Rack tooth tip relief 2

[steveren]

Does anyone know if there is a standard for the amount of tip relief required on a metric 2.5 module rack we are using this with a 20 tooth pinion.

 

[dimjim]

Do you mean tip radius instead of tip relief?
Tip relief is not standard. You would also
have to specify the pressure angle.

 

[steveren]

Thanks dimjim for your reply. I guess you could refer to it as tip radius, I'm not really sure whats its called.
We are using a 20deg pressure angle.
To give you a little more detail, we have in the past sub contracted some rack cutting and they have come in with a small chamfer on each rack tooth. We are now cutting these ourselves and we do not apply a chamfer and are getting running problems and noise with the mating pinion.
I have been trying to find a standard figure that I can apply to our process to avoid this problem. I was hoping someone could point me in the right direction.

 

[israelkk]

steveren

When you say "now cutting these ourselves" how do you cut them? Do you use gear dedicated cutting machines or general milling machine?
Can you give more information on the pinion and rack module/diametral pitch, number of teeth, accuracy and spec AGMA/ISO/DIN etc.

 

[steveren]

Hi israelkk

We are using a dedicated rack cutting machine, a Sykes vertical hobbing m/c. The rack and pinion are 2.5 module, pinion has 20 teeth. Pinion is hardened and ground to ISO 1328 grade 6, rack is to ISO 1328 grade 10. Pressure angle 20deg

 

[dimjim]

A standard rack should work with the pinion
if it has been generated with a rack type cutter
or hobbed.

If the pinion were shaped, then a standard
rack may not work. I guess you would have
to let us know how the pinions were generated
and its SAP Dia. and/or TIF Dia. and whether it has
any undercut. ie. the fillet information and
dedendum of the pinion. Often a protuberance
hob is used to cut the teeth, then hardened, and
then ground on both flanks after the hardening operation.
I am assuming your rack pinion has a 2.5mm addendum.

 

[steveren]

Thanks for your help.

We have been using this combination of rack and pinion for many years and had no real problems, its just now we have found that sub contracted racks have a chamfer along each tooth and we are not adding one. Clearly the chamfer or a radius on the rack tooth tip makes a differance, I just can't find a known standard to work to.

The rack is cut 0.1mm deeper than standard i.e. 2.6mm then we grind 0.2mm from the top surface of the rack, leaving a 2.4mm addendum. (Commercial reasons for grinding the surface).

 

[gearcutter]

As far as I understand the subject; there are three tooth tip modifications: Tip chamfering, tip rounding and tip relief. It sounds like you are talking about tip chamfering which is usually used for either removing the burred/sharp edge or for providing clearance on major diameter fit splines. Tip relief usually is combined with root relief and booth are calculated from the amount of tooth deflection under full load. There is no standard amount of relief as it needs to be calculated and kept to a minimum to maintain the contact ratio.
I doubt you need to have tip relief on a rack and pinion unless the set is being heavily loaded. It sounds to me like you might be having issues with how the rack is being generated. If I remember correctly, the Sykes uses a disc shaper for the cutting tool. I’ve used a Fellows gear shaper that used the same process. I remember that the slightest amount of run-out of the cutter caused an indexing error with a varying tooth to tooth space being generated. When old, these types of machines are notorious for generating large index & profile errors.
An easy test to see if there are profile errors is to “hard mesh” roll the pinion over the rack by hand and see if there is any interference. The pinion should roll freely.
I would also conduct a tooth profile analysis based on the cutters used for both the pinion and rack. If you can post the data I can do this for you with software.