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Straight Bevel Gear Question

Buma Boring Bar

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Bernard Wright21/03/2017 02:14:02
78 forum posts
16 photos

Hi Folks,

I hope someone can help me, I'm totally out of my depth with this.

I have a 'Buma' Boring Bar I'm trying to make accurate, the feed is controlled by an ACME thread of 0.75" x 6 TPI, the driven nut is bronze with a straight bevel gear machined into the top.

The threads have so much slop that a controlled cut is impossible.

I would like to try and make a new nut, but I'm struggling to calculate the bevel gear.

It has 24 teeth and I've measured the top down angle as 23.5 degrees, and the body is 1.612" in diameter.

I presume its of DP type not Module, due to it being British and quite old.

Ordering a suitable cutter shouldn't be too difficult, it's just working out by how much to tilt the diving head to get the taper tooth accurate, and calculating the root angle from the measured top angle, that the cutter needs to approach the cut.

I've a universal diving head and rotab to mount it on, to get the compound angles, if someone could point me in the right direction hopefully.

Thanks in advance for any help offered.


john feeney21/03/2017 09:33:09
25 forum posts
30 photos

Hi Bernard,

I have a similar gear waiting to be cut, 18 teeth 1.5 in. diameter. There is a full description of the process in Ivan Law`s book " Gears and gear cutting" although I believe there is an error in the formulae somewhere.

There is also a good description " Gear Cutting Practice" by Colvin and Stanley (1937). This is a Lindsay Publication book.

You can also find videos on U Tube. I suggest you make a blank in aluminium or plastic first. If you get round to doing it before me please let me know how you got on. I am waiting to find the time to have a full day at it without any interruptions!


John Feeney

JasonB21/03/2017 09:36:58
19564 forum posts
2142 photos
1 articles

Why not machine off the bevel and solder it to your new nut.

Rather than trying to measure angles from the gear can you not work them out based on the numbers of teeth on the two gears.

Andrew Johnston21/03/2017 11:11:52
5828 forum posts
662 photos

I assume that the bevel gear is driven by another bevel gear. In order to calculate the angles, and other parameters, we need to know the number of teeth on both gears.

There are two methods of machining a bevel gear on a mill, with involute cutters, neither of which results in a perfect tooth shape.Straight tooth bevel gears are normally designed using an integer value of DP at the outer diameter.

The first method uses an involute cutter of the correct DP and tooth count for the outer diameter. But in order to pass through the gap at the smaller diameter the cutter is thinner than standard. These special cutters were often stamped 'BEVEL'. As far as I know these cutters are no longer available commercially. Note that normal spur gear involute cutters cannot be used. The method results in a bevel gear with a tapered tooth shape, but the wrong curvature (not enough) at the small end. This is normally corrected by filing.

The second method is the parallel tooth depth method. This is the method described in detail in the Ivan Law book. As it implies the tooth depth is constant rather than tapered as with a true bevel gear. These type of gears mate well with each other, but I suspect they will not mate properly with a conventional bevel gear. The critical design point with paralllel depth bevel gears is that the DP, and design calculations, are specified at the inner diameter. So a standard involute cutter can be used. However, from experience I can say that it is darn near impossible to design a parallel tooth bevel gear, with integer DP, to fit an existing mechanism intended for use with conventional bevel gears. At least not without major changes to the mechanism.

I suspect that JasonB's advice to re-use the gear if possible is the easiest solution. If I was faced with making the gears I'd CNC them, rather than use a conventional milling machine; been there, done that. I could probably make the gears, but it wouldn't be in the immediate future due to other pressures. PM me if interested.

A picture of the gear/mechanism would be very useful.


Bernard Wright21/03/2017 12:08:24
78 forum posts
16 photos

Hi Folks,

Thank you all for your replies, the mating gear is 12 teeth at 90 degrees, so it would be 2:1 reduction at that point, the smaller bevel is driven through a worm.

JasonB, that is the route the replacement nut follows, but a bare nut is £80+ so I'd rather play with it myself.

Andrew, this is the knowledge I seek, before tripping myself up as usual, the offer of CNC'ing a nut is attractive, I have a lump of 2" dia Aluminium Bronze, which could be tried upon, I live in North Lincolnshire, if that's reasonably driveable.

John, strangely enough I have his book (Ivan Law), but it is stored away awaiting my moving.

As a trial I tapped a lump of hollow Bronze with a 3/4" x 6 TPI Acme tap I bought at Normous Newark Autojumble for £5, but it tapped too large for the existing leadscrew, I ended up with as much play as before.

I suppose I could tap out the nut with this tap and make new leadscrew, but I would like to try the bevel gear cutting.



Phil P21/03/2017 13:04:44
707 forum posts
175 photos

Hi Bernard

If you have never cut a bevel gear before, you might be biting off more than you can chew at this stage, you already have a problem with backlash and you could end up with even more if you make a pigs ear of it.

Jasons advice is your best option, but forget tapping the nut unless you have the correct Acme tap.

I would advise screwcutting the new nut, and then soldering or loctiting it into the existing bevel gear once you have removed the worn thread from it.

If you were planning on making a new bevel gear from scratch you would still have to produce the internal thread in it anyway, so you have nothing to lose by making a "repair insert" and trying that option first.

As mentioned above, a photo would help people to give you more informed advice.


Bernard Wright21/03/2017 13:26:33
78 forum posts
16 photos

Hi Phil,

The tap I used is a proper 3/4" x 6 TPI Acme tap, but I fear Buma made their leadscrew and nut a propriety 3/4" x 6 TPI Acme, the threads on the leadscrew do seem excessively deep, and the tap will not enter their nut, indicating a smaller core diameter.

I will post some pictures in my album.


Ian Taylor 327/06/2019 13:32:23
2 forum posts

Hi Bernard

i realise this is an old post but I wondered if you were successful and which method you used.

i now face exactly the same situation

regards Ian

Bernard Wright28/06/2019 21:45:09
78 forum posts
16 photos

Hi Ian,

Sadly I've done nowt with the Buma, I moved to Scunny May 2017, and I'm struggling to find everything.

Doing other jobs for myself that needed fixtures or mandrels, became a real chore, especially upon finding stuff I'd already done, after making bits anew.

Also I'm a bit limited doing recreational machining, as my Wife is really suffering with Osteoarthritis in her left hip, being her hubby I'm also her Carer, so the learning curve seems like a Mandlebrot.

I started making a universal dividing head for my Clarkson, but it is taking damned weeks, again everything seems to be a 10:1 job, but I've plenty of time.....



Ian Taylor 328/06/2019 22:34:09
2 forum posts


I made a acme 6 tpi sleeve, bored out the old thread and made it a 2 thou interference fit.

pushed it in with locative and prayed.

Rebuilt it and so far everything is ok, done 16 bores with it ( 4 x 4 cylinder big bore conversion)

if you ever get round to it I think that’s the easiest solution

If you ever need pics let me know.

best regards


Bernard Wright29/06/2019 09:52:18
78 forum posts
16 photos

Hi Ian,

Pleased to hear you managed to accomplish a sound repair, did you use a tap, or did you thread cut with a ground up tip.

I bought a 6Tpi Acme insert, only to find it needed a bar bigger than the root diameter of the blank, at 6Tpi they go on to 22mm tips as opposed to others at 16mm, bugga.

I do intend to sort it, but other things are in the way at the moment.

I would appreciate pics, might just give me the spur on.



vintage engineer30/06/2019 09:43:34
254 forum posts
1 photos

It could be a stub tooth gear form. There are common types Fellows and American Standard System. I had to make one for a 1910 Dennis engine. It turned out to be a metric version of the Fellows form. You have to use a bigger cutter than normal for the gear.Stub tooth formula

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