Tailstock DRO

This is the second version of a tailstock DRO for my HarrisonĀ M250. There are details of the first version below.

The M250 has graduations on the hand-wheel of the tailstock as well as on the barrel. This can be very useful for drilling holes to a particular depth, but the tailstock lead-screw is 2.5 mm pitch so it is a lot less convenient than one with a round number of millimetres per turn. Also, while it is easy to zero the hand-wheel dial, it is obviously not possible to zero the graduations on the barrel.

There were a couple of (albeit rather minor) issues with the first version of the tailstock DRO. The thing that prompted me to replace it was being offered a very cheap unbranded second-hand digital caliper. I already had a Mitutoyo digital caliper so I couldn't see me using the new one for its intended purpose. Like the Mitutoyo one, the new one had one key advantage over most digital calipers: it doesn't have an "inch/mm" button on it and hence is metric only. With the previous tailstock DRO, I found that I (quite frequently) accidentally touched the "inch/mm" button and found the display showing the tailstock position in inches. I can't imagine a situation where I would ever want that, so having a caliper that can only display millimetres is a massive benefit in my opinion.

The other minor issue with the first version was that very, very occasionally, the body would move. If it happened more often I would have considered it a major issue but it was so infrequent that it was quite easy to put up with.

For the new DRO I bit the proverbial bullet and drilled and tapped a hole in the top of the tailstock casting. The new bracket could thus be screwed down to the tailstock and hence cannot move.

If you look closely at the image at the top of the page, you'll see that I've shortened the right-hand end of the caliper. Due to the limited travel of the tailstock, there's no need for the full length of the caliper scale and by shortening it I can still get access to the tailstock's oiling point without having to remove the caliper.

This photo shows the rear of the mount, showing the screw that goes into the locking hole in the caliper and holds the caliper into the bracket.

The barrel clamp for the new tailstock DRO is unchanged from the previous version and is described below.

Original Tailstock DRO Description

I had a cheap digital caliper that I was pretty much guaranteed never to use as I had a much better quality one which didn't drain the battery as quickly as the cheap one. Therefore, I decided to hack it up and turn it into a tailstock DRO, which would be likely to be used far more often and hence hopefully be slightly less wasteful in terms of batteries!

These photos show the parts that make up the DRO. The large plate is 55 mm × 80 mm × 7 mm aluminium. The slot at the top is 4 mm × 18 mm (and accepts the sliding part of the caliper). As is obvious from the photo, the jaws of the caliper have been cut off (with a Dremel) and ground relatively flush (with a bench grinder). The clamping slot at the bottom (left in the photo) is 1 mm wide and is used to hold the plate on the end of the tailstock barrel, as shown in the photo at the top of the section. Both the tailstock barrel clamping screw and the caliper clamping screw are M4.

This (terrible quality, sorry) photo shows the rear of the caliper body. I have fitted a 3 mm thick aluminium plate to the back of the body. It has small holes located over the screws that hold the caliper body together and large holes for two 20 mm × 3 mm NdFeB magnets (positioned arbitrarily such that they don't get in the way of the screws. The plate and the magnets are held in place with superglue. The magnets hold the caliper body to the tailstock body and saved me from having to make holes in this casting.


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