Cutting my first gear

Hi all

Asking for help from someone experienced to check my calculations…

I eventually got a VDH dividing head - good thing I asked on here before diving in and buying a bigger one, because anything bigger would not fit on my horizontal mill. After lots of fixing and fiddling I have it on the mill and aligned.

I want to cut a gear as a test piece, to prove I can do it. I have a 26-35 14.5PA 10DP gear cutter than came with the mill so I thought I’d have a go with that, and try to cut a 26 tooth gear. I made a mandrel, broached keyways etc. See pic at the bottom for the setup.

Next step was to consult Machinery’s Handbook for the numbers. See the calcs on the pic below.

For a 26 tooth gear I think I need a 2.8 inch OD blank (Do)

The dedendum seems to work out at 0.12”, which gives a root depth Dr of 2.36”

So the cut depth is 2.8-2.36 = 0.44”. This seems a lot! Is this correct? Before I start cutting, even though it’s a throwaway piece, I might as well get it right.

Any thoughts appreciated.

Steve

Calcs

Setup

Edited By JasonB on 13/04/2022 16:18:00

The calculation for the OD is correct. The addendum is one over the diametrical pitch, so 0.1". In theory the dedendum is the same, but clearance is normally added. Common values are 1.157 or 1.25 divided by the diametrical pitch. Total tooth depth is addendum plus dedendum, so 2.157 or 2.25 divided by the diametrical pitch. The larger tooth depth is used for smaller tooth counts, but there is no hard and fast rule as to the changeover point. The tooth depth will be 0.2157" or 0.225" depending on the clearance allowance. If the cutter is old stock it should have the tooth depth for which it was made marked on it. If in doubt I'd start with a tooth depth of 0.2157"; you can always do another pass. But it's more difficult to add material. For scale these are 10DP gears:

Andrew

2.8 - 2.36 = .440 gives you the total tooth depth, on both sides. Depth of cut is half that. So .220" would be spot on the money for how deep to cut each tooth.

Do it the handy dandy way with this online calculator: **LINK**

Nice looking VDH you got there. It's worth getting GH Thomas's book on how to use the secondary adjuster dial to cut gears of all odd sizes such as 127 teeth etc. Also on mine I drilled the 60 tooth spindle gear with 24 holes to use it as a spin indexer for cutting hexagons etc without using the index plates.

Ivan Law's book Gears and Gear Cutting is a must have too. One of the Workshop Practice Series so cheaply available.

Edited By Hopper on 13/04/2022 12:33:07

Ahhhhh … I was reading Andrew’s post and desperately trying to work out where I’d gone wrong with my calculation but in fact I hadn’t really. Thank heavens I asked. That’s straightforward then.

On the VDH, it wasn’t well made really, the original builder had not really managed to drill the holes correctly on the far side, so I put some new dowel pins in to align it properly. It has holes drilled in the spindle gear for use as a spindexer, but it doesn’t have the plunger for that. I was thinking of getting the book you mentioned for the drawings somIncan make one.

Next challenge will be, I want to make some bevel gears, but obviously it needs an angle facility for that.

Steve

Apart from using a non-standard value for the dedendum.

Andrew

Yes bevel gears could be a challenge. You could take the chuck off and use a stub mandrel straight into the VDH spindle to mount the gear blank close to the VDH. It is made to be able to do this with removeable sleeve up the middle of the spindle hole. Then you would need an adjustable angle plate with minimal height to mount the VDH on.

If it gets too high, you might be able to cut on the side of  the cutter instead of the bottom. That's sort of how I cut pretty much all gears in the lathe using the VDH. It is mounted high on a swivelling vertical slide and cutting is done on the top of the cutter, as there is no room for a large gear blank between the bottom of the cutter and the lathe bed.

Both of G H Thomas's books are worth getting. Full of invaluable info on all manner of stuff. The plunger is just a spring loaded spindle in a housing, screwed into a small arm that mounts on the existing bracket that holds the big index plate mechanism.

You can just see it blurrily in the background of this pic:

Edited By Hopper on 13/04/2022 13:18:38

Edited By Hopper on 13/04/2022 13:21:29

Many gear cutters are marked with the depth of cut "D+f".

Ivan Law's book, "Gears and Gear Cutting" contains the formulae to calculate this, ,if it is not listed in one of the tables within the book

On page 66 he quotes the depth of tooth for 10 DP as 0.216", so it may be worth rechecking your calculations, to find the few thou difference..

I have only ever cut 20 DP, or 1.5 and 1.25 Mod gears, on a vertical hobby mill, so have no experience of 10 DP!.

With a full depth cut, (To avoid any positional errors from repeated increments ) the feed has to be very gentle.

If possible support both ends of the blank to minimise deflection.

Howard.

It is simply down to the choice of clearance added to the dedendum; 2.157/DP in the book versus 2.2/DP used by Steve.

In theory cutting forces when using a gear cutter are symmetric, so the blank shouldn't rotate. For larger diameter gears a positive method of preventing the blank from rotating can be useful, like the green clamp when cutting this 11" OD 6DP gear:

I use normal calculations for feedrates when cutting gears, based on a chip load of a few thou per tooth.

Andrew

Well, I’m not currently having much success.

The cutter is c. 2” with 13 teeth I think. Spindle speed is 250rpm, which I think is about right for mild steel @ 120 sfm. My calculator says the feed rate should be around 10 IPM.

But it’s not really making chips, more rubbing.

What would be a suitable depth of cut?

😞

Spindle speed of 250rpm is on the fast side, I'd be aiming for around 60 sfm. As setup 10 ipm gives a chipload of 3 thou per tooth, which is reasonable. Depth of cut is the full depth of the tooth; cut in one pass. Multiple passes just waste time and blunt the cutter.

Andrew

Does your mill run in either direction if not you may have the cutter going backwards

No Jason, even I know which way the cutter should run

Unfortunately that’s the slowest it will go Andrew, I thought about changing the motor for a VFD a while back, but couldn’t justify the cost really. Originally the mill had a geared motor, but that’s long gone. Whoever replaced it replaced it with a motor that runs too fast.

But …. Success… I was about to give up for the night, but thought I’d try another cutter that I have. I didn’t bother recalculating the tooth spacing, I had already failed with that blank. It went through it not quite like butter, but quite easily. So it seems simply that the first cutter was blunt. Is it possible to sharpen them? Can’t imagine it’s easy.

looks a bit like a gear…

Thanks for the advice all.

I assume the depth of cut isn't full tooth depth yet?

Involute gear cutters are designed such that grinding the front face of each tooth on a radius reduces the OD of the cutter but preserves the correct tooth form. To do so an indexing method would be needed. If the cutter isn't too blunt try giving each tooth face a lick with a diamond hone.

Andrew

No, the depth of cut in the pic is full depth. The reason it looks wrong is that it’s a different cutter, with a smaller tooth size, but I continued with the 13.7x  degrees I was using on the dividing head. Obviously it’ll never work as a gear, but I was just learning how to do it today. I will cut a new blank tomorrow, recalculates for this second cutter and try to cut a better gear.

I’ll get a diamond hone also.

it looks like there’s quite a lot of deburring to do. Is there a trick to that?

Edited By Steve355 on 13/04/2022 21:38:28

Just tedium with a round needle file.

If there is a sacrificial blank on the exit side of the real blank the burrs where the cutter breaks through can be almost eliminated.

Andrew

I make a lot of (prototype) gears in HPL, POM, PVC, and aluminium.

PVC is pretty soft and super easy to clean (deburring), just use a (nail) brush.

HPL is the cheapest and easily available. Cut it in one pass and use a second blank on the back to prevent chipping of the first blank. After cutting, the gear is done, no deburring to do.

UHMPE (old meat chopping plate from the kitchen) is soft and eays to cut, but deburring takes some time.

POM is super easy to cut but, compared to PVC, not as easy to debur. POM gears are pretty strong and durable. They are often used in small lathes.

Edited By Huub on 13/04/2022 22:12:57

Here's a few pics of the spring loaded plunger on my VDH. It's just a stepped spindle with a spring inside that body that screws into the pice of flat bar bolted to the existing index plate holding bracket.Easy to make without drawings.

I made this non-standard right-angle mounting for the plunger so it can engage with the outside teeth on the 60T gear, which allows you to index more numbers, including fives and tens etc.

Yes, you are right, that’s straightforward. I should be able to knock one up pretty easily.

I’ve been using a Myford chuck for mine, but is there a way to convert it for a collet (ideally mt2)?

Yes, have a look at the bottom of this page.

Thor

I don't have GHT's books in front of me -- they are out in the shed -- but from memory his book includes making a set of collets that fit inside the VDH parallel spindle bore. I think they use those three or four grub screws around the end of the spindle to close the collet down for grip. I'll have a look at the books tomorrow and refresh my memory. It is the last part of the VDH I have not made and really need to do because with a chuck there is way too much overhang unless using a tailstock, which I can't in the lathe with vertical slide. I am doing a job right now milling hexagons on some bolts for the tailstock lever attachment I am making and if I had those collets it would be a quick job. INstead I am faffing about clamping the job to the vertical slide and using a protractor to index 60 degrees at time for the flats.

I thought about making a sleeve to put an ER collet chuck's shank up the centre of the VDH spindle, but you still get way too much overhang. So I think GHT's collets are the way to go. Will take a look tomorrow.