7/16-14. 2017-01-08-17.56.26-300x225.jpg

Using a 3/4″ endmill to cut straight down into the cracked hole. 2017-01-08-12.08.02-300x225.jpg

After the 3/4″ endmill, I used a 1″ Forstner bit (really meant for wood) to cut a 1″ hole above the 3/4″ hole. It worked really (surprisingly) well and cut slightly undersize, which made it easy to make a -1″ press in bushing. 2017-01-08-13.34.17-300x225.jpg


Drilling the new aluminum to 3/8″ for the threaded stud: 2017-01-08-17.38.58-300x225.jpg

All but threaded! 2017-01-08-17.56.01-300x225.jpg

I had to be creative to fit such a large work piece on my BM’s table. I made some 5/16″ threaded rod into some 1/4″ threaded clamping bolts. (They thread into 5/16″ t-nuts in the table.) This was level enough to trust for drilling for the threaded studs. 2017-01-08-11.22.48-300x225.jpg

New mounting holes and threads were put into the casting. I’m not sure what the original mounting bolt size was, but the ones it came with were not that. I put in 7/16-14 threads and will eventually thread in and Loctite some studs to mount the motor.

Next up will be to polish the aluminum head and paint were needed. I plan to paint the inside and lowered sections of the logo’s area, and polish the outside. More on that later. Also, next step will be stripping down the cast iron lower section and determining what work is needed there.

Daniel K
Principal Timpanist, Greenville Symphony Orchestra Owner, DK Percussion Adjunct Professor of Percussion, Anderson and Clemson Universities http://www.dkpercusson.com/daniel View all posts by Daniel K

Posted onJanuary 9, 2017AuthorDaniel KCategoriesUncategorized
One thought on “Bridgeport M Head Rebuild (for the Benchmaster Mill) – Pt. 1”
  1. 2e6ce017555df46b5bbed06efb4b1794?s=56&d=mm&r=g.png Nelson Timkensays:
    January 10, 2017 at 7:18 pm
    Great job. I’ve been looking for a decent M head for a while.


Site Founder
Sep 22, 2010
Bridgeport M Head Rebuild (for the Benchmaster Mill) – Pt. 2
As work progresses, I hope to post here when there’s enough to write about. In the last few days I built the bench that will eventually hold the Benchmaster with the Bridgeport M Head. Currently it holds my newly restored Delta “Homelite” 11″ drill press, which is a “good enough” drill press to do the simple things not worth lining up on the mill. (Also, I use a tapping head a lot for a part I make regularly.)

The Bench
I ran the air hose to my extra air tank and the power cord for the air compressor. I used zip ties to keep the hose close to the pegboard wall.



I bought some “Steelmaster” (appropriate?) index files to install below the mill to hold cutters, tools, etc. They fit nicely, but I made a miscalculation and may have to only have three instead of four drawers due to the Benchmaster’s knee screw.


The structure of the bench is meant to hold the weight of the BM and BP machinery, which by my guess will be somewhere around 300 lbs. This should do just fine, and the BM will be lag screwed into the 2x3s that support the double plywood top.



Each piece of wood was confirmed level before being screwed into a wall stud.



The bottom shelf was cut into two sections to allow the boards to go into place.



The top was made of 3/4″ plywood, screwed into the structure, followed by a glued on piece of 1/2″ cabinet-grade birch.



The Missing Parts

This picture pretty much sums up what I got when I purchased this “complete head.”


Only two parts from that box even go to the mill head. Ridiculous.

Some other missing parts included small things I could make in the shop including a drawbar nut and a quill spring cover. Both made for fun projects.



Another missing part that is crucial to the head is the upper pulley spindle bearing housing. I found one on eBay but missed it by an hour. Luckily another one came up and I purchased it right away. I think the seller of the mill head should pay for it…



All the bearings have been ordered (six in total not including the motor) and will be replaced once the time is right.

The Motor
The first part of the head restoration was to get the motor up and running. As it is a 3 phase motor, I purchased a VFD to run it. When I initially tested it, it had a scraping sound and I figured it was the bearings. Turns out, it wasn’t. That didn’t stop me from replacing the open bearings with sealed, new bearings.

The scraping sound was actually from two things. The armature in the motor is unbalanced (the aluminum fans aren’t centered perfectly) and the portion on the middle wasn’t round. I dialed it in on the lathe to within .0005″ and took a skim cut along the length. I believe I removed about .030″. After putting the motor back together, it runs way nicer, however it still has a bit of a vibration from the unbalanced fans. I believe it will still work fine.

Some fresh paint and a skim cut on the lathe.



Before and after:

2016-12-24-12.42.32-e1483999051433-300x225.jpg 2017-01-14-16.41.25-e1484539970744-225x300.jpg

I plan to reuse the motor’s switch housing to contain some connections and possibly the potentiometer for the VFD.

The Aluminum Pulley Housing
This part… has enough evidence on it to indicate that the mill hadn’t been used in years. And it was probably removed from service when it fell over and broke. As mentioned in the previous post, there were cracked and stripped parts. I fixed all those using various methods: press fit and Loctite, aluminum 6013 stick welding electrodes, and sort-of-aluminum welding rods. At the time of this writing, the casting is completely repaired and is awaiting sanding and polishing.

The side that gave me trouble was corrected by first tack welding with some aluminum 6013 sticks, and then cracks filled using “aluminum welding” rods from Harbor Freight and Mapp gas. I wouldn’t trust these rods on something structural, but as far as a “filler” that can be polished later, they are great.



I didn’t stop to take pictures while welding or grinding, but afterwards…



Fixed Side (top), original (insert-repaired) on bottom:


The oiler port was securely attached but I didn’t like the way it stuck out. I tried to blend the casting and new part together using the “aluminum welding” rods and some grinding.



I plan to wire wheel, sand, do-whatever to get the oxidation and scratches removed from the aluminum casting and eventually bring it back to its original polished look. Should be a long term project, most likely. Any suggestion for chemical solutions to help brighten the metal and remove oxidation is certainly welcome…

The Quill
The quill seems to be in decent condition, but has a few age and use related marks on it. Hopefully it doesn’t have any effect on the accuracy.


While working on the internal parts of the quill, I kept it wrapped in paper towels to protect the outside surface.


Many people posted on various forums about trouble removing the top bearing nut. Rather than struggle with it, I decided to soak it in a special WD-40 penetrating oil over night and make a special removal tool in the morning. I bored out a simple “cup” on the lathe and tapped four holes 90 degrees apart and put set screws in that fit snugly inside the bearing nut’s slots. The advantage of set screws is that they can be adjusted to the depth needed for the nut and made to fit tight. I ended up putting them just right to require knocking the tool into place with a rawhide mallet. I also milled a flat to accept a 1-1/2″ wrench.



With the oil, large wrench, tight fitting tool, and two blocks of wood clamping down on the spindle, it came apart with very little effort. I did try the night before to use a brass punch to remove the nut and gave up quickly. So I know it was stuck on there. It was almost non-climatic how easy it came apart.



The quill is so dirty it’s hard to tell if the bearings are rusted stuck or just gunked up. I believe at one point this mill head was used for wood and not oiled for a while.



All the spacers were measured with a micrometer and all measured out evenly. The bearings, however, were originally ground at the BP factory to act as angular contact bearings. Measurements were written in case these would be needed later.



New bearings ordered included (all open and in pairs):

Lower (closest to cutter) spindle bearings:
NSK 7205 CTYNSUL P4 ABEC7 ($100 on eBay for a matched pair)

Upper spindle bearings:
Nachi 6205 C3

Pulley spindle bearings:
SKF 6206 C3

Why all the different brands? No reason other than price and quality, all purchased on eBay, all US or Japanese made. The matched pair are the only ones that need to be truly precise, where as the others can be electric motor quality or better.

The Head Casting
Not much to say here yet as I haven’t really dug into the head yet. However, I did remove about 1/8″ thick paint that had been slathered on over the years. It was so thick in some spots that I wondered if I was removing casting filler when I was chipping away with a screw driver and hammer.



The photo on the right shows the amount of grime on the quill. The shiny part was when I tested the movement of the quill.



Originally I had thought the quill feed pinion was missing a few internal parts. Luckily it was only missing the quill spring cover and one fiber washer. A quick test with some washers from my parts bin and it turns out the quill is great. The handle and the hand wheel shaft both work. I’ll replace the fiber washers when I reassemble the head.

I hope the next post shows a finished, polished aluminum pulley housing and perhaps a rebuilt quill! It’ll be a while before the head is actually installed on the Benchmaster and perhaps I’ll share my plans for that in the next post. Thanks for reading.

Daniel K
Principal Timpanist, Greenville Symphony Orchestra Owner, DK Percussion Adjunct Professor of Percussion, Anderson and Clemson Universities http://www.dkpercusson.com/daniel View all posts by Daniel K


Dec 20, 2014
Thank you Nel for posting this. I appreciate your interest in my project. I hope to post more updates to the blog in the future (maybe once a week or less often) and start to become an active member here. I've got plenty to post in the projects threads!


Site Founder
Sep 22, 2010
Welcome Dan!

A lot of people can use your project, because these M heads for small mills don't usually come in good shape to just install and use.
I will post your progress from your blog, or you can post it directly here if you want.


May 4, 2015
Hi Ti, welcome to the site. Nice job on the m head so far. Taking your time doing the rebuild is nice that your sharing with us. Thanks


Dec 20, 2014
From http://www.lathes.co.uk/bridgeport/page2.html - "The spindle could be ordered with a No. 2 Morse, B & S No. 7 or a B-3 taper." That link looks like a great deal and the seller seems to be including what you'd need to fix it up. ...Wish I had anywhere near that great of a sale. My sale stunk. I paid $500 for a "complete head" - the seller sent photos. Turns out the photos came from another website and I received a box of parts, maybe 75% complete. Looking back I should count myself lucky to have received that. (If I see that seller listing anything anywhere, I'll post a warning.)
Either way, I've enjoyed the challenges but I'm about ready to have a working mill with a quill ASAP. I'm waiting to disassemble my BM (I'll be moving it across the shop) so I can use it to make stuff. Just finished the worm and worm gear parts tonight for the head tilter.
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