Basic Tips on Profile Grinding
Basic tips on knife grinding for a beginner playing catch-up. November 26, 2008
Question
My boss bought a new Centek. Now there are two people that run it. Neither I nor my partner have ever ground before, but we've had a 2 hour lesson, and we have a few questions. We can grind knives pretty good. So far the major problem is when we tilt the head for a side cut, it cuts on both sides of the profile, ruining the opposite side. Now the head is at 25 degrees. We tilt 5 degrees and there is about an 1/8 inch to the tool rest. Am I not supposed to just control what I side cut, or should I be able to tilt left and right and grind the full profile without damage to opposing edges?
Forum Responses
(Solid Wood Machining Forum)
From Dr. David Rankin, forum technical advisor:
Profile grinding is not difficult as long as some basics are understood:
1) Use the correct grinding wheel for the steel that you are grinding.
2) Use a proper coolant that will keep the tool cool and not be dangerous to the operator.
3) Use a proper motion. I recommend grinding in a U shape as opposed to plunge grinding.
4) Use a dressed grinding wheel that is the same size and shape as the tracing pin.
5) Use a proper dressing stick to keep the grinding wheel clean.
6) Use the proper back clearance for the knife material and wood being run.
7) Side clearances need to be ground so that the clearance comes to the cutting edge. If the side clearance misses the cutting edge, the tool will either overheat or the profile will be changed. Either indicates a miss-set tool rest in height or a misaligned machine.
8) Assure that the profile on the template is made parallel to the base of the template.
In order to get complete understanding of the machine it is recommended to attend a class such as taught at Fox Valley Tech in Oshkosh, WI or to have a tech provide training in-house for you.
From contributor J:
It sounds like number 4 on Dave's list is your problem. You should have different thickness tracing pins for your grinder. Make sure the tracing pin is the same thickness as your grinding wheel. My guess right now would be your wheel is currently thicker than the pin you are using.
From the original questioner:
The wheel is dressed to 4mm round, same as pin. As we understood, after profile is finish ground, I should be able to just tilt the head and do relief cuts without any other changes... But when we do, it grinds the opposing side cutting edge.
What is the difference between coolant and lubricant? We used 20/1 of a liquid with Chinese written all over it, so I couldn't tell you what it was, but it was very thin and a little oily.
From contributor J:
Are you sure that the knife has been completely ground before trying the side clearance? I guess what I'm asking is, are you reshaping and resetting the wheel distance to the tool rest before grinding the side clearance angle? This would cause your problem. How much does it grind into the opposite side? Just a rub or a deep gouge?
From contributor R:
It sounds like you are trying to grind up the side of the profile opposite your relief cut. When applying side relief, on a bullnose for example, the wheel is tilted to the left when applying the relief to the left side. When you drop into the bottom of the profile you must ease out of the cut. Continuing on past this point will undercut the side of the knife as it rises in the opposite direction. Do yourself a favor, take Dave’s advice and get some training. You guys have more issues than side relief. Hope your boss saved a lot of money on his new Chinese machine, as you can already see it probably will not be such a bargain in the long run.
From the original questioner:
Now, you said I need to ease out of the cut and not continue up the opposite edge. I was told I could run the whole profile without damage to the other side. Well, if I stop, then I'm fine, but if I do go past on a tight arch, it will dig in. And I grind and dress till it's finished. You say I should redress for the side relief...
From contributor C:
My take on it is that it's a tool rest height issue. The side clearance grind should just reach the edge of the primary grind, no more, no less. This can be determined by the height of the tool rest. There should be shims at the bottom of the tool rest post which make the height adjustable. Is the 1/8" you mention the distance from the grindwheel to the tool rest? If it is, this is too much. 0.5mm to 1mm is better. Get yourself a "U" shaped or bullnose template, grind a knife (I keep one for just this purpose), tilt the wheel for side clearance, grind halfway down, back the knife out carefully and take a look at the side grind in relation to the edge - that will tell you a lot. When it's set up correctly, the left and right clearance grinds should meet perfectly at the bottom of the "U" or have a very small flat between them. I've never had to ease out of a cut to avoid the opposite side of a clearance angle.
One other answer could be that you are removing too much material at the finish grind and squaring the wheel a little, so by the time you are tilting it over, you are using a square wheel with a round pin. Re-dress the wheel to round and barely touch all the knives again before tilting.
I'm assuming here that the basic components of these grinders are the same as Weinig, as I'm not familiar with the brand.
From the original questioner:
Thanks for the useful tips. I'll play around on Monday. We were also thinking of the tool rest height. When I finish grind I usually click the temp back 5 lines, which I'm assuming is 5000's. I'll look into that. Also I'm going to put in a knife that was ground by an outside company to compare angle and tool rest distance.
From Dr. Dave Rankin, forum technical advisor:
I think that your problem is the height of the tool rest. It sounds like it is slightly low and needs to be shimmed up. This is a common problem on many of the grinders being made today.
From contributor R:
As others have pointed out, the height of the tool rest is critical to avoid over or under cutting both edges when applying side relief. However, I think his problem is more fundamental than that.
Read carefully what the questioner is saying:
“We can grind knives pretty good. So far the major problem is when we tilt the head for a side cut it cuts on both sides of the profile ruining the opposite side... I was told I could run the whole profile without damage to the other side.”
In figure 1 the wheel will be tilted to the left to cut the relief. In this case the entire length of the profile can be traversed without damage to the edge. In figure 2 the wheel is again tilted to the left. Grinding from left to right, if the wheel is allowed to travel much beyond point A, it will damage the opposing edge as it rises relative to the base line.
From contributor U:
Please beg your owner for some proper training. The money he saved on the machine should be enough to pay for some good training. You could be making matters worse by adjusting things you don't know about. Your tooling and your finished product will suffer as well as production time if you don't do things right. Get some formal training real soon.
From the original questioner:
Okay, I changed a few things. I did not adjust rest height, but I did bring it into 1mm and changed angle of grind wheel to 30 degrees, which matches a cutter done by another company.
I appreciate the help, and I might get sent for training, but until then I'm on my own.
From Dr. Dave Rankin, forum technical advisor:
The tool rest may be too low and needs to be shimmed or raised. My guess is that the tool rest needs to be raised about .020" or .5mm. The side clearance should come to the exact cutting edge of the tool. If you go too far, the profile will be changed, usually wider than you want.
From the original questioner:
Thanks. I raised the rest almost an 1/8 and now it's damn near perfect, not to mention I checked the head tilt where it locks at 90 and it was almost 3 degrees off.
I'm a little confused on cutter angle. I have the machine set to 30 degrees, which matches another cutter made by an outside shop. And I usually hog at 1800 fpm and finish at 2200. Does this sound right, or does it vary per machine and wheel?
From Dr. Dave Rankin, forum technical advisor:
For softwoods, the 30 degree angle should be fine. For hardwoods I would go down to 25 degrees. The rpm is fine for many grinding wheels. Ceramic will be faster - the white vitrified wheels run at the speed you are using.
