Cut a transverse tapered hole (Unimat milling column)

Here's a thought. Drill through 8mm and add the two counter bores then take a square file and open up the 8mm hole to a tapered rectangular one. Make a matching new cotter 

You only need the cotter to pull the column downwards so no real need for the hole to be round.

Edited By JasonB on 12/01/2020 07:51:49

Filing the taper is a good idea. I dont think the mount of metal to be removed would be too onerous and hand filing accuracy should be good enough for this job - thanks for that.

Out of interest I dug out some of the cutters I have and measured the included angle of each cutter. All the proper ones are 20 degrees and the Chinese Ti coated one is 17 or 17.5 degrees. (largest one is 30mm)

All are a quite a way from the 1:2 taper Andy measured (so 28.03 degrees inclusive).

Jason's hand filed option is definitely the best suggestion, best too because it can be fine tuned with blue to get good contact withe the cotter.

Ian P

This job has been progressing slowly mainly due to me not wanting to keep walking in and out of the shed in the wet weather. Now we have dry weather and, well... a certain amount more time on our hands so I've been moving things forward.

While discussing the 'D' bit I didnt mention that I also lacked anything suitable for counterboring the hole. A visit (long before current situation) to Coventry Grinders yielded a foot of 17mm silver steel. This was used to make a counterbore with a 3/8 hole to take a pilot bar.

The 'D' bit was also made from the same stock and my Bullfinch torch had its first two jobs to harden and temper both tools.

The column itself is 1 inch bar. The Unimat, being metric needs a 25mm column so I had to turn 0.4mm off the diameter. The finish on this is not great. I suspect it is down to backlash in the compound slide on the Faircut. It's the first time I've tried turning over almost a foot in length so it's a learning experience.

A photo of the partially machined column and the hole making tools complete with coloured oxides which probably won't show up very well. The pilot bar is brass in case you were wondering about the colour...

The transverse hole has now been drilled to 3/8 to match the pilot bar and the first counterbore done. I was uncertain whether my counterbore was going to cut steel but I put the Faircut into the lowest backgeared speed and did the job slowly. It went absolutely fine.

That'll do me for today.

The next session will be somewhat busy, needing the other counterbore to be done and then the tapered hole to be step drilled which will involve some careful setup and measurement to avoid taking too much off. The hole will be roughed out with a file and then (I hope) I will see if the 'D' bit works. The last op will be to take the step in the column back by about 2mm to fit it to the Unimat.

Job's a good un!

The 'D' bit worked just fine. I had my doubts because it seems to lack anything resembling a cutting edge, but what do I know.

The step drilling marks are still visible so I didn't get that quite right. I skipped the filing stage because the thing was set up and aligned on the lathe and I thought it would be easier to just see if the 'D' bit would do the job without any filing (it did).

Thanks again to everyone who offered advice on this. I got there in the end.

Well done mate, helpful lot on here arn't they.