In the end I decided to get a replacement spindle, since it was relatively cheap. If the bore on my lathe was large enough I would've had a crack at regrinding the taper. As it is I'd have had to try and get it set up in my flimsy steady rest.

Hey, thanks for all the great info! I got in touch with the supplier, turns out you can get a replacement spindle for not too much money. My plan is to give that a go and see if it solves the issue, if not I'd then have a go at fixing the taper. I did watch a video of Robin Renzetti hard turning the taper while installed in the mill, with a boring tool clamped in the vice. But as you say, setting the correct angle for MT3 would be tough. On the plus side, I don't think my spindle is actually hardened. I made a toolpost grinder attachment not so long ago, not sure if that would be accurate enough though. It would be just about within the capacity of my lathe, but my DIY steady rest is a bit flimsy and a proper one for my lathe costs a small fortune!

@@PaulsGarageProjects If you have a steady rest for your Boxford lathe, that seems large enough to have the power and strength to hard cut the taper. Does the cross slide have enough travel would be the next issue. Pretty easy to use a MT3 taper mounted between centers of the lathe to set up the angle for the cross slide. After seeing you make lapping plates, you have way more skill then me. I was surprised a few years ago, when I had a scrap piece of 1" round bar that had a stuck inner race of a ball bearing on it, and with carbide tooling was able to cut it off. But I know that tool deflection of a long boring bar will not behave the same as cutting from the outside of material held close to the 3 jaw chuck. Seems you might want to make a tool post grinder to do this precision machining of the spindle. That said, I don't want to know the runout in my R8 socket milling spindle. It's an old machine I got used at an auction, and had the collets spun by the previous users. So the problem I know I have is surface roughness of the R8 taper.

That's a good point - there'd be a fair amount of deflection with a boring bar long and thin enough to cut an MT3 taper. I'm guessing deflection wouldn't be as much of a problem with a tool post grinder? It's a bit outside my comfort zone to say the least, but with a spare spindle I might as well give it a go!

@@PaulsGarageProjects Although you would only take very light passes to take off mere microns, in order to achieve maximum accuracy you'll still want a grinding wheel with a shaft that's as stiff as possible in your tool post grinder. Another option might be to lap the MT3 taper of your spindle to better spec, though it still likely wouldn't be a simply procedure. The best way I can think of to do it would be to make a highly accurate MT3 tapered lap out of aluminium round stock, charge it with a small amount of very fine diamond paste and then lap your spindle with it, and every so often cleaning the spindle and lap then checking the progress by applying some machinists' blue ink to the spindle and pushing the cleaned lap against it to see where and how well they contact. Once you get a complete or nearly complete ink transfer from the spindle to the lap, you're done. How accurate your spindle with end up will depend on how accurate of a lap you make or purchase, how accurately you mount the spindle in the 4-jaw chuck of your lathe and the lap in your tailstock along with the accuracy of your lathe itself. Lapping is the most accurate and precise technique in the metalworking repertoire, surpassing even surface grinding. I have the same mini mill as you (in my case a HBM BF25) and also in the process of trying to get the best attainable out of it by carrying out the 3 bolt mod, upgrading the electric motor, scraping in the mating surfaces of the column and cross table, replacing the spindle bearings, plastic intermediate shaft gears, and I'm trying to see if I can somehow fill the base with epoxy granite to increase dampening and stiffness, perhaps brace up the back of the column as well. Another thing I put on my list of upgrades to carry out is bracing the head to the swivel base by means of a strong bracket. After all I don't really see myself using the mill with the head in anything other than vertical, and the connection points between the head and swivel base are all quite low down and below the center of gravity (which is the minimum height the mounting bolts should have been at), so adding a bracket at the top to connect the head to the base will significantly stiffen up and strengthen the connection.

Many thanks for all the great info! I got in touch with the manufacturer, who sold me a replacement spindle for a reasonable price. The runout is now less than 10 micron, not perfect, but good enough for what I do with it. I did a fair few upgrades to my old mini lathe, but haven't thought about them for the mill. There are some interesting ideas there, I'll have to investigate!

That's a very good point! I did measure the runout of the inside and outside of the MT3 taper, and they were both out by 0.03 mm, with the high and low spots matching. I got in touch with the manufacturer of the mill, and they were able to supply a replacement spindle for not too much money. With this installed, I now get less than 0.005 mm runout when measuring the internal MT3 taper.

A very good question! I've always just presumed they were for a c-spanner, so you can tighten/loosen the drawbar? I would love to upgrade to a bigger milling machine, perhaps someday...