Basic Grinding Technique

[MikeD]

Hi all, I have a couple of Grinding 101 questions in regards to the proper technique.

Application: prepping the upper and inner skins for recoring.
Tool : Porter Cable 5" RO Sander w/ 60 grit disk.

1. RPM of sander: Generally, do you grind with lower RPM setting (i.e. 1 or 2 on our Porter Cables) or higher? (So you can really fling the dust...)

2. Applied pressure: Is it better to gradually grind the surfaces down with minimal pressure, or do you just push down until the RPMs of the sander really slows down?

3. Angle of sander: I've been using a slight angle to get at specific areas, but should I keep the sanding pad flat and just grind away until the area gets ground, which would take longer but create a flatter surface?

THanks!

[CharlieJ]

Personal practice here-

# 1- no- one of my PC 5 inch ROS machines burned out it's variable control- I took it out- never used it very much anyway. Grinding glass I want the top speed anyway.

#2 - depends on where you are. Usually I'll let the machine do the job without forcing it. Sometimes ya just GOTTA push hard in some places, but usually not.

#3- it will really extend the life of your pad if you keep it flat. I try to use the ROS as flat as possible- again, sometimes ya gotta do what ya gotta do.

Having said that, since I found the flap sanders to go on a 4 inch angle grinder I seldom use my ROS in that sort of situation. The flap sander is SO much faster, smaller to get into tight places and the replacement is only 5 bucks. I used ONE flap wheel to grind out all the old glass, etc and grind edges of new tape, tabbing, etc on Tehani. I'm totally sold on them. I use 60, 80 and 120 grits.


They look like this and Walmart carries them even, made by Black and Decker. Lowes has a larger selection of grits though.



DON'T bump them into bare skin!! Don't ask how I know that *grin*

[Figment]

Charlie's right.
In this application you're only trying to give the surface some tooth for good adhesion. If you happen to even things out a bit as a result that's not a bad thing, but don't spend a lot of time at it. You needn't worry about making things smooth or fair.
Get in there, get nasty, get it done and get out. No points for politeness or prettyness.

In a more general sense, my experience is that the answer to your question no. 2 depends on the abrasive you're using and the surface you're attacking. Some are best just kinda floating along, others need some oomph. If you're not using a vacuum attachment it's easy to judge by the volume of dust created.

[Tim]

#1: Use full speed. I have never moved the speed control on my newer DA from "6" (full speed), and can't really see a reason to. I have several other sanders if I need a different result. My old DA had no speed control, and I never wanted or needed it. I honestly don't know why they bother.

#2: You shouldn't be pressing so hard that you're bogging down the sander. Experience will eventually show you what is the right technique, but it's somewhere between "letting the sander do all the work" and "pressing down hard". And don't be afraid of coarser grits on rough fiberglass grinding. 60 may seem coarse, but it's pretty tame for serious fiberglass grinding. I use the McMaster 40 grit discs for this.

When you get to more finishing work, though, it's best to begin with the lightest grit you think might do the job and go coarser only if the finer one isn't working. This eliminates coarse scratches that you can't get out, or excess and unnecessary sanding.

#3: With a DA sander, you will remove more material faster with a slight tip on the pad. For finer work, you can keep the pad flat on the surface and acheive a better finish, but the beauty of a DA is that it acts much like a grinder with the pad slightly tipped.

However, you still must use care with this technique if you're grinding a surface that will eventually be finished one way or another. Even with the soft pad, you can create dishes and unevenness in the surface if you employ the wrong technique with the tool.

Right angle grinders can remove a lot of material fast, but can also create a major disaster in short order if you're not used to them. I rarely use an angle grinder because of personal preference--I find the DA to be effective for most things when equipped with the appropriate paper, and my background involves much more use with the DA; therefore, I am most comfortable with it. But the flap wheels Charlie mentions are a good innovation and are extremely effective.

I do occasionally use a 4" or a 7" right angle grinder, but I get my best results with coarse paper on the DA. Sometimes, it won't do the job, but rarely.

There are many grinding butcher jobs out there, borne from inexperienced and uncaring hands. Beware of going too far, and work with your tool of choice until you feel comfortable with what it will--and won't do. Better to use a less aggressive grit (and/or tool) and get good results but with more work than to go grind-happy with the wrong tool for you and create a disaster!

You needn't worry about making things smooth or fair.
Get in there, get nasty, get it done and get out. No points for politeness or prettyness.

Sort of. I feel a need to expand on this, though.

This doesn't mean that you shouldn't take some care to do a good job, or keep whatever surface it is as smooth and fair as possible. Errors, sloppiness and unfairness all compound upon themselves and can create larger problems later in some situations.

That said, rough sanding is rough sanding, and certainly it doesn't have to be finish quality. But the overall quality of your finished product naturally builds upon each step you do along the way, so use the appropriate care at each step. Time and experience will soon begin to show what's right for you.

[cliffg]

What is DA? We need a glossary of acronyms!!! But only for me.

[dasein668]

Dual Action.

[CharlieJ]

which designation is normally used for air tools. The same basic thing in an electric is an ROS, or Random Orbit Sander.


Pretty much the same action on either.

[Tim]

What is DA? We need a glossary of acronyms!!! But only for me.

Sanders like the much-touted (on this forum) right angle Porter Cable 7335 have eccentric action in the pad mechanism that not only spins the disc in a circular motion (like a grinder), but also oscillates the pad around a basically oval pattern. This is the so-called "Dual Action". This motion helps prevent swirl marks or circular sanding marks, and alsp helps prevent the disc from digging in the way a true grinder would.

I typically call these right angle sanders "DAs" rather than "random orbit" because I feel it better differentiates them from the the other kind of random orbit disc sanders that have become common now--the small, palm-sized disc sanders that use a motion very similar to a DA, but much less aggressively. These "random orbit" sanders are good for fine finish work, but useless for stock removal of the sort frequently needed in boat repair.

"DA" is a term that really applies best to the air-powered version of the genre, which was in existence long before Porter Cable came up with the first electric version in the early 90s.

[MikeD]

Thanks guys!