Plunge line question

izafireman

izafireman

Well-Known Member
Every now and again when grinding I get the two plunge lines grind differently and I| cannot figure out why.
I like the plunge line to be sweeping rather then square so I dip the tip of the knife down when grinding, today was using a jig from a bit of angle iron.

All was going well 36, 60 120 ….grits , perfect symmetry on both sides then all started to go pear shaped, one side a lovely sweeping plunge but the other started to lose the sweeping affect and became very 'tight' and angular if that makes sense. I had the belt over hanging but just could rescue the plunge..... I should point out the plunge that went wrong was on left to right , my weaker side as right handed.

I realise its all about practice but just don't understand where it happens every so often and other times they are perfect.

Thanks in advance,
 
If all else was as even and symmetrical as possible, then its likely because of uneven pressure and/or torque applied in that area.

It can be especially exacerbated with finer grit belts because they react more dramatically when 'leaned into' in that plunge area.

Given your description, this is the likely culprit for me.
 
Motor City Mike said:
I'm going to throw a post on here for future reference because I'd like to know why this happens too
Click to expand...

So its not just me and an issue that occurs at times?...…….This occurred when I was flat grinding and the knife in a jig but when I bought my wheel for hollow grinding and did free hand grinding for the first time I was amazed as I did better plunge lines and if a plunge started to go wrong everything 'clicked' in my head and it was simple to resolve the issue.
 
J. Doyle said:
If all else was as even and symmetrical as possible, then its likely because of uneven pressure and/or torque applied in that area.

It can be especially exacerbated with finer grit belts because they react more dramatically when 'leaned into' in that plunge area.

Given your description, this is the likely culprit for me.
Click to expand...


That makes sense as t was as I progressed up the grits. I also never realised the finer belts react more in the plunge area. Would this be because they are more pliable and how can I resolve the issue?

Thanks
 
izafireman said:
That makes sense as t was as I progressed up the grits. I also never realised the finer belts react more in the plunge area. Would this be because they are more pliable and how can I resolve the issue?

Thanks
Click to expand...

Yes. The finer belts bend/fold/deform more and easier than heavier belts. Light touch in that area is the key and try to keep even pressure. Not easy since almost all of us are heavy handed on one side.
 
J. Doyle said:
Yes. The finer belts bend/fold/deform more and easier than heavier belts. Light touch in that area is the key and try to keep even pressure. Not easy since almost all of us are heavy handed on one side.
Click to expand...

I think the problem today was I was over thinking things maybe as my left side is weaker and maybe consciously trying to overcome that. So basically due to the extra pressure exerted a thinner belt will crease and so create a sharper angle ?
 
cnccutter said:
You didn’t say where your tracking your belt. Letting it hang over will soften the plung line. If your not tracking the same amount on both sides it can give you fits.
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^ this tends to be the root cause in my case.

You want the belt hanging off the side of the platen just a bit for a round transition. The hard part is tracking it off the edge the same amount on both sides. Also, your platen may have a slightly different radius on one side and this will cause the belt to act differently on one side than the other.

Angle is everything. In/Out/Up/down (the relationship of rhe blade to the belt) will definitely affect the angle of the plunge.
 
If your platen isn't square with the wheels, it results in higher belt tension on one side of the platen than the other. J flex belts when tracked over the side with less tension don't wrap around the edge of the platen and give you that predictable radius.

On the tighter side the belt wraps around the edge and the edge of the platen is what's creating the sharp geometry rather than the unsupported belt.
 
kuraki said:
If your platen isn't square with the wheels, it results in higher belt tension on one side of the platen than the other. J flex belts when tracked over the side with less tension don't wrap around the edge of the platen and give you that predictable radius.

On the tighter side the belt wraps around the edge and the edge of the platen is what's creating the sharp geometry rather than the unsupported belt.
Click to expand...
Good info...never checked this before.
 
cnccutter said:
You didn’t say where your tracking your belt. Letting it hang over will soften the plung line. If your not tracking the same amount on both sides it can give you fits.
Click to expand...

Hung over the sides of a glass platen ….though sometimes ive noticed belt wander.

Normally hungover 4 mm on both side
John Wilson said:
^ this tends to be the root cause in my case.

You want the belt hanging off the side of the platen just a bit for a round transition. The hard part is tracking it off the edge the same amount on both sides. Also, your platen may have a slightly different radius on one side and this will cause the belt to act differently on one side than the other.

Angle is everything. In/Out/Up/down (the relationship of rhe blade to the belt) will definitely affect the angle of the plunge.
Click to expand...
John Wilson said:
^ this tends to be the root cause in my case.

You want the belt hanging off the side of the platen just a bit for a round transition. The hard part is tracking it off the edge the same amount on both sides. Also, your platen may have a slightly different radius on one side and this will cause the belt to act differently on one side than the other.

Angle is everything. In/Out/Up/down (the relationship of rhe blade to the belt) will definitely affect the angle of the plunge.
Click to expand...


Ok I have the belt hung over the side of the platen say 4mm and try to replicate on both sides.

The platen itself it a ceramic platen.

Which parts do I need to double check as being square again as it is all still set up exactly how is was...….The other issue that might have occurred is the annoying issue of a belt sliding on the odd occasion from left to right or visa as the steel makes contact, never sussed out why that should happen on odd occasions only.

Belts are Cucbitron 2 to start with and then Gator Trizact belts if that helps.
 
Think you guys might have sussed my problem as just checked everything and realised I the table I am using is a new one that I made and I hadn't squared it to the glass platen.

On checking it was nowhere near square when checked with the square, so all sort now, just need to try it tomorrow.

Thanks to all and fingers crossed it will be ok now .

P
 
Best of luck. Just remember that nothing will ever be set up perfectly. Knowing your machine and using the appropriate finesse based on that is how you will get the results you’re looking for. I say finesse because as soon as you introduce your hands to the process it becomes a variable process.
 
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