A mandrel for long narrow tubes

I'd like to make a Delrin tube, of 2mm wall thickness, 12mm I.D., and around 210mm long.

I can drill out a thicker piece of Delrin rod fairly successfully with a pilot hole made with a single-point silver steel drill and following up with a standard, then long series 12mm drill.

To then turn the O.D. to final diameter, I want to mount the part on a mandrel both to stiffen the part while turning - it's wobbly stuff at this size) and to be able to finish the O.D. in one pass.

There doesn't seem to be much out there about mandrel designs for this specific application, so I have come up with a rough concept:

**LINK**

Four widgets slide on a shaft; the edges are chamfered to create a V-groove in which an O-ring sits. They are tightened by a nut on the end of the shaft which expands the O-ring and holds the workpiece.

So far, so theory! I'm finding it difficult to imagine the magnitude of forces at play here: would this thing actually grip against a cutting force? Obviously Delrin is soft stuff but I'd be taking a good bit off per pass, ideally.

Second, perhaps daft question: From my sketch, it seems to me an o-ring of 2mm thickness and 8mm internal diameter would be about right, but how would I go about being more exact about this? Presumably I can play around with the size of the chamfers and O-ring to achieve an optimum, but at this point I'm not necessarily sure what optimum even looks like!

I used an expanding oring to grip the bore of a 22mm I'd bronze sleeve whilst I machined the OD. No need for tapers, just squeeze it endways and it expands sideways, or tries to. However in this case if you can have the tube a tad overlength just slide a 12mm bar up it, grip one end in the three jaw, (4 jaw would be better), support the outer end of the 12mm bar on the tailstock and away you go

When the O-ring gets compressed, it will get larger in diameter. It will sure hold the delrin tube good enough for turning. I use the same method to place a chuck stop in the spindle boring.

You should not use chamfers. The O-ring should only expand in one direction. Turn a 3 mm long 8 mm shoulder at one end of the bar and a 4 mm deep 8.05 mm pocket for the shoulder at the end of the other bar.

The 8 mm diameter and 2 mm O-ring thickness will give a starting diameter of 12 mm. That should do the job.

You only need 2 O-rings, one at the beginning and one at the end of the mandrel.

Piece of 12mm rod threaded and ctr drilled at each end and two sacrificial aluminium nuts.(could use steel)

Screw a nut onto one end, slip the tube on and then a nut on the other and tighten. Mount between ctrs with a drive dog and tailstock support and turn to finished size cutting the tailstock end nut as you go but not all the headstock one.

Grip now round nut in vice and undo the other.

I've done similar using thick wall "rubber" hose eg car heater hose expanded into the bore. Albeit nowhere near that long. Pushing a stiff fit steel rod up the middle expanded the hose enough to grip the tube well enough for reasonable cuts. Maybe 0.5 mm +.

I held the excess material at the end of the tube in a collet and supported the other end of the rod in a tailstock centre.

Compressing the hose from both ends using suitable nuts will also expand it.

I figured that the large grip area meant that only a small expansion force would be needed to hold things reducing the chance of over expansion leading to wrong size or localised grip causing ripples in the bore. My eventual wall thickness was rather less than yours, 30 thou or so, making the whole thing weaker.

Obviously only works if you have hose and a rod of a suitable size.

Be careful if using compressed O rings. Even quite modest compression can produce serious expansion forces.

Clive

Another case where I think the optimum is more easily found by experimentation than calculation. Unknowns include:

• The coefficient of friction between Delrin (a slippery plastic), and the O-ring (made from 6 or 7 different materials including the very slippery PTFE. I guess a rubbery type would be used but...)
• The pressure that can be applied to a 2mm thick 12mm diameter Delrin tube without bulging it.
• How deep and fast Delrin can be cut without deforming due to heat. Although Delrin machines well, the operator has to keep it cool. I suspect this factorwill limit how much can be removed in a single pass as much as the gripping ability of the mandrel.

In principle I think the mandrel is a good idea, and it's useful to read Huub's practical experience, only two O-rings needed. I agree it's better not to squeeze the O-rings with chamfers because they squidge in the wrong direction.

I've used Jason's method successfully on Aluminium. but never bored a long plastic tube with any method. Jason's approach has the advantage of simplicity, which is always attractive! And I think the gripping pressure on the ends could be cranked up higher than an O-ring without distorting the tube. Have to try it to prove it though.

Please report back!

Dave

I've had good results with just o ring grooves cut in a bar.

The resulting push fit was enough to restrain the tube for machining.

Good luck.

jim

Finally got round to giving this a little more thought! Thanks all for the suggestions and input. I want to make something I can easily use repeatedly, and I was also trying to work out how to easily machine the whole length in one operation, so putting together your comments I came up with this:

Lots of o-rings, not so much to grip the part but so that the piece doesn't flex under cutting load - I did another long piece the other day at 16mm external diameter with a ~3mm hole down the middle and it has a huge bow in the centre of the piece where the cut was unsupported.

The internal shaft will be threaded at both ends so a nut can bear on the two end pieces - I've drawn these as a single part but I would think in reality a washer would be fine.

I have no help to offer, but I'm intrigued as to what it might be!

If you place a Dremel grinder in the tool post, you can grind the rod to final diameter. Due to the lower cutting forces, the rod will bend less. For Delrin, you need to run the Dremel at low RPM to prevent melting the Delrin.

More O-rings will support the rod better, especially if the inner diameter isn't close to the mandrel diameter.

In stead of drilling to final size, you could ream the rod (from 2 sides). That will give a pretty tight fit on a 12 mm mandrel. My standard length 12 mm reamer could ream about 120 mm deep.

Had some time today after putting together the Comically Oversized Vice (I'll update another thread with the gory details) so went ahead and had a go at putting this together.

Playing around with the 8mm shaft, I'd originally imagined I could hold it in the three jaw but it was pretty obvious the runout made it a non-starter...if only I had a collet holder of some kind - wait!

In the end I did the whole thing in the collet holder and it worked so well I am sorely tempted to consider making a backplated collet chuck to hold longer stock. Here's the finished parts:

The black piece of Delrin is the tube I want to machine down, already with the 12mm hole down it. The shaft is 8mm and the tubes, drilled out in the collet holder, are a nice sliding fit. An O-ring protects the face of the collet and the nut at the other end compresses the six pieces:

The holding force on the part is excellent; once installed, I couldn't begin to move the part, and the assembly was more than stiff enough; there's no measurable bulge due to deflection in the final part. Here it is being machined:

I mostly took 1mm depth of cut with fastest carriage feed; I'm not sure exactly what it is as my geartrain is set for screwcutting, not feeding, but it was Quite Fast. My last couple of cuts were slower with a 0.5mm DOC which produced a good finish (for Delrin!)

This whole setup worked so well I might consider replicating it for other workholding jobs - inevitably, woodwind instrument making involves holding an awful lot of things with holes up the middle...

Thanks to all for your helpful suggestions; getting to this point would have taken a lot longer without your advice!

It looks like the insert you are using isn't polished. Polished inserts for turning aluminium, work very well for Delrin and other plastics.

Also the insert seems to be a CCMT type (5° clearance angle). The thin Delrin chips can get easy between de part and the insert and will push the part away from the insert making the part thicker. I prefer a DCMT type insert for turning Delrin.

If you glue a piece of HPL, steel or aluminium under your turning tool, you never have to shim the tool to get it on centre height.
Do the same at the side of the tool and it is always positioned right below the bolts.

Nevertheless, I am glad you have a working setup!

Edited By Huub on 04/06/2022 23:20:28

Edited By Huub on 04/06/2022 23:21:39

Edited By Huub on 04/06/2022 23:21:52

6mm button inserts for profiling work well on plastics as well. They are reasonably sharp and the large radius helps polish the surface (no good for internal corners unless you want a 3mm or larger fillet).

Martin C

Yes, there are definitely better choices of cutting tool, though this did better than I expected. I have a bit of a daft idea to try grinding a bit of HSS to work like a skew chisel that a woodturner would use, so the cutting edge is at 45 degrees and the front edge rides on the part.