Cut a transverse tapered hole (Unimat milling column)

I've had my Unimat DB/SL for about 18 months but it lacks the steel column to allow it to be used for milling and drilling. I've tried watching eBay to see if any came up (without the adapter bracket - I already have that) but none have. Mine is the older cast iron model so the milling column fixing is different from most anyway.

Having now got a Faircut lathe in the shed I have the between centres capacity to make a new milling column so the job has been on my 'to do' list for a while.

Having taken the headstock off to see how the bottom of the column needs to look it seems that the job will be a bit tricky (see photo). The 25mm column needs to reduce to 20mm diameter where it fits into the base and needs to have a tapered hole to take the tapered fixing bolt. The hole is about 10mm at its minimum and has a taper that is about 1:2.

I can't put the column on the faceplate and bore it - the column is a foot long so I'm thinking that I need to hold it in the milling slide and use some sort of tapered cutter (or a very strange angled countersink) mounted to the lathe spindle.

Am I missing a better way?

Can such a cutter be had... or can I make something to do the job?

Regards, Andy

You could make a cutter from silver steel and then harden and temper it.

Or could you cut a section off the column and hold it in the four jaw chuck or faceplate and machine the tapered hole in the nomal manner, then join that piece onto the rest of the column? Maybe make it a male thread on one piece of the column and female on the other. A tapered shoulder on each might help location too. Then weld or silver solder it together. Or even Loctite? Dowel pin?

Any idea what such a cutter would look like? Figuring out the taper is easy but I'm not sure how the cutting edges would look or how to make them.

I had considered making the end as a separate piece and threading it into the main column. That approach has problems too, the biggest being that I would need to make and thread quite a big hole in the end of a foot long bar that won't fit through the headstock. Apart from the steadying challenge, fitting the job, a drill, a chuck and a tailstock into the available length might prove tricky.

So at the moment having the spigot as a separate piece doesn't look like it buys me much in terms of making the job easier so I'm preferring to tackle the transverse hole problem.

Regards, Andy

Do you have to match the standard taper and use the standard taper bolt or can you simply make your own taper and bolt for the milling column. If making your own pair is acceptable perhaps the taper on a small diameter plain cone cut bit will be close enough to work. Given the hole a matching taper on the bolt shouldn't be too hard to do as you can use the cutter as a gauge.

Perhaps something like this **LINK** would make a suitable hole.

From an engineering perspective I can't see the exact taper as being especially significant.

Clive

Thanks, yes making a new bolt with a matching taper would be an option (although the original uses an Allen key and I've never figured out a way to make a hexagonal hole).

I think the taper is just there to pull the column down into the socket so it probably just contacts with one side of the transverse hole.

The RS cutter quotes 2mm max thickness... which is a bit of a concern since the length of taper needed here is perhaps 12mm or more.

Regards, Andy

For convenient reference ... This ebay listing has pictures of what Andy needs to make:

**LINK**

https://www.ebay.co.uk/itm/Emco-Unimat-SL-Milling-Column-W-Mount-From-Early-SL-Cast-Iron-Model-Lathe/123771279762

MichaelG.

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P.S. for convenience of manufacture, a separate spigot, fixed with Loctite 638 would be my choice.

Edited By Michael Gilligan on 11/01/2020 12:33:20

I'd agree with Hopper, make a tapered D-bit. It's not very clear, but there's one bottom right in this picture, used to make a tapered blind hole in the check valves for my traction engine:

Simply machine the taper on the end of a piece of silver steel. Since one half of the taper already exists you may need to experiment with the top slide settings to match it. Then mill, or file, almost half the diameter away. Finally harden and temper. In use step drill to remove most of the material and then use the D-bit to clean up the taper.

Andrew

I would do what Clive suggests. Get the cone cutter and make a bolt to suit the taper it cuts.

It's unclear whether the cone bolt pulls the post down tight against the shoulder or if the 20mm diameter is a very close fit and the head of the cone bolt a close fit in it's hole too. I would suspect that the cone bolt bears only on the bottom of the taper to pull the post down hard. That would seem to be the most rigid setup.

First turn the 20mm diameter on the post but leave the shoulder 1/16" short or so for adjustment. Cross-drill the post using a crotch centre then use the cone cutter to make the taper. Once the taper is cut, turn a matching cone bolt and use blue to match the taper.

I would go for the cone cut option to create the tapered hole in the column. Whilst they are intended to be used on sheet metal they do cut over the whole cutting edge so with care can be used to create the hole you need.

I would definitely do the cross drilling and tapered hole before reducing the column diameter. The 20mm spigot needs to be accurately on axis so hold the bar in a collet if it will pass through the headstock, or use a fixed steady with the 'top' end of the column set true in the 4 jaw.

Getting the position of the shoulder in the right place will be easier than putting the tapered hole in the right place if you turn the spigot first.

Ian P

I agree Ian that's why I said leave it 1/16" short to begin with. Easy to fine-tune as a last op.

Thanks folks. I do fancy the D bit option - it's something I've not done before and I'm usually happy to invest a bit of time if I'm learning something as well. I'll keep the RS cutter as plan 'B' if I can't make the D bit work.

Since the minimum diameter is quite big I'm thinking of extending the small end and putting in a centre drill so that I can support that end with the tailstock during the cut.

Fine tuning the shoulder position as the last op seems like a good plan although I think that roughing the spigot first will reduce the amount of metal for the 'D' bit to remove. I might also do some step drilling to further reduce the amount of work or am I over-thinking this?

Regards, Andy

I appear to be in a minority of one ... but please let me put the ‘production engineering’ case for making this column in two parts:

All the machining of the spigot [including the cross-hole and its taper] could be simply accomplished in the lathe, without special tools.

MichaelG.

Actually Michael, that's a good idea. Make the spigot so that it has a fine thread to screw into the post.

Michael, thanks for your suggestions. They are perfectly good suggestions but none of the options suggested by anyone is spectacularly less difficult (for me with my existing tools) than the others. I need to pick one and having never so far made a cutting tool from silver steel I'd like to give it a try. I've even found a supplier nearby for the steel so I'll be popping over there to fetch some in the week - makes a nice change from ordering everything online.

Regards, Andy

Step drilling will definitely make it easier for the conical cutter. I just looked at the RS one suggested and its probably too small for this job if hole at the small end is about 10mm. I have collected an assortment of cone cutters so have a range of sizes (up to about 40mm) most of them are of cheap eastern origin but work as well as genuine conecut ones do.

I'm quite happy to loan you one (where are you?) as I think its a relatively lot of work to make a D bit.

Unless I misunderstood Michaels approach, I dont think a joint in a milling machine column is a good idea from a rigidity point of view.

I think Emco intended the shoulder where the column changes diameter to be the feature that in combination with a close diametrical fit, holds the column dead square to the base casting when it is pulled down tight by the wedging action.

Ian P

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You clearly don’t have the same faith in 638 as I do, Ian

... That’s fine: I have no intention of debating the matter, and Andy has already made his decision.

MichaelG.

Quick footnote about the 2 mm depth quoted for the RS cutter.

These cutters are primarily sold for adjusting the sizes of holes to make them a little larger when the original drilling wasn't quite big enough. The thicker the material the greater the difference in size between the big end and little end. Obviously too much difference and the fit of whatever goes in the hole becomes rather iffy. Variation over 2 mm thickness is considered small enough not to be a worry in the intended applications which, generally, are not exactly precision.

Many years ago I was told that using such on material up to 10 gauge - 1/8 " (3mm), was fine but it was advisable to go in from both sides on thicker material. Which reduced hole size variation. The ones I used had a screwdriver style handle drilled for a tommy bar. Quite effective as I recall matters but fairly hard work in thicker steel and needing a good deal of care to keep straight. But I never used them enough to nail the technique down.

If doing a longer hole step drilling first to ease the load sounds sensible.

Clive

Thanks again all.

I did some better measurements this evening. The taper is 11mm at the small end and 15mm at the big end. It is 8mm long. At each end the hole is counterbored to 17mm. I'm pretty sure it is a 1:2 taper.

I also made a start on the job - just centre drilling and facing each end of the bar that I hope will become the milling column.

While I was there I also checked up on a few questions regarding capacity. The good news is that my 4 jaw chuck has a hole big enough to admit the bar into the chuck body which buys me an extra inch and a half or so of lengthwise capacity.

The Faircut spindle hole is nowhere near big enough to allow this bar in (I already knew that).

There is enough length to allow the bar to be centre drilled using the tailstock chuck but not enough to allow a normal length drill of a suitable diameter to be used in the tailstock chuck. I do have a few 1MT imperial drills that will fit lengthwise (no chuck needed) but having scanned my collection I doubt that any of these will be a suitable tapping diameter to allow me to pursue the separate spigot option with my own facilities.

So plan A remains as before.

PM sent to Ian regarding his cutter collection.

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Just to be clear, Andy ... ‘though it’s only for academic interest.

I was suggesting a close sliding fit, fixed with Loctite 638 [no threading involved].

Thinking further ... I would probably reduce the spigot and hole to about 19mm for the insertion length, to give 3mm wall thickness on the column.

MichaelG.

Thanks Michael... but the limitation on bed length applies either way. I think the biggest I could fit in would be 1/2 inch by using a drill with an MT1 taper at t'other end. I'd be thinking in terms of 15mm plus as a sensible size - threaded or otherwise.