Gear cutting

[Gordon]

Has anyone tried cutting DP gears using MOD cutters? I need some 48 DP gears and I have .5 MOD cutters. the depth of tooth on 48 DP is .045 and the depth of tooth on .5 MOD cutters is .042. It would seem that a difference of .003 would not be significant on these small gears with a light load and relatively slow speed. I am thinking about using the .5 MOD cutters and cutting to .045 depth. The MOD cutters are pretty inexpensive compared to DP cutters. The alternative is to make a hob which I have done in the past but the cutter makes a cleaner cut than the hob.

Gordon

 

[SmithDoor]

I use single tooth cutter and grind to shape.
I made holder for my mill so keep a higher speed.

Dave

Has anyone tried cutting DP gears using MOD cutters? I need some 48 DP gears and I have .5 MOD cutters. the depth of tooth on 48 DP is .045 and the depth of tooth on .5 MOD cutters is .042. It would seem that a difference of .003 would not be significant on these small gears with a light load and relatively slow speed. I am thinking about using the .5 MOD cutters and cutting to .045 depth. The MOD cutters are pretty inexpensive compared to DP cutters. The alternative is to make a hob which I have done in the past but the cutter makes a cleaner cut than the hob.

Gordon

 

[Gordon]

I am sure that using the MOD cutter is closer to the correct profile than anything that I could grind on a single point cutter.

 

[SmithDoor]

Typically that small of cutter is low cost.
If you making both gears you use the MOD cutter
There are slightly different in DP and MOD.
If I was cutting gear for someone else I buy the right cutter.
For my own work I use cutter I had on hand.

Dave

PS
I have even made gear foundry patterns and timing gears for large engines.

 

[awake]

Gordon, for hobby use, I think this should work fine, so long as you match the pressure angle.

For the same number of teeth, DP and MOD do not by themselves indicate any difference in shape - involute is involute. However, pressure angle DOES indicate a difference in shape, and I have found that when it comes to inexpensive and widely available gear cutters (e.g., eBay imports), most MOD cutters are 20° while it seems easier to find DP in 14.5°.

So the shape of the teeth is the same for the same pressure angle and ... what about the size of the teeth? DP and MOD are reciprocal methods of describing the size. DP is typically based on a pitch diameter (PD) measured in inches, and the relationship between the number of teeth (N) and the pitch diameter (PD) is as follows:

DP = N / PD

Meanwhile, MOD is typically based on a PD measured in mm, and the relationship between N and PD is the inverse:

M = PD / N

So, taking into account the conversion from inches to mm, the relationship between DP and MOD is as follows:

M = 25.4 / DP or DP = M * 25.4

So, 48 DP is exactly the same size as MOD .53, assuming the same pressure angle, or conversely, MOD .5 is exactly the same size as 50.8 DP. As you note, the difference in depth of cut for a M .5 vs. 48 DP is only .003" ... and in fact, different reference works calculate the clearance differently, so the depth of cut for an M .5 could be given as .044" rather than .042 (and .047" for 48 DP rather than .045"). Thus, that .003" difference is down in the plus-or-minus error range.

All this to say: for hobby use, given the compromises inherent in gear cutters, etc., I can't see any reason it wouldn't work. Just make your OD based on the calculations for 48 DP, and cut the gears .003" less deep. Done!

 

[xpylonracer]

The different pressure angle will make no difference provided the mating gears are the same PA, e.g. crankshaft driving a camshaft, but on low tooth count pinion gears tooth form may be less than desired for strength.
Whichever way you go check your gear centres to ensure correct meshing.

xpylonracer

 

[tornitore45]

Low speed low load is OK, they will work. Just do not call them gears, would be an insult to real gears.

 

[TSutrina]

Note: the cutter is used for a range of number of teeth. Exact involute gear is different for each number of teeth gear. Thus the cutter only cuts a true involute for one gear of the range and an error occurs for the other gears.

 

[retailer]

Here is a pair of MOD 0.4 16T 45 deg bevel gears I made for my current IC engine project, I think MOD 0.4 is close to 64 DP. I sourced the cutters from Ebay seller was in Russia so I assume they are Russian made.

 

[xpylonracer]

Please describe briefly which method you used to cut the bevel gears.

xpylonracer

 

[retailer]

I think it is called the parallel tooth method and has been discussed on this forum before.

Calc the blank size - setup and cut the teeth (assume the job is being done in a vertical mill)

Roll the blank back and lift the cutter by the calculated amount - this will vary according to the size of the gear being cut - recut all teeth - return the cutter back to its original position and roll the gear blank back to its original position.

Repeat the previous operation but this time roll the blank FORWARD and do not lift but instead DROP the cutter by the calculated amount - recutting all of the teeth.

The attached Excel worksheet calculates offsets etc - I've used the info from Ivan Law's book on gear cutting so you should refer to this for explanations of second cut offset etc, last I looked it was still available as a download from the Internet Archive as are many other books on gear cutting - if you decide to use the worksheet I take no responsibility for any errors, for me Ivan Law's explanation was not 100% clear, also none of the cells in the worksheet are protected so it is possible to change the formulas/cell contents. If you decide to use the worksheet then only the GREEN coloured cells need to be changed - I don't have imperial cutters so I didn't bother with imperial dimensions.

The forum wouldn't accept files with a xlxs extension so I uploaded it as xls which is 97-2003 compatible.

EDIT - poorly worded Excel file removed !

 

[kiwi2]

I think it is called the parallel tooth method and has been discussed on this forum before.

Calc the blank size - setup and cut the teeth (assume the job is being done in a vertical mill)

Roll the blank back and lift the cutter by the calculated amount - this will vary according to the size of the gear being cut - recut all teeth - return the cutter back to its original position and roll the gear blank back to its original position.

Repeat the previous operation but this time roll the blank FORWARD and do not lift but instead DROP the cutter by the calculated amount - recutting all of the teeth.

The attached Excel worksheet calculates offsets etc - I've used the info from Ivan Law's book on gear cutting so you should refer to this for explanations of second cut offset etc, last I looked it was still available as a download from the Internet Archive as are many other books on gear cutting - if you decide to use the worksheet I take no responsibility for any errors, for me Ivan Law's explanation was not 100% clear, also none of the cells in the worksheet are protected so it is possible to change the formulas/cell contents. If you decide to use the worksheet then only the GREEN coloured cells need to be changed - I don't have imperial cutters so I didn't bother with imperial dimensions.

The forum wouldn't accept files with a xlxs extension so I uploaded it as xls which is 97-2003 compatible.

 

[kiwi2]

Gosh, the new system for replying to a post is complicated. Here's my second crack at it.
I have a couple of questions regarding the bevel gear calculator.
It says there is an error in Ian Law's book concerning the small end diameter calculation which should include the COS of the pitch cone angle rather than the SIN. In the "calculations" section however it is still shown as the SIN of the pitch cone angle. I'm confused.
In the spreadsheet, the rotation of the blank after the first cut is said to be:
"Roll the blank back by an amount equal to 0.25 times the number of teeth i.e. for 36 teeth this equals 10 degrees"
I can't make sense of this. Each tooth is 10 degrees apart so where does the 0.25 come in? Or does it mean that the blank should be rotated back by 0.25 X 10 = 2.5 degrees?
Regards,
Alan

 

[Drawfiler]

I have been an ’observing’ member for some time who makes a fair number of gears so have a bit of experience
Until the Gleason bevel machine was in vented by Miss Gleason - yes a lady - bevels we’re cut with form cutters as discussed above. The method proposed by Ivan Law seems to be that you Mill the first tooth and then turn the dividing head enough to cut the flank of the large end of the tooth and then correct the vertical height to keep the small end in line with the cutter.
Another method is to finish the first ‘middle’ cut and then position the cutter at the small end with one tooth of the cutter engaged with one tooth of the gear, slightly slacken the chuck and adjust the knee of the mill up or down down allowing the blank to turn, tighten the chuck and cut the second cut, then back to the small end and move the knee the other way. I have used this on small pinions and crown wheels.
Turning to spur gears, the form cutter method was used for all gears until bobbers were available, until the 1960s you could get intermediate cutters to give a more exact match to the shape for that tooth count. The hob for each dp or mod has a rack profile and generates the involute by relative movement, the pressure angle is the same as the angle of the tooth in a rack.
When you don’t have the right cutter it is possible to make a gear that will work by using the ‘wrong’ cutter, this is best with higher tooth counts, you take the next smallest Dp cutter and cut slightly deeper, some trial and error will make a good gear but with the tooth gullet a little deeper than theoretical.

 

[xpylonracer]

Thanks retailer for the info and spreadsheet they will be helpful, I thought you may have used the method described but wondered if there was another method.

xpylonracer

 

[tornitore45]

Is not 10 degree is 2.5 degree or 1/4 of the tooth angular spacing.
Angular Offset = +/- 0.25 x 360/Z = 90/Z Z is the number of teeth
The vertical movement = Pie x Diametral Pitch / 4 Z
Is a way to widen the teeth spacing at the periphery.

 

[retailer]

Yes the roll back ! it is poorly worded, the tooth spacing for a 36 tooth gear is 10 deg and 1/4 of this is 2.5 deg.
The pic on the left side is a reproduction of one page from Ivan Law's book the PDF copy I have is a bit fuzzy so I redrew it hoping it would make a bit more sense, the spread sheet calculation for the small end OD uses COS - I used this to calculate the blank diameters for the bevel gears I made, sorry for any confusion - I've edited the text to clarify and attached the file.

Thanks retailer for the info and spreadsheet they will be helpful, I thought you may have used the method described but wondered if there was another method.

xpylonracer

If there is another method using spur gear cutters I'm not aware of it, if you haven't already then take the time to download some of the books on gear cutting from the Internet Archive site - some date back to the early 1900's and the same method is described back then for cutting for bevel gears on a mill, I settled on Ivan Law's as it seemed to me to be the quickest way to achieve the calculations - I was keen to get into it and just wanted to be spoon fed a set of formulas with out wading through pages of theory that I knew I would forget a few months later.

 

[Richard Carlstedt]

I think the Bevel gear calculations ( sheet) were published before Laws book,
in Model Engineer in Oct 1991 by Mr Lammas
The Calculations are relatively easy for bevel gears , but it does take 3 x longer compared to spur gears as each gullet requires 3 passes.
Rich