Tube mitering is a fundamental part of custom frame building and I am sure there is always a need to be able to produce good results by hand and eye. Paper templates are a boon for speeding up the process and increasing accuracy but I decided to see how I would get on with some form of automation for the process. A milling machine, lathe or tube notcher can all be used but as I already have a lathe this seemed the way forward. I have described elsewhere making the tube blocks to enable this. My lathe is small with 500mm between centres.
I have obtained several Coba-Tech hole saws from Stakesy’s metalwork machinery suppliers (https://www.stakesys.co.uk/hole-punches-hole-saws/hole-saws-arbors/coba-tech-10-tpi-fine-tooth-holesaws). These are fine tooth saws as opposed to the more usual coarse tooth saws I have used in the past. For cutting good mitres in fine tubes I would say these are essential and Stakesy’s were the only supplier I could find. They also sell some tube notchers which are designed to be used with a conventional drill but I have no experience of using these.
The alternative to using hole saws, which usually cost less than £10 each, is to use frame tube cutters as sold by Ceeway (http://www.framebuilding.com/best%20quality.htm). Made by Silva these cost £95-£120 each. I imagine these would give a clean and accurate cut but are rather expensive and I am not sure how long they last or how they could be re-sharpened. The potential problem with hole saws is they are not reliably circular due to the way they are made. Also the teeth are necessarily splayed apart so they may not be the exact size they are supposed to be. They seem to be available only in exact metric sizes so you have to select the nearest size to the one you need. Even on websites where imperial sizes are listed, on scrutiny they are actually the nearest metric equivalent.
The arbors normally available from the suppliers for the saws have flats on them and are not suitable for a lathe chuck and in any case may not run true. This will inevitably lead to wobble and inaccurate holes or notches. I therefore made my own arbors using the lathe in an attempt to get them running as true as possible.
Hole saw arbors use a UNF thread, 5/8 x18 tpi for saw sizes 32mm and above and 1/2 x20 tpi for sizes below 32mm. I also faced the backs of the hole saws to ensure they were as square as possible when mounted on the arbors. Following this, using a dial gauge on the lathe I found the wobble on the various saws varied from 0.2 – 0.5 mm, which I therefore deemed acceptable.
Hole saw speeds
Stakesy’s provided a very useful chart of holesaw speeds which I reproduce below:
However, these speeds proved inaccurate during my experiments as they are generally too slow. I presume they really refer to sawing solid materials rather than very thin tubes. The main problem with drilling notches is snagging of the saw and jamming if the speed is too low. Also low speeds can produce an inferior finish on non-stainless tubes. I mostly used Columbus Gara tubing for my trials because I had a good amount of it and for the stainless tubing used some scrap pieces of Reynolds 953. For my earlier experiments I also used a 73 degree angle of cut. I used a general purpose cutting oil for lubrication throughout.
Overall I found the speeds used were similar for each diameter of tubing as of course there is very little difference in diameters anyway. Experimentation to some extent will always be needed as the dials on my lathe are not accurate enough to give an exact speed and only sophisticated set ups would have speed indicator. I obtained a reasonably clean cut on non-stainless tubes with a speed of 1,440 rpm to 1,850 rpm. The best finish at the lower end of this range was with the larger saws i.e. 40mm, as they would be rotating at a higher speed and the smaller diameters need the higher end of the range. As a general recommendation I would say 1,800 rpm is a good ball park figure. Additionally at lower speeds, and this could happen at the lower end of the range given, juddering of the lathe was a problem. A reasonable finish would require only light filing or sanding and possibly slight filing to fit if the mitre is not perfect so always be aware of this when measuring the tubes prior to cutting. Usually the best mitre fit comes from using the nearest size holesaw e.g. 32mm for 31.7 tubes. Oddly there was no 36mm size saw available and I obtained a pretty good mitre on a 36mm headtube with both a 35mm and 38mm holesaw!
The feed rate, i.e. the speed you feed the tube into the saw appears to be somewhat arbitrary and can be done manually or with an auto feed if there is one. The faster the cutting speed, the faster the feed rate that can be used.
Stainless tubing (and so far I have only used Reynolds 953) cannot be cut at high speed as it simply gets very hot and burns out the saw, additionally it gives a poor finish and I suspect doesn’t do much for the strength of the metal. Whilst low speed sawing on non-stainless material tends to cause jamming of the saw, on stainless it doesn’t once you have reached the minimum speed which I found was 200 rpm. Below this the saw snagged and anything above is unnecessary, doesn’t improve the finish and runs hotter. Use of a cutting oil is particularly important.
A slow feed is also necessary so can be done by hand, but I found that the slowest auto-feed speed on my lathe was perfect and in fact a more even feed speed seems to give a better result and is less likely to jam. If jamming occurs it is often at the end of the cut and speeding up right at the end can help. Also keeping the offset end short will reduce the risk of jamming with a large offcut.
These two short videos demonstrate the speeds used for cutting 953 tubing. 40mm holesaw for a bottom bracket mitre at 90 degrees. The offcut simply fell off after cutting.
Tube notchers are available, such as these: https://www.stakesys.co.uk/tube-notchers/tube-notchers. They are designed to be powered by a hand drill and are considerably cheaper than a lathe, though obviously are a one trick pony. The speed of the saws is set by the drill trigger speed and will be determined by feel rather than dials, but I am given to understand can be used quite successfully.
I am indebted to Mick Rushton of Ironworks Bikes (http://www.bespoked.cc/2013/bespoked-newbuil.html) for his advice and technical expertise in tube mitering.