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I bought some cheap 3/4-inch pipe clamps, and they seem to be working alright, except for one thing. Even though the pipes fit pretty snugly, when I get something clamped up, it seems tighter on the area of the face nearer to the pipe than it is at the far edge. Currently I'm using construction shims to workaround the problem.

Is this something common with pipe clamps? Or just cheap ones?

EDIT: I found more problems with the cheapies, so I replaced them with Bessey clamps, which I am finding to be much easier to work with.

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    Unrelated to your Question, I was looking at a few clamp comparisons recently and out of curiosity how much did the clamp heads + the length of pipe cost per clamp?
    – Graphus
    Commented Sep 23, 2022 at 13:49
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    Are you clamping particularly hard? If so, is that to overcome other issues, like stock preparation? Commented Sep 23, 2022 at 16:16
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    @AloysiusDefenestrate, well to be fair he should be clamping particularly hard. As should the majority of woodworkers :-)
    – Graphus
    Commented Sep 23, 2022 at 17:35
  • @Graphus I bought the first clamps (amzn.com/B07TFD51V5) for $6.67 each, and the replacement clamps (amzn.com/B0012YNJRO) for $17.77 each. As for the pipe, I picked up two 3/4" 2 foot pipes for about $10 - 20 each at a big box store.
    – mattalxndr
    Commented Jan 5, 2023 at 11:02
  • Thanks for the details, appreciate it!
    – Graphus
    Commented Jan 5, 2023 at 14:17

3 Answers 3

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Is this something common with pipe clamps?

Yes. There are a couple reasons for this. The first is that the screw on a pipe clamp usually meets the jaw at one point, and the jaw is free to pivot a bit on any direction. If the clamping force isn't centered right above the screw, the jaw may not stay parallel to it's counterpart on the other end of the clamp.

The second reason is that if you tighten the clamp enough, the pipe may flex. This is a lot more noticeable if you use 1/2" pipe, but even 3/4" pipe will flex under enough pressure, and when that happens then the jaws go out of parallel even if they were parallel to begin with.

Basically, pipe clamps are not parallel jaw clamps. Parallel clamps are designed to keep the jaws parallel even under high load; they have bars that are much more rigid and a mechanism that forces the jaw to move in line with the bar and not pivot.

Lots of people have used pipe clamps to put together a lot of projects, though. Two things that you can do are:

  • Use more clamps so that you don't have to tighten each one as much.
  • Alternate the clamps, so that half are on one side of the panel and the rest are on the other. That helps to prevent the bowing that can happen if all the clamps are on the same side.
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  • In case you're interested, as I just mentioned to the OP I happened to be looking at some clamp comparisons recently. The various detailed reviews highlighted that the Bessey K-bodies aren't as consistent as to how parallel the jaws are at rest and under load as one might expect from their reputation. Surprising, but given the Jets cost way more and are also subject to the same issue not really a big issue.
    – Graphus
    Commented Sep 23, 2022 at 13:58
  • @Graphus I saw a YouTube review indicating that the cheap Harbor Freight brand clamps maintained as much or more parallel faces under load than the Besseys for about 1/2 the price. I don't think they're available on your side of the pond, though.
    – FreeMan
    Commented Sep 23, 2022 at 15:51
  • FWIW I linked to Bessey's KRE not as a recommendation, but just as an example of a parallel jaw clamp. IMO they're the most widely known, pretty much an icon of the category. Also, out of curiosity after reading Graphus' comment, I tried clamping a narrow piece of wood near the end of the jaws in a K-body clamp. After tightening as much as I could, I measured about 0.4 mm difference between a point near the bar and ends of the jaws. That would be more if I clamped something large, because of some deflection in the bar, but it's a lot better than I'd get with a pipe clamp, which is the point.
    – Caleb
    Commented Sep 23, 2022 at 16:02
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    +1 for mentioning the over/under technique, which helps all the time -- not just with clamp faces. Commented Sep 23, 2022 at 16:19
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Is this something common with pipe clamps? Or just cheap ones?

You can expect this, but not for the reason you might expect. Even if your clamp heads are not cheaper or knock-off types and have perfectly parallel faces when they're not in use you can get this to some degree.

Flex is typical with pipe clamps, even at fairly modest lengths (where the pipe is stiffer) and not cranked up "too high" (more on this below). So even clamping faces that are parallel to begin with will end up non-parallel under load; this is why some clamps have heads with built-in toe-in to attempt to compensate.

This change in geometry under load is important to realise given the most popular woodworking glue is some form of PVA. The old thinking that joints "just need to be pinched tight" is based on older glues, but PVAs require high to very high clamp pressure to achieve their strongest bond1. And no, you don't need to worry about over-tightening of these powerful clamps yielding starved joints, this is a persistent myth when it comes to PVA2.

Pads or lining or both
So, given you can expect non-parallelism when the clamps are tightened as much as they typically should be you need to arrange something to compensate. Regardless of whether you make inserts or pads from wood, ply or MDF or just plan to use the jaws as-is, line them with something that has some yield — rubber, cork, crubber (cork/rubber composite), dense felt or thick leather. Or sort of do both in one by making slip-on pads from Homasote.

This also helps prevent marring of the edges of workpieces which pipe clamps are strong enough to do even on harder hardwoods, and without being cranked up to 11!

Other worthwhile tweaks or upgrades to pipe clamps:

  • Coat the pipe with shellac to help prevent black stains.
  • Wax3 the pipe well for the same reason as above (also makes them easier to adjust)
  • Make feet or spacers or stands
  • Use cauls to spread clamping pressure, reducing the number of clamps needed on large glue-ups

1 The difference in strength between joints just squeezed and clamped super-hard is considerable.

2 In actual fact it's impossible to over-clamp in a home workshop, something that you can confirm for yourself with a couple of quick experiments if you're interested on small samples using C-clamps (likely the strongest clamps you own).

3 In case you're a fan of James Wright, use straight wax for this, not a wax/oil mixture that he continually refers to as paste wax (it isn't).

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    Using Titebond III, this guy prepared a bunch of samples clamped at various pressures, tested them, and found that high clamping pressure yielded a weaker joint. That's consistent with what Matthias Wandel found ~10 years ago. I don't have enough info to say that you're wrong about clamping pressure, but I think providing some references to back up the claim would be really helpful.
    – Caleb
    Commented Sep 23, 2022 at 15:34
  • Not knowing how long "modern" formulations of PVA glue have been around (or if there is a "modern" vs "older" formulation or simply that non-PVA is the "older" glue mentioned), I too would like some references. Norm Abram (of New Yankee Workshop and This Old House fame) always talked about not over tightening clamps and starving the joint of glue. Of course, "as seen on TV" is no greater claim to legitimacy than "as seen on YouTube", so some verifiable references one way or the other would be nice.
    – FreeMan
    Commented Sep 23, 2022 at 15:55
  • @Caleb, I don't much care [actually scratch that, I care inversely] what Mattias's test showed given he acknowledged at the time that he didn't do the epoxy correctly.... and AFAIK (correct if wrong) in all the years since has never revisited it to revise the results. In terms of the methodology I can't recall at this point if he failed at the start line by not prepping the wood right. And anyway, with no apology, I'll take Franklin's word over any random YT pundit! (And for the record, this has been known industrially for decades, as Gene Wengert has had the opportunity to repeat endlessly.)
    – Graphus
    Commented Sep 23, 2022 at 17:15
  • @Caleb, so have to ask, have you done any tests of your own? I have, and using as many C-clamps as I could squeeze in to the surface area of small samples — so that's what, >1,000lb per square inch? — I was unable to starve a joint. Regardless of individual home tests, Franklin's info was released a few years ago and the subject of articles on FW (and possibly later on PW) so easy to look up that way.
    – Graphus
    Commented Sep 23, 2022 at 17:22
  • @FreeMan, the main older glue I was referring to was hot hide glue or Scotch glue, which actually can be used with no clamp pressure at all..... so obviously just cinching up tight is perfectly adequate there. With hide glue you can do the classic no-clamp joint, the rubbed joint. The way that works is it basically results in the joint being pressed down by atmospheric pressure.
    – Graphus
    Commented Sep 23, 2022 at 17:25
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Good pipe clamps will have parallel faces and non-marring mounting pads that don't have the problem you outline, at least at appropriate working tension. But any pipe clamp may show some non-parallelism if over-tightened, particularly if you're using relatively long pipe lengths. Iron pipe simply isn't rigid enough to prevent arching of the stretcher, and subsequent non-parallelism in the clamping surfaces at high tensions. But there are very few woodworking situations where you should need that kind of tension for long clamping runs

You can probably fix your problem by filing the clamping faces flat (being cheap clamps, they'll be cast iron, and easy to file), and then gluing wedge shaped hardwood pads onto the faces with a CA glue (Using CA glue makes them easy to remove if you need to replace them - a little heat on the clamp, and they'll pop right off).

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  • Can you please explain more about the hardwood wedges? Which way should the wedge face and why should they be there (what do they bring to the table)? I'm here to learn!
    – FreeMan
    Commented Sep 23, 2022 at 15:32
  • @Freeman - I merely meant that you could provide a non-marring clamping surface by glueing a hardwood pad to the cast iron surface, after you'd filed it flat. I assumed that filing the faces of the clamping surfaces flat would not be sufficient to make them perpendicular to the pipe and parallel to each other, so the pads might have to be somewhat wedge shaped. Commented Sep 24, 2022 at 17:59
  • Ah, gotcha! Thanks.
    – FreeMan
    Commented Sep 26, 2022 at 12:04

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