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DIY Milling Machine

I have often looked at my red pillar drill and thought "that should make a good mill". I'm not the first to convert a cheap drill into a home-made mill but if you search the web you won't find many. My experience has exceeded my expectations. I now have a very practical machine that did not take very long to convert and certainly did not cost much. I should probably say that while I use my lathe quite a bit, I don't really have a great need for a mill nor the space for one. I have no milling experience so spending significant amount on one is hard to justify.

In considering my options I could have bought a vertical slide to use with my lathe. However, these are not cheap nor have great capacity so I feared it might be a waste of money. A cheap bench mill was going to cost more money than I wanted to spend. It would take up space that I don't really want to tie up with something I seldom use. Also, the cheap ones tend to be 'rough diamonds' needing some work to clean up or modify. They also often have high speed DC motors and noisy gear drives; I prefer rumbling induction motors so back to the pillar drill...

To keep things simple my mill can only take cutters with a 6mm shank. I can still mount the original drill chuck which makes it fairly versatile. A 6mm end mill seems happy taking a 1mm cut across steel and can take decent cuts over most of the side of the cutter. Clearly I don't expect it to be as precise and rigid as a proper mill but has allowed me to start learning a new skill and do simple milling jobs.

Milling requires as much rigidity as possible. The column and head casting of a pillar drill are fairly solid but the quill is usually a loose fit. A slit 2/3rd the length of the front of the head and a 6mm bolt between the bearings fixes this. This allows you to vary how tightly it grabs the quill and to lock it for cutting. This is a useful mod for pillar drills too. A similar mod is needed for the rear hole to clamp the head against the column.

If you can find a pillar drill with an MT2 tapered spindle this would be ideal. However, these tend to be more expensive and the spindle in mine has a protruding arbor (which is more common). I bored this out to 6mm in the lathe using the technique described in the 'Shaft Hole' section on this machining tips page. For most of us the spindle/hole will probably be slightly off centre (depending on the quality of your chuck and ability to centre). The hole is also likely to open up over time. The grub screw to secure the cutter will tend to push the tool off centre so position where it has the greatest chance of pushing the tool back to the centre.

A mill's spindle needs to rotate smoothly withough having any play up, down or sideways. This requires pre-loading the bearings. While the standard two ball bearings would probably have sufficed initially, I took the opportunity to replace these with angular bearings of the same size. I then needed to do what for me was the most difficult part which was to thread the spindle above the top bearing and to make two 'nuts' to lock in the pre-load. A fine thread is needed and I chose a 1mm pitch. I had not cut threads before so practiced on some scrap to get the technique right. The threaded part is below the top of the quill when assembled so I also needed to make some special tools to 'reach in' to adjust the two nuts.

I have a BV20 lathe with an 8" swing and decent bed length. I was tempted to mount my mill on the end of the bed as this would give the greatest rigidity. However, it would limit travel and be a hastle to mount and remove the mill whenever needed. So I decided to mount it on the side of the bed which gives the greatest movement and convenience. This required making a bracket which is shown in the photos. By mounting the mill on the saddle I was able to surface the top of the bracket to make alignment of the mill to the lathe easier. I removed the paint on the side of the lathe where the bracket is mounted. The bracket is held in place with four 8mm bolts. I've not added them yet but a couple of tapered pins would reduce the risk of movement and allows disassembly and replacement without losing alignment.

The depth gauge on my quill had a 1mm pitch so I was able to make an aluminium disc (with a threaded hole in the middle) to provide fine adjustment of the Z axis. 10 or 20 divisions divides up the 1mm pitch nicely. I'm still using the plastic fitting around the quill and will need to machine one from steel in due course. Naturally this adjustment is not meant to be load bearing and can only be used with motor off and quill unlocked to set a depth for the next cut.

Some (proper) mills have round columns and some are rectangular. The problem with round ones are that the head can rotate left and right and lose alignment. However, it does have the advantage of allowing you to swing the mill's head out of the way for turning operations on the lathe. Note that in order to be able to lower the head down to the saddle, you need at least one hole in the belt guard through which the column passes. There's more I could to do to improve usability but for now it meets my simple needs admirably. Go build one!

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