Boring bar size ?

Hi,

Further to my posting on new member introduction,

I am currently working towards providing classic motorcycle repair and restoration services,

First thing that I have to do on my own test bike is to bore out a set of carbs from 26 to 28mm, I need advice on what size boring bar I should purchase for the job, the carbs are 90mm from front to back so it will be a depth slightly less than that, I am not sure what metal the carbs are made from but they are pretty sophisticated carbs and I think they are aluminium, my lathe is an Axminster C6 (Sieg ?) with the original standard tool post,

I need to know minimum and maximum size boring bar and what type of cutter to use, I guess that it's going to be an indexable type bar that is going to be the best way to go ?

Thanks in anticipation, Jeff

If you are not going to machine a standard boring bar, you will be limited by what you can grip in the tool post.

Why not make one to suit:-

Boring bar - home made

If going for an insert bar then I would say 16mm and get a CCGT insert with 0.2mm tip radius for it which will be less likely to deflect. Not sure of your toolposts capacity but there are ways to mount larger bars than will fit in a toolholder.

There are lots of things to consider. If you are going to pursue this as a business, and the job will repeat itself in the future, it is worth investing in the tooling to do it as fast and as easily as possible. That also extends to the method of workholding.

If it is a one-off, and you want to use the tooling more generally, than there are different considerations.

Inserted tool boring bars come in 16, 20, 25 and 32mm shanks as standard. The largest three of these four may not fit your toolpost, but you can make a dedicated holder (see https://www.youtube.com/watch?v=-55cbSUWSho).

If the things being bored are always 26mm minimum diameter, the most rigid off-the-shelf solution is a 25mm bar. But this is so tight to the bore that swarf management can become a challenge.

A 16mm bar 90mm deep is greater than 5:1 length to diameter. For a carb., you surely need a good surface finish for good gas flow and you have to take into consideration that it is an interrupted cut.

My suggestion would be a 20mm bar as it is just under 2 1/2 times more rigid than a 16mm one and it will still allow the cuttings a way out.

If the carbs. are die-cast aluminium, as Jason says, the sharp, polished inserts specifically for aluminium will help a lot in whatever bar you choose.

If doing a lot, it might be better to arrange for the carburettor to be mounted on the cross slide and bore between centres?

For boring a carb out a couple of mm,I woud make a boring bar from 19 (3/4 inch) mm dia silver steel and use a high speed steel tool bit with a lot of rake ,the cost is minimal and will last years,most of my home made bars have lasted 50 years. Bars to skim out aluminium die castings do not need to be that rigid,better to have a smaller bar so that you can see how the tool is cutting ,I do have a number of modern insert type bars I would not use one on a carb job.If you get serious about going into classic motor cycle resoration machining services,you will need a lot larger lathe with a large bore through the spindle, and get some product liability insurance or avoid making things such a wheel spindles or fork repairs,

Might be mostly Zinc, as in Mazak or Mazac used in every car carb in the 60's and 70's

Jason B recommends a 16mm boring bar, and I think that is the best size. The actual tip height of boring bars is somewhere near half the nominal tool height, so if the toolpost has enough depth to hold the bar, it can fit a lathe that could not manage a 16mm standard tool.

ARC sell a nice bar 16mm which takes CCMT09 type inserts. 060-325-39516

An insert suitable for zinc or aluminium CCGT09T308. 060-326-01938

The ARC site shows the dimensions of the tool. It will be long enough for your carb bore. I would recommend very small cuts and lubrication such as wd40 or ac90.

Back in the sixties, I had a pair of Amal monoblocks bored out to 1 3/16" for a Royal Enfield Constellation. The centre part of the carbs was in place and bored simultaneously. Some time later, one of the central parts failed, the cause was blowholes in the zinc casting.

Hi Jeff, Once you have what you think you need I would practice the job on several scrap carbs first and make sure you DO have all you need. Work holding and setting up to be true to the original axis will be important. Boring bar in the tool post or bar between centres and use the cross slide ? Mazak is the most likely material. Good luck Noel.

What carbs are they? Flange or spigot fitting? The old flange type had a habit of being overtightened and the face not being flat, which has to be addressed when mounting them. Keep the speed fairly low and check for any runout before cutting metal. The problem of tiny drillings into the bore getting clogged up and the problem of removing tiny burrs from the holes without changing their fuel or air flow is a worry.

 As already mentioned, the long stickout of a 16mm bar can cause problems, another reason for taking tiny cuts. I, too would prefer 20mm bar size, but it could be difficult to hold in your lathe.

Edited By old mart on 12/01/2021 21:28:51

Invest in a solid carbide boring bar with either a ground cutting edge as part of the tool or a carbide tool which accepts a TAT. I use several of these for jobs as you describe.

If you are opening out the throat are you doing other modifications to the carb as well? For instance - one area where the Amal carb can be improved is the slow run jet can be moved from its position in the underside of the fuel bowl facing to the bottom of the fuel bowl itself. There is an angled drilling for the fuel fed to the jet which can be tapped 2 BA to allow the slow run jet to be repositioned. Sinking the jet under fuel improves the transient signal when the jet comes 'on stream'.

Other carbs, such as Keihin can also be modified in ways to improve their transient responses.

HI,

well thanks for the responses ! there are a few of them ! I am unsure which route to take, I have yet to check the material that they are made of, as usual lots of other stuff going on at the same time,

Re what type of carb they are, they are Keihin TA09 carbs and they are spigot mounted, there are no engineers motorcycle or other engineers modifying the carbs other than the removal of a 1mm cut to take them to 28mm, this is done as part of a performance upgrade, the carbs work very well as a standard setup so it is just the bigger choke size / bore that is required,

I would like to go with a 20mm boring bar if my tool post can hold it or if I can modify or replace the tool post or the bar itself, I am thinking that the bar can have flats ground on it to facilitate fitting in the tool post if necessary,

I will repost when I have had chance to look at what my tool post can hold and what material the carbs are, I do not think they are mazak but will check,

Thanks

Nigel McBurney,

Re the needing a much larger lathe etc, I am not intending doing all of my own machining just the smaller stuff like the carb boring, spacers etc, I also have a small milling machine which I used to modify a top yoke on a trials bike to remove part of the casting that was the bottom of the handlebar clamps, I machined it flat and then was able to fit normal split handlebar clamps that are directly above the yoke rather than the tiller type that used to be the common thing on trials bikes,

It is just stuff like that I want to do myself and larger jobs like the wheel that I am modifying will be done by someone else,

My work will be repairs / restoration with the necessary sub contract work for larger machining, alloy welding etc which I have contacts for.

Jeff, whilst working on carbs, be vey careful when drilling as the Anzac has a tendency to grab hold of the drill and go much deeper than you intended. Example, drilling an oversized hole for the fuel banjo bolt !

You may just be OK with a 20mm bar, just check what the minimum diameter it will cut at is, typically 25mm for a 20mm bar and 20mm hole for a 16mm bar. So you would need to get it mounted dead right to stop the insert rubbing.

Most will already have flats on them but they can be machined depending on how hard the shank is but Carbide is likely to be needed to cut them.

Bigger is usually better with boring bars but you can get away with smaller with soft materials and a sharp cutting edge.

Martin C

Right idea but crappy workshop techniques..

I winced when his drill press vice rocked visibly under the drill and the his filing rocking and rolling was horrible to watch. Was there even a handle on the needle files?

Definitely a bodger.

* danny *

A solid carbide boring bar would certainly do the job, but that depth is at the limit of one of my 12mm ones and a 16mm one would probably cost more than a set of larger carburettors. If a 16mm steel bar is used, the compound and cross slide gibs should be tightened up a little and the minimum of tool overhang to reach the required depth is important.

Not necessarily a bodger, but most certainly crappy workshop techniques. Definitely not a good example for youtube video watching - a novice teaching other novices is not an ideal situation.🙂 Drilling way out from the vise jaws was the first thing, then the rocking. His filing would not pass if he were apprenticed! Finally showing his boring bar held with about twice the needed length unclamped was a poor example to his novice watchers. Most definitely still a novice, I might suggest?

Jeff,

Just noticed your reply that these are Keihin TA09's, from memory these are 'flat sliders' and you stated ...

"the carbs work very well as a standard setup ".....

Well, yes they do, but they can work even better by some other modifications. Changes to the spray bar and separating out the secondary air feeds from out of the bell mouths will give far better performance. The spray bar mods require the reshaping of the main jet well, and the secondary air feed mods improve the mass air flow capability through the carb, probably by as much as the boring out, if not more (just under 15% for boring, empirically around 18% to 20% for bell mouth changes). These are easy mods which can be performed on the lathe whilst set up to bore out the main airway.

Just as a reminder are these 'pumper' carbs?

Martin